JPH07327952A - Mri photographing method - Google Patents

Abstract

PURPOSE:To obtain distortion-free images in an echo planar photographing method. CONSTITUTION:This each planar photographing method of magnetic resonance imaging (MRI) comprises collecting image signals by impressing a high-frequency magnetic field for magnetization excitation, a gradient magnetic field for determining the region to be excited, a lead out gradient magnetic field for applying position information to magnetization and a phase encoding gradient magnetic field to the inspection object in a static magnetic field. a step of making the uniformity of the static magnetic field in the phase encoding direction within the excitation region better than the uniformity of the static magnetic field in a lead-out direction is executed prior to excitation of magnetization in the method.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging (hereinafter referred to as MRI) imaging method, and more particularly to an echo planar imaging method.

[0002]

2. Description of the Related Art The echo planar imaging method is one of ultra-high speed imaging methods used in an MRI apparatus which is a tomographic imaging apparatus utilizing a magnetic resonance phenomenon. The general shooting principle is outlined below.

An inspection target is placed in a static magnetic field, a high frequency magnetic field pulse is applied to excite magnetization of hydrogen atoms and the like, and then a phase encode gradient magnetic field and a read out gradient magnetic field are applied to provide position information to each magnetization. , Magnetic resonance signal (echo) is measured. Regarding the read-out gradient magnetic field, one echo is measured per one pulse while alternately applying a positive pulse and a negative pulse. As for the phase encode gradient magnetic field, a step pulse is applied immediately before the echo, or an offset gradient magnetic field having a small intensity is continuously applied. The number of sampling points is usually 64 to 512 per echo, and the number of echoes to be measured is 64 to 256.

After the measurement is completed, all echoes are arranged with the horizontal direction of the two-dimensional plane as the sampling direction and the vertical direction as the echo direction, and image reconstruction is performed by two-dimensional Fourier transform to obtain a tomographic image. The matrix size of the image is (the number of sampling points of one echo) × (the number of echoes). Besides this,
There is also a half Fourier method in which the number of echoes to be measured is reduced to about half and the remaining echoes are estimated from the measured echoes.

The details of the echo planar imaging method are described in the Journal of Magnetic Resonance (Journal of Magnetic Resonance).
Magnetic Resonance), vol.29, pp. 355-373, (1978)
It is described in.

It is desirable that the static magnetic field is uniform over the entire area to be imaged, but in reality, there are some inhomogeneities. The non-uniformity of the static magnetic field appears in the captured image as distortion or a false image (artifact) and has an adverse effect.

In order to solve this problem, it is necessary to correct static magnetic field inhomogeneity in advance. This correction is MR
This is performed by adjusting the current flowing through the shim coil for static magnetic field inhomogeneity correction prepared in the I device. This is called shimming. For more information on shimming, magnetic
Magnetic Resonancein Medicine
Medicine), vol.18, pp.335-347, (1991). Usually a radius of 15 to 20 from the center of the bore
Shimming is performed so that the static magnetic field homogeneity in the range of cm is the best on average. However, it is not possible to completely correct nonuniformity.

In the case of the spin echo imaging method, which is often used in general imaging, there have been proposed some methods for correcting the image distortion caused by the non-uniformity of the static magnetic field existing after shimming (for example, IEE). Transactions on Medical Imaging (IEEE T
ransactions on Medical Imaging), vol.11, No.3, p
p.319-329, 1992). By using this method, image distortion can be removed without depending on the direction of the static magnetic field inhomogeneity.

[0009]

On the other hand, in the echo planar imaging method, a method for correcting image distortion and artifacts caused by non-uniformity of the static magnetic field in the readout direction has already been proposed (for example, Magnetic Resonance in
Medicine, vol.23, pp.311-323, 1992). However, a method for correcting image distortion caused by non-uniformity of the static magnetic field in the phase encoding direction is not currently known. An object of the present invention is to reduce image distortions and artifacts caused by static magnetic field inhomogeneity in the phase encoding direction in the echo planar imaging method.

[0010]

The above problem is solved by including the step of making the static magnetic field homogeneity in the phase encoding direction better than the static magnetic field homogeneity in the readout direction.

[0011]

Since the static magnetic field inhomogeneity in the phase encoding direction is reduced, the image distortion and the artifacts caused thereby can be reduced. Further, image distortion and artifacts caused by non-uniformity of the static magnetic field in the readout direction can be corrected using the above-described correction method. As a result, it is possible to reduce image distortion / artifacts caused by non-uniformity of the static magnetic field in the entire image.

