JP2824656B2 - MRI equipment - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an MRI apparatus for creating a so-called off-center image in which a displayed image is shifted.

(Prior Art) When an nucleus is placed in a static magnetic field, the nucleus precesses at an angular velocity proportional to a constant that varies depending on the strength of the magnetic field and the type of the nucleus. When a high-frequency rotating magnetic field having the above-mentioned frequency is applied to an axis perpendicular to the static magnetic field, magnetic resonance occurs, and a group of specific nuclei having the constant generates transition between levels by a high-frequency magnetic field satisfying the resonance condition, and energy Transit to the higher level. Nuclei excited to a higher level after resonance return to a lower level and emit energy. MR
I is a device for observing a nuclear magnetic resonance (hereinafter referred to as NMR) phenomenon caused by this specific nucleus and capturing a tomographic image of the subject.

FIG. 2 shows a pulse sequence for applying a high-frequency magnetic field and a gradient magnetic field used in the Fourier transform method in MRI. In the figure, (a) is a diagram showing a state in which Gx, Gy, Gz gradient magnetic fields are applied to the x, y, and z axes, which are the lead axis, warp axis, and slice axis, respectively, and a high-frequency magnetic field is applied to the x axis. (B) is a diagram showing the timing of applying each magnetic field.
In period 1, the 90 ° pulse 1 and the slice gradient 2 selectively excite spins in a slice plane perpendicular to the z direction centered on z = 0. The rephase gradient 3 in the period 2 is for restoring the phase of the spin disturbed by the slice gradient 2. The dephase gradient 4 in the same period 2 is for giving a phase difference according to the place to the spin so that the center of the SE signal 5 coincides with the time center of the data read period 4. In period 2, a warp gradient 6 for shifting the phase of the spin in proportion to the position in the y direction is applied, and the work gradient 6 is applied with its intensity changed every period. Thereafter, a 180 ° pulse 7 is given to align the magnetic moments, and the SE signal 5 appearing thereafter is observed. In a period 4, a read gradient 8 is applied to the x-axis. Thus, the phase difference given by the dephase gradient 4 is canceled at the time center of the read gradient 8 in the period 4, and the SE signal 5 appears. This sequence is called a view, and 90 ° pulse 1 is added again after the pulse repetition period TR, and the next view is started.

In the above MRI, since the origin of the gradient magnetic field is fixed, imaging is performed centering on the origin of the gradient magnetic field. 4th normal imaging center
Shown in the figure. The figure shows a state in which a warp gradient 6 and a load gradient 8 are applied around the origin of the gradient magnetic field,
The imaging center is the origin O of this gradient magnetic field. In this imaging, the field of view (hereinafter FOV)
Is observed at the site of interest at the center of the FOV, but when the site of interest is, for example, above the FOV, it is often more convenient to move the site of interest to the center of the FOV.

(Problems to be Solved by the Invention) In the pulse sequence of the two-dimensional Fourier transform method (hereinafter referred to as 2DFT) shown in FIG. 2, the center position is shifted by shifting the frequency at the time of reception in the readout direction. The imaging can be shifted, but in the warp direction, the gradient magnetic field is offset as shown by the dashed line in FIG. 4 or the imaging (image reconstruction) is performed around the origin O of the gradient magnetic field. Above, there were only means such as rotating the pixels of the image. The same applies to the three-dimensional Fourier transform method (hereinafter referred to as 3DFT).

The present invention has been made in view of the above points, and an object of the present invention is to provide an MRI apparatus for obtaining an image in which an imaging center is moved without performing hardware improvement or complicated data processing in 2DFT or 3DFT imaging. It is to realize.

(Means for Solving the Problems) The present invention for solving the above problems receives a nuclear magnetic resonance signal from a subject by executing a predetermined pulse sequence, and performs image reconstruction processing using the nuclear magnetic resonance signal. In the MRI apparatus performing the above, in the frequency domain, the data of the nuclear magnetic resonance signal is multiplied by the phase e− ^jωt0 , and the calculation result is subjected to inverse Fourier transform to form an image in which the center of the nuclear magnetic resonance image is shifted by t0. It is characterized by having.

