JPH01238847A - Magnetic resonance image method - Google Patents

Abstract

PURPOSE:To shorten a collection time of a magnetic resonance signal, by a method wherein through regulation of a change amount of the strength of a phase encode magnetic field, the size of the photographing visual field of a fault surface is changed according to the size of a sample. CONSTITUTION:A value representing the size of a sample 1 or a photographing visual field L' in a phase encode direction is inputted to a calculator 16 through a console 17. The calculator 16 calculates the number N' of phase encodes responding to the photographing visual field L', and calculates the intensity of a phase encode magnetic field based on the number N' of the phase encodes. However, when the photographing visual field L' of a sample 1 is low than a visual field L responding to the number N of picture elements, the number N' of the phase encodes is decreased in proportion to the ratio therebetween. The number N' of the phase encodes is allotted so that, in the control waveform of a phase encode magnetic field, the position and the negative are approximately equal to each other based on a current value O. Through O-filling of (N-N') short phase encode data being short of picture elements in a phase encode direction, interpolation is effected, and a fault layer is formed in a picture.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is directed to MRI (Magnetic Re5ona
The present invention relates to a magnetic resonance imaging method called magnetic resonance imaging (CE imaging), and particularly relates to a magnetic resonance imaging method that shortens the acquisition time of magnetic resonance signals.

[Prior Art] Fig. 3 is a block diagram showing a general magnetic resonance imaging apparatus. In Fig. 0, (1) shows a subject such as a human body to be measured, and (2) shows a subject (1) A static magnetic field generator that applies a static magnetic field in the Z-axis direction to the patient (3) is an examination table that introduces the subject (1) into the static magnetic field generator (2).

(4) is a high-frequency coil for applying high-frequency energy such as a high-frequency magnetic field pulse to the subject (1) and receiving magnetic resonance signals from the subject (1); (5) is the impedance of the high-frequency coil (4); Convert, matching device,
(6) is a transmitting/receiving disconnection device connected to the matching device (5), (
7) is a transmitter for applying high frequency energy to the subject (1) via the transmission/reception switching device (6) and the high frequency coil (4), (8) is the high frequency coil (4) and the transmission/reception switching bag W (
6) is a receiver that receives magnetic resonance signals from the subject (1).

(9) is an X-axis gradient magnetic field coil that applies a magnetic field pulse tilted in the X-axis direction to the subject (1); (10) is an X-axis gradient magnetic field power supply that drives the X-axis gradient magnetic field coil (9); )
(12) is a Y-axis gradient magnetic field power source that drives the Y-axis gradient magnetic field coil (11), and (13) is a Y-axis gradient magnetic field coil that applies a magnetic field pulse tilted in the Y-axis direction to the subject (1). The Z-axis gradient magnetic field coil (14) applies a magnetic field pulse tilted in the Z-axis direction to the specimen (1), and the Z-axis gradient magnetic field coil (14)
13) is a Z-axis gradient magnetic field power supply that drives.

(15) is a sequence control device that controls the transmitter (7), the receiver (8), each gradient magnetic field power source (10), (12), and (14) in a predetermined sequence, and also controls the entire device. (16) is a computer connected to the sequence control device (15), which generates control data for image composition, processes magnetic resonance signals obtained from the receiver (8), and performs image composition processing. It has become. (17
) is an operation console that serves as an input/output terminal device for inputting parameters etc. necessary for image composition to the computer (16); (18)
is an image display device connected to the console (17).

Next, the conventional magnetic resonance imaging method will be explained with reference to the pulse sequence diagram of FIG. 4 as well as FIG. Here, the gradient magnetic field in the X-axis direction is a data readout magnetic field cP for frequency encoding, the gradient magnetic field in the Y-axis direction is a phase encoding magnetic field cB, and the gradient magnetic field in the Z-axis direction is a slicing magnetic field C8. Fourier transform (2D
FT> method using the gradient field (gradient magnetic field) echo method.

First, the subject (1) is inserted into the static magnetic field generator (2), the high-frequency coil (4), and each gradient magnetic field coil (9), (11), and (13), and the sequence control bag is opened! (15) executes the following sequence.

