JPH02156929A - Magnetic resonance imaging method - Google Patents

Abstract

PURPOSE:To make it possible to obtain a sectional image separated from the slicing gradient magnetic field center without using a special hardware by unifying the frequencies of a high-frequency magnetic field and a reference signal. CONSTITUTION:A high-frequency magnetic field necessary to generate a core magnetic resonance is applied to a substance to check 3 from a radiation coil 9. The high-frequency magnetic field is made from a high-frequency signal which has a necessary frequency in a signal generator 4. The core magnetic resonance signal is detected by a receiver coil 9, amplified by a preamplifier 10 and a receiver 11, and homodyne-detected by a phase detector 12. The homodyne-detected resonance signal is made into a signal which has a frequency equal to the frequency difference between the primary resonance signal and a reference signal. In this case, by scrolling the image data in the frequency encoding direction as making the high-frequency wave and the reference signal unified each other without making variation from the radiation of the high-frequency magnetic field to the receiving of the signal, a picture image in which the gradient magnetic field center and the image center are unified is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an imaging method in a magnetic resonance imaging apparatus, and relates to a slicing method.

[Conventional technology]

A magnetic resonance imaging apparatus is an apparatus that generates a nuclear magnetic resonance phenomenon within a specific plane of a sample or subject, and images the resonance signal generated at that time as area information. There is already a lot of literature on nuclear magnetic resonance phenomena, and I will not go into details here.

The important points regarding the invention are as follows.

■Nuclear magnetic resonance conditions are f=γ・H・・・・・・・・・・・・・・・・・・・
...... (Formula 1), where f: frequency of the resonance signal or frequency of the high-frequency magnetic field applied to cause resonance γ: gyromagnetic ratio (constant determined by the nuclide) I: Magnetic field strength ■ To cause nuclear magnetic resonance, a high frequency magnetic field that satisfies the above conditions may be applied. In other words, even if a high frequency magnetic field is applied, resonance will not occur unless the above conditions are satisfied.

On the other hand, imaging techniques are also known. In particular, there are many documents regarding the two-dimensional Fourier transform method, which is the basis of the present invention, so we will not go into details thereof, but the parts related to the present invention will be briefly explained below. To perform imaging, it is necessary to generate resonance in the spins within the plane to be imaged. This process of generating resonance in order to image a part of a sample or object is called slicing. The principle of slicing is shown in FIG. 1. Here, the case of slicing a cross section of five heads, that is, a plane perpendicular to the body axis will be described. In imaging, a sample or subject is placed in a uniform magnetic field (static magnetic field) within a space containing the sample or subject.

To perform slicing, a magnetic field that varies in strength depending on the position is superimposed on the uniform magnetic field in the direction of the space 1liIll (here, the body axis) perpendicular to the slicing cross section.

Generally, as the superimposed magnetic field, a magnetic field whose intensity changes linearly with respect to the position coordinates is used. This kind of magnetic field is called a gradient magnetic field.The shaded area in the figure represents the fault plane to be imaged.The magnetic field strength corresponding to the position of the fault plane is H'.Hl is the resonance condition. By substituting into the equation 1 expressing -, we get f' = γ·H', and it can be seen that the frequency of the high-frequency magnetic field necessary to cause resonance is f'. In reality, the fault plane has a certain thickness, and f' expands by that thickness, and the frequency component of the necessary high-frequency magnetic field is f' - Δf - f' plus Δf? It is necessary to have uniform intensity in the iF range. In order to apply such a high frequency magnetic field, a high frequency magnetic field obtained by amplitude modulating a carrier wave of frequency f' with a waveform based on the S inc function as shown in FIG. 2 is applied.

When slicing ends, application of the gradient magnetic field also ends. In reality, there are operations such as applying a gradient magnetic field with opposite polarity that is used after slicing in order to correct the spin phase shift during slicing, but this is irrelevant to the present invention and the details thereof will be omitted here.

When slicing is completed, the sample or object is placed in a uniform magnetic field again. As a result, the resonant frequency becomes f'→
fo, where fo is the resonant frequency corresponding to the uniform magnetic field strength.In order to identify the resonance signal from within the 0th order 118 layer plane according to its position, the
By setting two coordinate axes, capturing signals by making the position coordinates of each axis correspond to the frequency and phase of the resonant signal, and performing Fourier transform processing on the captured signals, the signal can be divided into spatial positions minute by minute. , a tomographic image can be obtained by arranging them on both sides corresponding to the positions. Corresponding position coordinates to the phase and frequency of a resonance signal is called right phase encoding and frequency encoding, respectively. Such a method is called two-dimensional Fourier transform imaging. A general timing chart of this method is shown in FIG. The horizontal axis represents the time axis, sGs represents the gradient magnetic field for slicing, GP represents the phase encoding, G represents the gradient magnetic field for frequency encoding, and the vertical direction represents the magnitude of the gradient. Gp represents a series of operations, or resonance. Until a high-frequency magnetic field is applied to capture the resonance signal, the magnitude is constant, but the magnitude is integrated over the period of application, and the above series of operations is performed while changing the integrated value by a constant amount. GF is applied when taking in a signal, and is always of a constant magnitude. Fig. 4 shows the OF, the frequency of the obtained signal, and the position on the image in the direction of the frequency encode axis.

