JPH0712352B2 - Image reproducing method in inspection apparatus using NMR - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method of measuring information inside an object using nuclear magnetic resonance (NMR), and in particular, imaging using a periodic change of a gradient magnetic field. Method, an image reproducing method for reproducing an image from a signal observed by using an arbitrary periodic waveform.

[Background of the Invention]

NMR imaging generally uses a time-varying magnetic field gradient. In particular, Echo Pla
nar) method [Reference: Journal of Magnetics]
Resonance (Journal of Magnetic Resonance) 29,355
(1978)] or a fast chemical shift imaging method using a reversing gradient magnetic field (reference: Proceedings of the 23rd NMR Conference, PP263-266, 1984), a gradient magnetic field is applied periodically.
Observe the signal. In these methods, it is necessary to use an accurate rectangular wave as the gradient magnetic field. However, it is difficult to accurately control the waveform of the actually generated gradient magnetic field, including the characteristics of the gradient magnetic field generation system. Such control inaccuracies have an unfavorable effect on image quality. To compensate for this effect, sampling at non-equal intervals [Reference: Journal of Physics E17,612 (1984)], sampling at equal intervals, and then multiplying by an appropriate weighting factor Method of T _ROPPER to Fourier transform from [Reference: Journal of Magnetic Resonance (Journal of Magnetic
Resonance) 42, 193 (1981)], a method of reproducing an image by taking an appropriate linear sum after Fourier transformation (see Applied Physics 22,
257 (1980)], etc. have been proposed. In all of these methods, the waveform of the gradient magnetic field actually generated needs to be known.

As a method of obtaining this waveform, a method using the ratio of the derivative dF (t) / dt of the NMR signal F (t) and F (t) has been proposed (see: Yamamoto et al .; The Institute of Electrical Communication, Medical Electronics). Bioengineering Research Group 84-P59). However, this method cannot eliminate the effects of static magnetic field inhomogeneity and lateral relaxation.
It is essential to use a small sample,
Since the demand for the shape is strict and the derivative is used,
There were problems such as being sensitive to noise.

[Object of the Invention]

The present invention has been made paying attention to such a point, and a general method for measuring the time dependence of a gradient magnetic field in a form suitable for image reproduction, and an actually measured gradient with respect to an arbitrary periodic waveform. An object of the present invention is to provide an image reproducing method for reproducing an image with little deterioration using the time dependence of a magnetic field.

[Outline of Invention]

According to the present invention, a phase-encoding gradient magnetic field is applied in the same direction as the time-varying gradient magnetic field in advance, and the result of Fourier transform only in the encoding direction is measured in the same manner without applying the time-varying gradient magnetic field. Focusing on the fact that information about only the time dependence of the gradient magnetic field can be extracted by removing the effect of static magnetic field inhomogeneity and lateral relaxation by dividing by the result obtained by using The essence is to reproduce the image from the signal observed under the periodic change of the gradient magnetic field.

The details will be described below. Time dependence G _x (t) = G _x g
The NMR signal intensity observed when an x-direction gradient magnetic field having (t) is applied is expressed by the following equation using a complex number.

Where ρ (x, y, z) is the spin density, mg _{l tl} is the encoding size (−M / 2 ≦ m <M / 2), G _x and g (t)
Is the maximum value of the gradient magnetic field that changes with time, and the normalized time dependence, H _I (x, y, z) is the inhomogeneity of the static magnetic field, and E (x, y, z, t) is Contribution of lateral relaxation, X ₁ , X ₂ , Y ₁ , Y ₂ , Z ₁ , Z ₂
Represents the upper and lower limits of the spin density distribution. Here, the following conditions are assumed.

(K-1) Δx ≦ x ≦ (k + 1) Δx (Δx≡1 / γg _l
t _l M, −M / 2 ≦ k <M / 2) …… (2) For x, ρ (x, y, z) ρ (kΔx, y, z) …… (3) E (x, y, z) E (kΔx, y, z) (4) The above conditions can be easily achieved under normal measurement conditions by setting g _l t _l M to be sufficiently large and appropriately selecting the size and shape of the sample.

Under the above conditions, the result of the discrete Fourier transform on the phase encode m can be easily integrated by x, and becomes the following.

By the way, the following equation is obtained by performing exactly the same processing even when the time-varying gradient magnetic field is not applied.

