JPH0345649B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a nuclear magnetic resonance imaging device (hereinafter referred to as NMR), and in particular, the present invention relates to a nuclear magnetic resonance imaging device ( _hereinafter referred to as _NMR ), in which a desired This paper relates to a method for obtaining images of images. (Prior art) Longitudinal relaxation time T ₁ , which is considered to be medically useful, has been measured from images measured by NMR imaging devices.
There are techniques to obtain images related to the value (T ₁ image), transverse relaxation time T ₂ value (T ₂ image), proton density (ρ image), etc. For example, the T ₁ image and T ₂ image were obtained by the following method. For example, the T ₁ image is calculated as follows. Inversion Recovery method (hereinafter abbreviated as IR method) as shown in Figure 6
and the IRSE method, which is a combination of the spin echo method (hereinafter referred to as SE method),
The saturation recovery method as shown in the figure
One original image is obtained using the SRSE method, which is a combination of the Recovery (hereinafter referred to as SR method) and the SE method, and calculations are performed using these two images and an approximation formula for signal strength. As shown in Figure 7, the SRSE method is a pulse sequence in which an echo signal is obtained by applying a 90° pulse and then a 180° pulse, and the time from the 90° pulse to the center of the echo signal is T _S , 90 The time from the application of the ° pulse to the application of the 90° pulse in the next view is defined as T _r . In addition, as shown in Figure 6, the IRSE method is a pulse sequence in which a 180° pulse for inversion recovery is applied before each 90° pulse of the SRSE method shown in Figure 7. T d is the time from the application of the 180° pulse to the application of the 90° pulse, T _s is the time from the 90° pulse to the center of the echo signal, and T _s is the time from the 90° pulse to the center of the echo signal.
The time from the application of the 180° pulse for recovery to the application of the 180° pulse for the next view is defined as T _r . _The theoretical formula for signal strength in the SRSE method I _SR is I _SR = I ₀・exp( _{−T s} /T ₂ )
_s /2T ₁ ) + exp (-T _r /T ₁ )} In addition, the theoretical formula for signal strength in the IRSE method I _IR is I _IR = I ₀・exp (−T _s /T ₂ ) {1−2・exp (−T _a /T ₁ )
+2·exp(−T _r /T ₁ +T _s /2T ₁ )−exp(−T _r /T ₁ )}. For this theoretical formula, here, as T _r ≫T ₁ ,
If exp(−T _r /T ₁ )=0, I _SR ≒ _IO・exp(−T _s /T ₂ ) I _IR ≒I _O・exp(−T _s /T ₂ ) {1 −2・exp(-T _d /T ₁ )} Therefore, I _IR /I _SR =1-2・exp(-T _d /T ₁ ) T ₁ =T _d /ln{2I _SR /(I _SR /T _IR )} Calculate the T ₁ value from this formula. If you are looking for T ₂ images, please refer to the publication "Eizo Information (M)" June 1984 issue (Vo1.16 No.11), for example.
