JPS62176444A - Method for measuring electrostatic magnetic field intensity distribution of nuclear magnetic resonance image pickup apparatus - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for measuring the distribution of static magnetic field strength in a nuclear magnetic resonance imager (NMR imager).

BACKGROUND OF THE INVENTION NMR devices are well known in the art.

The static magnetic field strength of NMR image capture affects the image quality of the NMR image, and non-uniformity of the magnetic field strength leads to a decrease in image quality. Although it is ideal that the static magnetic field strength distribution be uniform, it is not easy to make it uniform in reality.

Therefore, generally, a high-quality image can be obtained by measuring the intensity distribution and correcting the non-uniformity. Therefore, it is an important issue to accurately determine aI of the distribution of static magnetic field strength.

(Problem to be solved by the invention) Conventionally, small water phantoms were installed in different locations one by one, and the resonant frequency of protons was measured at each location.
Alternatively, the static 1a field strength distribution can be measured using a method used for chemical shift images, in which a large water phantom is installed, phase encoding is performed in each direction, and echo signals are read out without applying a readout gradient magnetic field. was.

Although the latter method can perform measurements in a shorter time than the previous method, it still takes a long time. Therefore, using the Fourier method (spin warp method) as shown in Figure 6, the static magnetic field strength is obtained as phase information from two types of images with different intervals between 900 pulses and 180@pulses and times from 180@pulses to the center of the echo. A method of calculating the distribution (q) was considered, but the phase has an ambiguity with a period of 2π, and since it is subject to large changes due to noise, it is necessary to obtain a continuous and correct static 11112 intensity distribution. Furthermore, when the fr4 region where the signal is weak is inserted, the continuity of the phase cannot be determined, so there is a problem that the i-field strength can only be estimated in a limited region.

The present invention has been made in view of these points, and its object is to provide a measurement method that can accurately measure the static field intensity distribution without ambiguity in a short measurement time.

(Means for solving the problem) A first invention for solving the above problem is NMR Ifn
In the image acquisition system, the readout gradient magnetic field is changed using a pulse sequence of the spin warp method to obtain multiple images of the same cross section of the subject using fi imaging, and the size of the positional shift of the same point is measured from these multiple images. The second invention is characterized in that a non-uniform distribution D(x, y) of the static magnetic field strength of J31 is determined at each pixel position. The readout gradient magnetic field and the axis of the warp gradient coordinates are rotated using the pulse sequence to image the same cross section of the object to obtain multiple images, and from these multiple images, the magnitude of the positional shift at the same point is measured. , the static magnetic field strength non-uniform distribution D (x, y) at each pixel position n is determined.

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

First, the principle will be explained. Here, we will take as an example a case where two images are used.

The non-uniform distribution of the static magnetic field strength is represented by D (x, y), and by the pulse sequence shown in FIG. 6, two images (real images) are generated by the readout gradient magnetic field Gy! and GV
2 (≠GV+), the positional distortion of each image is: 1/+=Vo+D(X,Vo)/GV+-Image 1V2-
110+D(X,Vo)/GV'z- The point that was originally at the coordinates (X, 110) is generated according to image 2, but it becomes the point (X, V+) in image f#1, and (X, V+) in image @2. 2
) point. Therefore, D(x,yo) =fGV+GV2/(GVz-GV+))X(V+
V2) ...(1) ;! By calculating this, we can find out the non-uniformity of the static magnetic field. Here, V+
Vz is the positional deviation of the same point in two images, and if the two images can be aligned pixel by pixel, the amount of positional deviation can be determined from the magnitude of the deviation at that time.

FIG. 1 is a block diagram showing an embodiment of an NMR imaging apparatus for carrying out the method of the present invention. In the figure, 1 generates a static magnetic field, applies the necessary gradient vA field and RF pulse to generate an NMR signal from the subject, and its NMR (,li
This is an NMR signal detection section that detects the signal. Reference numeral 2 denotes a control device, which is a control section that generates the static magnetic field, gradient magnetic field, and RF pulse at appropriate timings. 3 is NMR belief @(
Here, it is an image reconstruction device that receives an echo signal and reconstructs a cross-sectional image of a subject by two-dimensional inverse Fourier transform.

