TWI856562B - Elastography device and method - Google Patents

Abstract

An elastography device comprising: a probe that comprises a protruding part to be applied against the body of a subject, a low frequency vibrator arranged to move the protruding part, at least one ultrasound emitter and one ultrasound receiver; and an electronic unit. The electronic unit is adapted to alternatively control the elastography device so that it operates a) in a guidance mode (S1) to determine whether the probe is correctly positioned in front of a region of the body to be probed to carry out a measurement of a mechanical property of the probed region and b) in a measurement mode (S2). In the guidance mode, the vibrator delivers a plurality of successive probing pulses (PRB), each being a transient, low frequency mechanical pulse, and the electronic unit determines a propagation quality indicator (Q) representative of an aptitude of the probed region to transmit the probing pulse.

Description

Flexible imaging device and method

The disclosed technology relates to an elastic imaging device and method. More specifically, it relates to an elastic imaging device, which includes a probe, which includes: a low-frequency vibrator for transmitting a low-frequency mechanical pulse to a subject's body; and at least one ultrasound transmitter and an ultrasound receiver, which are configured to transmit ultrasound pulses and receive corresponding echoes to track how the low-frequency mechanical pulses travel in the subject's body, so as to characterize the tissue stiffness in the examined subject's body region.

For example, liver stiffness measured by Vibration-Controlled Transient Elastography (VCTE) has been shown to be a very useful tool in helping healthcare professionals detect or characterize liver disease or damage, and more generally in monitoring a subject's liver condition.

One well-known transient elastic imaging system is the FIBROSCAN® system (an ultrasound-based elastic imaging device for measuring the stiffness (or elasticity) and ultrasonic attenuation of tissues and organs) produced and sold by Echosens SA of Paris, France, which enables operators to non-invasively measure the stiffness of the liver or other organs to assess the health of the organ.

Using the FIBROSCAN® system, the operator places the tip of a probe with a relatively small diameter (usually between 5 and 10 mm) in contact with the subject's body, in front of the expected area of the subject's liver. The operator then presses a button to cause the head of the probe to deliver a transient low-frequency mechanical pulse to the subject (the spectrum of the pulse is centered around frequencies typically between 10 and 500 Hz). The pulse generates elastic waves that travel within the subject's body. The ultrasound transducer mounted on the tip of the probe is in contact with the subject's body and then emits a large number of ultrasound beams at a high repetition rate (e.g., 6 kHz) to the tissue. Echo signals corresponding to the backscatter of the different ultrasound beams emitted are obtained by the probe to track the slight movements of the tissue caused by the elastic waves passing through it. Tracking is performed using cross-correlation techniques applied to continuous echo signals. The detected movements allow one to synthesize elastic wave propagation images that show the deformation of the tissue as a function of both depth z and time t. Such maps are sometimes called "elastic maps" (Figure 1).

The mechanical pulse delivered by the tip of the FIBROSCAN® probe generates both shear waves and compression waves. In other words, the elastic wave described above combines shear waves and compression waves. However, these two waves have very different propagation speeds, and thanks to the transient nature of the mechanical excitation, they can be easily separated in time and identified in an elastic wave propagation image. For example, refer to Figure 1, which shows an elastic wave propagation image 105. In Figure 1, the compression wave is marked with reference numeral 105C, while the much slower shear wave is marked with reference numeral 105S. Figure 1 also shows a region of interest (ROI) whose boundaries are defined by two dashed lines at 25mm and 65mm, which correspond to the depth below the patient's skin where the liver is typically located. This elastic wave propagation image can therefore be used to accurately determine the propagation velocity of shear waves in the tissue to be characterized, and from this the stiffness of the tissue can be derived. These stiffness results are then provided to the operator.

Although FIBROSCAN® technology works well, it is sometimes difficult for the operator to know if he/she has correctly positioned the probe in front of an area of homogeneous liver tissue, or even if he/she has the probe aimed at the liver. Artifacts of ribs, blood vessels, pockets of fluid (ascites) or other heterogeneous tissue in front of the liver (such as cysts or tumors in liver tissue) can produce erroneous measurements of tissue stiffness and ultrasound attenuation. In addition, the operator may think he/she has the probe aimed at the liver, when in fact the probe is too close to the lungs or other internal organs. As a result, the system may not be able to obtain accurate measurements.

Document US2021022709 is a patent application assigned by the applicant, describing a method for an operator of such a transient elastic imaging device to find appropriate probe positioning. According to this method, the vibrator of the probe first provides continuous and periodic mechanical vibrations (such as sinusoidal vibrations) to detect the area of the subject's body at which the probe is pointed. Ultrasound pulses are emitted, and corresponding echo signals are obtained to track how this sinusoidal vibration travels through the detection area. A "harmonic elastic diagram" is then calculated and displayed, which represents the periodic deformation of the tissue caused by the vibration as a function of time t and depth z. It shows how the sinusoidal vibration travels in the area of interest. A harmonic elastic map in which diagonal stripes can be distinguished fairly clearly (such as the one in FIG. 16 of US2021022709) indicates that the medium in front of the probe is uniform (relative to the propagation of elastic waves), and therefore the probe is likely well positioned. In contrast, a harmonic elastic map showing disordered propagation in which diagonal stripes cannot be distinguished (such as the one in FIG. 17 of US2021022709) indicates that the probe is incorrectly positioned. Such harmonic vibrations are applied to the subject's body and the corresponding tissue strains are visualized (using ultrasound echo cross-correlation), thereby illustrating to the operator that the probe has been found to have the proper position and orientation. Once such a position is found, the operator can trigger the transient vibration-controlled transient elastic imaging measurement described above.

The periodic vibration guidance method described in US2021022709 substantially improves the operability of VCTE systems (such as FIBROSCAN®) (compared with conventional guidance based on A-mode and M-mode ultrasound imaging) and provides satisfactory guidance. In particular, due to the continuity of the vibration, the subject under examination has no unpleasant feeling, and this guidance can be continuous.

Nevertheless, the inventors have observed that in certain cases, although the guidance information obtained by harmonic vibration detection in this way leads to the conclusion that the probe is not properly positioned, high-quality transient vibroelastic imaging measurements can actually be made.

In this context, the disclosed technology relates to a flexible imaging device, comprising: a probe, which includes: a protrusion applied to the body of a subject, a low-frequency vibrator arranged to move the protrusion of the probe, at least one ultrasound transmitter, and an ultrasound receiver; and, an electronic unit, which includes an electronic circuit suitable for alternately controlling the flexible imaging device so that the flexible imaging device can a) operate in a guidance mode to determine whether the probe is correctly positioned on the body to be probed The electronic circuit is adapted to operate in a measurement mode to perform the following procedures: during the guidance mode: the electronic unit controls the low-frequency vibrator (5) to continuously and repeatedly transmit a plurality of detection pulses to the subject's body, each detection pulse being a transient, low-frequency mechanical pulse; for each detection pulse: the electronic unit controls the ultrasound transmitter to emit a series of ultrasound pulses, and obtains the ultrasound receiver's response to the received ultrasound pulses. The electronic unit determines a propagation quality index based on at least some of the echo signals, which represents the ability of the probe area to transmit the probe pulse and also represents the uniformity of the probe area relative to the propagation of the probe pulse; the electronic unit controls the elastic imaging device to transmit guidance information, which is based on the propagation quality index; during the measurement mode: the electronic unit controls The low-frequency vibrator transmits a measurement pulse to the detection area, the measurement pulse is a transient low-frequency mechanical pulse having an amplitude higher than the amplitude of each pulse in the multiple detection pulses; the electronic unit controls the ultrasonic transmitter to emit a series of ultrasonic pulses, and obtains the echo signal received by the ultrasonic receiver in response to track how the measurement pulse propagates through the detection area; and the electronic unit determines the above-mentioned mechanical properties from at least some of the echo signals, which are related to the propagation of low-frequency elastic waves.

By using the device, when it is operated in the guidance mode, the area located in front of the probe is probed by means of a transient mechanical pulse of reduced amplitude, repeated several times (instead of a harmonic vibration). When in the guidance mode, the subject's body is thus probed under conditions close to those in which the final transient measurement itself is made. The guidance information obtained thanks to the device therefore accurately predicts the measurement quality that can be obtained at the probe position under consideration and therefore constitutes very useful guidance information.

In particular, this guidance based on transient pulses is more similar to the measurement itself than guidance based on harmonic vibrations, and thus enables better guidance.

Guidance based on harmonic vibrations has some advantages (in particular, it may be easier to implement than guidance based on transient pulses), but the resulting guided "harmonic elastogram" may be distorted by elastic wave reflections within the tissue, which may produce resident wave patterns, as shown in Figure 2.

In some cases, this sensitivity to reflections is useful (e.g., for detecting the presence of a blood vessel in the probe shaft). In other cases, however, this high sensitivity to reflections (enhanced by the continuous, repetitive nature of the oscillations without downtime) results in a degraded “harmonic elastogram,” whereas the probe positioning actually allows for high-quality transient measurements. This occurs, for example, when the probe shaft is close to the edge of the subject’s liver (see Figures 4 and 5). In this case, the proximity to the liver border produces a stationary pattern that degrades the “harmonic elastogram,” but allows for a good transient elastogram.

This is illustrated by Figures 2 and 3. Figure 2 shows the harmonic elastogram obtained by applying 25 Hz harmonic vibration to a phantom (a synthetic elastic medium with mechanical properties close to that of the liver), with the probe tip close to the side wall of the phantom. Figure 3 shows the elastogram obtained by applying transient vibration (comprising a single cycle of a 25 Hz sine curve) to the same phantom, with the probe positioned the same as in Figure 2. The repetition rate of the ultrasound pulses used for both acquisitions was 2 kHz. Comparing Figures 2 and 3, it is clearly shown that transient pulse guidance can achieve more sensitive detection of suitable positions than guidance based on harmonic vibrations.