Here, if the ratios of equations (7) and (8) are taken, Becomes Therefore, with and without applying a time-varying gradient magnetic field, the measurement as described above is performed, and by taking the ratio as a complex number, the inhomogeneity of the static magnetic field and the effect of lateral relaxation are removed, It can be seen that only information on the time dependence of the gradient magnetic field can be extracted.

Now, according to the reproduction method of T _ROPPER described above, it is possible to immediately reproduce an image using the result given by the equation (9). That is, after performing measurement using a gradient magnetic field that periodically changes with an arbitrary waveform, the weighting factor F (kΔx of Eq. (9) obtained by using the same gradient magnetic field waveform for the obtained NMR signal is used. , t) and then Fourier transform to obtain a projection image in the direction perpendicular to x.

Furthermore, from equation (9) Other regeneration methods described above can also be performed by calculating

To obtain, the complex logarithm of F (kΔx, t) should be taken. At this time, the indeterminacy of 2nπ (n: integer) remains, but from the conditions (2) and (5), it is guaranteed that the phase angle is minute for small k, so it must be uniquely determined. You can Alternatively, the first-order approximation of the complex logarithm may be used by utilizing the fact that the phase angle is minute. Thus calculated for each k Can be obtained by taking an average or a weighted average with respect to. The various regeneration methods described above were defined here using the time integration of the gradient field, and were obtained here. Can be used to reconstruct an image with less deterioration. further By considering the time derivative of
_x g (t) can be determined.

Example of Invention

The configuration and operation of a typical device used when implementing the present invention will be described below. FIG. 1 shows a block diagram of this device. This device comprises a sequencer 12, a transmission system 13, a reception system 14, a magnetic field gradient generation system 16, a signal processing system 17 and a static magnetic field generation magnet 15, which operate under the control of the CPU 11 of the on-board device. The sequencer sends the various instructions necessary for the method of the present invention to each device. The transmission system includes a high frequency oscillator 131, a modulator 132, and a high frequency amplifier 133. A high frequency magnetic field (H ₁ ) is applied to the target object 20 by supplying a high frequency pulse amplitude-modulated according to a command to a high frequency coil 134. It The magnetic field gradient generation system is a gradient magnetic field coil 160 wound in three directions of z, y, and z.
And a driver 161 for each coil, and the gradient magnetic fields G _X , G _Y ,
G _Z is applied to the target object 20. The response due to the application of these magnetic fields is received by the receiving system 14 through the coil 134 described above. The receiving system is amplifier 141, quadrature detector 142, A / D converter 1
Two series of data, which have 43 and are sampled by quadrature detection at the timing instructed by the sequencer 12, are sent to the signal processing system. In the signal processing system 17, Fourier transform, correction coefficient calculation image reconstruction, etc. are processed, and the signal intensity distribution of an arbitrary cross section or the distribution obtained by performing an appropriate calculation on a plurality of signals is imaged, for example, on a CRT display 171. indicate.

The present embodiment is different from the conventional apparatus in that it has a function of actually measuring the time dependence of the gradient magnetic field according to the above-mentioned principle, and a function of reproducing an image based on the function. An example of a pulse sequence for measuring the time dependence of the gradient magnetic field is shown in FIG. Here, the 90 ° pulse and the 180 ° pulse are written as non-selective excitation pulses, but they may be selective excitation pulses. Sampling is performed while sequentially changing the magnitude of phase encoding according to this pulse sequence.
Further, subsequently, as shown in FIG. 3, sampling is performed without applying a time-varying gradient magnetic field. In this way, the two sets of data obtained by the quadrature detection are regarded as complex numbers, and the division is performed as described above to obtain the time dependence of the gradient magnetic field as expressed by the equation (9). Get information about. further,
From equation (9), by calculating the phase angle as described above, Is determined.

The results obtained in this way are stored in memory, on a disk, etc., and then Echo-Planar
Method or a method such as the above-described high-speed chemical shift imaging method is used to reproduce an image measured using a gradient magnetic field having the same time dependence as above. As previously mentioned,
This method is a method of reconstructing an image by multiplying the measured NMR signal by an appropriate weighting coefficient and then performing a Fourier transform. In the present embodiment, in order to determine the weighting coefficient, reproduction is performed using the above-mentioned measured value of the time dependence of the gradient magnetic field. The data measured by the Echo-Planar method or the high-speed chemical shift imaging method is stored in the memory as a complex number. The measurement is performed, for example, over 16 cycles, and the number of data points is, for example, 64 points per cycle of the gradient magnetic field change, and 1024 points in total. The 1024 points of data are multiplied by the weighting factor F (kΔx, t) of the equation (9) measured in advance as described above, and the Fourier transform is performed, so that 16 points around the zero frequency have the deterioration corresponding to the position of kΔx. Gives few projected images. For example, the above-described processing is performed for each of 32 k points, the result is stored in a memory as a two-dimensional array having a size of 32 × 16, and an image or a projected image is displayed through the display 171.
In this way, it is possible to reproduce an image with little deterioration by using the measured value of the gradient magnetic field change for an arbitrary waveform of the gradient magnetic field that changes periodically.