[0012]

FIG. 1 is a block diagram showing an example of an inspection apparatus using magnetic resonance according to the present invention (hereinafter simply referred to as an inspection apparatus). In the figure, 101 is a coil (magnet) that generates a static magnetic field, 102 is a coil that generates a gradient magnetic field, and 103 is an inspection target, which is installed in the coils 101 and 102. Further, the sequencer 104 sends a command to the gradient magnetic field power supply 105 and the high frequency magnetic field generator 106 to generate the gradient magnetic field and the high frequency magnetic field. The high frequency magnetic field is applied to the inspection target 103 through the probe 107. The signal generated from the inspection target 103 is the probe 107.
Received by the calculator 108 through the receiver 108.
The signal processing is performed there. The result is displayed on the display 110. If necessary, the storage medium 11
It is also possible to store signals and measurement conditions in 1.

When it is necessary to adjust the homogeneity of the static magnetic field, the shim coil 112 is used. The shim coil 112 is composed of a plurality of channels, and the shim power supply 113 supplies current. When adjusting the static magnetic field homogeneity, the sequencer 104 controls the current flowing through each coil. Sequencer 104
Sends a command to the shim power supply 113 to cause the coil 112 to generate an additional magnetic field for correcting the static magnetic field inhomogeneity.
The sequencer 104 performs control so that each device operates at a timing and intensity programmed in advance. Of the programs, those that describe the high-frequency magnetic field, the gradient magnetic field, and the timing and intensity of signal reception are called pulse sequences.

Next, FIG. 2 shows a typical example of the pulse sequence of the echo planar imaging method according to the embodiment of the present invention. A high frequency magnetic field (RF) pulse 202 for magnetization excitation is applied together with the application of the slice gradient magnetic field 201 to induce a magnetic resonance phenomenon in a certain slice in the target object. The two pulses 203 and 204 are dephasing gradient magnetic field pulses applied to make the magnetization phase negative once.

A magnetic resonance signal (echo) 205 is applied with a phase encode gradient magnetic field 206 for adding position information in the phase encode direction to the magnetization phase, and a read out gradient for adding position information in the read out direction. It is measured while applying the magnetic field pulse 207. The readout gradient magnetic field pulse is applied alternately in positive and negative directions, and a plurality of echoes are measured during that period.

First, a method of determining the phase encoding direction based on the static magnetic field distribution will be described. FIG. 3 shows an example of the static magnetic field distribution. The figure shows a static magnetic field distribution inside a circular inspection object placed in a square imaging field of view with a phase of magnetization proportional thereto. The static magnetic field inhomogeneity on the line segment AB is shown at the bottom of the figure. Static magnetic field inhomogeneity exists in the direction perpendicular to the stripes in the figure. Therefore, in this case, the direction of the arrow shown in the figure is the direction in which the static magnetic field homogeneity is the best. Therefore, the direction of phase encoding is determined to be the direction of this arrow. As a special case, there is a case where the direction with the best homogeneity of the static magnetic field cannot be set to the phase encoding direction due to the restriction of the gradient magnetic field generating coil. For example, when executing a pulse sequence that requires a gradient magnetic field with a very short rise time only in a certain axis direction, if the specifications are satisfied only in a specific axis of the gradient magnetic field generation coil, the phase encoding direction may be set arbitrarily. I can't decide. In that case, the direction in which the static magnetic field homogeneity is best may be determined from the directions in which phase encoding can be performed. It should be noted that shimming is usually performed before photographing to improve the uniformity of the static magnetic field. In that case, this phase encoding direction is determined after shimming.

Next, a method will be described in which after the phase encoding direction is arbitrarily determined, shimming is performed so that the static magnetic field homogeneity in that direction is maximized.

First, the shimming that is normally performed will be described. The shim coil is x, y, z, x2-y2, x
A multi-channel coil system having various characteristics such as y, z2, z3, ..., For example, x generates a magnetic field that linearly changes with respect to the x-axis, and the change rate is the current flowing through the coil. Almost proportional to the value. By superimposing these additional static magnetic fields on the static magnetic field generated by the main coil, a more uniform static magnetic field distribution can be obtained. Therefore, shimming is nothing but obtaining a set of shim current values that gives an optimum static magnetic field distribution.
This process is as follows.