(Operation) The off-center amount x or z is given to the MRI, and the calculation is performed by a previously stored calculation formula. To make the image off-center. Also to raw data To effect off center in the thickness direction.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

Prior to describing the method of the embodiment, the principle of the present invention will be described. Generally, data obtained by the 2DFT and 3DFT pulse sequences is obtained by applying a gradient magnetic field, so that a received signal from the subject is obtained as Fourier-transformed data. Therefore, the Fourier transform method is used for screen reconstruction. Among the general properties of the Fourier transform, there is the property of time transition as shown in the following equation (1).

F (ω) is the raw data obtained by MRI scan, f
Assuming that (t) is image data, the raw data has a primary phase The inverse Fourier transform of the product of
t ₀ can be considered to be shifted by image.

Now, let the in-phase component of the raw data be a (k ₁ , k ₂ ) and the quadrature component b (k ₁ , k ₂ ). Where, R: raw data FOV: field of view on display screen (cm) x: moving distance of image center on display screen (cm) N ₂ ... number of views k ₁ ... number of samples k ₂ ... 1, 2, ..., N (view number) In equation (3), if N ₂ is, for example, 256 views, k ₂
Is 1,2, ..., 256, the values in curly brackets are -128, -127, ... 0, ... 12
7 is the number in each view of the warp gradient. 2
π / FOV indicates an angle per 1 cm of the FOV when the entire FOV is 2π, and Expression (3) expresses the position on the screen for each view with the center of the FOV being 0 as an angle. Therefore, ωk ₂ · x is an angle representing the moving distance of the center of the image. x corresponds to t ₀ in equation (1), and (2)
The formula is the number of views N ₂ , the number of samples k ₁ , the moving distance of the image center
x, the field of view FOV, and the raw data R after Fourier transform
When (k ₁ , k ₂ ) is input and the calculation of equation (2) is performed, it is shown that an image whose image center is shifted from the center of the FOV by x corresponding to the given moving distance can be obtained. .

In the case of 3DFT, off-center imaging can be performed on the same principle. FIG. 3 shows a pulse sequence in the case of 3DFT. In the figure, parts equivalent to those in FIG. 2 are denoted by the same reference numerals. In the figure, reference numeral 9 denotes a second warp gradient applied to the slice axis, which sweeps in the thickness direction. this
For the direction of the second warp gradient in the case of 3DFT, it can be performed as shown in the following equation (4), similarly to equation (2).
In-phase component is A (k ₁ , k ₂ , k ₃ ), and quadrature component is B (k ₁ , k ₂ , k ₃ )
And each Where FOV ₂ FOFOV of the second warp (cm) N ₃ビ ュ ー view number of the second warp k ₃ビ ュ ー view number of the second warp z… center moving distance in the second warp direction (cm) In the off-center image configuration method of the embodiment, in the case of 2DFT, if the moving distance x on the display screen at the center of the image is input, other conditions have already been given. And then multiply by 2DFFT, you can make xcm off-center. In the case of 3DFT, z is input, the operation on the right side of the equation (4) is performed, and the result is multiplied by 3DFFT to make the zcm off-center.

Figure 5 shows the main configuration diagram of MRI for implementing the above method.
Shown in the figure.

In the figure, 11 has a space portion (hole) for inserting a subject inside, so as to surround this space portion,
A static magnetic field coil that applies a constant static magnetic field to the subject and a gradient magnetic field coil that generates a gradient magnetic field (the gradient magnetic field coils are x, y, z
3 axis coil. ), A magnet assembly in which an RF transmitting coil for applying an RF pulse for exciting spins of nuclei in the subject, a receiving coil for detecting an NMR signal from the subject, and the like are arranged. Static magnetic field coil, gradient magnetic field coil, RF transmitting coil and receiving coil are
Each is connected to a static magnetic field power supply 12, a gradient magnetic field drive circuit 13, an RF power amplifier 14, and a preamplifier 15. The sequence storage circuit 16 operates the gate modulation circuit 18 (modulates the RF output signal of the RF oscillation circuit 19 at a predetermined timing) in accordance with a command from the computer 17, and operates based on the pulse sequence shown in FIG.
An RF pulse signal is applied from the RF power amplifier to the RF transmission coil. The sequence storage circuit 16 also operates the gradient magnetic field drive circuit 13 with a sequence signal based on the pulse sequence of FIG. 1 to supply gradient magnetic fields to the three axes x, y, and z. Reference numeral 20 denotes a phase detector for detecting the output of the reception signal of the preamplifier 15 using the output of the RF oscillation circuit 19 as a reference signal. This output signal is converted into a digital signal by the AD converter 21 and input to the computer 17. Reference numeral 22 denotes an operation console for inputting instructions for realizing various pulse sequences and various setting values to the computer 17,
Reference numeral 23 denotes a display device that displays an image reconstructed by the computer 14.

