JPS61198043A - Static magnetic field variation detecting method for mr-ct device - Google Patents

Abstract

PURPOSE:To detect the dislocation of a static magnetic field by collecting a data by the first and the second pulse sequence for superposing the inclined magnetic field of the uniaxial direction and an inclined magnetic field of the direction opposite to said direction, onto the static magnetic field, respectively, and deriving its correlation. CONSTITUTION:An MR-CT device for generating an image by utilizing a nuclear magnetic resonance phenomenon of a body to be inspected is constituted so that the body to be inspected is placed in a magnetic field formed by a static magnetic field coil 1 and GZ, GY and GX coils 2-4, and an FID signal and an echo signal which are generated are received by an antenna 5, processed by a computer 28, and displayed as an image. In this case, the data is collected by two pulse sequences for superposing an inclined magnetic field of the uniaxial direction and an inclined magnetic field of the direction opposite to said direction, onto the static magnetic field, respectively, and from its correlation, a variation from a reference value of the static magnetic field is derived, and the static magnetic field coil 1 is controlled. Accordingly, a picture quality can be improved by eliminating a dislocation and a distortion of an image.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a method for detecting static magnetic field fluctuations in an MR-CT type device.

Conventional technology MR-CT type equipment, the object to be examined placed in a strong static magnetic field (
It uses the nuclear magnetic resonance (NMR) phenomenon of the human body to create images corresponding to various parameters of internal tissues. Since the resonance frequency is proportional to the magnetic field, spatial distribution information can be obtained by superimposing a gradient magnetic field on the static magnetic field and determining the frequency spectrum of the NMR signal. For example, the spinwave method is known as one of the imaging techniques (Japanese Patent Application Laid-open No. 158988-1988). MR-CT
The strength and uniformity of the static magnetic field of the molding device directly affects the accuracy of this spatial distribution information.

Problems to be Solved by the Invention Incidentally, conventionally, the strength and uniformity of the static magnetic field of an MR-CT type device are adjusted at the time of installation, but the static magnetic field strength tends to fluctuate due to the influence of magnet temperature. Fluctuations in the static magnetic field cause image distortion, blur, etc., and seriously affect image quality.

An object of the present invention is to provide a method for easily detecting fluctuations in a static magnetic field at any time, such as before imaging.

Means for Solving the Problems In the method for detecting static magnetic field fluctuations in an MR-CT apparatus of the present invention,
A first pulse sequence for collecting data while superimposing a gradient M field in a uniaxial direction on a static magnetic field, and a second pulse sequence for collecting data while superimposing a gradient magnetic field in an opposite direction with respect to the l-axis direction on a static magnetic field. are executed, and these first. Fluctuations in the static magnetic field from the reference value are detected from the correlation of data obtained in the second pulse sequence.

If the acting static magnetic field deviates from the reference value, both the frequency information obtained with the first pulse sequence and the frequency information obtained with the second pulse sequence will be the same by the frequency corresponding to the deviation of the static magnetic field. It will shift. Therefore, by inverting the frequency information of one of them with respect to the center frequency corresponding to the reference value of the static magnetic field and comparing it with the other, the correlation between the two can be established, so that the shift in the static magnetic field can be immediately known.

Embodiment FIG. 3 shows an outline of an MR-CT apparatus. In this figure, static magnetic field coil 1, Gz (Z direction gradient magnetic field) coil 2
, GV (Y-direction gradient magnetic field) coil 3, and Gx (X-direction gradient magnetic field) coil 4 are supplied with excitation currents from power sources 11 to 14, respectively. x.

As shown in FIG. 84, the Y and Z directions are defined as Z, which corresponds to the body axis direction of one human body, and the X and Y directions are directions in which the X-Y plane forms a plane perpendicular to the Z axis. A human body, which is a subject, is placed in a magnetic field formed by each of these coils 1 to 4, and an antenna coil 5 is placed around the human body.

The control computer 21 controls the static magnetic field power source 11 based on static magnetic field fluctuations detected by the method of the present invention, which will be described later, and also controls the waveform generation circuit 22 to execute a predetermined pulse sequence such as the spin warp method. The gradient magnetic field power supplies 12 to 14 are generated by the waveform generated from the waveform generation circuit 22.
And the RFF amplifier/high frequency power supply 23 is controlled according to a predetermined sequence. The RF pulse generated by the RF amplifier bulky high frequency power supply 23 is sent to the antenna coil 5 via the switch circuit 24, and a 180° pulse, a 90° pulse, etc. are applied to the human body.

FID (free induction damping) signals and spin echo signals from the human body are received by the antenna coil 5, and these NMR signals are sent to the RFF amplifier 25 by the switched switch circuit 24, and further sent to the phase discriminative detection circuit 26 and the interface. 27, and in this process, it is A/D sampled and converted into digital data, which is then taken into the main computer 28. A main computer 28 and an image processor 29 perform data processing mainly using fast Fourier transform to create an image, which is displayed on a display device.