The probe pulses delivered when the device is operating in the guided mode can also be used to perform a prediction of the mechanical property of interest, typically tissue stiffness (e.g., for each probe pulse). This initial estimate of tissue stiffness can guide the operator in positioning the probe (e.g., a preliminary stiffness value within a given interval confirms that the probe is probably located anterior to the liver). Tissue stiffness predictions performed using transient mechanical pulses are generally more accurate than those performed during periodic mechanical vibrations without downtime (even if the "harmonic elastogram" is of high quality). In fact, for periodic mechanical vibrations, there is a mixing phenomenon between shear waves and compression waves in the medium to be characterized (due to the repetitiveness and instability of the vibrations), and the elastic waves observed in the harmonic elastic diagram are a mixture of shear waves and compression waves, which propagate at very different speeds, resulting in a biased estimate of tissue stiffness. Therefore, guidance based on transient pulses can provide a more accurate estimate of tissue stiffness than guidance based on harmonic vibrations.

By using the disclosed apparatus, the value of the propagation quality indicator for a given probe pulse corresponds exactly to that probe pulse from a time perspective (without mixing between consecutive cycles of the periodic oscillations), thanks to the transient nature of each probe pulse. The propagation quality indicator represents the ability of an area to transmit the probe pulse and its uniformity relative to that transmission. Thus, by using this transient pulse-guided approach, the temporal monitoring of the propagation quality (and possibly the a priori estimation of the tissue stiffness) is more accurate than using harmonic continuous oscillations.

Additionally, because the probe pulses are transient with downtime between them, the ultrasound pulses fired to track how the probe pulses propagate can be fired for only a fraction of the total duration of the guided mode, which reduces the total amount of ultrasound fired and therefore the overall average acoustic power delivered to the patient.

Additionally, because the detection pulses are transient with downtime between them, the low-frequency oscillator requires less power and the driver amplifier generates less heat, thus requiring less heat sinking.

A transient pulse refers to a temporary mechanical vibration. The duration of the pulse, during which there is substantial movement of the protrusion (caused by the vibrator), is followed by a rest time, during which there is no or substantially no movement of the protrusion. Substantially no movement means, for example, that during the rest time, the displacement of the protrusion, which may be caused by the vibrator, remains less than 1/10 or even 1/20 of the peak displacement of the protrusion. For the above-mentioned transient pulses (detection pulses or measurement pulses), the actuation ratio (which is equal to the active time of the pulse divided by the sum of the active time and the subsequent rest time) is typically less than 50%, or even less than 20%. The downtime in question (if any) is the duration between the end of the active time and the subsequent significant movement of the protrusion (e.g. corresponding to a subsequent transient detection pulse).

A low-frequency pulse refers to a pulse whose center frequency is lower than 500Hz, or even lower than 250Hz. The center frequency of a pulse is, for example, the average frequency or median frequency of the spectrum corresponding to the displacement or displacement velocity of the pulse, or the peak frequency of the main peak of the spectrum, or the average value of the -3dB or -6dB cutoff frequency of the spectrum.

In an elastic imaging device according to the transient technique, the electronic unit may be configured to control the vibrator so that in the guidance mode, for at least some of the plurality of detection pulses, the center frequency of the detection pulses is lower than the center frequency of the measurement pulses delivered in the measurement mode. For example, the center frequency of these detection pulses may be at least 5% lower, even 10% or 20% lower (or possibly at least 50% lower) than the center frequency of the measurement pulses.

The device may be particularly configured to characterize the liver, the electronic unit being configured to control the vibrator, for example, so that in the guidance mode, for at least some of the plurality of detection pulses, the center frequency of the detection pulses is between 20 Hz and 45 Hz, and in the measurement mode, the center frequency of the measurement pulses is between 50 Hz and 200 Hz. As an example, the center frequency of each detection pulse may be 40 or 45 Hz, and the center frequency of the measurement pulse is 50 Hz.

The device may also be configured to characterize the spleen, the electronic unit being configured to control the vibrator so that in the guidance mode, for at least some of the plurality of detection pulses, the center frequency of the detection pulses is between 20 Hz and 90 Hz, and in the measurement mode, the center frequency of the measurement pulses is between 100 Hz and 200 Hz. As an example, the center frequency of each detection pulse may be 80 Hz, and the center frequency of the measurement pulse may be 100 Hz.

Reducing the central frequency of the probe pulses enables these pulses to propagate deeper in the medium being probed. In fact, the propagation depth increases with decreasing frequency, in viscoelastic media such as the liver or similar media. Increasing the propagation depth of the probe pulses is beneficial because it compensates for their low amplitude, which is intentionally limited so that these repetitive probe pulses are not uncomfortable for the subject being examined.

In addition, reducing the center frequency of the probe pulse helps alleviate the discomfort of the probe pulse. In fact, the pulse intensity experienced by the subject depends on the pulse amplitude and the pulse center frequency (because the displacement speed and even the acceleration experienced by the subject depend on the pulse amplitude and its frequency).

It can be noted that, on the contrary, very low frequencies are undesirable when measuring pulses. In fact, when the frequency decreases, the wavelength increases and the diffraction effects also increase. The presence of diffraction effects increases the value of the apparent shear wave velocity, which in turn leads to an overestimation of the hardness, which is not conducive to diagnostic purposes, as explained in the literature: "The role of the coupling term in transient elastography", Sandrin, L., D. Cassereau and M. Fink, (2004), J Acoust Soc Am 115 (1): 73-83. Therefore, with regard to the measurement itself, it is undesirable to use low or very low frequencies. For the probe pulse, the (pre)measurement accuracy will also decrease when using very low frequency probe pulses, but this decrease in accuracy is not important because the probe pulse is usually not used to obtain the final value of the mechanical properties of the tissue (e.g. the value of tissue stiffness can be used to characterize the condition of the tissue). Therefore, for the probe pulse, it is ultimately necessary to use a center frequency lower than the measurement pulse to take advantage of this low probe frequency (mentioned in the previous paragraph).

With respect to the amplitude of the detection pulse, in the guidance mode, for at least some of the plurality of detection pulses, the displacement amplitude of the protrusion of the probe may be at least 10% or 20%, or even 50% lower than the displacement amplitude of the protrusion of the probe during the measurement pulse delivered in the measurement mode. For the measurement pulse, for example, the peak-to-peak amplitude of the displacement of the protrusion of the probe may be between 1 mm and 4 mm. And for at least some of the detection pulses, it may be between 0.1 mm and 2 mm.

In the flexible imaging device according to the transient technique, the electronic unit may also be configured to control the ultrasound emitter so that in the guidance mode, for at least some of the plurality of detection pulses, the repetition rate of the ultrasound pulses emitted for tracking how the detection pulses propagate is lower than the repetition rate of the ultrasound pulses emitted during the measurement mode for tracking how the measurement pulses propagate (e.g., at least 20% lower, possibly at least two to three times lower).

For example, the measurement pulse can be tracked with an ultrasound pulse repetition rate between 2kHz and 10kHz. Each detection pulse can be tracked with an ultrasound pulse repetition rate between 0.5kHz and 2kHz or even 0.5kHz and 3kHz. For example, when characterizing the liver, the measurement pulse can be tracked with an ultrasound (hereinafter referred to as U/S) pulse repetition rate of 6kHz (or, for the spleen, a frequency of 8kHz), while each detection pulse can be tracked with a U/S pulse repetition rate of 1kHz, 2kHz or 3kHz.

Reducing the repetition rate of the ultrasound pulses used for the detection pulses (which themselves repeat frequently, typically several times per second) can reduce the total acoustic output power emitted by the flexible imaging device and reduce the calculation time. This is beneficial because the total number of ultrasound pulses will be reduced to cover the same duration, which will ultimately reduce the number of rows that need to be processed in the guided mode. Reducing the repetition rate of the ultrasound pulses is also beneficial because it reduces the amount of acoustic output power to which the patient is exposed during the examination and helps to meet regulations regarding ultrasound exposure from electronic equipment (such as the IEC 60601-2-37 standard for diagnostic ultrasound equipment). Reducing the acoustic output power is very important for equipment used by operators without specific ultrasound imaging certification. In this regard, it can be noted that high ultrasound pulse repetition rates are not necessary for tracking the probe pulses, since these pulses do not require a smaller temporal and spatial resolution than the measurement pulses themselves. In addition, the center frequency of the probe pulses is usually lower than that of the measurement pulses, so they can be tracked with a lower ultrasound pulse repetition rate (without having to reduce the tracking resolution). In fact, given the lower amplitude of the probe pulses, making the time difference between consecutive ultrasound pulses larger is beneficial in allowing the tissue to undergo a displacement sufficient to be measured between consecutive ultrasound pulses using the cross-correlation technique. In order to reduce the overall acoustic output power emitted by the flexible imaging device, the U/S pulses emitted in the guidance mode can be of a different waveform (more favorable in terms of radiated acoustic power) than when using the measurement mode.

In a flexible imaging device according to the transient technique, the electronic unit may be configured to control the oscillator so that in the guidance mode, the probe pulses are delivered at a rate of several pulses per second, for example more than five probe pulses per second (in practice, a rate of 10 pulses per second is usually suitable for such guidance).

For the operator, this enables almost continuous guidance.

The electronic unit can also be configured so that, in the guidance mode, for at least some of the multiple detection pulses, there is a time lag between the transmission of the detection pulse and the transmission of the corresponding guidance information to the subject's body, and the time lag is lower than 0.5 seconds or even lower than 0.3 seconds or 0.2 seconds, and/or lower than the repetition period of the repeated detection pulse.

Thus, when in guidance mode, the operator can be guided almost continuously and in real time.

While depression presents various characteristics above, this transient pulse guidance does not have the disadvantages of uncomfortable or discontinuous guidance that may appear at first glance, and provides many improvements over existing technologies.

Nevertheless, it can be noted that such real-time guidance based on transient probe pulses is very challenging from a computational point of view, since it requires on-site processing of each series of echoes, or in other words, on-site determination of the elastogram for each probe pulse, several times per second.