Δ” in the table. High-frequency magnetic field pulse RF with selective frequency drives the high-frequency coil (4) and the Z-axis gradient magnetic field coil (13).
At the same time, a slice magnetic field Gsl for specifying a tomographic plane is applied to the subject (1).

Input main curve - A phase encoding magnetic field GB is applied together with a slice magnetic field GS2 for aligning the phases of magnetic spins in the slice direction. Further, in order to read out the subsequent magnetic resonance signals, a data read magnetic field GP2 of opposite polarity is applied.

TOU IIL Subsequently, a data read magnetic field GP3 with reversed polarity is applied. This reversal of magnetic field polarity generates a magnetic resonance signal R by gradient magnetic field echo, which is acquired into the computer (16) via the high frequency coil (4).

The timing at which this magnetic resonance signal R is generated is the time when the areas of the hatched portions of the data readout magnetic fields GP2 and GP3 match, which is after the echo time TE after the application of the high frequency magnetic field pulse RF.

The above sequence is converted to the strength of the phase encode magnetic field GB (
Amplitude) is repeated at a predetermined pitch as shown by the broken line, and multiple magnetic resonance signals R are sampled and received as a pulse train.For example, the number of pixels is 256 x 256.
In this case, the minimum number of sampling points is 256, and the number of repetitions (number of signal acquisitions) N is 256.

Finally, the computer (16) performs two-dimensional Fourier transform on the pulse train of the magnetic resonance signal R to obtain a desired matrix size N.
Reconstruct the image of the tomographic plane of XN and use the image display device (18)
to be displayed.

In addition, in the case of the three-dimensional Fourier transform (3DFT) method, the fifth
As shown in the figure, the slice magnetic field G
This will change the strength of S2'.

[Problems to be Solved by the Invention] As described above, the conventional magnetic resonance imaging method performs magnetic resonance imaging at a predetermined number of repetitions N based on the imaging field of view corresponding to the pixel, regardless of the size of the subject (1). Since the signal R is acquired, there is a problem in that the power collection time of the magnetic resonance signal cannot be shortened.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to obtain a magnetic resonance imaging method that can shorten the acquisition time of magnetic resonance signals.

[Means for Solving the Problems] The magnetic resonance imaging method according to the present invention adjusts the size of the imaging field of view of the tomographic plane according to the size of the subject by adjusting the amount of change in the intensity of the phase encoding magnetic field. This is something I decided to change.

[Operation] In the present invention, when the subject is small, the data area for collecting magnetic resonance signals, that is, the imaging field of view is made small, and the number of times of collection is reduced without deteriorating the resolution.

[Example] Hereinafter, an example of the present invention will be described with reference to the flowchart shown in FIG.

The magnetic resonance imaging apparatus to which this invention is applied is similar to that shown in FIG. 3, and can be easily implemented by simply changing the program and memory contents in the computer (16). Further, the pulse sequence in one embodiment of the present invention is as shown in FIG. 4, and the only difference is the maximum intensity of the phase encoding magnetic field GB.

First, a value representing the size of the subject (1) or the imaging visual field L' in the phase encoding direction is input into the computer (16) via the console (17) (step S1).

The computer (16) calculates the number of phase encodes N' corresponding to the imaging field of view L' (step S2), and calculates the control waveform representing the strength of the phase encode magnetic field G based on this number of phase encodes N' (step S2). Step S3).

At this time, if the imaging field of view L' of the subject (1) is smaller than the field of view corresponding to the number of pixels N, the number of phase encodes N' decreases in proportion to the ratio.

That is, it is set to a minimum integer greater than or equal to the value determined from N'=N-L'/L...■. Further, the control waveform of the phase encode magnetic field GE is distributed by the number N' of phase encodes so that the positive and negative sides are approximately equal around the current value 0.

Thereafter, as described above, the pulse sequence shown in FIG. 4 is repeated while changing the phase encode magnetic field GB, and N' magnetic resonance signals R are collected (step S4). At this time, when the object (1) is small, If N'<N, the number of acquisitions (=N') of the magnetic resonance signal R will decrease.

Next, there is a shortage (N
-N') missing phase encode data are interpolated by filling them with 0, assuming that all 0 signals have been collected in a region corresponding to the outside of the imaging field of view L' (step S5).

For example, when obtaining an image with the number of pixels of 256 x 25B, if the size of the subject (1), that is, the imaging field of view L' in the phase encoding direction is 374 times the image field of view, then the number of phase encodes N' is N'=NX3 /4 = 256 x 3/4 = 192, and the magnetic resonance signal R is collected 192 times. Therefore, the missing phase encode data is the phase encode magnetic field GO corresponding to these 64 pieces, which is N - N '= 256-192 = 64.
If these are distributed almost equally to the outside of the imaging field of view L', 256 magnetic resonance signals R are obtained.

Based on the data interpolated by zero filling in this way, 2
Image the tomographic plane using the DFT method (step S
6).

In this way, by reducing the imaging field of view L', the data collection area for the magnetic resonance signal R can be saved, and the signal collection time can be shortened. Therefore, the time required for the patient to be examined (1) to be examined is shortened, and the number of patients that can be measured, that is, the throughput is also improved.

Further, at this time, if the number of data of the magnetic resonance signal R is interpolated to 256 by zero filling, the 2DFT method can be processed at high speed. This is because fast Fourier transform can be performed if there are 21 pieces of data.

In the above embodiment, the case of the 2DFT method was explained, but it goes without saying that it can also be applied to the DFT method as shown in FIG. 5. In this case, each step S in FIG.
As shown in 11 to S16, a similar operation for reducing the imaging field of view is performed for the encoded amount of the slice magnetic field Gs2'.

In addition, step S5 was provided to interpolate the missing phase encoded data by filling with 0, but 2DFT was performed using only the collected magnetic resonance signal R without providing this interpolation step S5.
In this case, it is sufficient to fill the image data corresponding to the pixels that are finally missing.

Furthermore, image data may be obtained by fast Fourier transformation by filling (2n-'-N') pieces of data with zeros, and then filling the missing image data of (2n 3n-1)/1g with zeros. For example, 1 for 110 pieces of collected data.
After 8 pieces of data are filled with zeros and subjected to fast Fourier transform as 128 pieces of data, 128 pieces of image data are filled with 0s to finally obtain 256 pieces of image data.

[Effects of the Invention] As described above, according to the present invention, by adjusting the amount of change in the intensity of the phase encoding magnetic field, the size of the imaging field of the tomographic plane is changed according to the size of the subject, and When the magnetic resonance signal is small, the data area for collecting the magnetic resonance signal, that is, the imaging field of view, is made smaller, and the number of times of collection is reduced without deteriorating the resolution. There is an effect that can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a flow chart diagram showing an embodiment of the present invention;
FIG. 2 is a flowchart diagram showing another embodiment of the present invention, FIG. 3 is a block diagram showing a general magnetic resonance imaging apparatus, FIG. 4 is a pulse sequence diagram showing magnetic resonance imaging method using the 2DFT method, and FIG. FIG. 5 is a pulse sequence diagram showing magnetic resonance imaging using the 3DFT method. (1)...Subject Gs...Slice magnetic field GR...Phase encode magnetic field GP...Data readout magnetic field RF...High frequency magnetic field pulse R...Magnetic resonance signal L'...Imaging field of view , in the figures, the same reference numerals indicate the same or equivalent parts. Kou 4 diagram

Claims (1)

[Claims] Magnetic resonance imaging, in which a slice magnetic field, a data readout magnetic field, a phase encode magnetic field, and a high-frequency magnetic field pulse are applied to a subject in a predetermined sequence, and a desired tomographic plane is visualized based on magnetic resonance signals obtained from the subject. A magnetic resonance imaging method, characterized in that the size of the imaging field of view of the tomographic plane is changed according to the size of the subject by adjusting the amount of change in the intensity of the phase encoding magnetic field. .