JP2001095773A - Magnetic resonance image diagnostic apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide magnetic resonance image diagnostic apparatus which can obtain metabolic image by high accurately suppressing useless signal such as fat and so. SOLUTION: A part range in the picture taking slice is two dimensionally excited by impressing vibration gradient magnetic field of 2 or 3 axis directions with RF simultaneously, and MRSI process is achieved. As the preliminary process, the scout image is taken, ROI is set colloquially on the image, and RF modulated waveform is automatically formed by the two dimensional Fourier transform of ROI shape. Minimum rectangular measuring matrix covering ROI is formed automatically. At MRSI process, the RF is used by modulating by RF modulated waveform obtained by the preliminary process, to measure image matrix obtained by the preliminary process. The metabolic image which useless signal is accurately removed, can be obtained since excitement is achieved by accurately along its shape avoiding the unnecessary part such as fat. ROI can be set simply since wanted ROI can be input colloquially on the scout image. The measuring time can be shortened by rectangular measuring matrix.

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 apparatus (hereinafter, referred to as an MRI apparatus), and more particularly to an MRI apparatus having a spectroscopic imaging measurement function.

[0002]

2. Description of the Related Art Spectroscopic imaging (hereinafter referred to as MRSI) using a medical magnetic resonance diagnostic apparatus (MRI) is a technique for imaging the distribution of a specific chemical species in a living body. Unlike morphological information, chemical information such as sugar metabolism and energy metabolism can be obtained, so that it is used for diagnosis at an early stage of disease.

As the MRSI method, there are various methods such as a 3D-CSI method in which a chemical shift, which is chemical information, is measured as a time coefficient.
Although the MRSI method has been attempted, clinical MRI apparatuses generally target protons ( ¹ H nuclei).

In proton-targeted MRSI, water and fat signals present in the living body are disturbed by two to four orders of magnitude more than metabolites, so it is necessary to use a technique to suppress these signals. . In such a technique, as a pre-process, the water spectrum is selectively excited, and then a gradient magnetic field is applied to dephase the magnetization of the water so that no signal is generated. It is excited and dephased by a gradient magnetic field. After an appropriate preprocess, spectroscopic measurement, which is the main measurement, is performed.

[0005] In the suppression of the fat portion, after exciting a slice perpendicular to the imaging section, a gradient magnetic field is applied to dephase the magnetization in the vertical slice (OVS: 0uter V).
olume Suppression). In this process, changing the slice,
Repeat until fat area is fully covered. Explaining the head transformer surface in FIG. 9 as an example, the excitation and dephase of slices 1 to 8 are performed while changing the direction so as to cover the roughly elliptical head surface. In FIG. 9, eight slices are used to cover the subcutaneous fat, and usually 4 to 8 slices are used.

FIG. 10 is a diagram showing an example of a sequence in which the above-described water suppression and fat suppression are incorporated in a conventional 3D-CSI. In the 3D-CSI method, spatial information in the x and y directions is phase-encoded, and spectrum information is encoded with the lapse of time at the time of signal measurement. The pre-process of combining water suppression and fat suppression needs to be repeated for each excitation of the main measurement by such a 3D-CSI method.

[0007]

In such a conventional fat suppression method, since a fat tissue is covered with a plurality of rectangles, it is difficult to cover a fat region accurately, and a signal from fat is sufficiently suppressed. I couldn't do that. On the other hand, in order to cover fat tissue sufficiently, it is necessary to increase the number of rectangles, which makes setting of a rectangular area complicated. When the number of rectangles is increased, the length of the previous process becomes longer, the magnetization of water is recovered by longitudinal relaxation, and the suppression of water becomes insufficient.

Further, in the 3D-CSI method, at least a double phase encoding loop is used to add spatial coordinate information of two or three axes to a signal, and it takes a long time for measurement.
The conventional fat suppression method requires about 20 to 50 ms in the previous process, and thus has a problem that the measurement time is further extended.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an MRI apparatus which can surely suppress an unnecessary signal and can realize the MRSI method without extending the measurement time.