To achieve this, the electronic unit may for example comprise two types of processors: a first, a dedicated processor such as an FPGA (“Field Programmable Gate Array”) for processing the echo signals acquired using the cross-correlation technique to determine tissue strain or, more generally, tissue movement parameters (as a function of time and depth); and a second, a general purpose processor.

This architecture significantly speeds up the processing of echo signals, especially because the (pre)processing implemented by the first processor greatly reduces (usually by a factor of 10 or more) the amount of data to be transmitted to the general purpose processor, thereby reducing the corresponding transmission time. In practice, this transmission time is often the most time-limiting step in the entire echo signal processing.

Nevertheless, implementing the discussed cross-correlation techniques in such a dedicated processor is inherently challenging. Indeed, before correlating the echo signals, displacements of the probe tip or head are desired to be compensated, and the commonly used techniques for compensating for such displacements (based on strong echo detection and Fourier domain compensation) are not easy or even impossible to implement in such a dedicated processor. To achieve such displacement compensation, for example, ultrasound pulse emission and/or reception times may be pre-compensated (at emission and/or reception) depending on the displacement of the probe tip or head, as explained in the applicant's assigned U.S. patent application US17/371,790, which has not yet been published.

The applicant therefore emphasizes that the real-time implementation of the above-mentioned transient pulse-based guidance method may require specific development work and not just correspond to the adjustment of operating parameters.

The above-mentioned acceleration of the processing of the ultrasound signal allows averaging the tissue strain of several consecutive detection (or measurement) pulses in order to improve the signal-to-noise ratio (SNR) of the elastogram (because this acceleration allows several elastograms to be determined per second). This can be used for display purposes, such as in the guidance mode. For example, the detection pulse can be repeated at a rate of 10 pulses per second (or more) and an averaged elastogram can be displayed, which corresponds to a rolling average calculated by averaging 3, 4 or 5 consecutive elastograms (corresponding to 3, 4 or 5 consecutive detection pulses, respectively).

This can also be used for measurement purposes. The fact that, when performing several very fast measurement pulses, the medium cannot move significantly between the pulses (relative to the probe, or relative to other organs of the subject) allows averaging and thus improves the signal-to-noise ratio of the measurement. For example, four measurement pulses can be performed at a rate of 10 pulses per second. Thus, the measurement pulse is completed in less than 400ms. Each measurement pulse is processed to obtain an elastogram. The shear wave velocity estimation algorithm is applied to the sum or average of the four elastograms. Another possibility is to perform, for example, four measurements per second for several seconds in order to accumulate a large number of measurements. It allows advanced statistics (other than the median) to be performed on the measurements (e.g. Gaussian distribution tests on the sample distribution).

At this point, the electronic unit can be configured so that when the device is operated in the measurement mode: the electronic unit controls the vibrator to transmit one or more subsequent additional measurement pulses to the subject's body in addition to the measurement pulse, each pulse being a transient low-frequency mechanical pulse; for each measurement pulse, the electronic unit determines tissue strain data within the detected area based on the echo signal collected to track how each measurement pulse propagates through the detected area; the electronic unit determines mechanical properties related to low-frequency elastic wave propagation by averaging and simultaneously considering the tissue strain data respectively associated with different measurement pulses transmitted to the subject's body.

The electronic unit may be configured such that in the guided mode, for each of the plurality of probe pulses: the electronic unit determines tissue strain data representing tissue strain within the probe region as a function of time and as a function of depth within the probe region, the tissue strain data being determined based on at least some of the echo signals collected to track how the probe pulse propagates through the probe region.

The electronic unit can then determine communication quality indicators based on the organizational strain data.

For different values of depth and time, the above tissue strain data can collect the tissue strain itself (i.e. its relative extension), or the displacement of the tissue at the location considered, or the strain rate, or the time derivative or integral of one of these quantities, or any other equivalent quantity. More generally, it is a tissue motion parameter representing the tissue motion at the time and depth considered. Moreover, in this article, "elastogram" refers to any representation of such a motion parameter as a function of depth and time.

It may be noted that in some embodiments, the communication quality indicator may be a graphical representation of organizational strain.

The guidance information may correspond directly to the transmission quality indicator itself. It may also correspond to an indicator derived from the transmission quality indicator; for example, the guidance information may take the form of a binary (e.g., red/green) indicator whose value specifies whether the transmission quality indicator is above or below a given quality threshold.

The guidance information may also take into account or represent one or more other parameters in addition to the propagation quality indicator. For example, the guidance information may combine the propagation quality indicator and ultrasound-based guidance information. The ultrasound-based guidance information may be determined from the ultrasound echo signal to represent the more or less uniform nature of the region located in front of the probe with respect to the U/S propagation and/or the fact that the U/S attenuation of this region is within the expected attenuation range of the organ to be characterized. For example, the ultrasound-based guidance information may be determined based on the coefficient of determination ( ^R2 ) of a linear regression applied to the envelope of the U/S echo signal (the closer the coefficient of determination is to 1, the higher the criterion). It can also be determined whether the U/S attenuation value is within an expected range, for example within the range of 100-400 dB/m (when the organ to be characterized is the liver), as described in U.S. patent application US2020390421 or in U.S. patent US9636085.

The guidance information may be transmitted via a visual transmission device, such as a display, an LED (e.g. a colored LED) or a group of LEDs, which is arranged on the probe. This enables the operator to access the guidance information while concentrating on positioning the probe he holds on the subject's body. In particular, the electronic unit may be configured to turn on one or more of the above-mentioned LEDs according to a transmission quality criterion to inform the operator and allow the operator to simultaneously remain focused on the probe.

Considering the device alone or in all technically possible combinations, the elastic imaging device proposed above may also include one or more of the following complementary, non-limiting features: in the guidance mode, for each of a plurality of probe pulses: the electronic unit determines the flight time of the probe pulse as a function of the depth in the detection area, the flight time of the probe pulse being determined by tissue strain data; and the electronic unit determines a propagation quality indicator (Q) in order to specify whether the probe pulse flight time varies linearly and smoothly with depth; in the guidance mode, the electronic unit is configured to determine the propagation quality indicator (Q) by averaging the tissue strain data of several probe pulses respectively corresponding to the plurality of probe pulses. The electronic unit is configured to calculate average tissue strain data; in the guidance mode, the electronic unit is configured to control the display device to display an average elastic diagram representing the average tissue strain data as a function of time and depth; the electronic unit is configured so that in the measurement mode: the electronic unit controls the vibrator to transmit one or more subsequent additional measurement pulses to the subject's body in addition to the measurement pulse, each pulse being a transient low-frequency mechanical pulse; for each measurement pulse, the electronic unit determines the tissue strain data in the detected area based on the echo signal collected in order to track how each measurement pulse propagates through the detected area; the electronic unit determines the tissue strain data by averaging and simultaneously considering the tissue strain data transmitted to the subject. The electronic unit is configured to determine mechanical properties related to the propagation of low-frequency elastic waves based on tissue strain data associated with different measurement pulses of the subject; the electronic unit is configured to adjust the center frequency of the measurement pulse to be transmitted to the subject's body according to the characteristics of the detected area of the subject's body detected by at least one of the multiple detection pulses; the characteristic represents the attenuation experienced by the detection pulse during the propagation in the detected area, wherein the electronic unit is configured to adjust the center frequency of the measurement pulse so that the higher the attenuation of the area, the higher the center frequency; the electronic unit is configured to: control the vibrator so that during the guidance mode, after at least some of the multiple detection pulses: there is A delay is provided during which the protruding portion of the probe is substantially not displaced; then there is a periodic mechanical vibration, which includes the same vibration pattern being repeated several times continuously over time, and during which the vibrator has substantially no downtime; a series of ultrasonic pulses are emitted and echo signals received in response are collected for tracking how the periodic mechanical vibration propagates through a detection area of a subject's body located in front of the protruding portion of the probe, and a periodic vibration propagation quality level is determined from at least some of these echo signals; an edge proximity indicator is determined by comparing the periodic vibration propagation quality level with a propagation quality indicator, and the edge proximity indicator is transmitted.

The instantaneous technique also relates to a flexible imaging method: the method is performed by a flexible imaging device, the device comprising a probe and an electronic unit, the probe comprising: a protrusion applied to the body of a subject, a low-frequency vibrator arranged to move the protrusion of the probe, at least one ultrasound transmitter and an ultrasound receiver, the electronic unit comprising electronic circuits suitable for alternately controlling the flexible imaging device so that it can a) operate in a guidance mode to determine whether the probe is correctly positioned in front of the body area to be probed; b) operate in a guidance mode to determine whether the probe is correctly positioned in front of the body area to be probed; c) operate in a guidance mode to determine whether the probe is correctly positioned in front of the body area to be probed; , to measure the mechanical properties of the detected area, and b) operate in a measurement mode to perform the measurement; the method includes: executing a guidance mode, the guidance mode includes: controlling the vibrator through an electronic unit to continuously and repeatedly transmit multiple detection pulses to the subject's body, each detection pulse is a transient low-frequency mechanical pulse; for each detection pulse: controlling the ultrasound transmitter through the electronic unit to emit a series of ultrasound pulses, and obtaining the echo signal through the ultrasound receiver to track the detection pulse. The invention relates to a method for controlling the detection area of the subject's body to transmit the detection pulse through the detection area of the subject's body located in front of the protruding part of the probe; determining a propagation quality index based on at least some of the echo signals by the electronic unit, the index representing the ability of the detection area to transmit the detection pulse and representing the uniformity of the detection area relative to the detection pulse propagation; transmitting guidance information, the guidance information is based on the propagation quality index; then, executing a measurement mode, the measurement mode comprising: controlling the vibrator to transmit the measurement pulse to the subject's body by the electronic unit , the measuring pulse is a transient low-frequency mechanical pulse, whose amplitude is higher than the amplitude of each pulse in the multiple detection pulses; the ultrasonic transmitter is controlled by the electronic unit to emit a series of ultrasonic pulses, and the echo signal is obtained by the ultrasonic receiver to track how the measuring pulse propagates through the detected area of the subject's body located in front of the protruding part of the probe: and the mechanical properties of the subject's body area related to the propagation of low-frequency elastic waves are determined by the electronic unit based on at least some of the echo signals.

