JP2000342552A - Method for correcting unequalness of static magnetic field in magnetic resonance imaging system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To precisely correct a static magnetic field by obtaining the unequal distribution of the static magnetic field via a specific step at the time of measuring the unequal distribution of the static magnetic field and generating a static magnetic field canceling this unequal distribution, so as to remove the discontinuity of a phase at the time of obtaining the distribution of the static magnetic field. SOLUTION: An MRI device is provided with a static magnetic field generating magnet 302, a transmission coil 314a for generating a high-frequency magnetic field, a detecting coil 314b for detecting an NMR signal, and a tilted magnetic field coil 309 for generating tilted magnetic fields Gx to Gz or the like. Then, a computer 308 measures the unequal distribution of the static magnetic field generated by the magnet 802 and generates the static magnetic field in the direction of canceling the unequal distribution. At this time, the unequal distribution of the static magnetic field is measured through a step including a step for obtaining the phase difference of an adjacent dot located on a prescribed line including one dot on the coordinates of the two-dimensional phase distribution of a desired area including information of the unequalness of the static magnetic field as a reference so as to eliminate the discontinuous point of the phase.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for correcting static magnetic field inhomogeneity in a magnetic resonance imaging (MRI) apparatus, and more particularly to a method for correcting static magnetic field inhomogeneity using an active shim.

[0002]

2. Description of the Related Art An MRI apparatus is an apparatus that obtains a tissue image of a subject using a nuclear magnetic resonance (NMR) phenomenon generated in the subject placed in a static magnetic field, and the uniformity of the static magnetic field determines the image quality. It is an important factor. Non-uniformity of the static magnetic field is one of the causes of image and spectral distortions. In particular, in the case of ultra-high-speed MRI such as echo planar imaging (EPI), high uniformity of the magnetic field is desired. In addition, in the case of high-precision calculation of a spectroscopic image or the like, in order to reduce non-uniformity of a static magnetic field caused by inserting a living body into a magnetic field, the non-uniformity of the magnetic field is corrected for each subject. It is necessary.

As a method of correcting the inhomogeneity of the static magnetic field,
A passive shim on which a magnetic material is arranged or an active shim for adjusting a current flowing through a coil (shim coil) for magnetic field correction is used. When correcting the non-uniformity of the magnetic field for each subject, an active shim is employed. In an active shim, the non-uniformity of the static magnetic field is measured first, and an additional magnetic field that cancels out the non-uniformity of the static magnetic field is generated by the shim coil, and this is superimposed on the static magnetic field generated by the static magnetic field magnet. Achieve a uniform magnetic field.

A phase distribution measurement method is known as a typical technique for measuring a non-uniform distribution of a static magnetic field. In this method, 90 °
Nuclear magnetic resonance (NMR) signals are measured and obtained using a spin echo method or a gradient echo method in which the time interval between the high frequency pulse and the 180 ° high frequency pulse and the time interval between the 180 ° high frequency pulse and the echo center are slightly different. From the arc tangent of the ratio of the real part to the imaginary part (Si / Sr) of the NMR signal, the phase distribution caused by the nonuniformity of the static magnetic field is obtained.

The static magnetic field distribution obtained in this way is shown in FIG.
As shown in (a), each time the ratio (Si / Sr) between the real part and the imaginary part increases or decreases by 2π, the value becomes discontinuous and cannot be used for calculating the shim current value as it is. The discontinuity is the arc tangent of (Si / Sr) and ｛(Si / Sr) ± 2n
This is caused by the fact that the arc tangent of π｝ is calculated to be the same value, and is called arc tangent aliasing.

[0006]

As a method of removing such discontinuities in the phase distribution, a method of sequentially finding and correcting discontinuities in the phase has been used. However, since the conventional phase correction is performed only in one direction or two directions including a direction orthogonal thereto, the non-uniform distribution is relatively simple as shown in FIG. It is good if it appears in stripes, but if the shape of the object to be photographed is complicated, the discontinuous point also has a complicated shape, so that it is impossible to cope with it, and there is a problem that uncorrected correction occurs.

