JPH0249103B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an ultrasonic diagnostic apparatus, and more particularly to an improved ultrasonic diagnostic apparatus capable of measuring the attenuation coefficient during propagation of ultrasonic waves in a living body.

[Conventional technology]

2. Description of the Related Art Conventional general ultrasonic diagnostic apparatuses obtain tomographic images of living tissues by a method usually called a pulse echo method.

This pulse-echo method emits an ultrasound pulse into the subject, receives the reflected waves at various depths during propagation, and displays the intensity of the received signal on a CRT as a function of the depth within the body. It is a thing,
Since ultrasound reflections occur in areas where acoustic impedance changes, the tomographic image obtained by this method is a visualization of the interface between organs and the like within the subject.

In other words, although images obtained by pulse echo imaging are an extremely effective means of obtaining morphological information about a subject, they are not sufficiently effective for diagnosing the unique characteristics of a subject. It was hot.

On the other hand, it is known that ultrasonic waves propagating in a specimen are attenuated based on frequency dependence specific to the tissue in which they propagate. That is, a subject generally has a characteristic that the attenuation rate for high frequencies is larger than that for low frequencies. It is known that the attenuation rate α(f) is proportional to the frequency f as shown in the following equation.

α(f)=α ₀ ·f (1) Here, α ₀ is an attenuation coefficient that varies depending on the tissue. It has also been reported that this attenuation coefficient differs between abnormal tissue and normal tissue, and by accurately measuring this attenuation coefficient, it becomes possible to accurately perform qualitative diagnosis of living tissues.

For this reason, the attenuation rate α(f) of the biological tissue is measured,
Ultrasonic diagnostic devices capable of qualitatively diagnosing tissues have been developed. One of them is ultrasound
There is CT (Computed Tomography). this is,
This method detects the ultrasonic waves that have passed through the subject and calculates the attenuation rate within the subject based on the intensity.

However, even with such improved ultrasound diagnostic equipment, there is a problem that effective measurements cannot be performed if there are parts of the subject that are difficult for ultrasound to pass through, such as bones or air. It was hot. In addition, the equipment used to measure transmitted ultrasound waves has become larger.
Furthermore, since the amount of calculation required when calculating the attenuation rate α(f) is enormous, a large-scale calculator is required, leading to the problem that the price of the apparatus increases.

On the other hand, an apparatus has also been developed that can measure the attenuation rate (f) from reflected waves of ultrasound waves from within the subject.

This device allows effective measurement even when there are parts of the subject that are difficult for ultrasound to pass through, such as bones or air, and the device can be made smaller because it measures reflected waves. It becomes possible to do so.

[Problem that the invention seeks to solve]

Problems with the Prior Art However, an ultrasonic diagnostic device that can measure the attenuation rate from such reflected waves requires a large-scale calculator like the conventional ultrasonic diagnostic device, making it an extremely expensive device. There was a problem with putting it away.

That is, in order to obtain the attenuation rate from the reflected wave, it is usually necessary to perform the following calculation. The power spectrum of the ultrasonic pulse incident on the probe is S ₀
(f), the power spectrum S(l,
f) is expressed by the following formula.

S(l,f)=S ₀ (f)e ^-2 〓 ^(f)l ...(2) Here, f represents the frequency of the ultrasonic wave.

Therefore, when the power spectra S(l ₀ , f) and S(l ₁ , f) of the ultrasound pulses reflected from tissues with different depths l are measured, the attenuation rate α can be calculated from the following equation.
(f) can be obtained.

α(f)=1/2(l ₁ −l ₀ )ln{S(l ₀ ,f)/S(
l ₁ , f)}...(3) However, as is clear from equation (2) above, in order to obtain the power spectrum of the received signal, the received signal must be Fourier transformed, so a fast Fourier transform device is required. (FFT), etc., resulting in an extremely expensive ultrasonic diagnostic device.

