JPS628046A - Method for measuring nuclear magnetic resonance signal - Google Patents

Abstract

PURPOSE:To easily calculate the accurate position of the DC component of a free induction decrement (FID) and the accurate amplitude of data in the vicinity of DC, prior to applying a gradient magnetic field for a reading gradient, by applying a gradient having a code opposite to that of the magnetic field for an appropriate time. CONSTITUTION:Prior to FID observation, a reading gradient is applied to an opposite direction only in slight quantity. By this sequence, a FID signal can be observed slightly before a time origin and the time origin can be decided as the peak of the signal. If DELTAT is set to proper length, the disturbance of the signal due to the rising of a gradient is also received in this time and a signal with accurate amplitude can be observed. DELTAT is different at every apparatus but may be several hundred micro-seconds-several mili-seconds and the increase in a signal observing time due to this can be neglected. This sequence is stored in a memory circuit and a gradient magnetic field driving circuit is controlled by said sequence and the application of a gradient magnetic field can be achieved.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is directed to nuclear magnetic resonance (nuclear magnetic resonance).
The present invention relates to an improvement in a method of measuring an NMR signal in an NMR tomographic imaging apparatus that utilizes the phenomenon of ic resonance (hereinafter abbreviated as NMR) to determine the distribution of specific atomic nuclei within a subject from outside the subject.

(Prior Art) Methods for observing NMR signals in an NMR1il imaging device include a method of observing an FfD signal (free induction decay signal) and a method of observing an echo signal. Figure 5 is flD
Diagram showing the pulse sequence when observing signals, No. 6
This figure shows a pulse sequence when observing an echo signal, and in FIG. 6, the state of applying a gradient magnetic field is not shown.

In the case of FID signal observation, two gradient magnetic fields are applied as shown in Figure 5 (B), and 9 as shown in Figure 5 (A).
01 pulse is applied to excite the spins of a predetermined slice (
selective excitation). Next, phase encoding is performed by applying a Y gradient magnetic field (its magnitude changes for each view). Subsequently, an X gradient magnetic field for the readout gradient is applied, and this point is F.
The time origin (t-0) of the ID signal is taken as the time origin, and the FID signal is observed for T time.

On the other hand, in the case of echo signal observation, the process from the application of the 90° pulse to the point where the NMR signal is generated is the same as the case shown in Figure 5, but when 1 hour has passed after the application of the 90° pulse,
An 80' pulse is applied to invert the spins during dispersion by 180°. As a result, the spins begin to converge, and the 180
° Generate an echo signal that reaches its maximum one hour after pulse application. Observation of the echo signal generated in this manner lasts for a period of Tmm from immediately after the application of the 180° pulse until the echo signal substantially disappears, as shown in the figure.

(Problems to be Solved by the Invention) Incidentally, since FED signals have several points that are difficult to use compared to echo signals, conventionally, observation of echo signals has been mainly used. However, when compared in terms of S/N per unit time, F
It can be seen that the ID signal is superior.

For these reasons, it is preferable to use the FID signal, but the use of the FID signal has the following problems.

When performing a two-dimensional Fourier transform, it is necessary to accurately capture the peak value of the FID signal and its 1ff (this position becomes the time origin when observing the FID signal) and process the data. When using an echo signal, there is no problem as the direct current component (DC component) in the two-dimensional Fourier transform of the NMR image can be accurately obtained as the peak of the echo signal (point P in the figure), as shown in FIG. When using an FID signal, although the DC component corresponds to the first data, it is difficult to determine the exact position of the peak in this part, and the waveform is disturbed by the rise of the observation slope, making it difficult to obtain the correct amplitude. There was also the problem that it was difficult to obtain.

In view of these points, an object of the present invention is to reduce the DC
It is an object of the present invention to provide an FID signal measurement method that allows an FID signal to be easily determined to accurately determine the exact position of a component and the accurate amplitude of data near DC.

