JPH0439858B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention utilizes the nuclear magnetic resonance (hereinafter abbreviated as NMR) phenomenon to detect the distribution of specific atomic nuclei within a specimen from outside the specimen. Regarding a nuclear magnetic resonance imaging device that reconstructs and outputs a cross-sectional image of a desired examination site of a subject in a destructive manner,
In particular, it concerns the optimization of composite pulses.

(Prior Art) The above-mentioned NMR imaging device has been well known, and a method of observing a tomographic image of a target region using an imaging method using a 180° pulse, particularly a multi-echo method, is also well known. There is.

By the way, when trying to obtain an image with no artifacts and a theoretical signal strength using such a multi-echo method, errors in the 180° pulse due to static magnetic field inhomogeneity and RF intensity inhomogeneity become a big problem. .

An effective method for reducing this 180° pulse error is to use a 180° composite pulse. 180° composite pulse means, for example, AJ
Shoka, and R. Freeman, “Composite Pulses.
with Dual Compensation,”J.Magn.Reson.
As described in Vol. 55, 487, 1983, it is a train of multiple pulses with different phases and works in the same way as a 180° pulse. For example, 90°x・
The pulse train of α°y and 90°x becomes a 180° composite pulse regardless of the value of α.

However, its characteristics vary greatly depending on the value of α, as shown in FIGS. 5 to 8.

5 to 8 show the characteristics (spin reversal efficiency) of each pulse with respect to static magnetic field non-uniformity and radio frequency pulse (RF pulse) intensity non-uniformity. In each figure, the horizontal axis is the error (variation amount) of the static magnetic field, and the vertical axis is the fluctuation error of the RF pulse intensity.
Figure 5 shows the characteristics for a simple 180° pulse, and Figures 6, 7, and 8 show the characteristics for 90°x, 180°y, 90°x, and 90°x, respectively. Characteristics of composite pulses of 243°y/90°x and 90°x/270°y/90°x are shown.

From these figures, it can be seen that the pulse error is greatly improved by using composite pulses. Also, if the RF intensity non-uniformity is large,
It can be seen that it is better to use a 90°x, 180°y, 90°x composite pulse, and a 90°x, 243°y, 90°x composite pulse when the static magnetic field inhomogeneity is large.

As described above, it is necessary to select the value of α depending on the non-uniform characteristics of the imaging region.

In view of these points, the object of the present invention is to
When using α°y/90°x format composite pulses, there are
It is an object of the present invention to provide a nuclear magnetic resonance imaging apparatus capable of optimizing the value of α of a composite pulse according to differences in imaging regions.

(Means for solving the problem) In order to achieve such an objective, the present invention has the following features:
NMR imaging equipment uses composite pulses of 90°x, α°y, and 90°x as 180° high-frequency pulses, and has a function to set the value of α to a value according to the imaging conditions. It is characterized by having been configured.

(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, reference numeral 1 denotes a magnet assembly, which has a space (hole) inside for inserting an object. A magnetic field coil 2, and 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 generate gradient magnetic fields individually ), an RF transmitter coil 4 that provides high-frequency pulses to excite the spins of atomic nuclei within the object, and a receiver coil 5 that detects NMR signals from the object.

The main magnetic field coil is connected to the static magnetic field control circuit 15, Gx,
Gy, Gz 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 is a memory for storing the phase-detected waveform signal from the preamplifier. In the waveform memory, here is A/
Contains a D converter.

A computer 11 has a function of performing predetermined signal processing on the signal read out from the waveform memory 21 to obtain a resonant frequency component, and a controller 13 with an RF
A function to set the pulse height and RF pulse length, the RF pulse height of the α° pulse in the 90°x, α°y, and 90°x pulses so that the component within the predetermined and bandwidth of the resonant frequency component is minimized. It also has the function of adjusting the RF pulse length and the function of reconstructing tomographic images from NMR signals. Incidentally, an input means (not shown) for inputting various information necessary for this apparatus is connected to the computer 11, so that the operator can input appropriate information.

12 is a display device such as a television monitor that displays the obtained tomographic image.

In such a configuration, an RF pulse and a magnetic field are applied to an object placed in the magnet assembly 1 to generate an NMR signal, which is phase-detected and processed by the computer 11 to identify the object. Reconstruct images related to the structure of objects,
The general operation of the imaging device for displaying information on the display is similar to that of conventional NMR imaging devices, and since this operation has little direct relation to the present invention, we will not explain the details of its operation here. Explanation will be omitted.

The following is a second explanation of the characteristic operation of the present invention.
This will be explained in detail with reference to the figures.

(1) Excite the magnetization of the object by applying a 90°x, α°y, 90°x composite pulse a along with a gradient magnetic field c. The relationship between the angle at which the magnetization falls and the position is shown in FIG. Note that the direction of the gradient magnetic field is the direction of the slice gradient in actual imaging.

(2) Stop applying the gradient magnetic field and apply a 90°x, α°y, 90°x composite pulse b (a simple 180° pulse may be used) to obtain an echo signal.

(3) A gradient magnetic field d having the same direction and the same polarity as the gradient magnetic field c is applied, and an echo signal e is received. The relationship between the frequency and intensity of the obtained signal is as shown in FIG. 3(e). In other words, the angle at which the magnetization falls is
When the angle is 90°, the signal strength is maximum, and when the angle of magnetization is exactly 180°, the signal strength is zero.