Purpose of the Invention The present invention has been made in view of the above-mentioned conventional problems, and its purpose is to measure the attenuation coefficient (attenuation rate) of ultrasonic waves by the subject from the reflected wave reception signal returned from the subject. The purpose of the present invention is to provide an inexpensive ultrasonic diagnostic device that can perform the following tasks.

[Means for solving problems]

In order to achieve the above object, the ultrasonic diagnostic apparatus according to the present invention detects when a received signal obtained by transmitting an ultrasonic pulse beam into a subject and receiving and amplifying reflected waves crosses a zero level. a zero-crossing detection section, a counting section that counts the number of zero-crossings, a zero-crossing density calculation section that converts this counted value into a zero-crossing density per unit time, and an average frequency that converts this zero-crossing density into an average frequency. an arithmetic unit, and an attenuation coefficient arithmetic unit that calculates a damping coefficient from the average frequency,
The zero point crossing detection unit detects that the received signal has passed from a negative trigger point set within a negative level to a positive trigger point set within a positive level.
a trigger circuit, detecting that the received signal passes from the positive trigger point to the negative trigger point;
DOWN trigger circuit, and the zero level of the zero crossing detection section has hysteresis characteristics.

[Effect]

As is clear from the above-described configuration, according to the ultrasonic diagnostic apparatus according to the present invention, the depth l measured by the zero-crossing detection unit, the counting unit, and the zero-crossing density calculation unit
From the zero _- crossing density N(l) of the reflected waves from
seek.

Here, σ ² (l) indicates the dispersion of the reflected wave spectrum.

Then, in the damping coefficient calculation section, this average frequency
From f _c (l), for example, the damping coefficient α ₀ is determined by the following equation.

α ₀ = 1/2σ ² (l) ・d/dl {f _c (l) − ε(l)} Here, ε(l) is a correction term specified from the depth l and the shape of the vibrator. , is a known value.

In the manner described above, it becomes possible to measure the attenuation coefficient α ₀ of the subject from the reflected wave of the ultrasonic pulse beam from within the subject.

In addition, the zero crossing detection section is equipped with an UP trigger circuit.
Since it is equipped with a DOWN trigger circuit, false detections can be eliminated even when high-frequency noise is superimposed on the received signal. In other words, the UP trigger circuit determines a zero crossing only when the received signal passes from the negative trigger point to the positive trigger point, while the DOWN trigger circuit determines a zero crossing only when the received signal passes from the positive trigger point to the negative trigger point. Since a zero crossing is determined, no matter how many times the received signal crosses one trigger point, the zero crossing will not be detected until it passes the other trigger point, and the zero crossing of the received signal can be accurately detected.

〔Example〕

Principle of the Invention The present invention calculates an attenuation coefficient from reflected waves of ultrasound obtained in time series without performing Fourier transform, and this is based on the following principle.

As mentioned above, it is well known that the attenuation rate α(f) of a living body is proportional to the frequency f of the ultrasonic wave, and is expressed as α(f)=α ₀ ·f (1).

Here, the power spectrum of the reflected ultrasound pulse is
Assume that S ₀ (f) is expressed by a Gaussian function of the following equation.

S ₀ (f)=exp{−(f−f ₀ ) ² /2σ ² } …(4) Here, f ₀ is the average frequency of the incident ultrasound pulse,
σ ² represents the spectral dispersion.

At this time, the power spectrum S(l,f) of the echo signal reflected from the part at depth l inside the subject
is expressed by the following equation.

S(l,f)=S ₀ (f)e ^- 〓 ₀ f・(2l) =exp{−(f−f ₀ ) ² /2σ ² −α ₀ f(2l)} =exp{−[f− fC(l)] ² /2σ ² } ・exp{−α ₀ l+2σ ² α ² ₀ l ² } …(5) However, f _c (l)=f ₀ −2σ ² α ₀ l …(6) .

That is, according to equations (4) and (5) above, the power spectrum of the reflected wave reflected from the site at depth l has the same Gaussian function as the incident wave, but its average frequency f _c (l) is 2σ ² α ₀ l shifts to the lower frequency side,
It is also understood that the amplitude is attenuated by −α ₀ l + 2σ α ² ₀ l ² .

