JP2006061235A - Magnetic resonance imaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide excellent images by correcting image distortion due to static magnetic field nonuniformity in post processing by using static magnetic field nonuniformity information possessed by an MRI (magnetic resonance imaging) device. <P>SOLUTION: Phase distribution data indicating the static magnetic field nonuniformity of the MRI device are acquired beforehand and preserved, a correction value for each pixel of image data is calculated by using the echo number of measurement space data after image pickup and the information on the phase distribution data preserved beforehand, and phase correction is performed by using the correction value. For the phase correction, the measurement space data (s) (kx, ky) are first Fourier-transformed in a frequency encoding direction, a phase error due to the static magnetic field nonuniformity of one pixel position E(xp, yp) for hybrid space data s(x, ky) after Fourier transformation is phase-corrected, and then image data I(x, y) obtained by further performing Fourier transformation in a phase encoding direction are obtained. The pixel data I(xp, yp) corresponding to the pixel are extracted from the image data and the pixel of a final image is attained. It is performed for all the pixels and phase-corrected image data I'(x, y) are obtained. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and more particularly to an MRI apparatus provided with means for correcting artifacts caused by static magnetic field inhomogeneity.

  An MRI apparatus irradiates a subject placed in a uniform static magnetic field with a high-frequency magnetic field, and reconstructs the subject image using a magnetic resonance signal (echo signal) generated thereby. In order to obtain the above, it is necessary that the static magnetic field is uniform. There are permanent magnets, normal conducting magnets, or superconducting magnets as static magnetic field generators for MRI apparatuses. However, there is a limit in performance in terms of uniformity. The uniformity is improved by incorporating a shim coil that generates a correction magnetic field that cancels the non-uniformity. To correct the static magnetic field inhomogeneity by the shim coil, for example, the phase rotation non-uniformity generated in the echo signal due to the static magnetic field inhomogeneity is obtained as phase distribution data (phase map), and then the static magnetic field inhomogeneity is calculated. A magnetic field that eliminates inhomogeneities is generated by shim coils. For example, Patent Literature 1 and Patent Literature 2 describe correction methods for static magnetic field inhomogeneity using shim coils.

  However, even when such active shimming using a shim coil or passive shimming using a magnet piece is employed, the non-uniformity cannot be completely removed, and a non-uniformity of about several ppm remains. Such static magnetic field inhomogeneity is caused by image distortion (such as echo planar imaging (EPI)) that cannot be ignored in an imaging method in which a gradient magnetic field pulse is inverted after a single excitation to measure a plurality of echo signals. (Artifact). This is because the magnetization is subjected to rotation more than the phase rotation originally received by the magnetization due to non-uniformity of the static magnetic field. FIGS. 7A and 7B show general static magnetic field inhomogeneity and image distortion caused thereby.

Various methods have been proposed for removing artifacts caused by such static magnetic field inhomogeneity by phase-correcting the signal after measurement. This is to perform phase correction corresponding to the amount of phase rotation caused by static magnetic field inhomogeneity on the measured signal, under the same conditions as when acquiring image forming data prior to acquiring image forming data. Correction data is created using the acquired echo signal (reference data), thereby correcting the image forming data. For example, in Patent Document 3, the difference between the reference data p (i, j, kb) and the other data p (i, j, k) is obtained for the reference data acquired as the correction data, and the phase component is obtained. And the change of the absolute value component, and this change is used as the correction data, and the image forming data q (i) having the same phase encoding i, frequency encoding j, and echo number k as the reference data p (i, j, k). , j, k). In such a conventional phase correction method, it is necessary to create a pre-scan for acquiring reference data and correction data using the pre-scan.
JP 2001-238866 A JP 2000-342552 A JP 2002-85376 A

  The present invention provides a phase correction method that eliminates the need for pre-scanning for each imaging that has been performed by the conventional phase correction method and can perform phase correction afterwards, thereby reducing the imaging time. For the purpose. It is another object of the present invention to provide an MRI apparatus that includes a post-phase correction unit and can obtain a good image.

  A correction method of the present invention that solves the above problem is a method of correcting image distortion due to static magnetic field inhomogeneity of image data, and acquiring and storing phase distribution data representing static magnetic field inhomogeneity of an MRI apparatus; Calculating a correction value for each pixel of the image data using the echo number of the measurement space data after imaging and the information of the phase distribution data; and image data phase-corrected for each pixel using the correction value And a creating method.