[0010]

Means for Solving the Problems To solve the above problems, in an MRI apparatus for performing spectroscopic measurement, as a control function, an oscillating gradient magnetic field in two or three axial directions is set to a predetermined high frequency magnetic field (RF). By adding the function of applying the signal at the same time, the fat suppression step in the phase encoding loop becomes unnecessary, and the signal from the fat tissue is greatly reduced.

Regarding the technique of performing spatially selective excitation by combining an oscillating gradient magnetic field and an RF waveform calculated from the shape function of the excitation, J. Pauly, D. Nishimura and A. Macovski
Paper "A k-Space Analysis of Small-Tip-Angle E
xcitation ", J. Magn. Reson., 81, 43-56 (1989), etc., but the MRI apparatus of the present invention specifically incorporates such a technique of spatially selective excitation in MRSI measurement. It gives the means.

That is, the MRI apparatus of the present invention comprises a magnetic field generating means for generating a static magnetic field, a gradient magnetic field, and a high frequency magnetic field (RF) in a space where a subject is placed, and a magnetic resonance signal generated by the subject. Control means comprising: detecting means for detecting; image reconstructing means for reconstructing an image using the detected magnetic resonance signal; display means for displaying the reconstructed image; and control means for controlling each of the above means. Is provided with means for setting a desired region of interest (ROI) within a predetermined region of the subject, and means for calculating in real time an RF modulation waveform that selectively excites the ROI. The RF is simultaneously applied with oscillating gradient magnetic fields in two or three axes, and control is performed to obtain a signal including spectrum information for a predetermined region of the subject.

[0013] Since a signal can be obtained by exciting only the inside of the ROI of any shape of interest, the influence of fat and water from outside the ROI can be effectively suppressed. This improves the quantitativeness of the spectrum and improves the accuracy of diagnosis. Further, since the excitation pulse is provided with region selectivity, it is not necessary to apply a fat suppression RF pulse in a pre-process performed every time phase encoding is repeated, and the total length of the pre-process can be reduced. As a result, the recovery of the water signal can be reduced.

In the MRI apparatus of the present invention, the means for setting an ROI displays a tomographic image taken of a desired slice as a scout image on a display means, and interactively sets the ROI on the displayed scout image. Having.

The means for calculating the modulation waveform is, specifically,
An excitation shape function is created from the shape of the ROI, and a modulation waveform of a high-frequency magnetic field is calculated almost in real time from a two-dimensional Fourier transform of the excitation shape function.

Although the RF waveform of the selective excitation varies depending on the shape of the ROI, a scout image is taken and displayed in advance, and the ROI is set interactively on the scout image. Can be set to In addition, since an RF modulation waveform is generated by a computer from the two-dimensional Fourier transform of the set ROI shape, an RF waveform corresponding to the ROI can be obtained almost in real time, and MRSI measurement can be performed immediately after setting the ROI. it can.

In a further preferred aspect of the MRI apparatus of the present invention, the control means sets a minimum rectangular area including the ROI,
The image matrix size is determined according to the rectangular area.

The image matrix size is the size of image data represented by the number of pixels (or voxels) arranged two-dimensionally or three-dimensionally (the number of rows × columns). It depends on the number of encodings. Therefore, by determining the image matrix size according to the smallest rectangle that covers the ROI shape, it is not necessary to increase the matrix size unnecessarily. As a result, unnecessary phase encoding is not repeated, and the imaging time of MRSI can be reduced.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail using embodiments. FIG. 4 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied. The MRI apparatus includes a static magnetic field generating magnetic circuit 402 for generating a uniform static magnetic field B0 inside the subject 401.
And a gradient magnetic field generation system 403 that generates gradient magnetic fields Gx, Gy, and Gz whose intensities linearly change in three orthogonal directions x, y, and z.
A transmission system 404 that generates a high-frequency magnetic field, a detection system 405 that detects a nuclear magnetic resonance signal generated from the subject, a signal processing system 406, and a computer that performs calculations for image reconstruction and controls the entire apparatus 408, a sequencer 407 that controls the gradient magnetic field generation system 403, the transmission system 404, and the detection system 405 according to an instruction from the computer 408, and a signal processing system 406.
And an operation unit 421 for sending necessary commands to the computer 408.

The static magnetic field generating magnetic circuit 402 includes an electromagnet or a permanent magnet, and the subject 401 is carried into the static magnetic field space.

The gradient magnetic field generation system 403 comprises a three-axis gradient magnetic field coil 409 and a power supply 410 for supplying a current to the gradient magnetic field, and a linear gradient magnetic field for giving spatial information to a nuclear magnetic resonance signal. Then, an oscillating gradient magnetic field is generated at the time of ROI selective excitation.

The transmission system 404 includes a synthesizer 414 for generating a predetermined high frequency, a modulator 412 for modulating the high frequency generated by the synthesizer 411, a power amplifier 413, and a transmission coil 41.
4a. The modulation waveform modulated by this modulator 412 is
The data calculated and stored by the computer 408 according to a procedure described later is provided from the sequencer 407. A high-frequency magnetic field is generated inside the subject 401 by supplying a high-frequency wave modulated with a predetermined modulation waveform to the transmission coil 414a,
Nuclear spin can be excited. The nuclear spin to be excited is usually ¹ H, but may be other nuclear nuclei having nuclear spin, such as ³¹ P and ¹² C.

The detection system 405 includes a receiving coil 414b for receiving a nuclear magnetic resonance signal emitted from the subject 401, and an amplifier 415.
And a quadrature detector 416 and an A / D converter 417.The nuclear magnetic resonance signal received by the receiving coil 414b and amplified by the amplifier 415 is subjected to A / D conversion after quadrature detection, and Input to 408.

The transmission coil 414a and the reception coil 414b may be separate as shown in the figure, or may be coils for both transmission and reception.

After signal processing, the computer 408 reconstructs an image corresponding to the nuclear spin density distribution, relaxation time distribution, spectral distribution, and the like, and displays the image on the CRT display 428. The operation unit 421 is an MRI built in the computer 408.
Commands necessary for executing a program for processing by the device and various settings are input to the computer 408. The processing executed via the operation unit 421 includes a process of setting an ROI (region of interest) in a predetermined region of the subject, a process of determining an image matrix size according to the ROI, and a transmission system based on the shape of the ROI. Processing for calculating an RF modulation waveform generated by 404 is included. The data or the final data in the middle of the calculations performed by the computer 408 are stored in the memories 424 and 425.

The computer 408 controls a gradient magnetic field generation system 403, a transmission system 404, and a detection system 405 via a sequencer 407 in order to perform imaging according to a predetermined imaging sequence.

Next, the MRSI using the MRI apparatus having such a configuration will be described.
The measurement will be described. In the embodiment described below, it is assumed that MRSI of protons is performed on the trans-plane (xy plane) of the lower abdomen. However, the present method can be similarly applied to other slice planes. In addition, although suppression of fat will be described as an example, a spectrum other than fat may be suppressed.

FIG. 1 is a diagram showing an MRSI flow according to the present invention, FIG. 2 is a diagram for explaining the setting of ROI, and FIG. 3 is a diagram showing an embodiment of an imaging sequence of the MRSI method according to the present invention.

In the MRSI measurement of this embodiment, a scout image of a desired slice is obtained prior to the main measurement (spectroscopic measurement) (FIG. 1, step 11). Scout image is FS
Imaging can be performed using a general MRI imaging method such as an E (fast spin echo) method or an EPI (echo planar) method. The slice selected here is an arbitrary slice included in the target area in the main measurement.

Next, after displaying this scout image on the display, the ROI of the closed area is set with a mouse or the like so as to avoid fat and completely cover the desired area (step 12).
FIG. 2 is a diagram schematically showing a scout image displayed on a display, and 61 indicates an ROI input by a mouse.