Therefore, if the average frequency f _c (l) can be measured,
From equation (6), the damping coefficient α ₀ is obtained as α ₀ =−1/2σ ² ·d/dlf _c (l) (7).

Here, the following method can be used to obtain the average frequency f _c (l) from the echo signal.

Considering an RF signal p(t) whose power spectrum is represented by P(f), the number of times p(t) crosses zero per unit time, that is, the zero crossing density N is P
(f) is known to have the following relationship.

N=2 [∫ ^∞ / _-∞ f ² P (f) df / ∫ ^∞ / _-∞ P (f) d
f] ^1/2 …(8) That is, N ² /4=∫ ^∞ / _-∞ f ² P (f) df / ∫ ^∞ / _-∞ P (f) d
f…(9) becomes.

When this relationship is applied to the echo signal, it becomes as follows.

In other words, if the number of times a reflected wave reflected from a site at depth l inside the subject crosses zero per unit time, that is, the zero crossing density, is expressed as N(l), then N ₂ (l)/4=∫ ^∞ / _-∞ f ² S (l, f) df / ∫ ^∞ / _-
∞ S (l, f) df…(10).

Also, S(l,f) has a variance of σ ² and an average of f _c (l)
Since it is a Gaussian function, the following formula holds.

σ ² =∫ ^∞ / _-∞ {f−f _c (l)} ² S(l,f)df/∫
^∞ / _-∞ S (l, f) df = ∫ ^∞ / _-∞ f ² S (l, f) df / ∫ ^∞ / _-∞ S (l
, f) df−f ² _c (l) = N ² (l)/4−f ² _c (l) …(11) Therefore, becomes.

In equation (12), σ ² is the dispersion of the power spectrum of the incident ultrasound pulse, and this is a known value.

Therefore, by measuring the zero crossing density N(l), it becomes possible to obtain the average frequency f _c (l) using the above equation (12), and furthermore, using the above equation (7), the attenuation coefficient α ₀
can be found.

By the way, the power spectrum of the incident wave in the ultrasonic pulse beam of an ultrasonic diagnostic device is generally
In many cases, it does not follow a Gaussian function.

Therefore, the above-mentioned principle cannot be directly applied to an ultrasonic diagnostic apparatus, and a case where the power spectrum is not a Gaussian function will be described next.

If the power spectrum is not a Gaussian function, the dispersion of the spectrum of the echo signal reflected from the site at depth l is a function of depth l, and this is expressed as σ ² (l). At this time, the equation corresponding to the above equation (12) is becomes.

Also, from the definition of average frequency, f _c (l)=∫f・S(l,f)df/∫S(l,f)dfdf
...(14) Then, similar to equation (5) above, S(l,f)=S ₀ (l,f)e ^- 〓 ₀ ^{f(2 l)} ...(15) Substitute into equation (14) above. However, by differentiating both sides with respect to l, the following equation can be derived. However, S ₀ (f) does not need to be a Gaussian function here.

d/dlf _c (l) = −2αf・{∫f ²・S(l, f) df/∫S(l
, f) df − (∫ fS (l, f) df/∫S (l, f) df) ²
} =−2α ₀ fσ ² (l) …(16) Therefore, as a formula corresponding to the above formula (7), α ₀ =−1/2σ ² (l)・d/dlf _c (l) …(17 ) can be obtained.

By the way, the above principle is based on the premise that ultrasonic waves propagate as plane waves.

However, a vibrator used in an ultrasonic diagnostic device has a focal point, and it is impossible for ultrasonic waves emitted from a normal ultrasonic diagnostic device to propagate as a plane wave. Therefore, it is necessary to modify the above equation (17) as shown in the following equation.

α ₀ =1/2σ ₂ (l)・d/dl{f _c (l)−ε(l)
} Here, ε(l) is a correction term related to the ultrasonic sound field that depends on the depth l and the shape of the transducer, and is a known value.