  In the correction method of the present invention, for example, the phase distribution data includes spin phase change information caused by static magnetic field inhomogeneity at a minute time α, and the step of calculating the correction value includes an echo time of measurement space data and the minute time A correction value for each pixel is calculated using the time α and the phase change information for each pixel.

  In the correction method of the present invention, for example, the step of creating the phase-corrected image data includes the step (1) of performing Fourier transform on the measurement space data s (nx, ny) in the frequency encoding direction, and the hybrid after Fourier transform. Step (2) for correcting phase error caused by static magnetic field inhomogeneity at one pixel position (xp, yp) for spatial data s (x, ny), obtained by Fourier transform of hybrid spatial data in phase encoding direction The pixel data I (xp, yp) at the pixel position is extracted from the obtained image data I (x, y) and used as the final image pixel. Step (4) is performed to obtain phase-corrected image data I ′ (x, y).

  The MRI apparatus of the present invention includes magnetic field generating means for generating a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field, and an imaging means for acquiring a nuclear magnetic resonance signal as an echo signal from a subject placed in the static magnetic field, Image processing means for processing the echo signal acquired by the image pickup means to generate image data, and image display means for displaying the image data as an image, the image pickup means being excited once by a high-frequency magnetic field. An imaging sequence for acquiring a plurality of echo signals is executed, and the image creating means uses the echo number and static magnetic field inhomogeneous information stored in advance for the measurement space data acquired by the imaging sequence. A correction value for each pixel is obtained, and image data whose phase is corrected for each pixel is created.

  According to the present invention, the MRI apparatus uses the phase distribution data acquired in advance to correct the static magnetic field inhomogeneity, thereby eliminating the need for pre-scanning for obtaining the amount of phase rotation caused by the static magnetic field inhomogeneity during imaging. Since the acquired data can be phase-corrected later, the imaging can be completed in a short time.

Hereinafter, embodiments of the MRI apparatus of the present invention will be described.
FIG. 1 is a block diagram showing an overall outline of an MRI apparatus to which the present invention is applied. This MRI apparatus mainly includes a static magnetic field generation circuit 1, a gradient magnetic field generation system 2, a transmission system 3, a reception system 4, a signal processing system 5, a control system (sequencer 6 and CPU 7), and an operation unit 8. It consists of. The static magnetic field generation circuit 1 generates a uniform static magnetic field in the body axis direction of the subject or in a direction perpendicular to the body axis in the space where the subject 9 is placed. A magnetic field generating means composed of a magnet or a superconducting magnet is disposed. The gradient magnetic field generation system 2 includes three gradient magnetic field coils 10 wound in three axial directions of X, Y, and Z, a gradient magnetic field power source 12 for driving each gradient magnetic field coil 10, and a shim coil 11 having a plurality of channels. And a shim power source 13 for applying a current to each channel of the shim coil 11, and by driving the gradient magnetic field power source 12 in accordance with a command from the sequencer 6, the slice direction, phase encoding direction and frequency encoding direction (reading direction) The gradient magnetic fields Gs, Gp, and Gf can be applied to the static magnetic field space. Further, by driving the power supply 13 of the shim coil 11, the non-uniformity of the static magnetic field can be corrected.

  The transmission system 3 includes a high-frequency oscillator 14, a modulator 15, a high-frequency amplifier 16, and a transmission-side high-frequency coil (transmission coil) 17, and a high-frequency pulse output from the high-frequency oscillator 14 is transmitted from the sequencer 6. After being modulated by a modulator 15 by a high-frequency magnetic field pulse, amplified by a high-frequency amplifier 16, and then supplied to a transmission coil 17, a nuclear magnetic resonance is caused in the atomic nucleus constituting the living tissue of the subject 9 to excite it. It is like that.

  The reception system 4 includes a reception-side high-frequency coil (reception coil) 18, an amplifier 19, a quadrature detector 20, and an A / D converter 21, and a subject 9 caused by electromagnetic waves irradiated from the irradiation coil 17. The NMR signal (echo signal), which is an electromagnetic wave of the response of, is detected by the receiving coil 18, amplified by the amplifier 19, and then phase-detected by the quadrature phase detector 20 using the high-frequency signal from the high-frequency oscillator 14 as a reference signal. 6 is sampled by the A / D converter 21 at the timing according to the command from 6 and sent to the signal processing system 5 as two series of digital data.

  In the figure, the irradiation coil 17 and the reception coil 18 are shown at positions away from the subject 9, but these are arranged close to the subject 9. One RF coil may serve as both the irradiation coil 17 and the receiving coil 18.

  The signal processing system 5 performs image reconstruction calculation using the collected data consisting of echo signals detected by the receiving system 4 and displays an image. The acquired data is subjected to Fourier transform, correction coefficient calculation, and image reconstruction calculation. And the like, a storage device such as a ROM (read-only memory) 22, a RAM (optional read / write memory) 23, a magneto-optical disk 24 and a magnetic disk 26, and a display 25. The ROM 22 stores, for example, a program for performing image analysis processing and imaging, and invariant parameters used for the execution thereof, and the RAM 23 stores collected data received by the receiving system 4 and images used for region of interest setting and region of interest setting. Parameters for such as are temporarily stored. In the MRI apparatus of this embodiment, phase distribution data (phase map) that is a result of measuring the non-uniformity of the static magnetic field is also stored in the RAM 23, and the shim coil 11 is driven to correct the static magnetic field non-uniformity based on this phase map. The current is determined. Further, using the phase map, phase correction for removing the influence of the static magnetic field inhomogeneity is performed on the data collected by a predetermined imaging method. The CPU 7 performs calculation of the current value supplied to the shim power supply 13 and calculation of phase correction of the image. Image data reconstructed by the CPU 7 is recorded on the magneto-optical disk 24 and the magnetic disk 26. The display 25 displays an image read from the data storage unit (the magneto-optical disk 24 and the magnetic disk 26), and displays a GUI for inputting commands and parameters to the apparatus via the operation unit 8.

  The CPU 7 has a function as a control unit for controlling the entire apparatus in addition to the function as the signal processing system 5 described above, and sends a command to the sequencer 6 to transmit the transmission system 3, the gradient magnetic field generation system 2, and the reception system 4. Are controlled to operate according to a predetermined pulse sequence. The pulse sequence is stored as a program in the signal processing system 5 (ROM 22), and variable parameters necessary for its execution can be set by the user via the operation unit 8. The operation unit 8 is used to input control information for processing performed by the signal processing system 5 and the control system, and includes a trackball, a mouse 27, a keyboard 28, and the like.

Next, the operation of the MRI apparatus having the above configuration will be described.
The MRI apparatus of the present invention has a function of correcting image distortion due to static magnetic field inhomogeneity as a function of the signal processing system 5, and particularly uses a phase map obtained for calculating a shim current for the correction calculation. It is a feature. The phase map is obtained by, for example, a known method disclosed in Patent Document 1 or Patent Document 2 described above, and is stored in the storage device of the signal processing system 5. A method known as a method for obtaining a phase map is that a deformed spin echo type sequence is executed using a homogeneous object such as a phantom, and α has elapsed from the echo time (2τ) at which the spin echo occurs. The echo signal is measured by, and the inhomogeneity of the static magnetic field generated during this minute time α is measured.

That is, when a 90 ° pulse 201 that excites nuclear spins as shown in FIG. 2 is applied, and a 180 ° pulse 202 that reverses nuclear spins after τ elapses, the phase of the nuclear spins changes after τ elapses from 180 ° pulse application. It's aligned. Thereafter, the phase rotation is caused by the non-uniformity of the static magnetic field, and the echo signal measured after the elapse of time α includes only the phase rotation amount caused by the non-uniformity of the static magnetic field. At this time, by using the slice gradient magnetic fields 203 and 204 and the phase encode gradient magnetic field 205, a phase map E ₀ (x, y) (x and y indicate coordinates in real space) is obtained as two-dimensional or three-dimensional data. be able to. The phase map E ₀ (x, y) thus obtained is used to determine the shim current that drives the shim coil. While passing the determined shim current, the phase map E (x, y) is obtained again as two-dimensional or three-dimensional data. Alternatively, the phase map E (x, y) when the determined current is passed is estimated based on the phase map E ₀ (x, y) obtained first. This phase map E (x, y) is stored in a signal processing system storage device and used for phase correction of acquired data after imaging.

  As an imaging sequence for correcting image distortion caused by static magnetic field inhomogeneity after data acquisition, there is an imaging method in which a plurality of echo signals are continuously acquired by one excitation. Specifically, there is an imaging method using a sequence such as single shot EPI, multi-shot EPI, GRASE (Gradient and Spin Echo Imaging).

  In the EPI sequence, as shown in FIG. 3, after applying a high frequency magnetic field pulse 101 together with a slice selective gradient magnetic field 102, a gradient magnetic field 103 that gives a gradient magnetic field offset in the phase encode direction is applied as necessary, and a gradient in the frequency encode direction is applied. The magnetic field 104 is repeatedly applied while reversing the polarity (106), the phase encoding gradient magnetic field 105 is applied in a blip shape, and the echo signal 108 generated every inversion of the frequency encoding gradient magnetic field is measured within the sampling time 107. . This EPI sequence may be a single-shot EPI that acquires all data necessary for image reconstruction in one measurement 109 or a multi-shot EPI that acquires all data by repeating measurement 109 a plurality of times.

  Data acquired by executing such a pulse sequence is stored in a storage device as measurement space data in which one coordinate axis is a phase encode amount ky (echo number) and the other coordinate axis is a frequency encode number kx, which will be described below. After phase correction, the image is reconstructed.

  FIG. 4 shows an image reconstruction procedure including phase correction, and FIG. 5 shows the relationship between acquired data and finally phase-corrected image data. First, the k-space data obtained in the above data measurement (step 402) is Fourier-transformed in the frequency encoding direction, and hybrid space data s (x, ny) (x is the position in the frequency encoding direction, ny is the echo number, ny = 1, 2, 3, ..., p) is created (step 403). On the other hand, a correction value is calculated using a static magnetic field inhomogeneity map (phase map E (x, y)) (401) previously stored in the storage device (step 404). The procedure for calculating the correction value is as follows.

First, the echo signal s (nx, ny) obtained by imaging can be expressed by the following equation (1), where B (x, y) is the static magnetic field inhomogeneity.
Where (nx, ny) is the position in the measurement space, (x, y) is the position in the real space, ρ (x, y) is the proton density, ω is the resonance frequency, and Gx is the frequency encoding gradient magnetic field. The application intensity, Gy is the application intensity of the phase encoding gradient magnetic field, Δt is the data sample time, and Δτ is the application time of the phase encoding gradient magnetic field pulse. This formula (1) can be further expressed by the following formula (2).

In general, since B (x, y) << Gx, it can be considered that Δx≈0, and the collection timing (te1, te2, te3,..., Tep of each echo signal 108 in the pulse sequence shown in FIG. The phase information of the static magnetic field inhomogeneity in) is expressed by the following equations.
That is, the phase change due to non-uniform static magnetic field becomes a phase gradient in the phase encoding direction when echo signals are arranged in the measurement space.

On the other hand, the phase map E (x, y) that the signal processing unit has as static magnetic field inhomogeneous data can be expressed by Expression (4), where B (x, y) is the static magnetic field inhomogeneity. Accordingly, the correction value θ (ny) of each echo signal is expressed by equation (5).
Here, t _eny is the echo time of the echo signal, and α is the deviation time from the echo time (2τ) when obtaining the phase map (α in FIG. 2), so if E (x, y) is known, θ (ny) can be calculated.

Therefore, the correction value for correcting the phase error due to the static magnetic field inhomogeneity at the predetermined pixel position (x _p , y _p ) is obtained from the following equation (6) from the above equation (5).

Using this correction value θ (ny), the phase error is corrected by the following equation (7) for the data s (x, ny) after Fourier transform in the frequency encoding direction (step 405).

Subsequently, the corrected data s ′ (x, ky) is Fourier transformed in the phase encoding direction (step 406). As a result, an image I (x, y) in which the static magnetic field inhomogeneity is corrected at the pixel position (x _p , y _p ) can be created. Next, a pixel I (x _p , y _p ) on the image I (x, y) is extracted and set as a pixel of the image I ′ (x, y) to be finally output (step 407). The correction value calculation 404, the phase correction process 405, the Fourier transform 406 in the phase encoding direction, and the pixel extraction 407 are changed for each pixel of interest (step 408) and executed for all the pixels on the image. Finally, it is possible to obtain an image I ′ (x, y) in which the image distortion due to the static magnetic field inhomogeneity is corrected. The corrected image is stored in the storage device and displayed on the display unit (step 409).