After smoothing the ROI 61 set in this way, a binary function is set to 1 for the inside of the ROI and 0 for the outside and set it as the shape function D (x, y) (step 13). Any shape can be specified in principle for the ROI shape. However, as the shape becomes more complex, the Fourier transform D '(kx, ky) of the shape function D (x, y) contains more high-frequency components (edge components of k-space). The k-space needs to be expanded. This means that the amplitude of the oscillating gradient magnetic field is increased or the application time is lengthened, which involves difficulties in terms of equipment and prolongs the imaging time. Therefore, it is practical to avoid unnecessary complexity when inputting the ROI shape. In addition, the sharper the rise of the boundary of the shape function, the sharper the fat region is suppressed, but the requirements on the amplitude characteristics and transient characteristics of the gradient magnetic field become strict. Therefore, the binary function D (x,
y) may be subjected to smoothing as required. For example
The boundary where the value of D changes from 0 to 1 has a certain width, and the function value smoothly decreases from 1 to 0 within this width.

Next, the shape function D (x, y) is subjected to a two-dimensional Fourier transform to create a k-space function D '((kx, ky). Calculate B1 (t) (Step 1
Four).

[0033]

(Equation 1) In the formula, γ is a nuclear magnetic rotation ratio, and G (t) is a gradient magnetic field vector. The gradient magnetic field vector G (t) can be obtained from gradient magnetic field waveforms Gx (t) and Gy (t) applied simultaneously with RF in the main measurement.

The gradient magnetic field waveforms Gx (t) and Gy (t) are predetermined as an imaging sequence of the main measurement.
Gradient magnetic field waveforms Gx (t) and Gy (t) that provide a spiral trajectory converging at a constant speed from the edge of the space to the origin are employed. Specific examples of such gradient magnetic field waveforms Gx (t) and Gy (t) are shown in the following equations.
These are sins whose amplitude decays as a linear function of time.
Waves, cos waves.

[0035]

(Equation 2) In the equation, T / n represents the rotation period of the spiral in space. Also A
Is a constant that determines the size of the spiral. Usually n is 10
Before and after.

The gradient magnetic field waveforms Gx (t) and Gy (t) are not limited to spirals, but may be zigzag trajectories such as the EPI method as long as the trajectories uniformly cover the k-space. What is given can also be used.

The RF waveform B1 (t) obtained in this way is saved in the memory, and the pre-process ends. Next, this RF waveform B1 (t)
The MRSI, which is the main measurement, is performed using the RF modulated as described above as the excitation pulse (step 15). At this time, RF waveform B1
The amplitude of (t) is experimentally adjusted so that its maximum flip angle is 90 °.

FIG. 3 shows an embodiment of the MRSI imaging sequence according to the present invention. In this imaging sequence, spatial selective excitation is applied to the 3D-CSI method. First, gradient magnetic fields 74 and 75 that oscillate in the x and y directions together with the excitation pulse 71 are applied to the inside of the two-dimensional region in the xy plane. Excites only the magnetization of
Here, the slice selection gradient magnetic field Gz is not used. The excitation pulse excites the inside of the ROI into a substantially D (x, y) shape. The accuracy of the excitation shape depends on the degree to which the gradient magnetic fields 74 and 75 include high-frequency components in the k-space. Next, the x, y spatial information is encoded into the phase of the magnetization by the phase encoding gradient magnetic fields 76, 77. Gradient magnetic field to select slice
Apply a refocusing pulse 72 together with Gz73, and after excitation TE
An echo 78 is generated after a lapse of time. The generated echo is received as a signal.

After that, the process waits for the recovery of the longitudinal magnetization, shifts to the next cycle, and repeats the signal measurement while changing the phase encode amount. The echo signal is subjected to three-dimensional Fourier transform as a function of phase encoding (kx, ky) and time t to obtain a spectroscopic image (metabolite distribution image). It is desirable that the MRSI method be of a spin echo type as shown. In general, when the frequency of the excitation pulse deviates from the resonance frequency, the shape of the region to be excited and the phase of the transverse magnetization are affected by the offset, but in the spin echo type, the refocus 180 ° pulse 72 causes the offset to be offset. The effects can be offset.