Reference numeral 4 denotes a first image processing device, which has a function of determining the maximum positional shift of each pixel for a plurality of images. Reference numeral 5 denotes a second image processing device, which has a function of two-dimensionally smoothing the positional deviation and estimating the deviation even for pixels that cannot be aligned, and calculates the static r4i from the positional deviation amount. It performs the function of determining the independent component (i) of the field intensity distribution and correcting the image appropriately. 6 is a display device that displays the obtained image.

The operation in such a configuration will be explained next.

The method of scanning the object to obtain an echo signal 1'11 and subjecting this echo signal to two-dimensional inverse Fourier transform to obtain a reconstructed image is the same as in the conventional method, so a detailed explanation will be omitted.

The operation of measuring the non-uniform distribution of static magnetic field intensity distribution, which is a feature of the method of the present invention, will be described in detail.

Using the Fourier method (spin warp method), the same cross section of the object is read out using the t control of the control device 2, and the gradient magnetic field Gy is
(Figure 6) is changed and the two images are 1q.

The first image processing device 4 correlates the positions of each pixel of the two images, and IFs the deviation 11. First, as shown in FIG. 2(a), pixels (i, j
), 2×IW, 1,001 pixels are handled in the readout direction j, and these are referred to as templates 1 to 1. This template is centered around the pixel (i, j >) on image 2, ±
j While shifting the force direction ΔW, the template and image 2
Calculate the cross-correlation coefficient of The maximum value among these is shown in Figure 2 (
As shown in (b), if the VA value is greater than or equal to the pre-given VA value T, fit a quadratic curve to 3 to 5 points including the value of (),
The amount of positional deviation between the two images at pixel (i, j) of image 1 is calculated from the symmetry axis position of the quadratic curve. Points that are below are labeled as points that cannot be aligned.

When the above processing is performed for all pixels, the sensitivity of the positional shift can be found on the two-dimensional surface.

Second image processing Lg! In the device 5, the first image processing device 4
The amount of misalignment obtained in step 2 is smoothed two-dimensionally as shown in FIG. For this purpose,
As shown in FIG. 3(B), the amount of deviation is cut out using the MXM finder centered on the pixel point (i, j). In this process, the misalignment of the points where alignment is achieved is tightened and leveled out.

The average value at this time is the smoothed output at the pixel point (i, j>).

This processing is performed for all pixels, and the result is represented as a two-dimensional smooth curved surface.

In this way, the positional deviation is measured, and the non-uniform distribution of static magnetic field strength at each pixel D(x, y) is calculated according to equation (1).
is required. According to experiments, 1W=10, ΔW=10
．． T=0.7. With M=51, D (x, y) could be found from a 320x320 pixel image.

Once the non-uniform distribution D (x, y) is obtained using the above method, by correcting the reconstructed image based on this, it is possible to obtain a high-quality image that is not affected by the non-uniformity of the static magnetic field strength distribution. The image can be displayed on the display device 6.

Incidentally, the method of the present invention is not limited to the examples, and can also be carried out as follows.

When using 03 or more images, the estimation accuracy can be improved by calculating the misaligned boats according to different combinations and averaging the misaligned boats.

■The alignment of two images (9) can be determined by using 5SDA (S eq
uantial S 1m1rarity D ete
There are many proposals such as ctionA Igoritbm), and you can choose an appropriate one from these.

■Alignment was performed using a one-dimensional template here, but if a two-dimensional template is used, a certain degree of smoothing can be achieved at the same time, and by performing two-dimensional matching, positional distortion in the warp axis direction can be reduced. It becomes possible to accurately measure positional deviation even in a certain image.

