JPH07255702A - Magnetic resonance imaging system - Google Patents

Abstract

PURPOSE:To enhance recognitionability for the dislocation of a relative position on a display and to improve accuracy and simplification for correction by displaying an addition object image on the display by performing differential processing between images in real time simultaneously as correcting the disloca tion of the relative position in the addition object image. CONSTITUTION:A CPU 1 expresses the signal strength of each picture element of image data in two-dimensional data, and expresses it in two kinds of image data arrangement I1(x, y) and I2(x, y), and displays the image A(I1(x, y)) that is a fundamental image on the display 18. Thence, the image B(I2(x, y)) to be added on the image is selected, and the differential processing Is(x, y)=I1(x, y)-I2(x, y) is performed on the image first without changing the relative position, and a result is displayed on the display 18. In such a case, when the relative position is dislocated, the dislocation DELTAx, DELTAy of the relative position of the image B for the image A are designated by the input device 21. Then, equation I's(x, y)=I1(x, y)-I2(x-DELTAx, y-DELTAy) is computed, and the result is displayed on the display 18.

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 MRI apparatus), and more particularly to an MRI apparatus capable of easily correcting a relative position when performing addition processing between images. .

[0002]

2. Description of the Related Art An MRI apparatus utilizes a so-called NMR phenomenon to measure the density distribution, relaxation time distribution, etc. of nuclear spins (hereinafter referred to as spins) in a cross section corresponding to a desired inspection site in a subject, An image of the inspection site of the subject is displayed as an image from the measurement data.

In such an apparatus, in order to perform magnetic resonance imaging, a high frequency magnetic field pulse (RF pulse) is applied in a state where a gradient magnetic field is applied to a static magnetic field, and an NMR signal emitted from an examination region of a subject is measured. . At this time, a gradient magnetic field is applied to encode (encode) the NMR signal as spatial information. An image is reconstructed based on the NMR signal data thus obtained. Since this MR image requires two-dimensional position information, normally, one magnetization vector excitation is performed for one NMR signal measurement, and this is repeatedly measured for an arbitrary spatial resolution. Generally, 192 to 256 times of repetitions are measured. This is called phase encoding. As a result, two-dimensional position information is obtained, but addition processing is performed to further improve the S / N of the image. This addition processing is conventionally performed by repeatedly measuring data of the same phase encoding and averaging the data.

However, the NMR signal measurement itself for phase encoding, for example, requires a long measurement time of 256 repetitions, and the measurement time is further extended for the addition processing. During this time, the subject needs to suppress body movements,
It is difficult not only for infants but also for adults for a long time, and breathing is stopped only for a very short time.
It is very difficult to secure / N.

On the other hand, the number of additions per phase encoding is reduced to shorten the image pickup time per image, a plurality of images are taken under the same condition, and then the addition processing is performed between the images so that the body movement There is a method of reducing the influence and ensuring S / N. However, in this method, a shift in the relative position due to body movement becomes a problem during image addition between the end of shooting each image and the start of the next shooting. Therefore, conventionally, as shown in FIG. 4B, the result of adding two images A and B {(A +
B) / 2} is displayed on the display and the relative position is corrected so as to eliminate the deviation between the two and the corrected 2
The result of addition between images was displayed in real time and corrected.

[0006]

However, in such a conventional MRI apparatus, the shift of the relative position on the added image (A + B) is recognized as a portion of the two images A and B which do not overlap and have low luminance. Therefore, the contrast with the image is small and it is difficult to recognize. The present invention has been made in view of such a conventional problem, and an object thereof is to improve the recognizability of the relative position shift on the display when correcting the relative position shift during the image addition processing. An object of the present invention is to provide an MRI apparatus with improved correction accuracy and simplicity.

[0007]

In order to achieve such an object, an MRI apparatus of the present invention has a memory for storing NMR signal data sequentially obtained by nuclear magnetic resonance signal measurement, and each memory stored in the memory. A signal processing unit for reconstructing an image based on the NMR signal data and a display unit for displaying the reconstructed image are provided, and the signal processing unit is an addition process for correcting a shift in relative positions of a plurality of images. When performing the above, the display unit is provided with an input unit that has a function of calculating a difference between the two images and that corrects the shift amount of both images based on the difference, and the display unit is corrected by the image of the calculated difference and the input unit. The difference image is displayed in real time. If you add more than two images,
One image to be added is an image obtained by adding a plurality of images.

[0008]

By correcting the displacement of the relative position of the images to be added and simultaneously performing the difference processing between the images in real time and displaying the result on the display,
The recognizability of the relative position shift can be enhanced, and the addition process between images including body movement can be performed accurately and easily.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. FIG. 3 is a block diagram of the overall configuration showing an MRI apparatus to which the present invention is applied. This MRI apparatus is roughly classified into a central processing unit (CPU) 1 for controlling the entire apparatus,
A sequencer 2 for controlling an imaging sequence, a transmission system 3 for transmitting an RF pulse, a static magnetic field generating magnet 4 for generating a static magnetic field in a space in which the subject 22 is placed, and a gradient magnetic field for generating. Gradient magnetic field generation system 5 and NMR
A reception system 6 for receiving signals and a signal processing system 7 for image reconstruction based on NMR signals are provided.