The different features of the elastic imaging devices described above can also be applied to this elastic imaging method.

1: Flexible imaging equipment

2: Probe

3: Probe housing

4: Protruding part

4’: Moving axis

5: Low frequency vibrator

6: Ultrasonic transmitter

7: Central unit

8: Subject's body

9: Connect the cable

10: Electronic unit

11: Displacement sensor

20: Control processing module

21: Calibration module

22: Sequencer

23: Controllable delay

24: Vibration control module

25:Interrelated modules

30: Motion controller

31: Amplifier

32:Signal conditioning module

40: Ultrasonic front end

41:U/S pulse generator

42:U/S receiver module

43:T/R switch

50: Processor

60: Processor

80: Detected area

101: Elasticity diagram

102: Display

103: Ultrasound echo M-mode display

105: Elastic wave propagation image

105C: Compression wave marker

105S: Shear wave marker

MSR: Measurement pulse

PMV: Periodic Mechanical Vibration

PRB: Detection pulse

USP: Ultrasonic Pulse

ROI: Area of Interest

RP ₁ ,RP ₂ : Pulse repetition period

Sc: control signal

Sd: measurement signal

S _TX : Transmit control signal

S _RX : Transmit receive signal

Seq_1, Seq_2: Series of ultrasonic pulses

A ₁ ,A ₂ : Amplitude

d: displacement

Q: Communication quality indicators

S ₁ : Guidance mode

S ₂ : Measurement mode

T _G : Repeating period

T ₁ ,T ₂ : Duration

T ₁ ': Delay (downtime)

T ₁ ”,T ₁ ''': duration

U/S: Ultrasound

t: time

z: depth

Figure 1 shows an exemplary elasticity diagram.

Figure 2 shows the elastic diagram obtained when a sinusoidal mechanical vibration (lasting several cycles) is applied to the elastic medium.

Figure 3 shows an elastic diagram obtained under the same conditions as Figure 2, except that the mechanical vibration is a transient mechanical pulse.

Figures 4 and 5 schematically illustrate the influence of organ boundaries when probing organs using transient pulses and harmonic mechanical vibrations, respectively.

FIG6 schematically shows a flexible imaging device according to the instantaneous technique.

FIG. 7 schematically shows some components of the electronic unit in the device of FIG. 6 .

FIG8 shows the control processing module 20 of the electronic unit in more detail.

FIG. 9 schematically shows a sequence of mechanical and ultrasonic pulses emitted by the device of FIG. 6 .

Figures 10 and 11 schematically show the information displayed when the device of Figure 6 is operated in the guidance mode and in the measurement mode, respectively.

Figure 12 shows different elasticity diagrams obtained when averaging or not averaging several consecutive detection pulses.

FIG. 13 schematically illustrates the displacement of the tip of the elastic imaging device over time when the elastic imaging device is in a guided mode according to an alternative embodiment.

As mentioned above, the transient technique involves an elastic imaging device configured to generate elastic waves propagating in a medium to be explored by moving an element in contact with the surface of the medium (e.g., a probe tip, or, more generally, a protruding portion of the probe of the device), and to track how the elastic waves travel in the medium (or, in other words, how the medium moves due to the vibrations applied thereto) by transmitting ultrasound pulses in the medium and recording echo signals received in response. The transient technique particularly involves such elastic imaging devices having special guidance features to enable the operator of the device to easily find a suitable probe position.

This guidance is achieved by delivering multiple probe pulses to the subject's body in succession and repetition, each of which is a transient, low-frequency mechanical pulse (low amplitude) that tests the ability of a region located in the front area of the probe to transmit a transient pulse and tests the uniformity of the region relative to the propagation of such a transient pulse. The use of such transient probe pulses allows the subject's body to be probed under conditions that closely correspond to actual vibration-controlled transient elastic imaging measurements, thus accommodating the positioning of the probe well.

FIG6 shows an exemplary embodiment of such a flexible imaging device 1. The flexible imaging device 1 comprises a probe 2, which comprises a probe housing 3 (which forms the body of the probe) to be held in hand and a protrusion protruding from the housing 3. Thus, the protrusion can be applied to the subject's body 8, delivering mechanical pulses thereto, and transmitting and acquiring U/S beams. In this embodiment, for example, the protrusion is a tip 4, such as a cylindrical tip (with a circular transducer 6 at its end).

Nevertheless, in other embodiments, the protruding portion may be an ultrasonic head (located at one end of the probe) comprising an array (e.g., a linear array of U/S transducers). In this regard, it may be noted that the proposed technique may be used with a single-element ultrasonic transducer (as shown in FIG. 6 ) or a multi-element ultrasonic transducer (e.g., an array of U/S transducers). A single-element ultrasonic transducer is suitable for displaying A- and M-mode ultrasonic imaging, while a multi-element ultrasonic transducer may also display B-mode images, making it easier to locate the tissue to be measured. In the case of a multi-element ultrasonic transducer, at least one beamformed ultrasonic ray is used to track the propagation of the detection and measurement pulses. For this purpose, it is beneficial to use a central beamformed ultrasonic ray (aligned with the probe axis) for symmetry reasons.

The probe 2 also includes a low-frequency vibrator 5 fixed to the end of the tip 4 and a U/S transducer 6. Here, the U/S transducer 6 acts as both an ultrasonic transmitter and an ultrasonic receiver (alternatingly). Nevertheless, in other embodiments, the probe may include a U/S transmitter and a U/S receiver that are different from each other. Here, the U/S transducer 6 is arranged on the z-axis of the vibrator. Nevertheless, in other embodiments, the U/S transducer may be located elsewhere in the probe, not necessarily on the axis of the vibrator.

The tip 4 is actuated by a low frequency vibrator 5. Here, the vibrator 5 is arranged to move the tip 4 relative to the probe housing 3. The vibrator 5 is arranged to move an axis 4', the end of which forms the tip 4 of the probe. Although, in other embodiments, the tip, or more generally the protruding part of the probe, can be integrated with the probe housing without moving relative to the probe housing, and the vibrator is then arranged to move the mass within the housing so that the entire probe is moved back and forth towards the tissue (by virtue of the recoil effect).

The vibrator 5 is a low frequency vibrator, where it moves the tip with an average center frequency of less than 500 Hz, or even less than 100 Hz (in contrast to ultrasound beams or echo signals, which typically have center frequencies above 1 MHz, e.g. between 1 and 5 MHz). The vibrator is a low frequency electromechanical actuator, e.g. with one or more coils and magnets, similar to a loudspeaker actuator.

In the device 1, the vibrator 5 is rotationally symmetrical about the vibrator axis, which coincides with the z-axis of the probe. When the vibrator 5 vibrates, the displacement it induces is mainly longitudinal, parallel to its axis. The axis 4' is centered on the z-axis and the vibrator 5 is arranged to move the axis along the z-axis.

In practical applications, the displacement of the ultrasonic transducer 6 is induced by the vibrator 5 with a peak-to-peak amplitude between 0.1 mm and 10 mm (e.g. between 1 mm and 4 mm or between 1 mm and 5 mm for the transient elastic imaging measurement itself, and smaller amplitudes can be used to guide the operator's probe pulses).

The probe 2 includes a displacement sensor 11, which is arranged to output a measurement signal _Sd representing the displacement of the ultrasonic transducer 6. In this embodiment, the measurement signal _Sd represents the displacement of the ultrasonic transducer 6 relative to the probe housing 3. A portion of the displacement sensor 11 is fixed to the above-mentioned axis, while another portion of the sensor is installed in the probe and does not move relative to the housing 3. The displacement sensor 11 can be a Hall effect sensor, an inductive displacement sensor, or any other suitable sensor.

The device 1 also includes an electronic unit 10 connected to the vibrator 5 and the U/S transducer 6. A block diagram of a possible implementation of the electronic unit 10 is shown in FIG7. The electronic unit 10 of FIG7 includes a control processing module 20, an ultrasonic front end 40, and a motion controller 30 for controlling the vibrator 5.

The ultrasonic front end 40 and the motion controller 30 are both connected to the control and processing module 20 (that is, they can receive instructions or control signals from the control and processing module 20 or send data or measurement signals to it). The electronic unit also includes a signal conditioning module 32 to condition and digitize the measurement signal Sd output by the displacement sensor 11. Here, the signal conditioning module 32 is part of the motion controller 30.

The motion controller 30 also includes an amplifier 31 for driving the vibrator 5. From an electrical point of view, the amplifier 31 is configured to convert the control signal Sc into a form suitable for driving the vibrator. The amplifier 31 can therefore be a current amplifier or a power amplifier (such as the LM3886 power amplifier provided by Texas Instruments).

The ultrasonic front end 40 includes an ultrasonic (U/S) pulse generator 41, a U/S receiver module 42, and a switch 43 for alternately transmitting and receiving ultrasonic signals. The U/S pulse generator 41 includes a circuit configured to generate an electric ultrasonic signal suitable for driving the U/S transducer 6 based on the transmission control signal S _TX output by the control processing module 20. The U/S receiving module 42 includes a circuit configured to obtain an electric ultrasonic electrical signal (echo signal) previously received by the U/S transducer 6 (and transmitted to the U/S receiver module 42 through the switch 43), and transmit the corresponding (digitized) U/S receiving signal _SR,X to the control processing module 20. The circuitry of the ultrasonic receiver module 42 may include a voltage amplifier, one or more filters, and an analog-to-digital converter (ADC), such as an 8 to 16-bit ADC having a sampling rate of 10 to 100 megasamples per second.

The control processing module 20 is a device or system including a circuit for processing data, such as a microprocessor coupled to a non-volatile non-transient memory including machine executable instructions, and/or a programmable microcircuit, such as an FPGA or other programmable circuit. The control processing module 20 may also include one or more RAM memories or registers. In any case, the control processing module 20 includes at least one processor, here including two processors 50, 60 and at least one memory.

