JPH0353937B2 - - 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 particularly relates to a signal used in various calculations such as when obtaining a calculation image. This paper concerns the improvement of the theoretical formula for strength.

(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 for obtaining images related to the value (T ₁ image), images related to the transverse relaxation time T ₂ value (T ₂ image), and images related to the proton density ρ.

These images are obtained using various pulse sequences, but in general, T ₁ , T ₂ , I am looking for a calculation image regarding ρ.

(Problem to be solved by the invention) However, the above theoretical formula does not include the effect of slicing (the effect of magnetization not tilting 90 degrees in the slicing direction), and it is said that accurate calculation images cannot be obtained. There was a problem.

In view of these points, the purpose of the present invention is to develop an NMR system that allows accurate values of T ₁ , T ₂ , and ρ to be obtained by using a theoretical formula for signal strength that includes the influence of slices.
An object of the present invention is to provide an imaging device.

In order to achieve such an object, in the present invention, a theoretical formula including the influence of slices is obtained by the following steps (a) to (d), and the relaxation time (T ₁ , T ₂ ) or proton density (ρ).

Note (a) Find the signal strength when the angle of magnetization tilt is α° with a 90° pulse.

(b) At the point of distance z from the center of the slice,
Find the angle α at which the magnetization falls due to the 90° pulse.

(c) Using the signal strength obtained in (a) above, use (b) above.
and calculate the signal strength including the influence of the slice.

(d) From the values in (c) above, find a theoretical formula that includes the influence of slicing.

(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 which a space (hole) is provided for inserting an object, and a uniform static magnetic field H _p is applied to the object surrounding this space. A main magnetic field coil 2, and a gradient magnetic field coil 3 for generating a gradient magnetic field (consisting 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 high-frequency pulses to excite the spins of atomic nuclei within the object, and a receiving coil 5 that detects NMR signals from the object.

The main magnetic field coil is connected to the static magnetic field control circuit 15 with 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 other.

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; and 21 stores the phase-detected waveform signal from the preamplifier. In the waveform memory, here is A/
Contains a D converter.

A computer 11 receives the signal from the waveform memory 21, performs predetermined signal processing, and obtains a tomographic image, and a display 12, such as a television monitor, displays the obtained tomographic image.

In such a configuration, the procedure for creating a calculated image will be described below as an example.

For example, let us consider a case where a calculated image is obtained from three original images in a pulse sequence using the SRSE method. Here, the SRSE method is a pulse sequence that combines a saturation recovery method (abbreviated as SR method) and a spin echo method (abbreviated as SE method).

Under the control of the controller 13 in which scan parameters and other conditions are set in advance, a Gaussian-modulated 90° pulse as shown in FIG. 4 to excite the object. At this time, a gradient magnetic field G _z is also applied (FIG. 2B) to selectively excite only the spins within a specific slice plane.

Next, phase encoding is performed using the gradient magnetic field _Gy , and at the same time, a gradient magnetic field _Gx is applied (d) to prepare for observing echoes.

Next, the application of the gradient magnetic field is stopped, and a 180° pulse is applied to reverse the spin. Thereafter, as shown in Figure D, an echo signal (Figure E) generated while applying G _x is detected by the receiving coil 5 and observed. The spin echo signal detected by the receiving coil is stored in the waveform memory 21 via the amplifier 19 and the phase detection circuit 20.

The original image (image data) is obtained from a large number of signals with different phase encoded amounts using the above method.

Apply a spatial filter to the image if necessary. That is, when the density resolution (S/N) is worse than the spatial resolution, the density resolution of the image is increased by using a spatial filter.

From the three original images obtained as described above (or) and the theoretical formula (details will be described later), calculation images of T ₁ , T ₂ , and ρ are obtained by, for example, the iteratively corrected least squares method.

By the above procedure, calculation images of T ₁ , T ₂ , and ρ can be obtained accurately.

Next, how to obtain the theoretical formula used above will be explained. The theoretical formula that does not include the effect of slices is
Let F _o (T _r , T _s , T ₁ , T ₂ , ρ).

F _o (T _r , T _s , T ₁ , T ₂ , ρ) = exp (−T _s / T ₂ ) {1−2exp (−T _r /T ₁ +T _s /2T ₁ ) +e×p(=T _r /T ₁ )}・p (a) When the angle of magnetization is α°, the signal strength is sinα/1+cosα.exp(−T _r /T ₁ )・F _o (T _r , T _s , T ₁ , T ₂ , ρ) ...(1).

(b) If a Gaussian 90° pulse is used, α at a point at a distance z from the center of the slice becomes α=(π/2) exp(−Z ² ) ……(2) h.

(c) By integrating equation (1) with respect to z using equation (2), the signal strength including the effect of slice can be found, resulting in the following equation.

F _o (T _r , T _s , T ₁ , T ₂ , ρ)∫sin {(π/2)e
xp(−Z ² )}/1+cos {(π/2)exp(−Z ² )}exp
(-T _r /T ₁ )dZ...(3) Since the integral of equation (3) is only related to (T ₁ /T _r ), this value is written as C _SRSE (T ₁ /T _r ).