■In general, the non-uniformity of the magnetic field strength changes quite slowly, so it is necessary to reduce the two images and align them.
It does not matter how many points J3 can be aligned or the correlation coefficients can be measured.

(2) The estimation function may be performed by applying a two-dimensional function to the whole or piecewise.

(2) Images may be obtained by rotating the coordinates of the warp axis and the readout axis to obtain a plurality of images. If the images in this case are reversely rotated and superimposed, the distortion will be the same in magnitude but in a different direction than before. If we align each pixel in four directions and find a point with a high -intensity, we can determine the degree of non-uniformity in the static magnetic field strength from that direction and the size of the positional shift. can. In this case, the same effect can be obtained by rotating the measurement object instead of rotating the coordinates.

To explain this in detail, since the direction in which distortion occurs is limited to the direction of the readout axis, there is a possibility of deviation in two directions, positive and negative. However, since the readout gradient Gy is the same for both images, the magnitude of the deviation is the same.

As shown in FIG. 4, if the rotation angle of the coordinates is θ, the point that was originally at Pa, for example at point 11 in image 1, will be at point Pl in image (!112, for example. , all possible deviations can be considered by performing alignment in the four directions of angles ±α and ±β from Pl.

Here, α=(π10〇＞/2゜ β=θ/2 It is confirmed that it will be.

Also, since it is known that the magnitude of positional deviation due to non-uniform distribution is D/Gy from the equation given earlier, Po Pz = Po Pl - D/Gy as shown in Figure 5.
There is F. Follow, 2 ・POP2 ・P+ P2
From the formula of cos (7U-(Z)), the magnetic field inhomogeneity distribution D(PO) at point Po is D(Po) = -Gy 4 (PI P2 /20
03(Z) is found.

Here, α is the direction in which the corresponding point was found, PI P2
Each represents the size of misalignment, and this method can be used as is by changing the alignment direction and angle.

(Effects of the Invention) As described above, the method of the present invention has the following effects.

(1) Since there is no 2π ambiguity, a magnetic field inhomogeneity distribution with good continuity can be correctly determined without adding continuity later as in the phase method.

(2) Even if there is a region in the target region where the magnetic field strength cannot be measured, since there is no ambiguity, the magnetic field strength can be estimated over a wide region by performing strong smoothing processing.

(3) Measurements can be made in a shorter time than methods that measure each point or chemical shift.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of an NMR decoy apparatus for carrying out the method of the present invention, FIGS. 2 and 3 are diagrams for explaining the present invention in detail, and FIGS. 4 and 5 The figure is a diagram for explaining another method according to the present invention, and FIG. 6 is a diagram showing a pulse sequence. 1...NMR signal detection unit 2...Control device 3...Image reconstruction device 4...First image processing device 5...Second image processing device 6...Display device patent application People Yokogawa Medical System Co., Ltd. Figure 2 (A) Movement of the tray 1 (O) Mutual phase Ijjl Kankin angle) Figure 3 (A) Window 4 Drawing 2 distortion direction Figure 5 A

Claims (2)

[Claims] (1) In an NMR imaging device, the same cross-section of the object is imaged by changing the readout gradient magnetic field using a pulse sequence of the spin warp method to obtain multiple images, and from these multiple images, the magnitude of the positional deviation of the same point is determined. 1. A method for measuring static magnetic field strength distribution in a nuclear magnetic resonance imaging apparatus, characterized in that a non-uniform distribution D(x, y) of static magnetic field strength at each pixel position is determined. (2) In the NMR imaging device, the readout gradient magnetic field and the axes of the warp gradient coordinates are rotated using a pulse sequence of the spin warp method to image the same cross section of the subject to obtain multiple images, and the same cross-section is obtained from these multiple images. 1. A method for measuring static magnetic field strength distribution in a nuclear magnetic resonance imaging apparatus, characterized in that the magnitude of positional deviation of one point is measured and a static magnetic field strength non-uniform distribution D(x, y) at each pixel position is determined.