JPS6038644A - Inspection equipment using nuclear magnetic resonance - Google Patents

Abstract

PURPOSE:To perfectly correct the distortion of projection data, by controlling at least one of the application start time or the amplitude of an oblique magnetic field so as to align the phases of signals from objects to be inspected to plural components of the applied oblique magnetic field prior to the detection of projection data. CONSTITUTION:The output of a high frequency pulse generator 5 excites a coil 7 through a power amplifier 6 under the control of a control apparatus 4. The signal from an object 19 is received by the coil 7 and converted to an image by a signal processing apparatus 10 through an amplifier 8 and a detector 9. On the other hand, pulse sequence is inputted to the pulse shaping device 23 in the control apparatus 3 prior to the detection of projection data and the delay or shaping of a pulse is performed on the basis of the information stored in a memory means 22 so as to align the phases of signals for the object 19 to be inspected. By this mechanism, the distortion of projection data caused by rising and falling characteristics of an oblique magnetic field at the time of signal detection is perfectly corrected.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an inspection device using nuclear magnetic resonance (hereinafter referred to as rNMRJ), and particularly relates to an inspection apparatus using nuclear magnetic resonance (hereinafter referred to as rNMRJ). The present invention relates to an inspection device using NMR that can completely correct data distortion.

[Background of the invention]

2. Description of the Related Art Conventionally, X-ray CT and ultrasonic imaging devices have been widely used as devices for non-destructively inspecting internal structures such as the head and abdomen of a human body. In recent years, attempts to perform similar tests using NMR phenomena have been successful, and it has become clear that information that cannot be obtained with X-ray CT or ultrasound imaging devices can be obtained. N.M.
In an inspection apparatus using the R phenomenon (hereinafter simply referred to as an "inspection apparatus"), it is necessary to separate and identify signals from an object to be inspected in correspondence with each part of the object. One way to avoid this is to apply a gradient magnetic field to the target object to vary the static magnetic field placed on each part of the target object, thereby varying the resonance frequency of each part of the target object to obtain position information. be.

FIG. 1 is a diagram for explaining the principle of the above method. When a gradient magnetic field Gt is applied to the target object 1, the signal intensity distribution 2, which is the integration of signals from all nuclear spins (hereinafter simply referred to as "signals") on a line perpendicular to the magnetic field G1, becomes equal to the static magnetic field In the N M It phenomenon, the relationship f = γH / (2π) holds, so the signal strength depends on the frequency f of the high-frequency magnetic field.
It is also a function of Here, r is the nuclear gyromagnetic ratio, which is a value specific to the nucleus. Next, change the direction of applying the gradient magnetic field and
2, a signal strength distribution of 3 is obtained. By changing the applied direction of the gradient magnetic field in various ways and obtaining similar signal intensity distributions, that is, projection data, the same algorithm as in X-ray CT can be used to determine the nuclear spin density distribution, relaxation time distribution, etc. of the target object 1. You can

In reality, the object to be inspected is three-dimensional, so the projection as described above is performed from the three-dimensional direction, or alternatively, first select a slice and limit the area to a two-dimensional plane, and It is necessary to decide from the direction. This decision takes into account the measurement time and the area 4 to be inspected. The following explanation will be given using the latter method of first selecting a slice as an example, but the present invention is not limited to this method (it goes without saying that υ is not the method).

In conventional NMR and imaging, the gradient magnetic field applied to obtain projection data is, for example, Lancet
Magazine (F, H, Doyle et al, p5
3-57 (1981)), the second
The sequence is as shown in the figure. That is, first slices perpendicular to two directions are cut out using a 90 degree selective irradiation pulse and a gradient magnetic field in two directions (Jz, and then a gradient magnetic field GII+ whose direction changes with each projection on the x-y plane is applied to generate various images. This method captures projection data from different directions.

In FIG. 2, the reason why the gradient magnetic field GE in two directions is reversed after the 90 degree pulse is due to the irradiation time of the pulse (1 to 10
This is to restore the phase dispersion of nuclear spins within the slice that has occurred.

By the way, the gradient magnetic field GR applied to perform the projection
does not necessarily show the ideal or ideal rise as shown in FIG. 2, but has a rise delay time of about 0.5 to 1 m1B+ at maximum. Further, the signal recovered by the reversal of the gradient magnetic field Gx begins to rapidly attenuate after the gradient magnetic field GR is applied. Since signal detection starts at 8 pm with the application of the gradient magnetic field GRO, conventional NM as described above
R imaging has a serious problem in that the rise delay time of the gradient magnetic field GR causes a significant deterioration in image quality.

In order to solve the above-mentioned problem, several methods can be considered, and one of them is a method of making the shape of the gradient magnetic field Gm as shown in FIG. 3.