The obtained resonance signal is subjected to homodyne detection using a reference signal having a reference frequency. This makes the resonance signal jI6
It is converted into a signal with a frequency equal to the quasi-frequency frequency difference and a phase of sly. Generally, a frequency corresponding to a uniform magnetic field to which no gradient magnetic field is applied is selected as the reference frequency. This is because the frequency is zero, that is, the center on the screen in the frequency encoding direction coincides with the center of the gradient magnetic field.

[Problem to be solved by the invention]

As already mentioned, when a resonance signal is subjected to homodyne detection, the phase of the obtained signal becomes the phase difference between the reference signal and the high-frequency magnetic field for generating resonance when the application of the slicing gradient magnetic field is stopped.

When the high frequency magnetic field and the reference signal have different frequencies,
The phase difference between them changes over time, and even if the timing is limited, the correlation between the mutual phases is not guaranteed at all.Therefore, the phase information is not guaranteed each time the series of operations is repeated. Since the position in the phase encoding direction cannot be identified and only images flowing in the phase encoding direction can be obtained, it is necessary to take some measures. Conventionally, for this reason, prior to applying the high-frequency magnetic field, the phases of the high-frequency magnetic field and the reference signal are aligned once, and the period from that timing to the timing of turning off the gradient magnetic field for slicing is made constant. Conventional methods have the disadvantage of requiring complex hardware ingenuity to align the phases of the high-frequency magnetic field and the reference signal.

[Means to solve the problem]

The above object is achieved by matching the frequency of the radio frequency magnetic field for causing resonance with the frequency of the reference signal.

[Effect]

By matching the frequencies of the high-frequency magnetic field and the reference signal, the phase difference between them is always constant, and there is no need to use a special device to match the phases of the two signals.

〔Example〕

A subject 3 is placed inside a magnet 1 that generates a uniform magnetic field. Magnet 1 is supplied with its required current by magnet type 'g2. On the other hand, a high-frequency magnetic field necessary to generate nuclear magnetic resonance is applied to the subject 3 from the irradiation coil 8. The high frequency magnetic field is generated by a signal generator 4 from a high frequency signal having a required frequency. The high frequency signal is amplified by the transmitter 43,
The signal is modulated by an amplitude modulator 6 to have the necessary band, and further amplified by a power amplifier 7 to obtain the necessary power, and then supplied to the irradiation coil 8. Power amplifier 7 is connected to power supply 23 . The nuclear magnetic resonance signal is detected by a receiving coil 9, amplified by a preamplifier 10 and a receiver 11, and subjected to homodyne detection by a phase detector 12 using a reference signal supplied from the transmitter 5. The resonance signal subjected to homodyne detection becomes a signal having a frequency equal to the frequency difference between the original resonance signal and the reference signal. This signal is amplified at an audio frequency by an audio amplifier 13 with a low-pass filter, converted into a digital signal by an analog/digital converter 14, and then converted to a digital signal via an interface 19.
It is taken into the calculator 20. The captured signal is subjected to calculations for image reconstruction such as Fourier transformation, and the result is output to the display device 2i21. A gradient magnetic field is applied by a gradient magnetic field coil 15 to cause a resonance phenomenon (slicing) only in a specific part of the subject or to perform separation according to the position where a signal is generated. The gradient magnetic field coil is composed of three pairs of coils that can generate magnetic field gradients in three orthogonal axes directions.As shown in the figure, the body axis direction of the subject 3 is Z, and the front-back direction (in the figure, the subject 3 is Since the person is sleeping, let the vertical direction of the magnet 1 be Y, and the horizontal direction be X. The gradient magnetic field coil is X.

Gradient magnetic field drivers 16 to 18 are independently connected to each of the Y and Z channels. Furthermore, the gradient magnetic field drivers 16 to 18 are connected to the calculator 2 via an interface 19.
0 and the calculator 2 according to the required time chart.
Controlled by 0. In addition, the operator
Control the entire system via.

Next, an outline of a method of controlling the apparatus when carrying out the present invention will be explained with reference to FIG. In the first step, a signal is acquired by the two-dimensional Fourier transform imaging method using the above-mentioned apparatus. At this time, consider the case where a cross section at a position far from the center of the gradient magnetic field is imaged. As shown in FIG. 7, the gradient magnetic field center is a position where the magnetic field strength does not change even if a gradient magnetic field is applied.

Assuming that the static magnetic field strength is Ha and the corresponding resonance frequency is fo, these are expressed by the following equation.

fo=γ・Ho・・・・・・・・・・・・・・・・・・
(Formula 2) On the other hand, the magnetic field strength H''' where the cross section is placed is expressed by the following equation.