The reconstructed metabolite distribution image is CR
Although it may be displayed on the T display, the spatial distribution is preferably corrected by multiplying by an inverse function of the excitation shape function D (x, y). Thereby, a spatial change in the flip angle of the excitation can be corrected, so that a metabolite distribution image having a quantitative property can be obtained.

As described above, the MRSI of the present invention does not require an OVS step for fat suppression, so that 3D-CSI having a double phase encoding loop can be shortened to the same degree as when no suppression step is included. In addition, the fat can be effectively suppressed.

FIG. 5 is a diagram showing another embodiment of the 3D-CSI method according to the present invention. This imaging sequence differs from the imaging sequence of FIG. 3 in that water suppression is also used. That is, the water spectrum is excited by the well-known CHESS pulse 81 for each phase encoding cycle, and the crusher gradient magnetic field 82 is applied to dephase the magnetization of the water. Sin for CHESS pulse
There are a c function, a Gaussian function, and the like, which are appropriately selected and used in consideration of the shape of the excitation spectrum and the application time.

The measurement following the application of the crusher gradient magnetic field 82 is the same as the imaging sequence of FIG. That is, based on the ROI set interactively on the scout image in advance, the RF modulation waveform is calculated and saved in the memory, and the RF modulated with this modulation waveform is used together with the gradient magnetic field oscillating in the x and y directions. To selectively excite a previously set ROI, apply a refocus pulse 72 after phase encoding, and echo
Generate 78.

In the above-described imaging sequence (two-dimensional MRSI) shown in FIGS. 3 and 5, a three-dimensional spectroscopic image can be obtained by performing measurement while changing the slice position. In this case, depending on the slice position
When the ROI shifts, the ROI can be moved in parallel from the reference position by applying phase modulation to the RF waveform B1 (t).

That is, to shift the position of the ROI by x0, phase modulation as shown in Expression 3 may be applied to RF.

[0046]

(Equation 3) In the equation, G (s) is the gradient magnetic field vector, and T is the application time of B1 (t). If you specify the position shift x0 on the scout image,
The phase-modulated RF pulse waveform can be calculated by Equation 3. Here, the gradient magnetic field waveform G (s) is determined in advance.

Next, another embodiment of the MRSI according to the present invention is shown in FIG.
This will be described with reference to the flow of FIG. Also in this embodiment, the steps from the shooting of the scout image (step 11) to the calculation of the RF waveform B1 (t) (step 14) are the same as those in the flow of FIG. 1, but in this embodiment, the ROI is displayed on the display of the scout image. Following the setting, the image matrix size in the main measurement is set (steps 16 and 17). For this reason, as shown in FIG. 2B, first, the ROI 61 is interactively specified on the image, and then the minimum rectangular area 62 covering the ROI is automatically generated (step 16). An image matrix size such as × 12 is set (step 17). The main measurement (space selective excitation MRSI) is performed by a phase encoding step (kx, ky) corresponding to the image matrix size.

In the ordinary 3D-CSI method, the phase encoding step is usually 16 × 16 or 32 × 32.
As shown at 63, the field of view is square. In contrast,
In this embodiment, since the image matrix is set according to the minimum rectangular area 62 covering the ROI, it is not necessary to measure external pixels in which magnetization is not excited, and the number of iterations of the phase loop can be reduced. In addition, as shown in FIGS.
The direction of the coordinate axis of the image measurement matrix may be rotated according to the rectangular direction at the inclination of I61. This allows ROI
Since the region can be set along the shape, the number of repetition steps can be minimized. The rest is the same as the embodiment shown in FIG.

In the above embodiment, the case where the present invention is applied to a two-dimensional MRSI has been described. However, the present invention can also be applied to a three-dimensional MRSI which performs phase encoding in the slice direction. FIG. 7 shows an example of a three-dimensional MRSI imaging sequence. This imaging sequence includes water suppression (application of a water spectrum excitation pulse 81 and a crusher gradient magnetic field 82) as in the sequence of FIG. 5, and a phase encode loop 91 in the slice direction is added. Note that the gradient magnetic field in the slice direction applied simultaneously with the refocus pulse 72 is a slab selection gradient magnetic field, and can be omitted.