In this method, a pulse with the polarity of the gradient magnetic field GR reversed is first applied for a certain period of time, and the time at which the signal is generated is delayed, and the peak of the signal will occur at time t6, which satisfies the following equation. .

Therefore, the signal is detected by regarding the time t6 as the peak of the signal, but this has the following serious problem. This is a problem in that when the time (tl or t13 described above) at which a drive command is issued to the gradient magnetic field drive power supply is fixed, the time t6 changes depending on the amplitude of the gradient magnetic field GR. This will be explained in detail below.

Fall time of gradient magnetic field: ff-12t, Rise time:
tr=14-13 is generally the amplitude GR of the gradient magnetic field Gy+
Since it is proportional to o, when changing the gradient magnetic field per unit time (slew rate) from negative to positive: K s
(G/IiX”), from positive to negative: Kz
(G/FEE'), then from the above equation (1), t
6 can be rewritten as follows.

Since the above Ki + K2 is a value specific to the inspection device, from the above equation, t6 is 1. ,．． 13 and the gradient magnetic field GWI
It can be seen that it is a function of the amplitude GRO. Therefore, for the amplitude GRO at the time t1. Once t3 is determined, the peak of the signal will be at time t6 shown by the above equation (2), and if the signal is processed with this as the center, correct projection data will be obtained. however,
The amplitude G of the gradient magnetic field in order to perform projection from another angle
If io is changed, the time t6 will be a different value, and the signal peak will occur at a different point. Therefore, if this signal is processed in the same way as before, there is a problem that a phase shift occurs in the projection data, and the image constructed from it becomes distorted. Note that in reality, in order to perform projection from various directions on the x-y plane, two mutually orthogonal gradient magnetic fields G, , Ga are combined, so a similar situation occurs for each of them. That's what happens.

[Purpose of the invention]

The present invention has been made in view of the above circumstances, and its purpose is to solve the above-mentioned problems in conventional NMR-based inspection equipment, and to improve the rise and fall characteristics of gradient magnetic fields during signal inspection. The object of the present invention is to provide an inspection device that can completely correct distortion of projection data caused by the distortion.

[Summary of the invention]

The gist of the present invention is that, prior to the detection of projection data, the application start time of the gradient magnetic field is adjusted so that the phases of the signals from the inspection object for each of the plurality of components of the applied gradient magnetic field are aligned;
The point is that one of the amplitudes or waveforms is controlled.

[Embodiments of the invention]

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 4 shows the configuration of an inspection device that is an embodiment of the present invention. In the figure, 4 is a control device, 5 is a high-frequency pulse generator, 6 is a power amplifier, 7 is a coil for generating a high-frequency magnetic field and at the same time detecting the signal generated from the target object 19, 8 is an amplifier, and 9 is a detector. , 10 is a signal processing device. In addition, 11, 12 and 13 leaves each have 2
Coils 14, 15, and 16 for generating gradient magnetic fields in the direction and in the direction perpendicular thereto are the coils 11゜1, respectively.
This is the power supply section that drives the 2.13.

The control device R4 has a function of outputting various commands to each device at a constant timing, and the storage device in the control device 4 is also equipped with a pulse shaping function of the present invention, which will be described later. The output of the high frequency pulse generator 5 is amplified by a power amplifier 6 and excites the coil 7. The coil 7 also serves as a receiving coil as described above, and the received signal component passes through the amplifier 8 and is detected by the detector 9, and then converted into an image by the signal processing device 10.

Note that the static magnetic field is generated by a coil 17 driven by a power source 18. The human body 19, which is the object to be inspected, is on the bed 20.
The bed 20 is configured to be movable on a support base 21.

The operation of this embodiment configured as described above will be explained below.

When applying an inverted gradient magnetic field prior to detection of projection data and aligning the phases of the signals from the inspection object with respect to each of the plurality of components of the applied gradient magnetic field, from equation (2) above, the gradient Gradient magnetic field application start time tl according to the magnetic field amplitude GRO! A possible method is to change at least one of '3 or to change the negative amplitude Goo of the gradient magnetic field.

Now, let us consider the case where time t1 shown in FIG. 3 is taken as the origin and time t3 is changed according to the amplitude GRO of the gradient magnetic field. According to the above formula (2), it becomes. Here, if s + K1 + K2 is constant and GRO changes to cHto, the above formula (
In order for 2) to hold, the above time t3 must be set to another value t3'.
It is necessary to do so. Formula (2), above formula (3)
From Equation 1 (4), the difference Δt between the above times t3 and t3 is as follows. Therefore, the amplitude of the gradient magnetic field is oW from GRO.

If the above time 1≦ is set as bow=t3+Δt, the time at which the signal reaches its peak will not change.