Step 1: The current-magnetic field characteristic (shim characteristic) of each shim coil in the adjustment area including the photographing area is examined.
The shim characteristics may be obtained by calculation, or a water sample or the like may be inserted into the magnet to obtain the change amount of the static magnetic field distribution with respect to the change in the shim current value for each channel. Step 2: Measure the static magnetic field distribution in the imaging area. At this time, it is desirable to use an inspection target that is actually photographed as the measurement target. Step 3: Using the shim characteristics of each channel obtained in step 1, find a set of shim current values that cancel the static magnetic field distribution obtained in step 2. Step 4: The shim current obtained in step 3 is passed through the shim coil.

In step 3, when a set of shim current values that cancels the static magnetic field distribution is found, the static magnetic field distribution before shimming and the shim coil are created because the static magnetic field distribution matrix number is larger than the shim coil channel number. The magnetic field distribution cannot be made completely equal. Therefore, usually, an approximate solution is obtained by using the least square method or the like. That is,
Make the static magnetic field inhomogeneity small on average over the entire region.

On the other hand, in the shimming of the present invention, step 3
In (1), emphasis is placed on the phase encoding direction arbitrarily determined previously, and a set of shim current values that minimizes the change in the static magnetic field distribution in that direction is found. Here, the method will be described taking the case where the phase encoding direction is the y direction as an example. When a current Ik is applied to the shim channel k, the static magnetic field strength of the pixel (i, j) in a certain plane is given by Equation 1 when the static magnetic field strength when Ik = 0 is expressed by a matrix [Esij].

[0022]

[Equation 1]

Here, Nx and Ny are the numbers of pixels in the horizontal and vertical directions, and each element of the matrix [A] is a shim channel k.
Pixel (i, j) when the current value of is changed by a unit amount
3 is a shim characteristic that represents the amount of change in the shim magnetic field at. Number 1
When only the [Eij] column of is expanded and expressed, the formula 2 is obtained.

[0024]

[Equation 2]

This equation represents the magnetic field strength in one line of x = i. In order to improve the uniformity of the static magnetic field in the y direction, the current vector [Ik] that minimizes Equation 3 may be obtained.

[0026]

[Equation 3]

This equation is the sum of the square error of the static magnetic field strength of each pixel based on the static magnetic field strength of j = Ny / 2. Thereby, the static magnetic field homogeneity of the line of x = i can be optimized. In order to maximize the static magnetic field homogeneity in the y direction in the entire plane, it is sufficient to minimize Equation 4 which is the sum of Equation 3 also in the x direction.

[0028]

[Equation 4]

By the above method, the static magnetic field homogeneity in the phase encoding direction can be optimized. This method
The same applies when the phase encoding direction is other than the y direction.

The static magnetic field distribution in steps 1 and 2 can be obtained, for example, from the phase distribution or the resonance frequency distribution of the magnetic resonance signal of the water sample or the inspection object. When the static magnetic field distribution is obtained from the phase distribution of the magnetic resonance signal, for example, Journal of Physics E: Scientific Instrument
ment), vol.18, pp.224-227, (1985). Moreover, when obtaining the static magnetic field distribution from the resonance frequency distribution, for example, the Jounal of Ma
The method described in gnetic Resonance, vol.85, pp.244-254, (1989) can be used.

In MRI, there are cases in which different regions are excited by a high frequency magnetic field to continuously capture images. At this time, prior to a series of shooting, the phase encoding direction of each area is arbitrarily determined, the shim current value is stored by performing the shimming of the present invention, and the current value of the shim coil is set before shooting each area. Be sure to switch to that value. This makes it possible to realize the optimum static magnetic field homogeneity for each image.

The phase encoding direction may be determined based on the image actually taken. In this case, a plurality of images with arbitrary phase encoding directions are photographed prior to the main photographing, and the phase is corrected and reconstructed. The one with the highest image quality is selected from the images, and the phase encoding direction is adopted in the actual shooting.

The effect of improving the static magnetic field homogeneity in the phase encoding direction by any of the methods described above will be described. FIG. 4 and FIG. 5 show images of the lattice model taken by the echo planar imaging method when the static magnetic field is uniform and when it is not. 5A shows an image when secondary static magnetic field inhomogeneity exists in the readout direction, and FIG. 5B shows an image when no secondary magnetic field inhomogeneity exists in the phase encoding direction. It can be seen that the lattice is distorted in both cases compared to FIG.

FIG. 6 shows the result of applying the correction in each case. When the static magnetic field inhomogeneity is present in the readout direction of (a), the correction is completed, but (b)
If there is non-uniformity in the phase encoding direction of, there is no correction effect. In this way, image distortion caused by static magnetic field inhomogeneity in the readout direction can be completely corrected, so that the effect of static magnetic field inhomogeneity can be reduced by reducing static magnetic field inhomogeneity in the phase encode direction. Can be completely removed.

In the above description, the case of a two-dimensional image is described, but the same applies to the case of a three-dimensional image. When measuring a three-dimensional image, the second phase encoding gradient magnetic field is applied in the direction of the slice gradient magnetic field in FIG. At this time, the same procedure may be performed paying attention to one of the two phase encoding directions.

[0036]

According to the present invention, by making the static magnetic field inhomogeneity in the phase encoding direction, which is difficult to correct, smaller than the static magnetic field inhomogeneity in the correctable read-out direction, image distortion and artifacts can be obtained as compared with the prior art. It is possible to take images with few.

[Brief description of drawings]

FIG. 1 is a block diagram showing an example of a device configuration according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a pulse sequence according to an embodiment of the present invention.

FIG. 3 is a static magnetic field distribution chart used for determining a phase encoding direction.

FIG. 4 is an explanatory diagram showing an echo planer image when a static magnetic field is uniform.

FIG. 5 is an explanatory diagram showing an echo planar imaging image when the static magnetic field is not uniform.

FIG. 6 is an explanatory diagram showing an echo planer image obtained by performing phase correction when the static magnetic field is nonuniform.

[Explanation of symbols]

101 ... Coil, 102 ... Coil, 103 ... Inspection target,
104 ... Sequencer, 105 ... Gradient magnetic field power supply, 106 ...
High frequency magnetic field generator.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Etsuji Yamamoto Etsuji Yamamoto 1-280 Higashi Koikekubo, Kokubunji City, Tokyo Inside Central Research Laboratory, Hitachi, Ltd. (72) Hiroshi Nishimura 1-14 1 Kanda, Uchida, Chiyoda-ku, Tokyo Inside the Hitachi Medical Co.

Claims (8)

[Claims] 1. A high-frequency magnetic field for exciting magnetization, a gradient magnetic field for determining a region to be excited, a read-out gradient magnetic field for giving position information to the magnetization, and a phase-encoding gradient magnetic field for an inspection target in a static magnetic field. In an echo planer imaging method of magnetic resonance imaging (MRI) in which a signal is applied to collect a signal, the static magnetic field homogeneity in the phase encode direction in the excitation region immediately before or after the magnetization excitation is compared with the static magnetic field homogeneity in the readout direction. An MRI imaging method characterized by executing a step for improving. 2. The method according to claim 1, wherein the step of making the static magnetic field homogeneity in the phase encoding direction higher than the static magnetic field homogeneity in the readout direction comprises the step of determining the phase encoding direction based on the static magnetic field distribution. MRI imaging method including. 3. The MRI imaging method according to claim 2, wherein the step of determining the phase encoding direction based on the static magnetic field distribution includes the step of matching the direction of the smallest static magnetic field inhomogeneity with the phase encoding direction. 4. The method according to claim 1, wherein the step of making the static magnetic field homogeneity in the phase encode direction higher than the static magnetic field homogeneity in the read-out direction includes the step of arbitrarily determining the phase encode direction and the phase encode direction. An MRI imaging method including a step of performing shimming so that the static magnetic field homogeneity is maximized. 5. The method according to claim 1, wherein the step of making the static magnetic field homogeneity in the phase encode direction better than the static magnetic field homogeneity in the read-out direction includes the step of arbitrarily determining the phase encode direction of the excitation region, The method includes a step of determining a shim current value that maximizes the homogeneity of the static magnetic field in the phase encoding direction, and a step of storing the shim current value. Furthermore, immediately before or after the magnetization excitation, it corresponds to each excitation region. MRI imaging method including a step of switching to a shim current value to be performed. 6. The method according to claim 1, wherein the step of making the static magnetic field homogeneity in the phase encode direction higher than the static magnetic field homogeneity in the read out direction includes a step of measuring a plurality of images in different phase encode directions. An MRI imaging method including the steps of: (1) correction and the step of obtaining an image with minimum distortion. 7. The MRI imaging method according to claim 2 or 3, wherein the static magnetic field distribution is obtained from a phase distribution of a magnetic resonance signal. 8. The MR according to claim 2 or 3, wherein the static magnetic field distribution is obtained from a distribution of resonance frequencies of magnetic resonance signals.
I shooting method.