When obtaining a tomographic image on the X-Y plane shown in Fig. 4 using a two-dimensional Fourier transform method (Fourier fist imatography) such as the spin warp method or a projection restoration method, the pulse sequence shown in Fig. 1 is used. As in (1), 90
When performing A/D sampling of the FID signal generated after the ° pulse, Gx (X-direction gradient magnetic field) is applied to obtain projection data on the X-axis of the object on the X-Y plane. Gy, G
Although z (Y, Z direction gradient magnetic field) is omitted,
Gz is given along with an RF pulse to excite a narrow range in the Z direction corresponding to the x-Y plane by selective irradiation method,
GV is given to perform phase encoding in the Y direction in the spin warp method (or two-dimensional Fourier transform method).

In order to measure static magnetic field fluctuations, in addition to the pulse sequence shown in Fig. 1, pulse sequence (2) shown in Fig. 1 is executed. This pulse sequence (2) is the same as the pulse sequence (1). The other parameters are exactly the same except that the direction of the X-direction gradient magnetic field Gx is exactly opposite.

Here, if we look at the combination of the static magnetic field and the X-direction gradient magnetic field Gx, it will be as shown in A in FIG. Assume that the static magnetic field is oriented in the Z direction and its strength is a preset reference value Ha as shown in FIG. 4 - Xs and X2 are the boundaries of the set region of interest in the X direction.

Xo is the midpoint. The gradient of the X-direction gradient magnetic field Gx is h
o / (X2 Xl)7! l: Then, in pulse sequence (1), the combined magnetic field strength at each position in the X direction becomes as shown by the solid line in FIG. 2A. Therefore, if the collected data is Fourier transformed to obtain frequency information, it will be as shown in the upper part of FIG. 2B.

fo is the frequency determined by Ha, ft-fo
=fo−f□ is determined by hO. When data is collected using pulse sequence (2), the combined magnetic field strength at that time is as shown by the broken line in Figure 2A, so the frequency information is
It will look like the bottom of Figure B. In this case, the relationship between the frequency information (top and bottom of Figure 2B) obtained with both pulse sequences is that the information of pulse sequence (2) (bottom of Figure 2B) is
The relationship is such that when the left and right sides are reversed at O, the information becomes exactly the same as the information of pulse sequence (1) (upper part of FIG. 2B).

However, the static magnetic field fluctuates and its strength changes from the reference value Ho to H.
If it deviates to o', such a relationship will not hold.

In this case, in pulse sequence (1), the combined magnetic field strength is as shown by the solid line in FIG. 2C.

The frequency information is as shown in the upper part of the second diagram, and in pulse sequence (2), one composite magnetic field strength is as shown by the broken line in FIG. 2C, and the frequency information is as shown in the lower part of the second diagram. Therefore, even if the frequency information shown at the bottom of the second diagram is horizontally inverted around fO, it no longer matches the frequency information shown at the top of the second diagram. Therefore, if we calculate the cross-correlation function between the inverted information at the bottom of the second diagram and the non-inverted information at the top, and find fo' from the frequency value that becomes the maximum value, we can calculate Ha' from fo' - fo. -Ha, that is, the amount of variation in the static magnetic field can be found.

Therefore, the static magnetic field can be corrected by controlling the static magnetic field power supply 11 according to the detected amount of variation.

In addition, in the above, the fluctuation of the static magnetic field was corrected, but
When one line of data is collected in one scan, the position of each line may be corrected based on the amount of variation in the static magnetic field detected as described above.

In addition, in the spin warp method, the orientation of the gradient magnetic field Gy in the Y direction is the first line and the nth line (n is the number of lines (views)), the second line and the (n-1)th line, etc. Since they are opposite to each other, it is also possible to determine static magnetic field fluctuations during scanning based on them.

Furthermore, immediately before and after the actual scan, static magnetic field fluctuations are determined using the methods described in pulse sequences (1) and (2), and if the fluctuations are within the range, the image is constructed as is, and if the fluctuations are outside the range, the data is It is also possible to rearrange the images (in this case, the amount of data is halved) to create a Kali image.

Effects of the Invention According to the present invention, fluctuations in the static magnetic field can be easily detected, and image shift, distortion, and blurring can be prevented when imaging is performed while the static magnetic field remains deviated from a reference value.

[Brief explanation of drawings]

Fig. 1 is a time chart for explaining one embodiment of the present invention, Fig. 2 is for explaining the relationship between composite magnetic field strength and frequency information, and Fig. 2 A and C are time charts for explaining an embodiment of the present invention. A graph of the composite magnetic field strength distribution, Figures 2B and D are diagrams showing the position of frequency information in the frequency range, Figure 3 is a block diagram showing an overview of the MR-CT device, and Figure 4 is a diagram showing each direction with respect to the human body. It is a schematic diagram for explanation. 1... Static magnetic field coil 2... Z direction gradient magnetic field coil 3... Y direction gradient magnetic field coil 4... X direction gradient magnetic field coil 5... Antenna coil

Claims (1)

[Claims] (1) A first pulse sequence that collects data while superimposing a gradient magnetic field in one axis direction on the static magnetic field, and a second pulse sequence that collects data while superimposing a gradient magnetic field in the opposite direction with respect to the one-axis direction on the static magnetic field. A method for detecting static magnetic field fluctuations in an MR-CT apparatus, comprising: detecting fluctuations in a static magnetic field from a reference value from a correlation between data obtained in the first and second pulse sequences.