In other words, when the ultrasonic wave is a plane wave, the average frequency of the reflected wave reflected from a reflector placed in an unattenuated propagation medium such as water does not depend on the depth l of the reflector and is always A constant value.

However, if the ultrasound is not a plane wave, the average frequency changes depending on the depth l. Correction term ε
(l) is a constant for correcting such changes depending on the depth l, and can be determined by calculation or experimentally.

As described above, in a normal ultrasonic diagnostic apparatus, the spectrum of an incident ultrasonic pulse does not follow a Gaussian function, so the attenuation coefficient is determined from equations (13) and (18).

That is, once the dispersion σ ² (l) of the spectrum of the reflected wave reflected from the site within the subject is determined for various depths l within the subject, the zero-crossing density (l) of this reflected wave can be measured. and the center frequency f _c
(l) is obtained, and furthermore, this f _c (l) is calculated from the above (18).
By substituting into the formula, it is possible to obtain the damping coefficient α ₀ .

Here, σ ² (l) is the dispersion of the spectrum of the reflected wave reflected from a site at depth l within the subject, and originally varies depending on the attenuation coefficient of the subject.

However, since the frequency bandwidth of the reflected wave is relatively narrow, its dependence on the attenuation coefficient is considered to be extremely low in practice.

Therefore, using a material with an attenuation coefficient that approximates that of a living body, reflectors are placed at various depths within the material, and the above σ ² (l) can be determined experimentally from the reflected waves reflected from the material. It is possible to ask for
Furthermore, it is well known that the ultrasonic waveform in the damping medium can be calculated from the shape of the transducer and the position of the waveform observation point, and it is also possible to determine σ ² (l) by calculation.

Specific Embodiments Preferred embodiments of the present invention will be described below based on the drawings.

FIG. 1 shows a block diagram of the main parts of an ultrasonic diagnostic apparatus according to the present invention.

In this embodiment, the ultrasonic diagnostic apparatus includes a probe 100, an ultrasonic wave transmission/reception control section 200, a zero crossing detection section 300, a counting section 400, and a zero crossing density calculation section 50.
0, average frequency calculation section 600, attenuation coefficient calculation section 7
00 and a display section 800.

In this embodiment, the ultrasonic wave transmission/reception control section 200
The configuration is similar to that of an ultrasonic wave transmission/reception control section of a conventional ultrasonic diagnostic apparatus using the pulse echo method, and includes a control circuit 210, an oscillator 220, and an amplifier 230. Then, under the control of the control circuit 210, the oscillator 220 outputs a high-voltage electric pulse, and by applying this to the probe 100, an ultrasonic pulse is emitted from the probe 100.

The reflected wave reflected from inside the subject is transmitted to probe 1.
00 is received and converted into an electrical signal, and after being amplified by the amplifier 230, the zero crossing detection unit 300
is output to.

The zero crossing detection section 300 outputs an electric pulse to the counting section 400 every time the input signal from the ultrasonic wave transmission/reception control section 200 crosses zero.

In the counting section 400, a zero crossing detection section 30
Sequentially integrates and counts pulse signals starting from 0. This count value is output to the zero crossing density calculation section 500.

In this embodiment, the zero crossing density calculating section 500 includes latch circuits 510 and 520 and a subtracter 530, and the count signal from the counting section 400 is input to the latch circuits 510 and 520.

These two latch circuits 510 and 520 are controlled by timing signals s ₁ and s ₂ from the control circuit 210, and temporarily store the counting results from the counting section 400 until the timing signals s ₁ and s ₂ are input. to hold.

A time difference is provided between the timing signals s ₁ and s ₂ by an appropriately set time τ, and the count value held at the input interval of these timing signals s ₁ and s ₂ is input to the subtracter 530. The subtracter 530 calculates the difference between the counts held by both latch circuits 510 and 520, which is the number of times the input signal crosses zero during time τ. Therefore, if time τ is taken as unit time, this difference value represents the zero crossing density.