FIG. 5 shows the configuration of a portion of the control device 4 that executes the above functions. In the figure, 22 is a storage means (ROM) that stores the value of Δt according to the projection angle.
or I'LAM), 23 is a pulse shaper. The value of Δt stored in the storage means 22 is, for example, expressed by the above formula (
5) It is a calculated value or an actually measured value, and x+
The value corresponds to the gradient magnetic field in the Y*" direction. Therefore, it is necessary to prepare a maximum of three circuits as shown in FIG.

In the above circuit, a conventional sequence of pulses is input to the pulse shaper 23, and the pulses are delayed and shaped based on the information from the storage means 22 according to the projection angle, and a distortion-free signal is detected. This has the effect of making it possible.

In the above embodiment, the case where the gradient magnetic field is reversed in order to measure a signal is given as an example, but the present invention is not limited to this. For example, even when using a 180 degree pulse, the same effect as in the above embodiment can be achieved by making the integrated values of the gradient magnetic fields before and after the 180 degree pulse equal.

Further, in the above description, it is assumed that the signal peaks always occur at the same time, but this condition does not necessarily have to be satisfied. What is important is that in each projection, x, y
, the integral value of the gradient magnetic field applied in each direction of z all becomes substantially zero at a certain time, and it is acceptable that this time varies slightly for each projection. It is from. In practice, it is considered that the above tolerance should be sufficiently small compared to the transverse relaxation time of the object to be inspected and the decay time due to non-uniformity of the static magnetic field. In addition, in this case, it is necessary to process the occurrence time of the signal peak as shifted according to the projection, but the amount of shift can be calculated using the above equation (5), and can be easily corrected. It is possible to do so.

Note that depending on the device, the slew rate 11K2 may not be constant and may depend on the amplitude of the gradient magnetic field, but even in that case, the Δt may be calculated so as to satisfy the above-mentioned conditions, for example, the equation (1). It is possible to shape the pulse sequence by actually measuring the pulse sequence.

Also, on the contrary, + to the above slew rate! By varying the amplitude of the gradient magnetic field, it is also possible to align the peak occurrence times of the signals.

Although the above description has been made with the projection-reconstruction method in mind, the present invention is not limited to this, and is widely applicable to all imaging methods that use gradient magnetic fields to form echo signals. It goes without saying that there is.

〔Effect of the invention〕

As described above, according to the present invention, the phases of the signals from the inspection object for each of the plurality of components of the applied gradient magnetic field are aligned prior to the detection of projection data, so that the rising edge of the gradient magnetic field at the time of signal detection is Furthermore, it is possible to realize an inspection apparatus that can completely correct the distortion of projection data caused by the falling characteristic.

[Brief explanation of the drawing]

Figure 1 shows the principle of imaging, Figure 2 shows the pulse sequence of the conventional imaging method, and Figure 3:
FIG. 4 is a block diagram showing an embodiment of the present invention, and FIG. 5 is a block diagram showing the main part thereof. 4...Control device, 5...High frequency pulse generator, 6.
...Power amplifier, 7, 11, 12.13... Coil,
8... Amplifier, 9... Detector, 10... Signal processing device, 22... Storage device, 23... Pulse shaper,
Gz...2 directions 1st ward 2 Figure 4 Eye 5 1 ■- 111\, →Output''22 Continuation of page 1 @ Inventor Take 1) Ryuobu [Co., Ltd.] Inventor Tsu 1 ) Munetaka @ Inventor Yoshio Tajima

Claims (1)

[Claims] 1. Magnetic field generating means for a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field, a signal detecting means for detecting a nuclear magnetic resonance signal from an object to be examined, and calculating a detection signal of the signal detecting means. In an inspection apparatus using nuclear magnetic resonance that includes a computer and means for outputting calculation results by the computer, the gradient magnetic field is adjusted such that the phase of the signal from the inspection object for each of the plurality of components of the applied gradient magnetic field is aligned. An inspection apparatus using nuclear magnetic resonance, characterized in that it is provided with means for controlling at least one of the application start time, amplitude, or waveform of each component. 2. The control means is configured to apply a gradient magnetic field having a polarity opposite to that of the gradient magnetic field applied when measuring a signal, prior to applying the gradient magnetic field when measuring a signal. An inspection device that uses nuclear magnetic resonance. 3. The control means is configured to apply a gradient magnetic field of the same polarity as the gradient magnetic field applied when measuring the signal, before applying the 180 degree high frequency pulsed magnetic field for signal formation. An inspection device using nuclear magnetic resonance according to claim 1, characterized in that: 4. An inspection apparatus using nuclear magnetic resonance according to claims 1 to 3, wherein the control means is configured to control the slew rate of the gradient magnetic field.