(Means for solving the problem) In order to achieve such an object, the present invention provides a nuclear magnetic resonance system in which a gradient magnetic field for a readout gradient is applied after selective excitation, and the generated FID signal is observed. In the signal measurement method, prior to applying a gradient magnetic field for the readout gradient, a gradient with the opposite sign to that of the gradient magnetic field is applied for an appropriate time, and the FID signal is generated a little before the time origin, and the time The present invention is characterized in that the accurate position of the origin and the accurate amplitude of data in the vicinity thereof can be obtained.

(Example) The present invention will be described in detail below with reference to the drawings. FIG. 1 shows an NMR section Fm for carrying out the method of the present invention.
FIG. 2 is a configuration diagram of main parts of an m-image device. In the figure, 1 is a magnet assembly 1, which has a space (hole) inside for inserting an object, and a main magnetic field that surrounds this space and applies a constant magnetic field to the object. coil and
Gradient magnetic field coils for generating gradient magnetic fields (X gradient magnetic field coil configured to be able to generate gradient magnetic fields individually, y gradient (i & field coil and 2 gradient!! field coils) and An RF transmitting coil that provides a high-frequency pulse to excite the spin of the nucleus of the
A receiving coil and the like for detecting MR signals are arranged.

The main magnetic field coil is connected to the main magnetic field power supply 2, G X + G y *
Each Gz gradient magnetic field coil is connected to the Gx, Gy, Gz gradient magnetic field drive circuit 3, the RF transmitting coil is connected to the RF power amplifier 4, and the NM
The receiving coils for the R signal are each connected to a preamplifier 5. 1o is a sequence memory circuit that controls the generation sequence of gradient magnetic fields and high-frequency magnetic fields and stores the obtained N
Controls the timing when converting MR signals into A/DI.

6 is a gate modulation circuit, and 7 is an RF oscillation circuit that generates a high frequency signal. The gate modulation circuit 6 modulates the high frequency signal output from the RFR oscillation circuit 7 using the timing signal from the sequence storage circuit 10 to generate a high frequency pulse.

This high frequency pulse is given to the RF power amplifier 4.

A phase detector 8 detects the phase of the NMR signal detected by the receiving coil and sent via the preamplifier 5 with reference to the output signal of the RF oscillation circuit 7.

11 is an A/D converter that converts the NMR signal (analog) obtained through the phase detector 8 into a digital signal. 13 is a processing device including a computer, which converts scan conditions to realize various scans. a function of supplying information to the sequence storage circuit 10, an arithmetic processing function of reconstructing information on spin density distribution etc. into an image from observation data inputted from the A10 transducer 194, a function of sending and receiving information to and from the operation console 12, etc. have

The reconstructed image obtained by the processing device 13 is displayed! ! 9.

The operation in such a configuration will be explained next.

In the conventional sequence shown in FIG. 5, the moment when the readout gradient (X gradient magnetic field) is applied corresponds to the time origin. However, in reality, it takes time for the readout gradient to rise, and the spin phase changes during this time, so by the time the gradient has completely risen (and therefore the signal can be observed), it has already shifted from the time origin. It's gone. Further, immediately after the gradient rises, the received signal may be disturbed due to the coupling state of the gradient magnetic field coil and the receiving coil, and the accuracy of the data is not guaranteed.

In the present invention, as shown in FIG. 2, accurate data near the time origin is obtained by scanning in a sequence in which the application of a readout gradient (X gradient magnetic field) is different from the conventional sequence. That is, prior to the FIDI2 measurement, a read gradient is applied in a slightly opposite direction. This sequence makes it possible to observe the FID signal from a little before the time origin, and it becomes possible to determine the time origin as the peak of the signal. Furthermore, if ΔT is set to an appropriate length, signal disturbances due to the rise of the slope, etc. will be contained within this time, and a signal with accurate amplitude can be observed. ΔT
4. Depending on the device, it takes several hundred μsec to several msec.
c is sufficient, and the signal W due to this? The increase in 11 hours is negligible.