Some submodules of the control and processing module 20 for implementing the pre-compensation technique for transducer displacement are shown in more detail in FIG. 8 . They will be introduced later.

Some components of the electronic unit 10 (e.g., the signal conditioning module 32) may be mounted in the probe 2, while other components of the unit 10 (e.g., the general purpose processor 60) may be remote. Alternatively, the entire electronic unit may be mounted in the probe 2, or conversely, it may be completely external to the probe.

The probe 2 is operatively connected to a central unit 7, which has a computer structure (and can be a laptop, a smartphone or a dedicated electronic device for controlling and docking the probe and processing the acquired signals). The central unit comprises at least a memory and a processor. Here, it also comprises a user interface, such as a touch screen. The probe can be connected to the central unit 7 via a connecting cable 9 or via a wireless link. Here, some components of the electronic unit 10 (in particular the general processor 60) are part of the central unit 7.

The electronic unit 10 (more specifically, the control processing module 20 of the electronic unit 10) is configured to control the electronic device 1 (for example, by means of instruction programming stored in a memory) so that the electronic device 1 alternately a) operates in a guidance mode to determine whether the probe 2 is correctly positioned in front of the area 80 of the body 8 to be probed to measure the mechanical properties of the probed area 80, or b) operates in a measurement mode to perform measurements.

When the elastic imaging device 1 operates in the guidance mode (stage S1, as shown in FIG9 ), the electronic unit 10 controls the low-frequency vibrator 5 to sequentially and repeatedly transmit a plurality of probe pulses PRB to the subject's body 8, each probe pulse being a transient, low-frequency mechanical pulse. As described above, transmitting (and tracking) these probe pulses enables testing the ability of the region 80 located in the front region of the probe to transmit transient pulses, and enables testing the uniformity of the region for propagating such transient pulses. This, in particular, enables determination of a propagation quality index Q, which represents the ability of the region to transmit the probe pulse and the uniformity of the region for the propagation. This enables the operator to know if the probe is in front of and pointing toward a homogeneous organ of sufficient size and size, and therefore if it is properly positioned for measurement with vibration-controlled transient elastic imaging.

The following first introduces the operation of the elastic imaging device in the guidance mode. Then the measurement mode will be introduced.

In the guidance mode (phase _S1 , as shown in FIG. 9), as described above, the electronic unit 10 controls the low-frequency vibrator 5 to sequentially and repeatedly transmit a plurality of detection pulses PRB to the subject's body 8, each detection pulse being a transient, low-frequency mechanical pulse.

The guided mode can be triggered in response to a manual trigger by the operator, for example by means of a touch screen of the central unit 7.

In the embodiment described herein, once in the guidance mode, when the electronic unit 10 detects that the tip 4 is applied to the subject's body 8, the electronic unit 10 controls the vibrator so that the detection pulse is repeated continuously and automatically (i.e., without the need for manual re-triggering) as long as the tip remains in contact with the body of the subject 8. The electronic unit 10 can detect whether the tip 4 is applied to the subject's body based on the contact force level F1 (measured by a force sensor (e.g., a strain gauge) not shown in the figure, or inferred from the position of the shaft 4' pushed into the housing when the tip is pressed against the subject's body). Regulating the generation of the probe pulse based on this contact detection is beneficial because it prevents the probe from vibrating when it is not actually probing (and is only supported by the operator's hand).

Regardless, if the generation of the probe pulse is conditioned upon such contact detection, the probe pulse is continuous and automatically repeating (i.e., no manual triggering is required) when in the pilot mode.

The electronic unit 10 uses a motion controller 30 to control the vibrator 5 so that it transmits a detection pulse. More specifically, the displacement d of the shaft 4' is controlled according to a predetermined command signal. Here, the displacement is controlled by a control loop, which includes an amplifier 31, a displacement sensor 11, a signal conditioning module 32, and a vibration control module 24 (Figure 8), and the vibration control module 24 can be, for example, a PID (proportional, integral, derivative) corrector (although, in an alternative embodiment, the vibrator can be controlled by an open loop, i.e., without sensor feedback).

In the guidance mode, the probe pulses are transmitted at a rate of several pulses per second, for example 5 or even 10 pulses per second or more. Therefore, the probe pulse PRB is repeated with a fairly short repetition period _TG , typically less than 0.2 seconds or even 0.1 seconds. Transmitting the probe pulses at such a rate is beneficial because it allows the operator to perform almost continuous guidance. It also allows the results of several consecutive probe pulses to be averaged and the corresponding (averaged) guidance information to be transmitted with low lag time. Furthermore, due to this rather high repetition rate, the average detection results are therefore obtained in a rather short time compared to the usual time it takes for an organ to move within the subject's body (due to respiratory movement or heart pulse).

Each detection pulse PRB has a duration _T1 during which the protrusion has substantial movement induced by the oscillator, followed by a dead time before another detection pulse PRB is generated during which there is no or substantially no movement of the protrusion. Therefore, the actuation ratio (in other words, the duty cycle, which is equal to the duration _T1 of the pulse divided by the repetition period _TG of the detection pulse) is less than 1, for example, the actuation ratio is lower than 50%, or even lower than 20%.

Each probe pulse has a finite amplitude and is smaller than the amplitude of the measurement pulse MSR delivered in the measurement mode. Among many advantages, this reduced amplitude makes the navigation process relatively comfortable for the subject being examined. For example, each probe pulse may correspond to a peak-to-peak displacement _A1 of 1 mm of the tip 4, which is twice smaller than the peak-to-peak displacement _A2 of the tip to which each measurement pulse MSR corresponds.

Regarding the center frequency of each probe pulse PRB, it is slightly reduced compared to the center frequency of the measurement pulse MSR to improve the comfort of the subject receiving the probe pulse and to increase its penetration depth in the medium, as explained in detail in the "Content of the Invention" section. Here, the elastic imaging device is a device suitable for characterizing the liver. The center frequency f _c,1 of each probe pulse PRB is between 20 Hz and 45 Hz, and its duration T ₁ may be lower than 5/f _c,1 as here, or even lower than 3/f _c,1 .

Each detection pulse can be composed of a sine wave of one cycle or several cycles (usually less than 2 or 3 cycles), and its frequency (equal to or close to the center frequency mentioned above) is between 20Hz and 45Hz.

For example, in the example of FIG7 , each probe pulse consists of one cycle of a 40 Hz sine curve, so its duration _T1 has a value of 25 ms. In this example, there is a 75 ms downtime after each probe pulse (so T _G =0.1 s); the probe pulse is repeated at a rate of 10 pulses per second (as described below, the transmission quality indicator Q is calculated by a rolling average, averaging the results corresponding to 3 to 5 consecutive probe pulses). Alternatively, the downtime can be, for example, 25 ms (T _G =0.05 s), and then the probe pulse is repeated at a rate of 20 pulses per second.

For each probe pulse PRB, the electronic unit 10 controls the ultrasonic transducer 6 (with the help of the U/S pulse generator 41 of the U/S front end 40, etc.), so that the U/S transducer 6 emits a series of ultrasonic pulses Seq_1, and obtains the echo signal received by the ultrasonic transducer 6 in response to track how the probe pulse PRB propagates through the probe area 80 of the subject's body 8 located in front of the probe tip 4.

For this sequence Seq_1, and for the ultrasound pulse sequence Seq_2 emitted in the measurement mode (for tracking how the measurement pulse MSR propagates), the center frequency of each ultrasound pulse USP is, for example, between 0.5 and 10 MHz. The ultrasound pulses of the sequence Seq_1 or Seq_2 can be transmitted one at a time, two consecutive pulses being separated by a pulse repetition period _RP1 , _RP2 , which is typically between 100 microseconds and 2 milliseconds (corresponding to a pulse repetition rate between 0.5 kHz and 10 kHz). The ultrasound pulses of the above sequence can also be transmitted in groups, for example, two pulses are emitted as a group (to calculate the correlation between two corresponding echo signals). The two pulses of each group can be separated by a duration between 50 and 200 microseconds, and the pulse groups themselves are separated by longer durations, such as higher than 0.2 or 0.5 ms. It is understood that other transmission sequences can also be considered in various embodiments. The total duration of the U/S pulse sequences Seq_1 and Seq_2 can be between 25 ms and 200 ms. This duration can be selected based on the propagation speed of the slower elastic wave and the depth of the area to be observed. For example, for a depth of 80 mm and a propagation velocity of 1 m/s (typical of shear waves in a subject's liver), the duration of the sequence may be 80 ms.

For the echo signals acquired in order to track the propagation of the mechanical pulse under consideration, each echo signal consists of a signal received by the U/S transducer 6 at time t after the emission of a U/S pulse USP. More precisely, the signal is received within a given time window starting after the emission of the above-mentioned U/S pulse USP and having a given duration.

In the embodiments described herein, in the guidance mode, the repetition rate of the U/S pulse (i.e., 1/RP ₁ ) is lower than the repetition rate of the U/S pulse in the measurement mode (i.e., 1/RP ₂ ), for example, at least 20% lower, and may be at least twice lower. For example, in sequence Seq_1 (in the guidance mode), the U/S pulse repetition rate may be between 0.5kHz and 3kHz (e.g., 2kHz), while in sequence Seq_2 (in the measurement mode), the U/S pulse repetition rate may be between 2kHz and 10kHz (e.g., 6kHz). As explained in detail in the "Content of the Invention" section, the use of a lower U/S pulse repetition rate in the guidance mode is beneficial to the acoustic wave output power and calculation time emitted by the elastic imaging device. And it is well adapted to the detection pulse PRB whose center frequency and amplitude _A1 are lower than the center frequency and amplitude _A2 of the measurement pulse MSR.

In the steering mode, for each probe pulse PRB, the electronic unit 10 determines the above-mentioned propagation quality indicator Q. This indicator is determined by the acquired echo signal to track how the probe pulse PRB travels in the detection area 80.