The CPU 1 controls each of the sequencer 2, the transmission system 3, the reception system 6, and the signal processing system 7 according to a predetermined program. Sequencer 2 is CP
It operates based on the control command from U1, and sends various commands necessary for data acquisition of the tomographic image of the subject 22 to the transmission system 3, the gradient magnetic field generation system 5, and the reception system 6.

The transmission system 3 has a high frequency oscillator 8, a modulator 9 and an irradiation coil 11 as an RF coil, and the modulator 9 amplitude-modulates the RF pulse from the high frequency oscillator 8 in accordance with a command from the sequencer 2. By amplifying the amplitude-modulated RF pulse via the high-frequency amplitude unit 10 and supplying the amplified RF pulse to the irradiation coil 11, a predetermined pulse-shaped electromagnetic wave is applied to the subject 22.
I am trying to irradiate.

The static magnetic field generating magnet 4 is for generating a uniform static magnetic field around the subject 22 in an arbitrary direction. Inside the static magnetic field generating magnet 4, in addition to the irradiation coil,
A gradient magnetic field coil 12 for generating a gradient magnetic field and a receiving coil 14 of a receiving system are installed. The gradient magnetic field generation system
A gradient magnetic field coil 12 having a configuration capable of independently applying a gradient magnetic field in Cartesian coordinate axis directions orthogonal to each other;
The gradient magnetic field coil 13 is configured by a gradient magnetic field power supply 13 that supplies a current.

The receiving system 6 includes a receiving coil 14 as an RF coil, an amplifier 15 connected to the receiving coil 14,
It has a quadrature detector 16 and an A / D converter 17, and when the receiving coil 14 detects the NMR signal from the subject 22, the signal is amplified by the amplifier 15, the quadrature detector 16, A /
While converting to a digital amount through the D converter 17,
At the timing according to the command from the sequencer 2, the quadrature detector 16 converts the collected data into two series of collected data and sends the collected data to the CPU 1.

The signal processing system 7 has an external storage device such as a magnetic disk 20 and an optical disk 19 and a display 18 such as a CRT as a display unit. When data from the receiving system 6 is input to the CPU 1, CPU1 performs signal processing,
While executing processing such as image reconstruction and displaying a desired cross-sectional image of the subject as a result thereof on the display 18,
The data is stored in the magnetic disk 20 or the like of the external storage device.

Here, the CPU 1 has a function of adding or subtracting a plurality of reconstructed image data in addition to a calculation such as Fourier transform, displaying the addition result and the subtraction result on the display 18, and the subtracted result. Based on the above, the input means 21 for correcting the deviation of the relative position of the image is provided. This input means includes a trackball,
A joystick, a keyboard, etc. are used.

Further, for example, a sequence as shown in FIG. 2 is incorporated in the sequencer, and an NMR signal is obtained based on this sequence. Incidentally, FIG.
Is a timing diagram for applying an excitation pulse and a gradient magnetic field pulse for normal gradient measurement. In the figure, RF indicates the timing of RF pulse irradiation and the envelope for selective excitation. Gs indicates that the measurement is performed by changing the timing of applying the gradient magnetic field in the slice encoding direction and its amplitude. Gp indicates that the measurement is performed by changing the timing of applying the gradient magnetic field in the phase encoding direction and its amplitude. Gf indicates the timing of applying the gradient magnetic field in the frequency encoding direction, and Signa1 indicates the measured NMR signal. The segmentation of S0 to S5 is segmentation of the time sequence for applying the high frequency magnetic field pulse and the gradient magnetic field pulse in the signal measurement.
Note that Gs, Gp, and Gf are applied in directions orthogonal to each other.

In this sequence, after the preparatory period S0 before the start of measurement, an excitation pulse is irradiated in S1 and a gradient magnetic field in the slice direction is applied to excite nuclear spins (hereinafter referred to as spins) in a predetermined slice section. .
Further, after the rest period S2, a gradient magnetic field in the phase encoding direction is applied in S3 to add a rotation of spin depending on the position in this direction. At the same time, a frequency encode gradient magnetic field is applied to spin the phase in advance in order to obtain an echo signal when the NMR signal is measured in the section S4. In the section S4, the dephased spins are rephased by applying the frequency encoding gradient magnetic field, and the NMR signal is measured. S5 is a rest period.