Such a sequence is stored in the sequence storage circuit 10, and the gradient magnetic field drive circuit 3 is controlled by the sequence storage circuit 10, thereby achieving application of the gradient magnetic field as described above.

Now, the time origin is the position where the signal peaks, but generally that point does not always coincide with the sampling point.

Therefore, as shown in Figure 3, several points near the peak (S+
, 82.83), it is desirable to obtain the true peak Pt by approximating it with an appropriate function (eg, quadratic leap number, etc.) Fn. Also, in the Fourier transform method, Warp! Since the time origin can be detected only in the view at l zero, the values determined in the warp binding view are used in other views.

Accurate FID data obtained in this way can be visualized by an appropriate method. There are various imaging methods, and one method for obtaining an image with fewer artifacts using Fourier reconstruction is the following method, for example.

ml when measuring the FrD signal by embedding a zero value in the negative time domain of the FrD signal or assuming that there is a zero value
Depending on the direction of the gradient, one side is left as is, while the other is Fourier-transformed to the complex image so that the readout time axis is reversed. Furthermore, the position and position in the readout direction caused by the main magnetic field inhomogeneity are
Correct density distortion. By adding complex numbers to the two images obtained in this manner, the influence of the main magnetic field non-uniformity is reduced, and a real value image is obtained by image processing such as absolute value processing or phase rotation.

Note that the method of the present invention uses not only the Fourier method but also the projection method (
It can be similarly used for PR method).

An example of the pulse sequence at that time is shown in FIG. In the PR method, scanning is performed by changing the direction of the projection gradient for each view (by changing the X gradient magnetic field and the Y gradient magnetic field), but in order to implement the present invention, the direction of the projection gradient is changed for each view (by changing the X gradient magnetic field and the Y gradient magnetic field). It is sufficient to apply gradients having opposite signs at angles. That is, for example, as a Y gradient magnetic field, Gpr
When applying +, apply -fiGpr+ before signal observation and use GP? as the X gradient magnetic field. 2, apply -AGP1''2 before signal observation. However, k is a positive constant, and GP?IIGp?2 is the strength of the magnetic field that changes for each view.

(Effects of the Invention) As explained above, according to the present invention, it is possible to obtain accurate FID data of the signal amplitude at and around the time origin, thereby creating a wi image based on the FID signal and a FrD signal. It is possible to obtain an image with a good S/N ratio per unit time, which is a characteristic of .

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of an NMR tomography apparatus for implementing the method of the present invention, FIG. 2 is a diagram showing a pulse sequence for implementing the method of the present invention, and FIG. 3 is an F
A diagram for explaining how the peak value of the fD signal is approximated by a curve, FIG. 4 is a diagram showing a pulse sequence when the method of the present invention is applied to the PR method, and FIG. A diagram showing a pulse sequence. FIG. 6 is a diagram showing a conventional pulse sequence related to echo signal observation. 1... Magnet assembly, 2... Main magnetic field power supply,
3... Gradient magnetic field drive circuit, 4... RFF force amplifier,
5... Preamplifier, 6... Gate modulation circuit, 7...
・RF generation note circuit 8...Phase detector, 9...Display device, 1o...Sequence storage circuit, 11...A/D
Converter, 12... operation console, 13... processing device. Second diagram. Figure 3 Figure 4 Figure 5 qD'

Claims (1)

[Claims] In a method for measuring a nuclear magnetic resonance signal, which selectively excites a desired slice plane of a subject, applies a gradient magnetic field for a readout gradient, and measures an FID signal used to obtain a reconstructed tomographic image of the subject. , Prior to applying the gradient magnetic field for the readout gradient, a gradient with the opposite sign to the gradient magnetic field is applied for an appropriate time, and the FID signal is generated a little before the time origin, so that the time origin is accurately determined. A method for measuring nuclear magnetic resonance signals, characterized in that accurate amplitude of data at and around the position can be obtained.