As described above, the propagation quality indicator Q represents the ability of the detection region 80 to transmit the detection pulse PRB, that is, to have the detection pulse propagate through the region and deep into the region (e.g., at least to a given depth), even if the detection pulse may be attenuated and may be partially distorted during this propagation process. The propagation quality indicator Q also represents the propagation uniformity of the detection region 80 relative to the detection pulse PRB; in other words, it represents the absence of substantial propagation inhomogeneities, such as bounces, discontinuities, or variations/steps in the propagation speed of the detection pulse.

The propagation quality indicator Q can specify whether the spatiotemporal characteristics of the tissue strain induced by the transmission of the probe pulse to the subject's body (i.e., the characteristics representing the change in tissue strain as a function of both time and at least one spatial coordinate) correspond to the propagation of a low-frequency mechanical transient pulse in a homogeneous medium.

In the embodiment described herein, in the guided mode, for each probe pulse PRB, the electronic unit determines tissue strain data representing tissue strain within the probe region 80 as a function of time t and depth z within the probe region 80. The tissue strain data is determined by the acquired echo signals to track how the probe pulse PRB propagates through the probed region. As described above, when graphically represented, the tissue strain data is formed into an elasticity map as a function of time and depth (as shown in FIG. 1 , FIG. 10 , or FIG. 11 ).

The strain data of the tissue are determined from the echo signals using a cross-correlation technique or another pattern matching algorithm to determine how parts of the tissue move under the influence of elastic waves passing through it (elastic waves are generated by periodic mechanical vibrations transmitted by the system). For example, for each pair of two consecutively received echo signals, the two echo signals are correlated with each other by a cross-correlation module 25 (Figure 8), which is able to determine the tissue displacement as a function of depth and a given time (i.e., the tissue displacement that occurs between two U/S pulses). The electronic unit then determines the transmission quality indicator Q based on the tissue strain data corresponding to the detection pulse.

The propagation quality index Q can be determined in order to specify whether the elasticity diagram representing the tissue strain data includes one or several regular fringes (in the t-z coordinate plane).

As here, a propagation quality indicator Q can also be determined to specify whether the flight time of the probe pulse varies linearly and smoothly with the depth z. To this end, the propagation quality indicator Q can be derived from a coefficient of determination R ² applied to the linear regression of the probe pulse flight time as a function of depth. The propagation quality indicator Q can even correspond directly to this coefficient of determination. The probe pulse flight time in question is the time it takes for the probe pulse to propagate from the surface of the subject's body 8 to a given depth within the probe region 80 (in other words, to reach the depth under consideration). The flight time can be determined by calculating the Fourier transform of the tissue strain at the depth of interest (tissue strain as a function of time at a fixed depth) and then deriving the flight time from the phase of a component of the Fourier transform (usually the frequency of this component is the center frequency of the probe pulse delivered by the probe). Other techniques can also be used to determine the propagation quality indicator Q, such as the zero-crossing technique or the pattern matching technique. When the zero-crossing technique is used, the flight time can be determined as the time for the tissue strain to cross the zero point (cross zero and then stay on the same side of the zero line for a period of time to avoid irrelevant zero crossings caused by noise) for the depth of interest. When pattern matching techniques are employed, the time of flight can be determined as the time offset that best matches a given (reference) pulse curve to the temporal variation of tissue strain at the depth under consideration.

In the embodiments described herein, the propagation quality indicator Q is a numerical value (e.g., between 0 and 1). Nevertheless, in other embodiments, the propagation quality indicator Q can directly take the form of an elasticity diagram representing tissue strain data. In fact, such an elasticity diagram enables the operator to easily determine whether the tissue is homogeneous and suitable for elastic wave propagation, just like the numerical value mentioned above.

In the guidance mode, for each detection pulse PRB, the electronic unit 10 controls the flexible imaging device 1 to transmit guidance information, which is based on the transmission quality indicator Q. In practice, the guidance information is transmitted and communicated to the operator of the device 1 (visually and/or using sound signals).

The guidance information may include elasticity plots representing tissue strain data. It may also include the transmission quality indicator Q itself (i.e., maintained as is, without further processing). The guidance information may take the form of a binary (e.g., red/green) indicator whose value specifies whether the transmission quality indicator Q is above or below a given quality threshold. The guidance information may also be a composite element that aggregates the different features described above (and possibly other features), which are kept distinct from each other (i.e., not fused) in the guidance information. In this example, the guidance information provided to the operator includes: an elasticity diagram 101 (probe or guidance elasticity diagram), which is an average elasticity diagram, which will be explained below; a display 102 showing the quality level Q1 in the form of an indicator indicator or a bar scale indicator or equivalent; the quality level Q1 can be the transmission quality indicator Q itself, or, as here, a level combining (merging) the transmission quality indicator Q and the ultrasound-based guidance information in the form of a single level (for example, by calculating the average of these two quantities); and a binary (green/red) indicator determined by comparing the quality level Q1 with a preset threshold; the binary indicator is displayed by turning on or, conversely, turning off an LED arranged on the probe housing 3.

The elasticity diagram 101 and the quality level Q1 are displayed on the screen of the central unit 7. The force level F1 and the ultrasound echo M-mode display 103 are also displayed on this screen, as they help the operator to adequately position the probe. The ultrasound echo M-mode display 103 is a two-dimensional image in which each column represents one of the acquired ultrasound echo signals, and the continuously acquired U/S echo signals are displayed side by side.

As mentioned above, the quality level Q1 combines the propagation quality indicator Q and the above-mentioned ultrasound-based guidance information. As described in the "Content of the Invention" section, the ultrasound-based guidance information can be determined from the ultrasound echo signal in order to represent the more or less uniform nature of the area located in front of the probe in terms of U/S propagation and/or the fact that the U/S attenuation in this area is within the expected attenuation range of the organ to be characterized. It is worth noting that the ultrasound-based guidance information cannot predict the propagation of shear waves well, because the ultrasound signal is not sensitive enough to the mechanical properties of soft tissue.

As described above, the electronic unit 10 determines the propagation quality indicator Q for each probe pulse PRB in the guidance mode. For each new probe pulse PRB delivered to the subject's body, the corresponding propagation result (e.g., tissue strain data) is taken into account to determine a new value of the propagation quality indicator Q.

This new value of the propagation quality indicator Q can be determined based solely on the propagation results of this new probe pulse.

This new value can also be determined based on the propagation results associated with more than one probe pulse, for example by averaging these different propagation results and then calculating the propagation quality indicator Q. In this case, the propagation quality indicator Q can be updated after each probe pulse (for example, by using a rolling average), thereby providing the operator with real-time (or almost real-time) monitoring. Alternatively, the propagation quality indicator Q can be updated only after several new probe pulses have been delivered (in this case, a classic non-rolling average can be used).

In the embodiment described here, the propagation quality indicator Q is determined based on propagation results corresponding to several detection pulses (usually between 3 and 6 consecutive pulses, for example 5), and these different propagation results are averaged by rolling average. For each new detection pulse, the corresponding elasticity diagram (or in other words, the corresponding tissue strain data) is determined (by mutual correlation), and then averaged with the elasticity diagrams corresponding to the previous four detection pulses to obtain an average elasticity diagram 101, which is then displayed. A new value of the propagation quality indicator Q is then calculated from the average elasticity diagram (average tissue strain data).

This averaging of the results corresponding to several rapidly repeated detection pulses is beneficial since it increases the signal-to-noise ratio of the averaged detection elastogram (see Figure 12, inserts a) and a')) while allowing almost continuous monitoring of a more or less adequate positioning of the probe 2.

Figure 12 shows different elastic graphs measured in vivo with/without averaging (when directed at the subject's liver). The average elastic graph is obtained by averaging 5 consecutive elastic graphs obtained at a rate of 10 elastic graphs per second. Elastic graph a) is one of the elastic graphs in which the 5 elastic graphs are averaged together to obtain the average elastic graph a'). This example illustrates the signal-to-noise ratio improvement of this averaging technique (especially in the portion of the graph surrounded by the dotted line). Elastic graphs b) and c) are also single non-averaged elastic graphs corresponding to the average elastic graphs b') and c'). Elastic graphs b) and b') show that this averaging technique also increases the depth at which the detected pulse can be tracked and visualized. The elasticity plots c) and c’) show that the technique also reduces the impact of possible artifacts, such as those caused by the presence of blood vessels or other inhomogeneities in the region of interest.

The operation of the flexible imaging device 1 in the measurement mode (phase S ₁ in FIG. 9 ) will now be described in more detail.

The elastic imaging device 1 can be configured to switch from operating in the guidance mode to operating in the measurement mode ( _S2 stage in Figure 9) in response to manual triggering by the operator. For example, this manual triggering can be achieved by driving a button switch arranged on the probe housing 3, or by driving a foot switch. In this case, when the operator believes that the position and orientation of the probe 2 are appropriate based on the guidance information, he can trigger the measurement mode to measure at least one mechanical property of the detection area related to the propagation of low-frequency elastic waves (such as its Young's modulus).

The flexible imaging device can also be configured to automatically switch from the guidance mode to the measurement mode when a given criterion is met. For example, this switching may occur when the transmission quality indicator Q (or the above-mentioned quality level) exceeds (e.g., is higher than) a given quality threshold. Other criteria that take into account other parameters can also be used as a supplement or alternative to the transmission quality indicator Q to automatically switch the flexible imaging device from the guidance mode to the measurement mode. The automatic switch described in this article can be implemented using one or more electronic circuits, including components such as comparators.

Once the measurement mode is entered, the electronic unit 10 controls the low-frequency vibrator 5 to transmit at least one (here, multiple consecutive) measurement pulses MSR to the detected area, each of which is a transient low-frequency mechanical pulse. The electronic unit 10 also controls the U/S transducer 6 to emit a series of ultrasonic pulses USP of Seq_2, and obtains the echo signal received in response to track how each measurement pulse MSR propagates through the detection area 80.

For each measurement pulse, the electronic unit 10 processes the acquired echo signal as described above with respect to the detection pulse to determine tissue strain data representing tissue strain in the detected area as a function of depth z and time t.

The electronic unit 10 then determines the mechanical properties of the detection area 80 from the tissue strain data.