Due to the non-uniform intensity of the RF pulse, the echo signal
Center frequency component of RF pulse (component of frequency fo)
will not become zero. The smaller the pulse error due to non-uniform intensity of the RF pulse, the smaller the center frequency component of the RF pulse of the echo signal becomes.

On the other hand, if it deviates from the point Z=0, the static magnetic field is shifted due to the gradient magnetic field c when the pulse is applied, so its frequency component does not become zero. The smaller the pulse error due to static magnetic field inhomogeneity, the smaller the frequency component.

(4) Predetermined bandwidth (Δf shown in Figure 3 E)
Adjust (optimize) α in the composite pulse so that the frequency component within the width) is minimized.

The band width in this case is determined according to the non-uniformity of the static magnetic field in the imaging range of the device; if the non-uniformity is large, the band width Δf is wide, and if the non-uniformity is small, it is narrowed.

Note that although an echo signal was used in the above example, other NMR signals, such as FID (free
An induction decay) signal may also be used.

Also, as shown in Figure 4, 90°x, α°y, 90°x
It is also possible to apply the composite pulse a plurality of times, focus only on the n-th echo signal, and adjust the value of α so that it becomes the maximum. The larger n is, the more accurate the adjustment can be.

In addition, the inversion efficiency of the composite pulse at each point within the imaging range can be determined from data on the static magnetic field non-uniformity, RF pulse intensity non-uniformity, and characteristic data (FIGS. 6 to 8) of the device. Therefore, optimization can be achieved, for example, by selecting a composite pulse that maximizes the sum of inversion efficiencies in the imaging range.

In addition, the optimal value of α for the imaging range can be determined in advance (this value may be stored in the memory of the computer), and the value of α corresponding to the imaging range specified during imaging operation can be calculated in advance. You may read and use it.

(Effects of the Invention) As explained above, the present invention has the following effects.

Since the optimal composite pulse can be selected for each imaging, the effects of the device's static magnetic field non-uniformity and RF pulse intensity non-uniformity can be minimized.

For example, if the RF pulse intensity non-uniformity is large on the axial plane, the angle will be α180°. On the other hand, if the static magnetic field inhomogeneity is large in the sagittal plane, the pulse error will hardly be improved even if a composite pulse of α180° is used for imaging the sagittal plane. However, according to the device of the present invention, a composite pulse of α243° can be used, and the pulse error is greatly improved.

Reducing the pulse error is important in the multi-echo method and in each calculation image of T ₁ , T ₂ , and proton density. According to the present invention, it is possible to accurately reduce the pulse error, so the practical effects are great.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of main parts showing an embodiment of the NMR imaging device according to the present invention, Figure 2 is a waveform diagram of each part to explain the operation, and Figure 3 is the relationship between the angle of magnetization and signal intensity. FIG. 4 is a waveform diagram of each part showing another embodiment of the present invention, and FIGS. 5 to 8 show static magnetic field non-uniformity and RF pulse intensity non-uniformity when a 180° pulse is applied. FIG. 3 is a diagram showing spin reversal efficiency with respect to uniformity. 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 circuit, 18... Power amplifier, 19... Preamplifier,
20... Phase detector, 21... Waveform memory.

Claims (1)

[Scope of Claims] 1. Means for applying a magnetic field to an object, means for applying a high frequency pulse to an object, means for receiving a nuclear magnetic resonance signal, and a means for applying a magnetic field to a target object, a means for receiving a nuclear magnetic resonance signal, and a means for applying a magnetic field to a target object, It is equipped with a computer to obtain an image of the object, a display to display the obtained image, and a controller to generate the control signals necessary to control each part, and applies high frequency pulses and a magnetic field to the object. In a nuclear magnetic resonance imaging apparatus capable of generating a nuclear magnetic resonance signal using a nuclear magnetic resonance signal, using this signal to obtain an image regarding the tissue of a target object, and displaying the image on a display, the computer and the controller are configured to generate a 180° high-frequency A core characterized in that it uses composite pulses of 90°x, α°y, and 90°x as pulses, and has a function of setting the value of α to a value according to imaging conditions. Magnetic resonance imaging device. 2. As the value of α, the optimum value of α determined based on the information of the imaging range from the values of the static magnetic field non-uniformity and RF pulse intensity non-uniformity of the device and the characteristics of the composite pulse is used. A nuclear magnetic resonance imaging apparatus according to claim 1, characterized in that: 3. Under the control of the controller, apply the composite pulse together with a gradient magnetic field, then apply a gradient magnetic field in the same direction as this gradient magnetic field, observe the NMR signal generated at that time, and use the computer to detect the NMR signal. the value of a predetermined frequency component or a component within a predetermined bandwidth is determined, and the value of α in the composite pulse is adjusted by the controller so that the determined value is minimized. A nuclear magnetic resonance imaging apparatus according to claim 1, characterized in that: 4 Under the control of the controller, apply a selected 90° pulse to excite magnetization in the slice plane, then apply the composite pulse multiple times, and receive a spin echo signal due to the magnetization in the slice plane generated thereby. and the computer is configured to determine the intensity of the spin echo signal, and the controller adjusts the value of α in the composite pulse so that the intensity of the determined spin echo signal is maximized. A nuclear magnetic resonance imaging apparatus according to claim 1, characterized in that: 5 Based on the measured NMR signal, the computer determines the optimal value of α for each imaging range, stores it, and reads out the value of α corresponding to the specified imaging range at the time of imaging and uses it. A nuclear magnetic resonance imaging apparatus according to claim 1, characterized in that it is configured as follows.