  As described above, according to the MRI apparatus of this embodiment, the phase correction value for correcting the static magnetic field inhomogeneity is calculated using the phase map that the apparatus already has for calculating the shim current. There is no need to perform pre-scanning to obtain information, imaging can be performed without extending the time, and a good image in which image distortion due to static magnetic field inhomogeneity is corrected can be obtained.

In the embodiment described above, in order to calculate the correction value θ in the equation (6),
The echo time t _eny (t _e1 , t _e2 , t _e3 , ... t _ep ) used in this calculation is determined by the effective echo time, the echo acquisition interval, and the number of echo signals, and is a constant value. To increase. Therefore, it is also possible to calculate using these parameters.

It is also possible to calculate the correction value θ using the following equation (8) instead of the equation (6). As already described, the phase change due to the static magnetic field inhomogeneity becomes a phase gradient in the phase encoding direction as shown in FIG. 6 when the echo signals are arranged in the measurement space. Therefore, if the manually input value is In, θ (ny) can be calculated by Equation (8).

  As described above, the method for correcting the phase of the acquired data using the phase map in the signal processing system of the MRI apparatus has been described as an embodiment of the present invention. The phase correction of the present invention is a signal provided in the MRI apparatus. It can be executed not only as a processing system but also as an independent image processing apparatus. For example, when image data is transferred to a remote place and displayed, the static magnetic field inhomogeneity information of the MRI apparatus can be transferred along with the image data, and phase correction can be performed as part of image processing.

  According to the present invention, image distortion caused by static magnetic field inhomogeneity can be corrected by post-processing using static magnetic field inhomogeneity information held by the MRI apparatus, and image distortion can be corrected without affecting imaging time. No good image can be obtained.

The figure which shows the whole outline | summary of the MRI apparatus with which this invention is applied. The figure which shows an example of the pulse sequence for obtaining the phase map showing the static magnetic field inhomogeneity The figure which shows an example of the imaging sequence which the MRI apparatus of this invention performs The figure which shows the procedure of the phase correction process in the MRI apparatus of this invention Diagram showing the relationship between acquired data and phase-corrected image data The figure explaining another embodiment of the phase correction of this invention Figure showing static magnetic field inhomogeneity and resulting image distortion

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Static magnetic field generation circuit, 2 ... Gradient magnetic field generation system, 3 ... Transmission system, 4 ... Reception system, 5 ... Signal processing system, 7 ... CPU (control part), 8: Operation unit.

Claims (4)

A method for correcting image distortion caused by non-uniform static magnetic field of image data captured by a magnetic resonance imaging apparatus,
Obtaining and storing phase distribution data representing static magnetic field inhomogeneities of the magnetic resonance imaging apparatus;
Calculating a correction value for each pixel of the image data using the echo number of the measurement space data after imaging and the information of the phase distribution data;
Creating a phase-corrected image data for each pixel using the correction value.   The phase distribution data includes spin phase change information caused by static magnetic field inhomogeneity at a minute time α, and the step of calculating the correction value includes an echo time of measurement space data, the minute time α, and a phase change for each pixel. The correction method according to claim 1, wherein a correction value for each pixel is calculated using information.   The step of creating the phase-corrected image data includes a step (1) of Fourier transforming the measurement space data s (nx, ny) in the frequency encoding direction, and 1 for the hybrid space data s (x, ny) after the Fourier transform. Step (2) for correcting phase error due to static magnetic field inhomogeneity at two pixel positions (xp, yp), from image data I (x, y) obtained by Fourier transform of hybrid space data in phase encoding direction The pixel data I (xp, yp) at the pixel position is extracted and used as a pixel of the final image (3), and the steps (1) to (3) are executed for all the pixels, and the phase-corrected image data I 3. The correction method according to claim 1, further comprising a step (4) of obtaining '(x, y). An imaging unit including a magnetic field generating unit configured to generate a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field, and acquiring a nuclear magnetic resonance signal as an echo signal from a subject placed in the static magnetic field; and an echo signal acquired by the imaging unit In a magnetic resonance imaging apparatus comprising: an image creating means for performing signal processing and creating image data; and an image display means for displaying the image data as an image.
The imaging means executes an imaging sequence for acquiring a plurality of echo signals by one excitation by a high-frequency magnetic field,
The image creation means obtains a correction value for each pixel of the image data using the echo number and static magnetic field inhomogeneous information stored in advance for the measurement space data acquired by the imaging sequence, and calculates the phase for each pixel. A magnetic resonance imaging apparatus characterized by creating corrected image data.