By changing the intensity of the phase-encoding gradient magnetic field of such a sequence and measuring the NMR signal repeatedly, for example, 256 times, it is possible to obtain the NMR signal data necessary for the construction of the image. The gradient magnetic field in the frequency encode direction is applied with the same intensity for each phase encode, and the spatial coordinates in the frequency encode direction are encoded on the frequency axis. That is, the phase encode gradient magnetic field strength applied in the section S1 is

ΓGp × D × Tp = − (Np / 2) × π, −
{(Np / 2) +1} × π, ... {(Np / 2) -2} × π, {(N
The phase encode gradient magnetic field strength Gp is changed so that the phase encode amount γGp × D × Tp is changed by π so that p / 2) −1} × π. In the above equation, γ is the gyromagnetic ratio of the proton as the target nucleus, D is the field diameter, and Np is the phase encode number.

The NMR signal data may be obtained by adding and averaging measured data obtained by measuring a plurality of times for one phase encoding, but in order to suppress artifacts due to body movements, addition at the time of measurement is performed. Is preferably reduced, eg, one NM per phase encode
Obtaining an R signal and changing the amount of phase encoding
R signal data may be obtained. The decrease in S / N due to the reduction of the addition at the time of measurement can be compensated by the addition of images at the time of image processing described later.

The NMR signal data obtained by the above sequence is stored in the memory in the CPU 1 in the order based on the phase encoding, and becomes the measured raw data for image processing. Normally, this raw data uses the QPS (quadrature phase detection) method, so that an NMR signal is stored as data including an imaginary part.
Therefore, an image can be obtained by reconstructing it by a two-dimensional Arier transform.

In the present invention, a plurality of images are obtained by repeating such image pickup a plurality of times under the same condition, and these images are added by the CPU. Next, this addition processing will be described. For simplicity, let us consider a case where addition processing between two images is performed.

First, the CPU 1 expresses the signal intensity of each pixel of the image data as two-dimensional data, and the data array of the two images is as shown in FIG.

I ₁ (x, y) (x = 1, 2, ..., X) I ₂ (x, y) (y = 1, 2, ..., Y) is expressed. Of these, the basic image A (I ₁ (x, y))
Is displayed on the display 18. Next, the image B (I ₂ (x, y)) to be added to this image is selected, and first the following difference processing is performed without changing the relative position (101).

Is (x, y) = I ₁ (x, y) −I ₂ (x, y) Then, the result is displayed on the display 18 (10
2). Here, if the relative positions are not correct,
As shown in (a), in the tomographic image of the subject, the signal of the portion (edge) where the signal intensity spatially suddenly changes is strongly displayed. The relative position shift is corrected so that this difference is minimized (103). To correct the displacement, the displacement of the relative position of the image B with respect to the image A is designated as Δx in the x direction and Δy in the y direction by the input device 21 such as a trackball or a joystick. And immediately after designation

I's (x, y) = I ₁ (x, y) -I ₂ (x-Δ
After the calculation, x, y-Δy) is displayed on the display (102). Repeat this operation as needed, and when the deviation is minimized,

I (x, y) = {I ₁ (x, y) + I ₂ (x−ΔX, y−ΔY)} is added (104) and the result is displayed on the display 18.
Displayed above and saved on the magnetic disk 20 (1
05, 106). When three or more images are added, the added image (A + B) is used as a basic image, and the difference between the added images is calculated / displayed (101, 102) to correct the relative position. The addition process can be realized by repeating the above operation.

[0028]

As is apparent from the above description, according to the MRI apparatus of the present invention, a difference between images is obtained during image addition processing, the result is displayed, and the deviation is corrected and corrected in real time. Since the difference is displayed, the deviation of the relative position between the images can be corrected accurately and easily.

[Brief description of drawings]

FIG. 1 is an overall view showing an embodiment of an MRI apparatus of the present invention.

FIG. 2 is a diagram showing an example of a pulse sequence for acquiring image data in an MRI apparatus.

FIG. 3 is a diagram for explaining the operation of the MRI apparatus of the present invention.

FIG. 4A is a diagram for explaining correction of relative position shift of images in the MRI apparatus of the present invention, and FIG. 4B is a diagram for explaining correction of relative position shift of images in the related art.

[Explanation of symbols]

 1 CPU (Signal Processing Unit) 18 Display (Display Unit) 20 Magnetic Disk (Memory) 21 Input Means

Claims (2)

[Claims] 1. N sequentially obtained by nuclear magnetic resonance signal measurement.
Magnetic resonance imaging apparatus including a memory for storing MR signal data, a signal processing unit for reconstructing an image based on each NMR signal data stored in the memory, and a display unit for displaying the reconstructed image In the above, the signal processing unit has a function of calculating the difference between the first and second images when performing the heating process for correcting the displacement of the relative positions of the plurality of images, and both the images based on the difference. A magnetic resonance imaging apparatus comprising: an input unit that corrects a shift amount of the difference, and the display unit displays the calculated difference image and the difference image corrected by the input unit in real time. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the first or second image is an image obtained by adding a plurality of images.