Also in the case of this three-dimensional MRSI, a scout image is taken as a pre-process, and an ROI is set on the scout image,
Calculating the RF waveform from this shape function is the same,
Further, it is possible to determine the image matrix size in the xy direction according to the set ROI.

In the above embodiment, the spin echo type
Although we explained MRSI and explained that the spin echo type is desirable to eliminate the influence of the offset from the resonance frequency, the FID type can be used for the measurement of short T2 substances.

FIG. 10 shows this example. In FIG. 10, after applying the excitation RF pulse 71, the phase encodes 76 and 77 are applied in a short time,
Measure the FID immediately.

Although the 3D-CSI method has been used as an example to describe MRSI by spatially selective excitation, MRSI using other EPI, FSE, etc.
Spatial selective excitation can also be performed with the SI sequence.

[0054]

As described above, according to the present invention,
It is possible to obtain a signal by exciting only unnecessary portions while avoiding unnecessary portions, such as fat, exactly along its shape. This makes it possible to obtain a metabolite image from which the fat signal has been accurately removed. In addition, fat is added to the OVS
Since there is no need to suppress in the inside, the previous process can be shortened, and the recovery of water due to longitudinal relaxation can be reduced. Further, since a desired ROI is interactively set on the scout image and an excitation RF waveform is automatically obtained from the ROI shape by calculation, MRSI can be executed immediately after setting the ROI. Furthermore, since the minimum rectangular matrix is set according to the ROI, unnecessary measurement loops of MRSI can be reduced, and the speed can be increased.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a procedure according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a method of interactively setting an ROI and a method of setting a matrix.

FIG. 3 is a diagram showing an example of a sequence of a selective excitation 3D-CSI method to which the present invention is applied.

FIG. 4 is a diagram showing the overall configuration of an MRI apparatus according to the present invention.

FIG. 5 is a diagram showing another example of the sequence of the selective excitation 3D-CSI method to which the present invention is applied.

FIG. 6 is a flowchart showing a procedure according to another embodiment of the present invention.

FIG. 7 is a diagram showing a sequence of a selective excitation 4D-CSI method to which the present invention is applied.

FIG. 8 is a diagram showing a sequence of the FID type 3D-CSI method to which the present invention is applied.

FIG. 9 is a diagram illustrating conventional fat suppression.

FIG. 10: 3D-CSI combining conventional water suppression and fat suppression
The figure which shows the sequence of a method.

[Explanation of symbols]

 401 Subject 402 Static magnetic field generation magnetic circuit 403 Gradient magnetic field generation system 404 Transmission system (high-frequency magnetic field generation means) 405 Detection system 406 Signal processing system (image reconstruction means) 428 Display (display means) 408 Computer (control means) 61 Region of interest 71 Spatial selective excitation high-frequency magnetic field 74,75 Oscillatory gradient magnetic field

Claims (3)

[Claims] A magnetic field generating means for generating a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field in a space where a subject is placed;
Detecting means for detecting a magnetic resonance signal generated by the subject, image reconstructing means for reconstructing an image using the detected magnetic resonance signal, display means for displaying a reconstructed image, and each of the means A magnetic resonance imaging diagnostic apparatus comprising: a control unit that controls a unit that sets a desired region of interest within a predetermined region of the subject; and a high frequency that selectively excites the set region of interest. Means for calculating a modulation waveform of a magnetic field in real time, wherein a high-frequency magnetic field modulated by the modulation waveform is applied simultaneously with an oscillating gradient magnetic field in two or three axes, and includes spectrum information for a predetermined region of the subject. A magnetic resonance imaging apparatus for performing a control for acquiring a signal. 2. The setting of the region of interest is characterized in that a tomographic image taken of a desired slice is displayed on the display means as a scout image, and the region of interest is set interactively on the displayed scout image. The magnetic resonance imaging apparatus according to claim 1, wherein 3. The magnetic resonance image according to claim 1, wherein the control unit sets a minimum rectangular area including the region of interest and determines an image matrix size according to the rectangular area. Diagnostic device.