H〃:： Ho+ΔZ−Gz・・・・・・Old and old・・
(Equation 3) However, ΔZ: Inclination inclination m * Distance to center cross section m Gz Varnish Gradient magnetic field strength for slagging From the irradiation of the high frequency magnetic field to the reception of the signal, the high frequency and the reference signal are made to match and do not change. Conventionally, the high frequency magnetic field After irradiation, the frequency of the reference signal was changed to a frequency corresponding to the static magnetic field.

Assuming that the frequency of the high-frequency magnetic field for causing resonance and the frequency of the reference signal are both equal and fI+, they are expressed by the following equation.

fn=γ・HIT=γ・(FIo+ΔZ−Gz) =γ・Ho+γ・ΔZ−Gz=fo+γ・ΔZ−Gz・・・・・・・・・・・・
...(Formula 4) Figure 8 shows the details (timing chart) of device control.
Exactly the same argument can be made for gradient end echoes that do not use 0 degree pulses. Figure 9 shows a time chart for the conventional method.

Next, the second step in FIG. 6 will be explained.

The image captured by the method of the present invention is an image shifted from the center of the image in the frequency encoding direction, as shown in #9@.

The second step is to correct this deviation. The correction amount ΔN is given by the following equation.

ΔN=N・(fR−fo)/L−Gx・γ=N・ΔZ
-GZ/L -G.

・・・・・・・・・・・・・・・(Formula 5) However, N number of data points in the two-layer wave number encoding direction 58 Width of field of view in the frequency encoding direction Gx: Magnitude of gradient magnetic field in the frequency encoding direction As shown in FIG. 10, by scrolling the image data by ΔN points in the frequency encoding direction, the image is changed to an image in which the center of the gradient magnetic field in the frequency encoding direction coincides with the image center, as in the conventional case.

〔Effect of the invention〕

According to the present invention, it is possible to obtain a cross-sectional image far from the center of the slicing gradient magnetic field without using special hardware, which has the effect of simplifying the apparatus and reducing costs.

[Brief explanation of the drawing]

Figure 1 is a diagram of the slicing principle, Figure 2 is a diagram of high-frequency magnetic field waveforms, Figure 3 is a timing diagram of two-dimensional Fourier transform imaging method, Figure 4 is a diagram of the relationship between reference signal frequency and position on the image, and Figure 5 is a diagram of the relationship between reference signal frequency and position on the image. The figure is a hardware configuration diagram of the present invention, Figure 6 is a processing flow diagram of the present invention, Figure 7 is a diagram of the relationship between slicing position and magnetic field strength when away from the center of the slicing gradient magnetic field, and Figure 8 is a diagram of the present invention. FIG. 9 is a detailed diagram (timing chart) of the device control in the conventional system, and FIG. 10 is an explanatory diagram of the second step in FIG. 6. 1... Magnet, 2... Magnet power supply, 3... Subject, 4
... Signal generator, 5... Transmitter, 6... Amplitude modulator, 7... Power amplifier, 8... Irradiation coil, 9...
...Receiving coil, 10...Preamplifier, 11...Receiver, 12...Phase detector, 13...Audio amplifier with low-pass filter, 14...Analog/digital converter, 15... Gradient magnetic field coil, 16...
X channel gradient magnetic field driver, 17...X channel gradient magnetic field driver, 18...Z channel gradient 1t
lj field driver, 19...interface, 20.
...Calculator, 21...Display device. TE=Riz'nia°γ；7+ Fig. Now Slice cP1 S Fig. 3 Funeral Fig. Fig. Zhou liquid tun (Break word 3. (B) Tee Oe, Shoume O Kuginami and Masashima right figure 7 lol figure 6 figure 8 take:, 7"l;7" figure 9 figure 10

Claims (1)

[Claims] 1. A high-frequency generator and a high-frequency irradiation coil for placing a sample in a static magnetic field and applying a high-frequency magnetic field to the sample to cause a nuclear magnetic resonance phenomenon, and a resonance signal from the sample that has caused nuclear magnetic resonance. A receiving coil for receiving the received signal, a receiving device that amplifies the received signal, performs phase detection using a reference signal, and converts it to an audible frequency, a calculator that captures and processes the output of the receiving device as data, and a calculator. A display device that performs processing such as image reconstruction processing and displays the results, and a display device that selectively causes the nuclear magnetic resonance phenomenon based on a difference in spatial position, or a display device that performs processing such as image reconstruction processing and displays the result. When obtaining a cross-sectional image at a position far from the center of the gradient magnetic field, a nuclear magnetic resonance imaging system is equipped with a gradient magnetic field generator and a gradient magnetic field coil that generate magnetic fields with different strengths depending on the spatial position to reflect the the frequency of the reference signal remains consistent with that of the high-frequency magnetic field until the high-frequency magnetic field is irradiated and the resonance signal is received, and the image thus obtained is scroll-processed. Resonance imaging method.