Next, the operation of the apparatus configured as described above will be described for the case of 2DFT. The device timing of gradient magnetic field due to the pulse sequence of the conventional Fourier method of FIG. 2, the RF pulse amplitude, pulse width, number of views N _2, the sample number k _1, FOV
Etc. are given as normal operating conditions. The computer 17 generates a control signal based on the operation condition and sends the control signal to the sequence storage circuit 16. The sequence storage circuit 16 controls the gradient magnetic field drive circuit 13 based on the signal to generate a gradient magnetic field of a predetermined pulse sequence, and controls the gate modulation circuit 18. The gate modulation circuit 18 modulates the RF signal oscillated and output by the RF oscillation circuit 19 into a signal having a set pulse width and amplitude, and supplies a modulated RF pulse to the RF power amplifier 14. The modulated RF pulse is amplified in the RF power amplifier 14 and excited in the static magnetic field generated by the static magnetic field power supply 12 in the magnet assembly 11 together with the gradient magnetic field applied to each axis by the gradient magnetic field drive circuit 13. Resonates spin. The SE signal generated by the resonance is received,
The signal is amplified by the preamplifier 15 and input to the phase detector 20. The phase detector 20 performs phase detection of the input SE signal using the output of the RF oscillation circuit 19 as a reference signal, and sends the output complex image signal to the AD converter 21. The SE signal converted into a digital signal by the AD converter 21 is Fourier-transformed by the computer 17, image-reconstructed, and displayed on the display device 23.

If it is necessary to move the center of the image in the warp direction on the screen, the operation console 22 is operated to input the center movement distance x to the computer 17. Since the calculation formula shown on the right side of Expression (2) is stored in the computer 17 in advance, when the center movement distance x is input, the computer 17 starts calculation and calculates a cos (ωk ₂ × x) of the in-phase signal. ) + B sin (ωk ₂ · x) and the orthogonal signal
b cos (ωk ₂ ×) −a sin (ωk ₂ ×) is obtained. calculator
17 performs Fourier transform on the calculation result to reconstruct an image, and performs off-centering of the image displayed on the display device 23.
In the case of 3DFT, the description is omitted because it is exactly the same as above.

FIG. 1 is a flowchart of a method including 3DFT according to one embodiment of the present invention described above.

Step 1 MRI scans based on input operating conditions, FOV, number of views, and number of samples. The obtained data is subjected to phase detection in the phase detector 20 and is converted to a digital signal in the AD converter 21. The computer 17 performs preprocessing such as offset correction on the input complex image data.

In step 2 the pretreatment of finished complex image data calculator 17, each based on the moving distance z input view, i.e. changes depending on the view number k ₃ of the (5) .omega.k ₃ performs the calculation of formula calculated, cos (ωk ₃ · z) and seeking _{sin (ωk 3 · z),} (4) by adding or subtracting after multiplying the in-phase component a and the quadrature component b according to formula raw data R (k _1, k ₂ , k ₃ ) To output pixel data off-centered by z.

Step 3 For the data obtained in Step 2,
Fourier transform in the warp direction is performed to obtain image data in the time space.