That is, if the timing signals s ₁ and s ₂ are sequentially generated at intervals of Δt, the generation times t ₁ and t ₂ of the timing signals s ₁ and s ₂ can be expressed by the following equation.

t ₁ = Δt _i i=0,1,2... t ₂ = Δt _i +τ (19) At this time, the zero-crossing density obtained using the i-th timing signal is expressed as , the zero-crossing density (hereinafter referred to as _{N i}
). Also, the variance σ ² corresponding to this depth l=(Δt _i +τ/2)・C/2
(l) and correction term ε(l) are expressed as σ _i and ε _i , respectively.

The output N _i of this zero crossing density calculation section 500 is output to the average frequency calculation section 600.

The average frequency calculation unit 600 includes an average frequency converter 610 and an adder 620, and the output N _i of the zero crossing density calculation unit 500 is calculated by the average frequency converter 610.
It is input to 0.

The converter 610 converts the zero crossing density N _i to the corresponding average frequency f _ci using the relationship of equation (13), and performs the calculation √ _i ² 4− _i ² Become.

It is also preferable that the converter 610 be configured as follows.

That is, since σ _i ² is a known constant that depends on the depth l (=i), the average frequency f _ci corresponding to various σ _i and N _i is calculated in advance, and this is stored in the ROM. put. And, this converter 610 is
By reading out f _ci from the ROM according to the values of σ _i and N _i and outputting it, it is possible to convert it into an average frequency f _ci .

Furthermore, if the average frequency f _ci after sound field correction is calculated in advance, including the correction term ε _i , and stored in the ROM as described above, the average frequency converter 610 can correct the sound field. becomes possible.

f′ _ci = f _ci −ε _i =√ ² 4− _i ² −ε _i …(20) In other words, since σ _i and ε _i depend only on the depth i in the living body, this depth i and the zero crossing density If a two-dimensional table with N _i is created in the ROM, it is possible to obtain the average frequency f _ci from this table.

The average frequency f _ci calculated by this average frequency converter 610 is input to an adder 620.

That is, the ultrasonic reflection coefficient of a living body, which is a subject, varies depending on time and position, and is considered to have no fixed value.

Therefore, it is expected that the average frequency f _ci (or f' _ci ) obtained from the reflected waves reflected from such a living body will fluctuate around its expected value. Therefore,
The adder 620 adds and averages the average frequency f _ci (or f' _ci ) to reduce error magnification due to fluctuations in the reflection coefficient of the living body.

The average frequency f _ci (or
f′ _ci ) is output to the damping coefficient calculating section 700.

The attenuation coefficient calculation section 700 includes a memory 710 and a calculation unit 720, and the output from the average frequency calculation section 600, that is, the average frequency f _ci (or f' _ci ) is stored in the memory 710.

The arithmetic unit 720 performs an operation corresponding to the above equation (18), and this equation (18) includes differentiation. However, since the data in the memory 710 is discrete, it is sufficient for the arithmetic unit 720 to perform the arithmetic operation expressed by the following equation in which the differential of the equation (18) is replaced with a difference.

α ₀ =−1/2σ _i ²・f′ _ci +1−f′ _ci /Δt・C…(2
1) That is, read the corrected average frequencies f′ _ci and f′ _ci+1 from the memory 710, divide these difference values by the known constant Δt・C, and further calculate the difference by the spectral dispersion σ _i ² . The arithmetic unit 720 may be configured to perform division. In addition, σ _i ² is required to perform the calculation shown in equation (21), but this can be obtained by storing it in advance in the memory 710 as described above and reading it from there as necessary. Can be done.

Note that when the data in the memory 710 is the average frequency f _ci for which no sound field correction has been performed yet, the memory 710 stores a correction term ε _i in addition to the variance σ _i ² . Before performing the calculation of equation (21), ε _i may be subtracted from the average frequency f _ci .

Furthermore, measurement accuracy can be improved by finding the average of the attenuation coefficients found for various depths i in equation (21).