CPMG described on pages 570 to 576 of
T ₁ and ρ are removed from a plurality of echo data by the method, and the T ₂ value is determined by the least squares method. In addition, in the CP method, which allows multiple echo data to be taken out continuously in one data collection to obtain the _T2 value, if the length of the applied pulse is incomplete, the error will increase as the echoes are acquired. However, the _pulse sequence called CPMG method solves this problem by applying a 180° pulse after a 90° pulse as shown in Figure 8. n
This is a pulse sequence that is applied repeatedly in rotation to generate n echoes. (Problem to be solved by the invention) However, the T ₁ value and T ₂ value obtained in this way
As for values, ρ values _, etc., it may not be possible to separate the tissue of interest from other tissues based on these _values alone;
There was a problem that the contrast of the image was weak when using the ρ image alone. In view of these points, it is an object of the present invention to obtain a high-contrast image of a tissue of interest, and to obtain a nuclear magnetic field image with an arbitrary scan parameter using an arbitrary pulse sequence from a set of scan data. An object of the present invention is to provide a resonance imaging device. (Means for solving the problem) In order to achieve such an object, the present invention applies a high frequency pulse and a magnetic field to an object to generate a nuclear magnetic resonance signal, and uses this signal to detect the object. In a nuclear magnetic resonance imaging device that can obtain and display images related to the composition, a T ₁ , T ₂ , and ρ calculation image is obtained, and the T ₁ , T ₂ , and ρ values obtained from this calculation image are The present invention is characterized in that it includes a means having a function of performing calculations for each image to effectively separate the tissue of interest and obtaining a recombined image of the tissue of interest with a high contrast. (Example) The present invention will be explained in detail below using the drawings. FIG. 1 is a diagram showing the configuration of essential parts of an embodiment of an NMR imaging device according to the present invention. In the figure, 1 is a magnet assembly, inside of which a space (hole) is provided for inserting an object, and a uniform static magnetic field H ₀ is applied to the object surrounding this space. The main magnetic field coil 2 includes a gradient magnetic field coil 3 for generating a gradient magnetic field (composed of an x gradient magnetic field coil, a y gradient magnetic field coil, and a z gradient magnetic field coil configured to be able to individually generate gradient magnetic fields). ), an RF transmitting coil 4 that provides a high-frequency pulse to excite the spins of atomic nuclei within the object, a receiving coil 5 that detects NMR signals from the object, and the like are arranged. The main magnetic field coil is connected to the static magnetic field control circuit 15, G _x ,
G _y , G _z each gradient magnetic field coil is gradient magnetic field control circuit 1
4, the RF transmitting coil is connected to the power amplifier 18, and the NMR signal receiving coil is connected to the preamplifier 19.
are connected to each. 13 is a controller that controls the sequence of generation of gradient magnetic fields and high-frequency magnetic fields.
Performs necessary control to capture the NMR signal into the waveform memory 21. 17 is a gate modulation circuit, and 16 is a high frequency oscillator that generates a high frequency signal. Gate modulation circuit 17
appropriately modulates the high frequency signal output from the high frequency oscillator 16 using a control signal from the controller 13 to generate a high frequency pulse with a predetermined phase. This high frequency pulse is applied to the RF transmitting coil 4 through the RF power amplifier 18. 19 is a preamplifier that amplifies the NMR signal obtained from the detection coil 5; 20 is a phase detection circuit that phase-detects the NMR signal by referring to the output signal of the high-frequency oscillator; 21 is a phase detection memory;
Contains a D converter. 11 is a computer that receives the signal from the waveform memory 21 and performs predetermined signal processing to obtain a tomographic image;
12 is a display device such as a television monitor that displays the obtained tomographic image. A console 30 is connected to the computer 11 and is input means for inputting various information necessary for the apparatus. The procedure for creating a recombined image in such a configuration will be explained. (1) Here, three images obtained using the IR3SE method,
From a total of 7 images, 4 images obtained using the SR4SE method,
An example of calculating T ₁ , T ₂ , and ρ images will be explained. In addition, the IR3SE method, as shown in Figure 2,
This is a pulse sequence based on the IRSE method, in which a 180° ₍₂₎ pulse is repeatedly applied three times in one view to obtain three echo signals. The SR4SE method is similar to the SRSE method, as shown in Figure 3, and is a multi-echo method in which 180° ₍₂₎ pulses are applied four times to obtain four echo signals per view. . In Figures 2 and 3, the 90° pulse and the first
The interval Ts ₁ to the center of the echo signal, the interval T _s2 between the centers of each echo signal after the first echo signal, the interval T _d between the 180° ₍₁₎ ° pulse and the 90° pulse, and the repetition time T _r are each arbitrarily determined. You can choose. These time managements are performed by the controller 13, and the time settings can be performed using the console 30. 180° ₍₁₎ pulse is a 180° pulse for spin reversal,
180° ₍₂₎ is a 180° pulse for Spi and echo, both of which are 90° _-45 to reduce pulse errors.
Composite pulses of 270° ₄₅ and 90° _-45 are used. The number of 180° pulses for each view is determined by the IR3SE method.