Step 4 With respect to the separated (or resampled) data after the processing of step 3, the computer 17 sets each view based on the input FOV, N ₂ , x, that is, according to the changing view number k _2. (3) performing the calculation of equation calculates ωk _2, cos (ωk ₂ ·
x) and sin (ωk ₂ · x) are obtained, and the in-phase component a and the quadrature component b are alternately multiplied and then added or subtracted according to the equation (2) to obtain raw data R (k ₁ , k ₂ ). ^{−jωk · x}
To output image data off-centered by x.

Step 5 Fourier transform is performed on each of the separated (or resampled) two-dimensional data to obtain image data on a time space.

Step 6 Post-processing such as distortion correction of the off-centered image data and conversion of the format for image display is performed.

Step 7 The off-centered image is displayed on the display device 23. Unless three-dimensional data is obtained in each of the above steps, steps 2 and 3 are omitted.

As described above, according to the present invention, it is possible to bring the image center to an arbitrary position by giving a calculation formula to the computer and inputting the off-center amount without modifying the hardware. it can.

In the conventional method of moving the center position by moving a pixel on a reconstructed image, complicated processing such as interpolation is required, but according to the present embodiment, complicated data processing is not required.

Moving to an arbitrary position by image processing after reconstruction requires some kind of data interpolation, and the image deteriorates to a greater or lesser extent than the Fourier method. There is no deterioration at all.

The present invention is not limited to the above embodiment.
The following modifications are possible.

(1) In the embodiment, the spin echo method has been described.
The Fourier transform method such as an iterative recovery method and a field echo method can be generally used.

(2) Similar results can be obtained for the multi-echo method by changing the sign of ω in the equations (2) and (4).

(3) The same processing may be performed not only in the warp direction but also in the lead direction. However, in this case, the image is simply shifted in a short time while the band is limited and the end of the image is cut by the reduction filter or the like.

(4) When the image of water and fat is separated by the chemical shift, only the fat component is subjected to inverse Fourier exchange to return to raw data, and then the distance is calculated from the phase difference between water and fat. If Δε is the chemical shift amount (3.5 ppm), ∴R ′ (k) = R (k) e− ^jωΔl (7) where H ₀ … main magnetic field (G _s ) G _pr … readout gradient (G _s / cm) Δl… position shift due to chemical shift (cm) ) K: sample point R '(k): data obtained by shifting the raw data R (k) of the fat by Δl According to the formula (6), the displacement is calculated by the chemical shift between water and fat, and is represented by the formula (7) To fat data
^-JωΔl can be used to correct the positional shift caused by the fat component and create combined water and fat image data.

(Effect of the Invention) As described in detail above, according to the present invention, 2DFT, 2DF
In T imaging, an image in which the center of the image is shifted can be obtained without performing hardware improvement or complicated data processing, and the practical effect is large.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flowchart of a method according to an embodiment of the present invention, FIG. 2 is a diagram of a pulse sequence of the 2DFT method of MRI, and FIG.
FIG. 4 is a diagram of a pulse sequence of the RI 3DFT method, FIG. 4 is an explanatory diagram of a normal image center and a warp gradient offset, and FIG. 5 is a diagram of an apparatus for implementing the method of the embodiment. 1 90 pulse, 2 slice gradient 3 rephase gradient 4, dephase gradient 5 SE signal, 6 warp gradient 7 180 pulse, 8 lead gradient 9 Second warp gradient 11 Magnet assembly 12 Static magnetic field power supply 13 Gradient magnetic field drive circuit 14 RF power amplifier 15 Preamplifier 16 Sequence storage circuit 17 Computer 18 Gate modulation circuit 19 RF oscillator circuit 20 Phase detector 21 AD converter 22 Operation console 23 Display device

Claims (1)

(57) [Claims] 1. A nuclear magnetic resonance signal is received from a subject by executing a predetermined pulse sequence on the subject placed in a static magnetic field, and data obtained from the nuclear magnetic resonance signal is added to the pulse sequence. An MRI apparatus for creating and displaying a display image of the subject based on the MRI apparatus, wherein data ^obtained by multiplying the nuclear magnetic resonance signal by a phase e- ^jwt0 are subjected to Fourier transform, and the center of the display image is shifted by t0 and displayed. An MRI apparatus comprising: