JPH02274228A - Magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To eliminate the surface picture dislocation of an image due to an error in an RF center frequency by conducting preliminary image scanning for measuring the current dislocation of the center frequency of a device, calculating the image dislocation and then correcting an image display position. CONSTITUTION:Parameters such as a pulse sequence, RF pulse width and pulse width are set and an instruction for correcting a positional dislocation is inputted via the operation of a console 32, thereby conducting preliminary image scanning to measure the current dislocation of the center frequency of a device, prior to a scanning process. Then, MR I scans the image with an RF signal using the obtained RF center frequency on the basis of the set up parameters, and receives an NMR signal from a subject via an RF coil. Thereafter, the signal is inputted to a calculator 27 via a pre-amp 25, a phase detector 30 and an AD converter 31. The calculator 27 operates to reconstitute the image on the basis of the inputted data, and the dislocation of the center frequency on the basis of preliminarily scanned data. Furthermore, the calculator 27 calculates the image dislocation from the aforesaid dislocation amount and makes necessary correction for the image data.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as MRI), and more particularly to an MHI that prevents image display shifts due to system center frequency errors.

(Prior Art) MRI, which uses the phenomenon of nuclear magnetic resonance (hereinafter referred to as NMR) to obtain a tomographic image of a subject focusing on specific atomic nuclei, has been known for a long time. An outline of the principle of this MRI will be explained below.

An atomic nucleus can be seen as a spinning top that is magnetic, but if it is placed in a static magnetic field Ho in the Z-axis direction, for example, the atomic nucleus will have an angular velocity ω expressed by the following equation. to precess. This is called Larmor precession.

ω0″″γHO However, γ is... nuclear gyromagnetic ratio Now, if a high-frequency coil is placed on an axis perpendicular to the Z-axis with a static magnetic field, for example, the X-axis, and rotates within the X+3' plane, the above-mentioned angular frequency ω . When a high-frequency rotating magnetic field is applied, magnetic resonance occurs,
After the static magnetic field H8, the population of atomic nuclei undergoing Zeeman splitting causes a transition between levels due to the high frequency magnetic field that satisfies the resonance condition, and transitions to a higher energy level. Here, since the nuclear gyromagnetic ratio γ differs depending on the type of atomic nucleus, the atomic nucleus can be specified by the resonance frequency. Furthermore, by measuring the strength of that resonance, we can determine the amount of that nucleus present. During a time determined by a time constant called the post-resonance relaxation time, the atomic nucleus excited to a higher level returns to a lower level and radiates energy. MHI
is an apparatus that observes the NMR phenomenon caused by this specific atomic nucleus and captures a tomographic image of a subject. FIGS. 3 and 4 show pulse sequences for applying a high-frequency magnetic field and a gradient magnetic field used in the Fourier transform method, which is a popular MRI imaging method. Figure 3 is a diagram showing a state in which gradient magnetic fields of Gx, Gy, and G2 are applied to the X+ y+ Z axes, which are the lead axis and warp axis slice axis, respectively, and a high frequency magnetic field is applied to the X axis.
FIG. 4 is a diagram showing the timing of applying each magnetic field. In FIG. 4, in period 1, 90@pulse 1
With the slice gradient 2, spins in the slice plane perpendicular to two directions centered on z-0 are selectively excited. The rephase gradient 3 in the period 2 is for restoring the phase of the spins disturbed by the slice gradient 2. Same, period 2
The day phase gradient 4 is for giving a phase difference to the spins depending on the location so that the center of the SE signal 5 coincides with the temporal center of the data read period 4. In period 2, a warb gradient 6 is further applied to shift the phase of the spin in proportion to the position in the y direction, and the warb gradient 6 is applied with its intensity changed every cycle. then 180
Apply pulse 7 to align the magnetic moments, and then observe the SE signal 5 that appears. In period 4, a lead gradient 8 is applied to the X axis. As a result, the phase difference given by the day phase gradient 4 is canceled out at the temporal center of the lead gradient 8 in the period 4, and an SE signal 5 appears. This sequence is called a view, and after the pulse repetition period TR, 90° pulse 1 is applied again to start the next view.

(Problem to be Solved by the Invention) If there is an error of Δf, Hz in the center frequency of the current device, an image shift of ΔJ7cm will occur in the lead axis direction (frequency direction) of the CRT screen, as shown in Figure 5. . In the figure, 11 has no deviation in center frequency, that is, Δfc=
o is the true image, 12 is the deviation image when the center frequency is shifted, and there is 8ΩCm between the two center positions.
This is causing a misalignment. The amount of deviation of this image ΔJ1c
m is expressed by the following formula.

... (2) Here, Δfc...center frequency error (Hz)G,
...Amplitude of readout gradient (G/am) γ...Nuclear gyromagnetic ratio (KHz/G) Now, display the tomographic plane perpendicular to the body axis on the CRT screen, and cut in the body axis direction from this screen. When trying to position a scan to
However, since the longitudinal section position 13 is outside the true image 11, no image will appear. FIG. 7 is a diagram of the screen on which the vertical section tomographic image that should originally appear is displayed, and in FIG. FIG. If the center frequency of the RF is shifted by Δf in this way, causing a shift in the image on the CRT screen, positioning for scanning another cross section becomes inaccurate.

Even if an attempt is made to set the center frequency to the correct frequency in order to prevent this problem, it is difficult to adjust the center frequency accurately. Even if it could be adjusted accurately, it would be difficult to maintain the center frequency at a constant value due to frequency stability issues.

The present invention has been made in view of the above points, and its purpose is to realize an MRI in which there is no shift of the image display screen due to an error in the RF center frequency when displaying the image on the display screen. .

(Means for Solving the Problems) The present invention for solving the above-mentioned problems includes means for performing a preliminary scan to determine the deviation from the center frequency in the current device prior to the start of scanning, and a measuring means for the same. , a calculation means for calculating an image shift amount from the frequency shift amount, a correction means for correcting the image shift amount and displaying the image in a correct position, and an input means for manually inputting an instruction to perform the preliminary scan. It is characterized by comprising the following.

(Operation) Before performing a regular scan, a preliminary scan is performed to measure the amount of shift in the center frequency of the current device, and the amount of image shift is calculated from this amount of frequency shift to correct the display position of the image.

(Embodiments) Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention.

In the figure, 21 has a space (hole) into which the subject is inserted, and the space is surrounded by a
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 coil has x, y
Equipped with Sz 3-axis coil. ), an RF transmitting coil that provides an RF pulse to excite the spins of atomic nuclei within the subject, and a receiving coil that detects NMR signals from the subject, etc., are arranged. The static magnetic field coil, gradient magnetic field coil, RF transmitting coil, and receiving coil are connected to a static magnetic field power supply 22, a gradient magnetic field driving circuit 23, an RF power amplifier 24, and a preamplifier 25, respectively. The sequence storage circuit 26 operates the gate modulation circuit 28 (modulates the RF output signal of the RF oscillation circuit 29 at a predetermined timing) according to a command from the computer 27 in an arbitrary view, and converts the RF pulse signal based on the Fourier transform method. is applied from the RF power amplifier 24 to the RF transmitting coil. Also, the sequence storage circuit 26 stores the gradient magnetic field drive circuit 2 using a sequence signal based on the Fourier transform method.
Figure 3.

As shown in Figure 4, gradient magnetic fields are supplied to each of the three axes, x, y, and z. Further, the sequence storage circuit 26 changes the oscillation frequency of the RF oscillation circuit 29 according to instructions from the computer 27. A phase detector 30 detects the phase of the received signal output from the preamplifier 25 using the output of the RF oscillation circuit 29 as a reference signal. This output signal is sent to the AD converter 31
The signal is converted into a digital signal and input to the computer 27. 32 is an operation console for inputting instructions and various setting values to the computer 27 for realizing various pulse sequences, and 33 is a display device for displaying images reconstructed by the computer 27.

34 is a frequency meter that determines the frequency of the RF multiplied signal oscillated by the RF oscillation circuit 29 by δP1 and sends the information to the computer 27.

Next, the operation of the apparatus configured as described above will be explained.

The operator operates the operation console 32 to set the timing of the pulse sequence, the amplitude of each gradient magnetic field, the amplitude of the RF pulse, the pulse width, etc., and manually inputs a signal to the computer 27 based on the set values. The computer 27 generates a control signal based on the set value and sends it to the sequence storage circuit 26. The sequence storage circuit 26 controls the gradient magnetic field driving circuit 23 to generate a gradient magnetic field of a predetermined pulse sequence based on the above signal, and also controls the gate modulation circuit 28.

The gate modulation circuit 28 modulates the RF multiplied signal oscillated and outputted by the RF oscillation circuit 29 into a signal having a set pulse width and amplitude, and supplies the modulated RF pulse to the RF power amplifier 24. Furthermore, the sequence storage circuit 26 can control the oscillation frequency of the RF oscillation circuit 29 according to instructions from the computer 27. The above modulated RF pulse is transmitted to the RF power amplifier 2.
In the static magnetic field generated by the static magnetic field power source 22 in the magnet assembly 21, a specific atomic nucleus is caused to resonate in conjunction with the gradient magnetic field applied to each axis by the gradient magnetic field drive circuit 23. SE caused by resonance
the doubled signal is received and amplified by a preamplifier 25;
It is input to phase detection H30. The phase detector 30 is an RF
The phase of the input NMR signal is detected using the output of the oscillation circuit 29 as a reference signal, and the output signal is sent to the AD converter 31. A
NM converted into a digital signal in the D converter 31
The R signal is image-reconstructed by the computer 27 through predetermined processing according to the scan sequence and displayed on the display device 33.
Displayed by The computer 27 is a sequence storage circuit 2.
6 can be rewritten, thereby realizing various scan sequences.