SUMMARY OF THE INVENTION It is an object of the present invention to effectively eliminate phase discontinuities in obtaining a static magnetic field distribution and perform highly accurate static magnetic field correction. Another object of the present invention is to realize high uniformity of a static magnetic field and to improve image accuracy in ultra-high-speed imaging such as spectroscopic imaging and EPI.

[0008]

According to the present invention, there is provided a static magnetic field inhomogeneity correcting method for achieving the above object, comprising: a static magnetic field generating means;
A correction means for correcting non-uniformity of a static magnetic field generated by the static magnetic field generating means, wherein the non-uniformity of the static magnetic field generated by the static magnetic field generating means is corrected. Measuring a uniform distribution, and generating a static magnetic field that cancels out the non-uniform distribution of the static magnetic field by the correction unit, and measuring the non-uniform distribution of the static magnetic field, Determining a two-dimensional phase distribution of a desired area including information; determining a phase difference between adjacent points on a predetermined line including the point with reference to one point on the coordinates of the phase distribution; Performing a process of obtaining a phase discontinuity point in a plurality of lines, and performing a process of eliminating the phase discontinuity point in the plurality of lines, Spiral is any or a combination of these radial.

In the method of correcting static magnetic field inhomogeneity according to the present invention, the step of obtaining a discontinuity of the two-dimensional phase distribution corresponding to the inhomogeneous static magnetic field distribution is performed in various directions, so that the accuracy can be improved even in a complicated shape. Discontinuities (arc tangent aliasing generated in the phase distribution) can be removed well. As a result, a more accurate static magnetic field non-uniformity map can be obtained, so that the shimming accuracy is improved.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method for correcting static magnetic field inhomogeneity according to the present invention will be described in detail with reference to an embodiment shown in the drawings.

FIG. 3 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied. The MRI apparatus includes an electromagnet or a permanent magnet 302 for generating a uniform static magnetic field H0 in a space where the subject 301 is placed, a transmission coil 314a for generating a high-frequency magnetic field in the space, and an NMR signal generated from the subject. A detection coil 314b for detection, gradient magnetic field coils 309 (309a, 309b) for generating gradient magnetic fields Gx, Gy, Gz whose intensities linearly change in three orthogonal directions x, y, and z, and correcting inhomogeneous magnetic fields And a shim coil 318 for performing the operation.

The gradient magnetic field coil 309 and the shim coil 318 are connected to power supplies 310 and 319 for supplying current, respectively, and constitute a gradient magnetic field generating system 303. Shim coil 318
Consists of static magnetic field generating coils of multiple channels that generate magnetic fields orthogonal to each other, for example, magnetic fields corresponding to each term of the spherical harmonic function, eliminating static magnetic field inhomogeneity by changing the current value supplied to these coils An additional magnetic field can be generated.

The transmission coil 314a is connected to a synthesizer 311 via a power amplifier 313 and a modulator 312, and forms a transmission system 304. The high frequency generated by the synthesizer 311 is modulated by the modulator 312, amplified by the power amplifier 313, and supplied to the coil 314a to generate a high frequency magnetic field inside the subject 301 to excite nuclear spins.

The nuclear magnetic resonance signal emitted from the subject 301 is received by the coil 314b, passes through the amplifier 315,
Quadrature phase detection is performed by a detector 316, and input to a computer 308 via an A / D converter 317. The transmission coil 314 is shown in FIG.
Although a and the detection coil 314b are described separately, they may be used for both transmission and reception.

The gradient magnetic field generation system 303 and the transmission system 304 are controlled by a sequencer 307.
5 is controlled by the computer 308. Computer
Reference numeral 308 includes a signal processing system 306 and an operation unit 321 including a keyboard 322, a mouse 323, and the like, and is controlled by a command from the operation unit 321. The signal processing system 306 performs signal processing on the detected NMR signal, and displays an image corresponding to the nuclear spin density distribution, relaxation time distribution, spectrum distribution, and the like on the CRT display 328. Data during calculation or final data is stored in memory 32
4, stored in 325. Further, in the present invention, a signal processing system
In step 306, calculations necessary for correcting the static magnetic field inhomogeneity described below are performed using the NMR signal.

Next, in the MRI apparatus having such a configuration,
A method for correcting static magnetic field non-uniformity will be described. As described above, a uniform static magnetic field is formed in the space where the subject 301 is placed by the static magnetic field generating magnet 302. Is disturbed. Such static magnetic field inhomogeneity caused by the subject differs depending on the measurement site of the subject. According to the correction method of the present invention, high-precision correction is selected and performed according to the measurement site.

The procedure for correcting the inhomogeneity of the static magnetic field includes the following four steps. <Step 1> Measure the characteristics of each shim channel. <Step 2> Obtain the static magnetic field distribution of the subject. <Step 3> Eliminate discontinuities in the static magnetic field distribution. <Step 4> Using the shim coil characteristics measured in <Step 1>, a shim current value for generating a magnetic field that cancels out the inhomogeneity of the static magnetic field is calculated. <Step 5> The current calculated in <Step 4> is passed through the shim coil.

Hereinafter, each step will be described in detail. [Step 1] Shim coil (shim channel) characteristics mean a change in static magnetic field distribution per unit current flowing through the shim coil. Therefore, if the amount of change ΔBj of the magnetic field at each position (denoted as pixel j) is calculated by changing the current flowing through the shim coil k by a small amount ΔI, the characteristic of the shim coil k with respect to the pixel i is expressed by the equation (1). It can be represented by a matrix.

[0019]

(Equation 1)

Since the shim coil characteristic is usually a value inherent to the apparatus, it is measured in advance on a homogeneous sample such as water, and is developed as a matrix for each slice thickness, slice interval, and slice position, or expanded with a spherical harmonic function. It is stored as a coefficient. This is used to calculate the current value (corrected shim current value) flowing through each shim coil in step 4.

[Step 2] In step 2, a signal is measured by a pulse sequence as shown in FIG. 2, and a static magnetic field distribution is obtained by calculating the obtained signal. That is, first, a 90 ° radio frequency pulse 201 (hereinafter, referred to as a 90 ° pulse) together with the application of the gradient magnetic field Gs203 for slice selection.
Excites the spin of the subject. Next, a gradient magnetic field Gp205 for changing the phase of the spin is applied. Further, after applying a 180 ° high-frequency pulse 202 (hereinafter referred to as a 180 ° pulse) together with the slice gradient magnetic field Gs204, the readout gradient magnetic field Gr20
6, 207 is added to generate a spin echo.

At this time, the reversal timing of the gradient magnetic field Gr is adjusted, and the interval 212 between the point at which the echo signal becomes maximum and the 180 ° pulse is changed to the interval τ211 between the 90 ° pulse and the 180 ° pulse.
And by a small time ε. When the time τ elapses after the 180 ° pulse is applied, the phase change of the spin caused by the static magnetic field inhomogeneity E (x, y, z) is completely canceled immediately after the 90 ° pulse is applied. An image S (x, y, z) obtained by Fourier-transforming the obtained signal contains only phase information generated due to the non-uniformity of the static magnetic field during the minute time ε.

Here, the gyromagnetic ratio is γ, and the complex image S (x,
The real and imaginary parts of y, z) are Sr (x, y, z) and Si (x, y,
Expressed as z), the non-uniform static magnetic field distribution E (x, y, z) is obtained by equation (2).

[0024]

(Equation 2)

[Step 3] Arc tangent aliasing is removed from the static magnetic field inhomogeneous distribution E (x, y, z) obtained in step 2. The inhomogeneous distribution of the static magnetic field is shown in FIG.
A phase diagram represented by a phase diagram 101 as shown in FIG.
At 101, arc tangent aliasing occurs because the phase change exceeds the range of -π to π. Looking at this at the analysis position 102, the one-dimensional phase distribution has 11
A phase jump occurs between 2 and 113 or between 117 and 118. In general, since the phase change does not change discontinuously in this way, the discontinuity is corrected on the assumption that arc tangent aliasing has occurred.

The detection of arc tangent aliasing (discontinuous points) is performed by comparing the phase values of adjacent pixels. If the phase values of adjacent pixels are Pa and Pb,
If | Pa-Pb | is equal to or greater than a certain value (for example, π), it is considered that arc tangent aliasing has occurred, and Pa <P
If b, 2π is subtracted from Pb, and if Pa> Pb, 2π is added to Pb. Such subtraction or addition is performed until | Pa-Pb | becomes a certain value or less, and phase correction is performed.

In the correction method of the present invention, such processing is performed in various directions. FIG. 4 is a diagram showing one embodiment of the present invention.
In this embodiment, a reference point 403 is first determined, and the above-described arc tangent aliasing is detected and corrected for a one-dimensional phase distribution on a line 404 including the reference point.
The reference point is the center point of the image in the illustrated example,
A point at the center of the shim or a point with a high signal strength may be used.

With the reference point as a starting point, the above processing is sequentially performed toward the right side (or left side) in the figure, and then the above processing is also performed on the opposite side (FIG. 2A). Next, the above processing is performed in the vertical direction in the figure, using each point on the line 404 as a reference point (FIG. 2B). Thereafter, the above processing is performed in an oblique direction using each point on the line 404 as a reference point (FIGS. (C) and (d)).

FIG. 5 shows an example of an algorithm for processing in the oblique direction.
8 to FIG. In these figures, P (k, l) represents the phase value of the pixel (k, l) of the matrix size NX × NY, and k is 1 to NX
And l is an integer of 1 to NY. In the illustrated example, first, processing is performed on pixels existing on a diagonal line that extends diagonally to the upper right from the reference point (1, lc) at the left end of the line 404. While moving the reference point (1, lc) to the reference point (NX, lc) at the right end of the line 404, processing is performed on a line parallel to the oblique line (FIG. 5). Next, processing is performed from the left end to the right end on a line extending obliquely upward from the line 404 (FIG. 6). Similar processing is performed for the lower right direction and the diagonally lower left direction (FIGS. 7 and 8), and the diagonal processing is completed.

Finally, processing is performed in the left-right direction to remove the influence of malfunction due to noise or the like (FIG. 4E). By performing the processing in all directions in this way, FIG.
As shown in (1), the arctangent aliasing can be completely removed.

When the shape of the subject is complicated and high-precision correction is required, it is preferable to perform all of the above processing.
When the shape is simple, it is not necessary to perform all of the processes in FIGS. 5 to 8 and it is possible to combine them as appropriate. For example, only the processing in the diagonally right direction (FIGS. 5 and 6) or the diagonal left direction (FIGS. 7 and 8) may be performed. Since the processing may be performed on the entire area to be subjected to the aliasing processing, the processing direction or a combination of various directions can be arbitrarily set.

Another embodiment is shown in FIGS. 9 (a) and 9 (b). FIG. 9A shows a case where processing is performed radially with one point as a reference point. Since such processing has high processing accuracy in the central portion, it is suitable when high-precision correction is required in the central portion. Radial processing is performed as shown in FIG.
It may be carried out in combination with each processing shown in (b) to (e).
FIG. 9B shows an example in which the processing is performed spirally. In this case, the processing may be combined with the processing in each direction described above.

[Step 4] A shim current value is calculated using the nonuniform static magnetic field distribution obtained in this way and the shim coil characteristics measured in step 1. The shim current is a current necessary for the shim coil to generate a magnetic field having the same magnitude in a direction opposite to the magnetic field inhomogeneity, and can be calculated by Expression (3).

[0034]

(Equation 3)

The shim current is obtained for each channel of the shim coil. [Step 5] The finally calculated current is supplied to the shim coil. Thereby, the non-uniformity of the static magnetic field caused by the subject is corrected with high accuracy. By performing MRI imaging in this state, the accuracy of the image is improved.

[0036]

As described above, according to the present invention, the arc tangent aliasing generated in the phase distribution can be effectively removed. As a result, a more accurate static magnetic field non-uniformity map can be obtained, so that the shimming accuracy is improved.

[Brief description of the drawings]

1A and 1B are diagrams illustrating arc tangent aliasing, wherein FIG. 1A is a phase diagram before correction, and FIG. 1B is a phase diagram after correction.