Both SR4SE methods have even numbers. Note that the suffix value attached to the frequency of the composite pulse represents the phase difference with the 90° excitation pulse, and these pulses are non-selective pulses. The excitation 90° pulse is Gaussian modulated. Application of such a 90° pulse or a 180° pulse is performed as follows. That is, a predetermined 90° pulse or 180° pulse signal obtained through the gate modulation circuit 17 under the control of the controller 13 is applied to the RF transmitting coil 4 via the power amplifier 18,
Generates an RF magnetic field that is applied to the object. On the other hand, the gradient magnetic field is as follows. x
The gradient magnetic field G _X in the direction is the projection gradient,
a is a spoiler for eliminating noise outside the slice plane due to the 180° pulse. The z-direction gradient magnetic field _Gz is a slice gradient, the y-direction gradient magnetic field _Gy is a warp gradient, and b is a spoiler for eliminating artifacts due to 180° pulse errors. Further, c is a spoiler for removing correlation between views. Application of each gradient magnetic field is controlled by a controller 13. Each echo signal generated by the above pulse sequence is detected by the receiving coil 5. The spin echo signal detected by the receiving coil is sent to the waveform memory 2 via a preamplifier 19 and a phase detection circuit 20.
It is stored in 1. From the seven images thus obtained, T ₁ , T ₂ , and ρ images are calculated using the nonlinear least squares method. (2) Next, select a pulse sequence and scan parameters that allow the tissue of interest to be clearly distinguished from the T ₁ , T ₂ , and ρ calculation images. (3) Next, using the theoretical formula for the signal strength of the pulse sequence and scan parameter selected in (2) above,
Calculations are performed for each pixel from the T ₁ , T ₂ , and ρ calculation images to obtain a re-synthesized image of the tissue of interest with high contrast, which is displayed as appropriate. In this way, a contrast image that allows the tissue of interest to be clearly distinguished can be obtained. According to this method, it is possible to render images even under conditions that cannot actually be scanned due to limitations of the device. For example, from the T ₁ , T ₂ , and ρ calculation images observed using a combination of the SRSE method and the SR2SE method, it is also possible to obtain a recombined image corresponding to the image scanned using the IRSE method. In this case, it is also possible to obtain images under conditions that cannot be realized in actual scanning, such as an infinite repetition time T _r . (Other Examples) The present invention is not limited to the above-mentioned embodiments, but can also be implemented as follows. (1) Obtain T ₁ , T ₂ , and ρ calculation images. (2) Remove the effect of shedding from the ρ image.
For example, this is performed by division processing using a water phantom image. (3) Find the linear combination coefficients (C _T1 , C _T2 , C〓) that maximize Fisher's discriminant function from the mean and variance of the tissue of interest. (4) Calculate C _T1 ·T ₁ +C _T2 ·T ₂ +C〓·ρ from T ₁ , T ₂ , and ρ images using the above coefficients. (5) Perform the calculation in (4) above for each pixel to obtain an image and display it. Although an image can be obtained by such a method, a specific example will be described below in which two tissues of interest are better separated, for example, by linear combination of T ₁ and T ₂ values. In principle, if the ranges of T ₁ and T ₂ values of A and B are distributed as shown in Figure 4 (for simplicity,
T ₁ or T ₂ (two-dimensional case), T ₁ or T ₂
It is based on the method that although it is not possible to separate and confirm A and B independently, it is possible to separate and confirm T ₁ + T ₂ . A more specific example will be explained as follows. Here, an example will be described in which a T ₁ T ₂ linear combination image is obtained by focusing on two tissues A and B in a sagittal image as shown in FIG. Note that the ρ value was not used because it includes the influence of shading. For the tissues A and B of interest, the average and variance were calculated from each 10 x 16 pixel area, and the values shown in the following table were obtained.

[Table] This average and variance are used in pattern recognition.