(d) Since C _SRSE is a function of only T ₁ /T _r , the necessary
Find C _SRSE by numerical integration in the range of T ₁ /T _r ,
From this value, C _SRSE can be determined as a polynomial of T ₁ /T _r .

From the above, the theoretical formula F _s that includes the influence of slicing
(T _r , T _s , T ₁ , T ₂ , ρ) is determined as the product of F _o and a coefficient C _SRSE representing the influence of slicing.

F _s (T _r , T _s , T ₁ , T ₂ , ρ) = F _o (T _r , T _s , T ₁ , T ₂ , ρ) C _SRSE (T ₁ /T _r ) Here, C _SRSE is , for example, if 0.2<T ₁ /T _r <10.0, C _SRSE = 8.1537E−6(T ₁ /T _r ) ⁶ −2.95086E−4(T ₁ /T _r ) ⁵ +4.27675E −3(T ₁ / T _r ) ⁴ -3.17902E-2 (T ₁ / T _r ) ³ +1.29262E-1 (T ₁ / T _r ) ² -2.8554E -1 (T ₁ / T _r ) + 1.0557 Above is SRSE The theoretical formula for the case where the pulse sequence is one 90° pulse and an odd number of
If it consists of 180° pulses,
The coefficient representing the influence of slices is the same as the _CSRSE described above.

If the pulse sequence consists of one 90° pulse and an even number of 180° pulses (for example, two 180° pulses are added for each view in the SRSE method, as shown in Figure 3). If the theoretical formula that does not include the influence of slices is F _o , , the signal strength when the angle at which the magnetization falls is α° is sinα/1−cosα·exp(−T _r /T ₁ )·F _o , which can be calculated in the same manner as in the above case.

For example, if we use a Gaussian 90° pulse,
When 0.2<T ₁ /T ₂ <10.0, the coefficient C representing the influence of slice is C=-2.4203E-5・(T ₁ /T _r ) ⁵ +5.6861E −4・(T ₁ /T _r ) ⁴ − 3.6523E −3・(T ₁ /T _r ) ³ −1.0071E−2・(T ₁ /T _r ) ² +3.2162E−1・(T ₁ /T _r ) +0.9178.

The above is a calculation using a Gaussian 90° pulse, but even if other 90° pulses are used, if the angle α at which the magnetization falls due to the 90° pulse at a point at a distance z from the center of the slice is found, the same calculation can be performed. Can calculate.

This angle α may be determined experimentally from a signal when a gradient magnetic field is applied in the slice direction in an NMR imaging device.

The method of obtaining a calculated image using the theoretical formula obtained as described above is such that the calculated image obtained using the theoretical formula that does not include the influence of slices is later corrected due to the influence of slices. This has the advantage that calculations are simpler and more accurate calculated images can be obtained than in the case of In particular, when calculating by the least squares method, curve fitting will be performed using the wrong function unless a theoretical formula that includes the influence of slices is used.

Although the embodiment has been described using an example in which a calculated image is finally obtained, the final result does not necessarily have to be an image.

(Effects of the Invention) As explained above, according to the present invention, values such as T ₁ , T ₂ , and ρ can be calculated accurately by using a theoretical formula for signal strength that includes the influence of slicing. be.

In addition, especially when the pulse sequence consists of one 90° pulse and one 180° pulse (the 180° pulse does not have to be present), the above theoretical formula for signal strength indicates that the deviation due to the effect of slicing is T ₁ / It becomes a function only of T _r and is easily expressed by a polynomial of T ₁ /T _r , and has the advantage that such a theoretical formula can be easily obtained.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the NMR imaging device according to the present invention, FIG. 2 is a time chart for explaining the operation, and FIGS. 3 and 4 are diagrams showing other pulse sequences. . 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 circuit, 19... Preamplifier, 20... Phase detection circuit, 21... ...Waveform memory.

Claims (1)

[Scope of Claims] 1. A nuclear magnetic resonance imaging apparatus that applies a high-frequency pulse and a magnetic field to an object to generate a nuclear magnetic resonance signal, and uses this signal to obtain an image regarding the tissue of the object, as follows: By steps (a) to (d), we obtain a theoretical formula that includes the effects of slicing, and we also have a function that allows us to calculate the values of relaxation time (T1, T2) or proton density (ρ) from the original image and this theoretical formula. What is claimed is: 1. A nuclear magnetic resonance imaging apparatus characterized by comprising a control means having the following features: Note (a) Find the signal strength when the angle of magnetization tilt is α° with a 90° pulse. (b) Based on the Fourier transform of the shape of the 90° pulse to be applied, or based on the NMR signal obtained by applying a gradient magnetic field in the slice direction after applying the 90° pulse, at a point at a distance z from the center of the slice, Find the angle α at which the magnetization falls due to the 90° pulse. (c) Integrate the signal strength obtained in (a) above over the distance z using (b) above to find a signal strength that includes the influence of the slice. (d) From the values in (c) above, find a theoretical formula that includes the influence of slicing.