Here, as a method of determining the weighting factor, a method of using an actual value of the equation (9) as it is, a method of using an appropriate power of the equation (9), or There is a method to calculate and obtain based on the value of. In either case, the reliability is improved by taking the weighted average of each period by using the periodicity. In this way, an image with little deterioration can be reproduced based on the time change of the actually measured gradient magnetic field.

The other regeneration methods mentioned above were also obtained from actual measurements. Can be done using.

Now, according to the present invention, in the time domain where the NMR signal is small due to lateral relaxation, the presence of noise reduces the reliability of the result. This problem can be alleviated by using a sample having a long transverse relaxation time. Furthermore, by changing the timing of the 180 ° pulse and performing the measurement with the echo peak position shifted to a region with a large time t, the time dependence of the gradient magnetic field can be measured accurately over a wide time range. it can. Further, by applying the phase encode to not only one direction but also two directions or three directions, it is possible to measure the time dependence of the gradient magnetic field two-dimensionally or three-dimensionally. is there.

〔The invention's effect〕

According to the present invention, the inhomogeneity of the static magnetic field eliminates the effect of lateral relaxation, and it becomes possible to easily measure only the time dependence of the gradient magnetic field. Therefore, an Echo- Pla
The nar method or the high-speed chemical soft imaging method can be performed. As a result, it becomes possible to perform the measurement by the above method without performing the high-speed switching of the gradient magnetic field, which has been a practical problem in the past.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIGS. 2 and 3 are diagrams showing examples of pulse sequences using the present invention. 11 ... CPU, 12 ... Sequencer, 13 ... Transmission system, 14 ...
… Reception system, 15 …… Static magnetic field generation magnet, 16 …… Magnetic field gradient generation system, 17 …… Signal processing system.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Etsuji Yamamoto 1-280 Higashi Koigakubo, Kokubunji City, Tokyo Inside Hitachi Central Research Laboratory (72) Inventor Hideki Kono 1-280 Higashi Koigakubo, Kokubunji City, Tokyo Hitachi Ltd. Central Research Center

Claims (2)

[Claims] 1. Magnetic field generating means for static magnetic field, gradient magnetic field and high frequency magnetic field, signal detecting means for detecting a nuclear magnetic resonance (NMR) signal from an inspection object, a computer for calculating the nuclear magnetic resonance signal, In an image reproducing method in an inspection apparatus using NMR having output means for outputting a result, and sequence control means for controlling a measurement sequence including control of the gradient magnetic field and the signal detecting means, the gradient is provided in the static magnetic field. A sample used for measuring the time change of the magnetic field is placed, the gradient magnetic field and the signal detecting means are controlled by the sequence control means, and the application intensity or application time of the gradient magnetic field in at least one direction is set. And then applying a gradient magnetic field that changes with time in the same direction as the gradient magnetic field for which the applied strength or the application time is set. A first step of detecting a first magnetic resonance signal, the second from the sample without applying a gradient magnetic field that changes the time in the first step
The second step of detecting the nuclear magnetic resonance signal of the
And a fourth step of repeating the second step by changing the applied strength or application time of the gradient magnetic field, and in the fourth step, replacing the sample with the inspection target and the third nucleus from the inspection target. A third step of detecting a magnetic resonance signal, extracting a temporal change of the gradient magnetic field based on a ratio between the first nuclear magnetic resonance signal and the second nuclear magnetic resonance signal, and extracting the temporal change. An image reproducing method in an inspection apparatus using NMR characterized in that an image is reproduced from the third nuclear magnetic resonance signal by using a temporal change of a gradient magnetic field. 2. The Fourier transform is performed by multiplying the third nuclear magnetic resonance signal by the ratio of the first nuclear magnetic resonance signal and the second nuclear magnetic resonance signal. An image reproducing method in an inspection apparatus using NMR according to the first section.