The above operation is a general operation, but when correcting a shift in image position due to an error in the RF center frequency, the operation is performed according to the flowchart shown in FIG. This operating procedure is explained by Calculator 2.
7 as a program, and by inputting an instruction regarding the execution of positional deviation correction from the operation console 32, the correction is automatically performed.

Step 1 The operator operates the console 32 to perform a pulse sequence.
Input commands to set parameters such as RF pulse amplitude and pulse width, and to perform positional deviation correction.

Step 2 Prior to actual scanning, perform a preliminary scan of several views in order to reduce the amount of shift in the center frequency of the current device by a total of 11 FJ.

Step 3 The MHI performs scanning based on the set parameters and the RF multiplied signal using the RF center frequency obtained in Step 2.

Step 4: Receive the NMR signal radiated from the subject by the RF coil. The received signal is amplified by the preamplifier 25, detected by the phase detector 30, and sent to the computer 2 via the AD converter 31.
7 is input.

Step 5 The computer 27 performs image reconstruction calculations based on the input data.

Step 6 Calculate the amount of shift in the center frequency based on the data obtained from the preliminary scan.

Step 7 The calculator 27 further calculates the image deviation amount by 61c from the deviation amount of the RF center frequency obtained in Step 6 using equation (2).
Calculate m.

Step 8 A correction calculation is performed on the image data obtained in Step 5 using the image shift amount Δg obtained in Step 7, and the image data is corrected.

Step 9 An image is displayed on the display device 23.

It is determined by inputting an execution command to the operation console 32 whether the above-mentioned preliminary scan is performed every time an MRI scan is performed or a reduced number of times. The amount of shift in the center frequency can be corrected by changing the frequency at the time of reception, but if continuous frequency conversion cannot be performed from the hardware, it is also possible by performing phase shift correction processing. The above operation can be applied to 2DFT (two-dimensional Fourier transform method) and 3DFT.

As explained above, by following the steps in this example, even if there is an error in the frequency from the theoretical value, the positional shift on the display screen can be corrected and the image shift shown in Figure 6 can be prevented. Even when the obtained image is used for positioning on a different surface, the image shift due to the center frequency shift is corrected, so accurate positioning can be performed.

In addition, even if there is a temporal fluctuation in the center frequency, if the operating procedure shown in Figure 2 is programmed in advance to perform when the detected value of the NMR signal decreases, the image on the display screen will automatically change. Corrections are made and there is no need to consider the stability of the device.

(Effects of the Invention) As described above in detail, according to the apparatus of the present invention, even if there is an error in the center frequency when displaying an image on the display screen, it can be corrected so that there is no shift on the display screen. is possible,
The practical effects are significant. 4,

[Brief explanation of drawings]

FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention, FIG. 2 is a flowchart of the operating procedure of the present invention, and FIG. 3 is an MRI
Figure 4 is an illustration of the standard pulse sequence of MHI. Figure 5 is an illustration of image deviation caused by center frequency error. The figure is an explanatory diagram of the influence of image shift on the positioning of scanning in different directions. Figure 7 is an illustration of the image of scanning in different directions obtained by positioning without image shift. Figure 8 is the case with image shift. FIG. 3 is a diagram of an image of scanning in different directions obtained by positioning. 1...90'' pulse 5...SE signal 7...
・180° pulse 11...True image 12・
...Image of displacement 13...Vertical section position 21...
- Magnet assembly 22... Static magnetic field power supply 23... Gradient magnetic field drive circuit 24... RF power amplifier 26... Sequence storage circuit 27... Computer 29... RF oscillation circuit 32... Operation console 34... Frequency needle 28... Gate modulation circuit 30... Phase detector 33... Display device 25... Preamplifier

Claims (1)

[Claims] means for performing a preliminary scan to determine the deviation from the center frequency in the current device prior to the start of scanning; a measuring means for the same; a calculation means for calculating the amount of image deviation from the amount of frequency deviation; A magnetic resonance imaging apparatus comprising: a correction means for correcting the amount of deviation of the image and displaying the image at a correct position; and an input means for inputting an instruction to perform the preliminary scan.