Here, the electronic unit 10 determines the mechanical properties by averaging and taking into account the tissue strain data respectively associated with the different measurement pulses transmitted to the subject's body. In practice, the number of measurement pulses MSR transmitted as a result of switching to the measurement mode can be between 2 and 10, for example, and they can be transmitted at a rate of between 3 and 20 pulses per second. The tissue strain data respectively associated with these different measurement pulses are averaged together to obtain average tissue strain data and a corresponding average measured elastogram, as shown in FIG. The mechanical properties of the detection area are then determined based on this average tissue strain data. The number of consecutive measurement pulses MSR considered in this average tissue strain data may be the same as the number of consecutive detection pulses PRB averaged in the guided mode to produce the average guided elasticity map 101 (and the corresponding repetition rate may also be the same).

Mechanical properties of tissue related to low frequency shear wave propagation can be quantities related to tissue stiffness, such as the propagation velocity of shear waves _Vs , the shear modulus of the tissue, or the Young's modulus E of the tissue (which can be derived from the slope of the bars identified in the elasticity diagram, or from the variation of the flight time of the measured pulse with depth). Mechanical properties can also be quantities related to the attenuation of low frequency shear waves in the tissue, such as viscosity.

As previously described, when operating in the guidance mode, the amplitude _A2 of each measurement pulse MSR is higher than the amplitude _A1 of each detection pulse PRB. For example, for the measurement pulse, the peak-to-peak amplitude _A2 of the displacement of the probe tip 4 can be between 1 mm and 4 mm. In addition, as here, the center frequency of each measurement pulse MSR is higher than the center frequency of any detection pulse PRB. For example, as here, when the elastic imaging device is configured to characterize the patient's liver, the center frequency of each measurement pulse MSR can be between 50 Hz and 200 Hz. Here, each measurement pulse consists of one cycle of a sine wave with a frequency between 50 Hz and 200 Hz, which is equal to 50 Hz here. Each measurement pulse is followed by a dead time, the duration of which is higher than the duration T ₂ of the measurement pulse (possibly higher than two to three times this duration). As already described, in the sequence Seq_2, the ultrasound pulse repetition rate is higher than in the U/S sequence Seq_1 emitted to track one of the detection pulses. For example, in the Seq_2 sequence, the U/S pulse repetition rate can be between 2kHz and 10kHz. Here, for example, it is equal to 6kHz.

According to an optional feature, the electronic unit 10 can be configured to adjust the center frequency of the measuring pulse based on a characteristic of the detection area 80 previously determined using at least one detection pulse in a pilot mode (the characteristic being determined based on how the detection pulse propagates through the detected area). More generally, the measuring pulse or the characteristics of the pulse (in terms of frequency and amplitude) can be adjusted based on a preliminary characterization of the detected area 80 obtained by the detection pulse.

For example, the characteristic may represent the attenuation experienced by the detection pulse during its propagation in the region. In this case, the electronic unit may be configured to adjust the center frequency of one or more measurement pulses so that the higher the attenuation in the region, the higher the center frequencies of all (this enables the measurement pulses to obtain the desired penetration depth even if (for example for the liver) the elastic wave attenuation is on average high in the detection region, i.e. higher than expected).

Referring now to FIG8 , the optional pre-compensation technique described above (see FIG7 ) is illustrated. When processing the acquired ultrasound echo signals, it is desirable to compensate for the displacement d of the tip in order to determine the tissue strain. In fact, when the ultrasound pulses for detecting the displacement of the medium are emitted by the tip, the very important tip displacement is added up to the displacement of the tissue to be measured. In order to reduce the cross-correlation calculation time and improve the signal-to-noise ratio, it is therefore necessary to compensate for this displacement. Known compensation techniques are based on post-processing of the echo signals, in which strong echoes are identified and used to realign these signals in time. However, this technique is very time consuming and is not well suited for implementation in a dedicated processor such as the processor 60 (which may be an FPGA, for example). Therefore, in order to compensate for this displacement d, here, the electronic unit 10 (more specifically, its processor 60) is configured to implement the following pre-compensation technique.

Ultrasonic waves for tracking detection pulses and measurement pulses are emitted under the following conditions: with an emission time offset δt _TX , by which the emission of the ultrasonic _pulse is offset; and/or, with a reception time offset δt _RX , by which the echo signal obtained in response to the emitted ultrasonic pulse is offset; thereby compensating for the time _offset of the echo signal relative to other obtained echo signals caused by the displacement d of the ultrasonic transducer 6 (or multiple ultrasonic transducers); the emission time offset δt _TX and/or the reception time offset δt _RX are adjusted so that the difference is equal to Δt _o -2. d /v _us , Δt _o is a constant delay, v us _us is the speed of ultrasound in the tissue being examined.

Therefore, the displacement of the sensor is compensated from the beginning and no special post-processing is required.

In the case of FIG. 8 , the flexible imaging device is more specifically configured such that the transmit time offset is equal to δt _TX,o + d /v _us , δt _TX,o being a constant delay when transmitting, and the receive time offset is equal to δt _RX,o - d /v _us , δt _RX,o being a constant delay when receiving.

To introduce this delay when transmitting, the control module 20 can generate a reference transmit control signal S _TX,O (e.g., based on a predetermined transmit sequence stored in the control module memory), and when it is necessary to track the detection (or measurement) pulse, this signal is delayed in a controlled manner using a controllable delay 23, thereby generating a transmit control signal S _TX sent to the U/S front end 40. The receive time offset δt _RX can be obtained using a controllable sequencer 22, for example, using a shift register or another digital buffer to select a suitable sequence of values in the digitized signal output by the amplifier and ADC 42. And the correction module 21 can determine the variable delay d / _vUS from the digitized signal output by the signal conditioning module 32 (which represents the digitized signal output by the displacement sensor 11). In the embodiments of FIG6 to FIG8 , the displacement d of the transducer 6 is its displacement relative to the probe housing 3.

With regard to the mechanical vibrations applied to the subject's body in the guided mode, in order to detect the area facing the probe, it can be noted that in an alternative embodiment, the vibrator can be controlled in a hybrid manner so as to deliver transient pulses (i.e. the above-mentioned detection pulses PRB) and periodic mechanical vibrations PMV. In Figure 13, as an example of such a hybrid detection, the displacement d of the tip 4 is represented over time t.

For this hybrid detection/guidance, each detection pulse PRB is followed by a delay T ₁ ' (down time) in which the tip 4 is essentially not displaced, and then a periodic mechanical vibration PMV, which consists of the same vibration pattern repeated several times in succession over a period of time, and during which the vibrator has essentially no down time. The periodic mechanical vibration PMV may include at least 3, or even at least 5, occurrences of the vibration pattern (e.g., one cycle of a sine curve). As shown in FIG13 , the periodic mechanical vibration includes 4 occurrences of the vibration pattern (in this example, 4 consecutive sinusoidal cycles). The duration of the periodic mechanical vibration PMV is T ₁ ''. After the periodic mechanical vibration and before delivering any new detection pulse, there is a rest time of duration T ₁ '''.

The duration of the pulse downtime after detection T ₁ ' can be higher than 1/f _c,1 , or higher than T ₁ or even higher than twice T ₁ , and T ₁ ''' is also the same.

By using both transient mechanical pulses (probe pulses PRB) and periodic mechanical vibrations on the probe area 80, it is possible to detect that the probe axis z is close to the edge of the organ to be characterized. (The proximity observed from the side; that is, close to the edge of the organ).

In fact, as explained in the "Content of the Invention" section, with reference to Figures 2 and 3, when probing a medium with periodic (e.g. harmonic) vibrations, proximity to walls, edges, or discontinuities in the medium often disturbs the propagation of the vibrations, favoring the resident mode. In contrast, the propagation of transient mechanical pulses is less disturbed by such edges. Therefore, the fact that transient mechanical pulses can propagate through the probe area while periodic vibrations cannot (or at least propagate with very strong distortion) indicates that the probing axis z may be close to the edge of the organ to be characterized.

In this alternative embodiment, when in the guidance mode, the electronic unit controls the ultrasonic transducer 6 to emit a series of ultrasonic pulses and obtains echo signals received in response to track how the detection pulses and periodic mechanical vibrations propagate through the detected area of the subject's body.

In addition to the above-mentioned (transient) propagation quality indicator Q, the electronic unit then determines from at least some of these echo signals an indicator of the propagation quality level of the periodic vibrations.

The periodic vibration propagation quality level specifies whether the elasticity plot associated with this vibration includes well-defined diagonal fringes. It also specifies whether the phase delay of this vibration varies roughly linearly and smoothly with depth.

The electronic unit then determines an edge proximity indicator based on a comparison of the periodic vibration propagation quality level and the propagation quality indicator Q, and transmits this edge proximity indicator (communicating it to the operator of the elastic imaging device). The edge proximity indicator can be a binary indicator, for example, switching from off to on when the difference between the propagation quality indicator Q and the periodic vibration propagation quality level is higher than a given threshold.