Determine the coefficients of the linear combination using Fisher's discriminant function. That is, the variance of the classes is made as small as possible and the distance between the average values of the classes is made as large as possible so that Fisher's discriminant function J expressed by the following equation becomes maximum. J=|m ₁ −m ₂ | ² /S ₁ +S ₂ where m ₁ , m ₂ , S ₁ , and S ₂ are the average value and variance of the class after transformation. The coefficients of the linear combination are determined as follows. −0.15T ₁ +0.98T _2In this way, the linear combination image is obtained. (Further other embodiments) (1) If it is not necessary to display all pixels, it is also possible to designate a region of interest and calculate and display only that part. (2) If the displayed image is insufficient for the intended diagnosis, etc., use the already obtained T ₁ , T ₂ , and ρ images without rescanning,
A different image may be obtained by changing the arithmetic expression. Normally, only one set of scans is performed, and the operations for the above-mentioned diagnosis, that is, attempts to change and display the calculations are performed many times, so a calculation device (calculator), a means for specifying the method of calculation, a display device, etc. are required. A plurality of them may be provided in parallel as necessary. (3) The pulse sequences to be actually scanned are not limited to the IR3SE method and SR4SE method of the embodiment, and other pulse sequences may be used. (Effects of the Invention) As explained above, the present invention has the following effects. By appropriately combining the T ₁ value, T ₂ value, and ρ value, it is possible to separate tissues that cannot be separated alone. By combining the T ₁ image, T ₂ image, and ρ image, it is possible to obtain a high contrast image of the tissue of interest. Images with arbitrary pulse sequences and scan parameters can be displayed from the T ₁ image, T ₂ image, and ρ image without actually capturing images (including those under impossible conditions). It is possible to designate a region of interest and perform predetermined calculations only within that region, thereby reducing calculation time. In particular, pulse sequences using the IR3SE method and the SR4SE method, or the IR3SE method and the FR4SE method (FR is
By using a pulse sequence based on Fast Recovery (which is a combination of the FR method and the SE method), highly accurate T ₁ , T ₂ , and ρ images can be obtained, and the images after calculation are also of high quality. be. If shading is removed from the water phantom image, the ρ image will be accurate and the image after calculation will also be of high quality. The T ₁ , T ₂ , and ρ images calculated from a set of scan data can be used to obtain various images as needed without rescanning.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the main part configuration of an embodiment of the nuclear magnetic resonance imaging device according to the present invention, Fig. 2 is a diagram showing a pulse sequence of the IR3SE method, Fig. 3 is a diagram showing a pulse sequence of the SR4SE method, and Fig. 4 is a diagram showing a pulse sequence of the SR4SE method. The figure is a diagram to explain how two tissues can be separated and recognized by linear combination, Figure 5 is a diagram showing the tissue part of the head of interest, and Figures 6 to 8 are conventional It is a figure showing an example of a pulse sequence. DESCRIPTION OF SYMBOLS 1... Magnet assembly, 2... Main magnetic field coil, 3... Gradient magnetic field coil, 4... RF transmitting coil, 5... Receiving coil, 11... Computer, 12... Display, 13... Controller ,1
4... Gradient magnetic field control circuit, 15... Static magnetic field control circuit, 16... High frequency oscillator, 17... Gate modulation circuit, 18... Power amplifier, 19... Preamplifier, 20... Phase detection circuit, 21... Waveform memory, 30...operation console.

Claims (1)

[Claims] 1. A nuclear magnetic resonance signal generated by applying a high-frequency pulse and a magnetic field to an object, and using this signal to obtain an image of the tissue of the object and to display this image. In magnetic resonance imaging equipment, IR3SE method and SR4SE method, or IR3SE method and
The T ₁ , T ₂ , and ρ calculation images are obtained from the images obtained by the FR4SE method, and the influence of shedding is removed from this ρ calculation image. Next, from the mean and variance of the tissue of interest, the discriminant function of Fischer is maximized. Find the linear _combination coefficients ( _{C T1} , C _T2 , C〓) that give A nuclear magnetic resonance imaging apparatus characterized by comprising means having a function of obtaining a composite image.