The attenuation coefficient α ₀ obtained as described above is displayed on the display section 800 as a specific numerical value or the like.

As explained above, according to the ultrasonic diagnostic apparatus according to the present embodiment, it is possible to obtain an attenuation coefficient from reflected waves without using a fast Fourier transform device or the like.

Note that the zero crossing detection unit 300 outputs an electric pulse every time the input signal from the ultrasonic wave transmission/reception control unit 200 crosses zero, and it outputs an electric pulse every time the input signal actually crosses zero. If such a circuit configuration is used, the following problems may occur.

In other words, as shown in Figure 2A, if the input signal is a sine wave with noise superimposed on it, if we simply output an electric pulse every time it crosses zero, the result will be as shown in Figure 2B. As shown, there may be a situation where a plurality of electrical pulses are output before and after the signal that should originally be obtained crosses zero.

Therefore, in the present invention, in order to provide a hysteresis characteristic to the zero level as shown in FIG. 3A, two positive and negative levels +L and -L are set, and when the input signal changes from positive to negative, the -L level is set. It is preferable to consider that it has crossed zero when it crosses , or when it crosses the +L level in the case of moving from negative to positive.

An example of such a circuit configuration is shown in FIG. 4.

In the figure, when the input signal is decreasing from a large positive value, the output of the comparator 310 is a negative value, and the negative value obtained by dividing this voltage by resistance is compared with the input signal. Ru. On the other hand, when the input signal increases from a negative value, the output of comparator 310 is positive, and therefore the input signal will be compared with a positive voltage. Monostable multivibrators 320 and 330 are connected to the comparator 310, and the monostable multivibrator 320 outputs an electric pulse when the input voltage rises, and the monostable multivibrator 330 outputs an electric pulse when the input voltage falls. do.

By inputting both of these electric pulse signals to the OR gate 340, it is possible to obtain an electric pulse signal when the input signal crosses +L or -L.

Therefore, extremely accurate measurement is possible even when noise is superimposed on the sine wave.

〔Effect of the invention〕

As explained above, according to the ultrasonic diagnostic apparatus according to the present invention, it is possible to inexpensively obtain an ultrasonic diagnostic apparatus that can calculate an attenuation coefficient at high speed without performing frequency analysis on an echo signal. .

Then, a UP trigger circuit is installed in the zero crossing detection section.
Since a DOWN trigger circuit is provided, it is possible to measure the average frequency extremely accurately even when noise is superimposed on the received signal, and with this, it is possible to improve the accuracy of the calculated attenuation coefficient. be.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the main parts of an ultrasonic diagnostic apparatus according to the present invention, FIGS. 2 and 3 are explanatory diagrams of an improved state of the zero-crossing detection section, and FIG. 4 is a diagram according to a second embodiment of the present invention. FIG. 3 is a block diagram of a zero crossing detection section. 100... Probe, 200... Ultrasonic wave transmission/reception control section, 300... Zero crossing detection section, 400... Counting section, 500... Zero crossing density computing section, 600... Average frequency computing section, 700... ...Attenuation coefficient calculation section, 8
00...Display section.

Claims (1)

[Claims] 1. A zero-crossing detection unit that detects that a received signal obtained by transmitting an ultrasonic pulse beam into a subject and receiving and amplifying reflected waves crosses a zero level, and a zero-crossing detection unit that detects the number of zero-crossings. A counting section that counts, a zero-crossing density calculation section that converts this count value into a zero-crossing density per unit time, an average frequency calculation section that converts this zero-crossing density into an average frequency, and a damping coefficient that is calculated from this average frequency. an attenuation coefficient calculation unit to obtain, and the zero point crossing detection unit detects that the received signal has passed from a negative trigger point set within a negative level to a positive trigger point set within a positive level. a trigger circuit; and a DOWN trigger circuit that detects that the received signal has passed from the positive trigger point to the negative trigger point, and the zero level of the zero crossing detection section has hysteresis characteristics. Ultrasound diagnostic equipment.