MSR: Measurement pulse

PRB: Detection pulse

USP: Ultrasonic Pulse

RP ₁ ,RP ₂ : Pulse repetition period

Seq_1, Seq_2: Series of ultrasonic pulses

A ₁ ,A ₂ : Amplitude

d: displacement

Q: Communication quality indicators

S ₁ : Guidance mode

S ₂ : Measurement mode

T _G : Repeating period

T ₁ ,T ₂ : Duration

U/S: Ultrasound

t: time

Claims (18)

A flexible imaging device (1) comprises: a probe (2), the probe (2) comprising: a protruding portion (4) applied to a subject's body (8), a low-frequency vibrator (5) arranged to move the protruding portion (4) of the probe, at least one ultrasound transmitter (6), and an ultrasound receiver (6); and an electronic unit (10), the electronic unit (10) comprising an electronic circuit suitable for alternately controlling the flexible imaging device (1) so that the flexible imaging device (1) a) operates in a guidance mode ( _S1 ) for determining whether the probe (2) is correctly positioned in front of a body region (80) to be detected so as to measure the mechanical properties of the detected region, and b) operates in a measurement mode ( _S2 ) operation, for performing the measurement, the electronic circuit is adapted to execute the following procedure: in the guiding mode (S ₁ ) period: the electronic unit (10) controls the low-frequency vibrator (5) to continuously and repeatedly transmit a plurality of detection pulses (PRB) to the body of the subject, each detection pulse being a transient, low-frequency mechanical pulse; for each detection pulse: the electronic unit (10) controls the ultrasound transmitter (6) to transmit a series of ultrasound pulses (Seq_1), and obtains the echo signal received in response by the ultrasound receiver (6) to track how the detection pulse (PRB) propagates through the body region (80) to be detected of the body (8) of the subject located in front of the protruding portion (4) of the probe; The electronic unit (10) determines a propagation quality indicator (Q, 101) based on at least some of the echo signals, wherein the propagation quality indicator represents the ability of the body region (80) to be detected to transmit the detection pulse (PRB) and represents the uniformity of the propagation of the detected region relative to the detection pulse; the electronic unit (10) controls the elastic imaging The device (1) transmits guidance information (Q, Q1, 101), wherein the guidance information is based on the transmission quality indicator (Q); during the measurement mode: the electronic unit (10) controls the low-frequency oscillator (5) to transmit a measurement pulse (MSR) to the detected area, wherein the measurement pulse is a transient low-frequency mechanical pulse, and the amplitude (A) of the measurement pulse is ₂ ) is higher than the amplitude (A ₁ ) of each pulse of the plurality of detection pulses (PRB); the electronic unit (10) controls the ultrasound transmitter (6) to transmit a series of ultrasound pulses (Seq_2) and obtains an echo signal received in response by the ultrasound receiver (6) to track how the measurement pulse (MSR) propagates through the body region (80) to be detected; and the electronic unit (10) determines the mechanical performance (E) based on at least some of the echo signals, the mechanical performance being related to the propagation of low-frequency elastic waves. An elastic imaging device (1) according to claim 1, wherein the electronic unit (10) is configured to control the oscillator (5) so that in the guidance mode ( _S1 ), for at least some of the multiple detection pulses (PRB), the center frequency of the detection pulse is lower than the center frequency of the measurement pulse transmitted in the measurement mode ( _S2 ). According to any one of the preceding claims, the elastic imaging device (1) is configured to characterize the liver, wherein the electronic unit (10) is configured to control the oscillator (5) so that in the guidance mode ( _S1 ), for at least some of the multiple detection pulses (PRB), the center frequency of the detection pulses is between 20 Hz and 45 Hz, and in the measurement mode ( _S2 ), the center frequency of the measurement pulse (MSR) is between 50 Hz and 200 Hz. According to any one of claims 1 to 2, the elastic imaging device (1) is configured to characterize the spleen, wherein the electronic unit is configured to control the vibrator so that in the guidance mode, for at least some of the multiple detection pulses, the center frequency of the detection pulse is between 20 Hz and 90 Hz, and in the measurement mode, the center frequency of the measurement pulse is between 100 Hz and 200 Hz. An elastic imaging device (1) according to claim 1, wherein the electronic unit (10) is configured to control the vibrator (5) so that in the guidance mode ( _S1 ), for at least some of the multiple probe pulses (PRB), the displacement amplitude ( _A1 ) of the protruding portion (4) of the probe is at least 20% lower than the displacement amplitude ( _A2 ) of the protruding portion during the measurement pulse (MSR) transmitted in the measurement mode ( _S2 ). A flexible imaging device (1) according to claim 1, wherein the electronic unit (10) is configured to control the ultrasound emitter (6) so that in the guidance mode ( _S1 ), for at least part of the multiple detection pulses (PRB), the repetition rate of the ultrasound pulse (USP) emitted for tracking how the detection pulse propagates is lower than the repetition rate of the ultrasound pulse (USP) emitted during the measurement mode ( _S2 ) for tracking how the measurement pulse (MSR) propagates. According to the flexible imaging device (1) of claim 1, the electronic unit (10) is configured to control the vibrator so that in the guidance mode ( _S1 ), the detection pulse (PRB) is transmitted at a rate of several pulses per second. The flexible imaging device (1) according to claim 1, wherein the electronic unit (10) is configured so that in the guidance mode, for at least some of the multiple detection pulses, the time lag between transmitting the detection pulse to the body of the subject and transmitting the corresponding guidance information is less than 0.5 seconds and/or less than a repetition period ( _TG ) of repeating the detection pulse (PRB). The elastic imaging device (1) according to claim 1, wherein the electronic unit (10) is configured to, in the guidance mode, for each of the plurality of detection pulses, determine tissue strain data, the tissue strain data representing tissue strain in the body region (80) to be detected as a function of time (t) and as a function of depth (z) in the detected region, the tissue strain data being determined based on at least some of the echo signals obtained in order to track how the detection pulse (PRB) propagates through the detected region. The elastic imaging device (1) according to claim 9, wherein in the guidance mode, the electronic unit (10) determines the propagation quality index (Q) based on the tissue strain data. The elastic imaging device (1) according to claim 10, wherein in the guidance mode, for each of the plurality of detection pulses: the electronic unit (10) determines a detection pulse flight time as a function of a depth (z) within the body region (80) to be detected, the detection pulse flight time being determined based on the tissue strain data; and the electronic unit (10) determines the propagation quality index (Q) so that the propagation quality index (Q) specifies whether the detection pulse flight time varies linearly and smoothly with the depth (z). The elastic imaging device (1) according to any one of claims 9 to 11, wherein in the guidance mode, the electronic unit (10) is configured to calculate average tissue strain data by averaging the tissue strain data corresponding to several detection pulses in the multiple detection pulses (PRB). The elastic imaging device (1) according to claim 9, wherein in the guidance mode, the electronic unit (10) is configured to control the display device to display an average elasticity map (101) representing the average tissue strain data as a function of time (t) and depth (z). The elastic imaging device (1) according to claim 1, wherein the electronic unit (10) is configured so that in the measurement mode: the electronic unit (10) controls the vibrator (5) to transmit one or more subsequent additional measurement pulses in addition to the measurement pulse (MSR) to the subject's body, each of the one or more subsequent additional measurement pulses being a transient low-frequency mechanical pulse; for each measurement pulse The electronic unit determines tissue strain data within the body region (80) to be detected based on echo signals obtained to track how each measurement pulse propagates through the detected area; the electronic unit determines the mechanical properties (E) related to the propagation of low-frequency elastic waves by averaging and simultaneously considering the tissue strain data associated with different measurement pulses transmitted to the body of the subject. The flexible imaging device (1) according to claim 1, wherein the electronic unit (10) is configured to adjust the center frequency of the measurement pulse (MSR) to be transmitted to the body of the subject based on the characteristics of the body region (80) to be detected of the body of the subject detected by at least one of the multiple detection pulses (PRB). The flexible imaging device (1) according to claim 15, wherein the characteristic represents the attenuation experienced by the detection pulse (PRB) during propagation in the body region (80) to be detected, and wherein the electronic unit (10) is configured to adjust the center frequency of the measurement pulse (MSR), the higher the attenuation of the resulting region, the higher the center frequency. The elastic imaging device (1) according to claim 10, wherein the electronic unit (10) is configured to: control the vibrator (5) so that during the guidance mode, after each of at least some of the multiple detection pulses (PRB): there is a delay ( _T1 '), during which the protruding portion ( ₄ ) of the probe (2) is substantially not displaced; and then there is a periodic mechanical vibration (PMV), the periodic mechanical vibration (PMV) comprising the same vibration pattern being repeated several times continuously over time, during which the vibrator has substantially no downtime; emit a series of ultrasonic pulses, and obtain responses received for tracking the periodic mechanical vibrations (PMV) determines how echo signals of the body area (80) to be detected are propagated through the body (8) of the subject located in front of the protruding portion of the probe, and determines a periodic vibration propagation quality level based on at least some of these echo signals; determines an edge proximity indicator by comparing the periodic vibration propagation quality level with the propagation quality indicator (Q), and transmits a prime edge proximity indicator. A flexible imaging method is performed by a flexible imaging device (1), the flexible imaging device (1) comprising: a probe (2) and an electronic unit (10), the probe (2) comprising a protruding portion applied to the subject's body, a low-frequency vibrator (5) arranged to move the protruding portion of the probe, at least one ultrasound transmitter, and an ultrasound receiver, the electronic unit (10) comprising an electronic circuit suitable for alternately controlling the flexible imaging device so that the flexible imaging device (1) a) operates in a guidance mode for determining whether the probe is correctly positioned at a certain position. The method comprises: a) controlling the vibrator to transmit a plurality of detection pulses to the body of the subject continuously and repeatedly through the electronic unit, each detection pulse being a transient, low-frequency mechanical pulse; and for each detection pulse: controlling the ultrasound transmitter to transmit a series of ultrasound pulses through the electronic unit, and obtaining an echo signal through the ultrasound receiver to track how the detection pulse propagates through the body located at The method comprises: determining the detected area of the body of the subject in front of the protruding portion of the probe; determining a propagation quality index based on at least some of the echo signals by the electronic unit, the propagation quality index representing the ability of the detected area to transmit the detection pulse and representing the uniformity of the propagation of the detection area relative to the detection pulse; transmitting guidance information, the guidance information being based on the propagation quality index, and then; executing the measurement mode, the measurement mode comprising: controlling the vibrator to transmit the measurement pulse to the body of the subject by the electronic unit, The measuring pulse is a transient low-frequency mechanical pulse having an amplitude higher than the amplitude of each of the multiple detection pulses; the ultrasonic transmitter is controlled by the electronic unit to emit a series of ultrasonic pulses, and the echo signal is obtained by the ultrasonic receiver to track how the measuring pulse propagates through the detected area of the body of the subject located in front of the protruding part of the probe; and the mechanical properties of the area of the body of the subject related to the propagation of low-frequency elastic waves are determined by the electronic unit based on at least some of the echo signals.

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