FIG. 2 is a diagram showing a general sequence for measuring a static magnetic field distribution.

FIG. 3 is a diagram showing the overall configuration of an MRI apparatus to which the present invention is applied.

FIG. 4 is a diagram showing an embodiment of arc tangent aliasing correction according to the present invention.

FIG. 5 is a diagram showing an example of an algorithm for arc tangent aliasing correction according to the present invention.

FIG. 6 is a diagram illustrating an example of an algorithm for arc tangent aliasing correction according to the present invention.

FIG. 7 is a diagram illustrating an example of an algorithm for arc tangent aliasing correction according to the present invention.

FIG. 8 is a diagram illustrating an example of an algorithm for arc tangent aliasing correction according to the present invention.

FIG. 9 is a diagram showing another embodiment of the arc tangent aliasing correction according to the present invention.

[Explanation of symbols]

 302 ・ ・ ・ Static magnetic field generating magnet 303 ・ ・ ・ Gradient magnetic field generating system 304 ・ ・ ・ Transmitting system 305 ・ ・ ・ Detecting system 306 ・ ・ ・ Signal processing system 308 ・ ・ ・ Computer 318 ・ ・ ・ Shim coil (correction means)

Claims (4)

[Claims] 1. A method of correcting non-uniformity of a static magnetic field in a magnetic resonance imaging apparatus, comprising: a static magnetic field generating unit; and a correcting unit that corrects non-uniformity of a static magnetic field generated by the static magnetic field generating unit. Measuring the non-uniform distribution of the static magnetic field generated by the static magnetic field generating means, and generating a static magnetic field that cancels out the non-uniform distribution of the static magnetic field by the correction means, The measuring step includes a step of obtaining a two-dimensional phase distribution of a desired region including information on the inhomogeneity of the static magnetic field, and a step of determining an adjacent point on a predetermined line including the point with reference to one point on the coordinates of the phase distribution. Determining the phase difference between the points to be performed, and performing a process of obtaining a discontinuous point of the phase on the line for a plurality of lines, and a step of performing a process of eliminating the discontinuous point of the phase Wherein the plurality of lines are oblique, spiral, radial, or a combination thereof. 2. A method of correcting non-uniformity of a static magnetic field in a magnetic resonance imaging apparatus, comprising: a static magnetic field generating unit; and a correcting unit that corrects non-uniformity of a static magnetic field generated by the static magnetic field generating unit. Measuring the non-uniform distribution of the static magnetic field generated by the static magnetic field generating means, and generating a static magnetic field that cancels out the non-uniform distribution of the static magnetic field by the correction means, The measuring step includes a step of obtaining a two-dimensional phase distribution of a desired region including information on the inhomogeneity of the static magnetic field, and a step of determining an adjacent point on a predetermined line including the point with reference to one point on the coordinates of the phase distribution. Determining the phase difference between the points to be performed, and performing a process of obtaining a discontinuous point of the phase on the line for a plurality of lines, and a step of performing a process of eliminating the discontinuous point of the phase Wherein the plurality of lines are point targets with respect to a reference point or line targets with respect to a line including the reference point. 3. The static magnetic field non-uniformity correction method according to claim 1, further comprising the step of performing the processing for obtaining the discontinuous point performed on the plurality of lines a plurality of times while changing the direction of the line in a desired area. . 4. A magnetic resonance imaging apparatus comprising: a static magnetic field generating means; and a correcting means for correcting non-uniformity of a static magnetic field generated by the static magnetic field generating means. A two-dimensional phase distribution of the area is obtained, a phase difference between adjacent points on a predetermined line including the point is determined with reference to one point on the coordinates of the phase distribution, and a discontinuity of the phase on the line is determined. Non-uniform distribution measurement in which the processing to be performed is performed on a plurality of lines composed of any one of oblique directions, spirals, radials, or a combination thereof, and processing for eliminating the discontinuity of the phase is performed to measure the non-uniform distribution of the static magnetic field. A magnetic resonance imaging apparatus characterized by comprising means.