JPS6246243A - Nmr imaging apparatus using weighting projection - Google Patents

Abstract

PURPOSE:To make it possible to selectively enhance the S/N of the frequency component having a high contributory ratio to the formation of a good quality image, by changing the integrating number of times of a signal along with projection. CONSTITUTION:High frequency generating a nuclear magnetic resonance (NMR) phenomenon is sent to an irradiation coil 3 from a transmitter 7. The NMR signal generated from an examine is detected by a receiving coil 4 to be sent to a receiver 8 and the phase relation between the transmitter and the receiver is accurately synchronized through a receiver gate signal 12. An inclined magnetic field power source 9 comprises a three-channel constant current power source and an inclined magnetic field is applied in a pulse like state and the generation of the pulse is controlled by an inclined magnetic field control part 20. The operation of the system is performed by using an operation table 10 and two CRTs are mounted to the operation table 10 other than various keys. Further, the control of the whole system and the high speed operation for constituting an image are performed by a computer 21 and the control of various pulse sequences is performed by a sequence control part 16.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention is based on NMR (Nuclear Magnetism).
The present invention relates to an imaging device that utilizes the nuclear magnetic resonance (clesonance) phenomenon, and particularly to an NMR imaging device that noninvasively images the inside of a human body and diagnoses lesions such as tumors.

[Behind the invention]

NMR and imaging techniques include a projection reconstruction method using polar coordinates and a two-dimensional Fourier method using an orthogonal coordinate system. The present invention is especially effective when applied to two-dimensional Fourier methods.

The NMR signal generated from the atomic nucleus of a sample is extremely weak, and an analysis device typically integrates the degree of variation several thousand times, and an imaging device usually integrates it 2 to 10 times to increase the S/N ratio. In addition, to improve image quality, we experimentally determined the number of filters that remove the high-frequency components of noise contained in the signal, or the number of filters that improve the image quality of visual overlays.

It has been used in conventional equipment.

Examples of known columns include *J, M, S, F (utc
Hison.

W, A, Edelstein and G, Jo
hnson: AWhole-body NMRIm
aging Machine “J, Phys, E, 1
3,947 (1980).

[Purpose of the invention]

An object of the present invention is to provide a nuclear resonance apparatus using weighted projection that can selectively increase the 8/N ratio of frequency components that contribute highly to the formation of high-quality images.

[Summary of the invention]

The present invention improves image quality by incorporating a relatively large amount of frequency component information that is effective for image quality using a method completely different from conventional filtering.

[Embodiments of the invention]

One embodiment of the present invention will be described below.

Figure 1 shows an outline of the NM imaging system. Magnet 1 forms the main part of the system. The magnet.

There are two types: superconducting type, normal current groove type, and permanent magnet type.
The normal conductivity type is shown as an example. As a normal conductive type.

In order to obtain high magnetic field uniformity, an air-core electromagnet is usually used. In this example, the magnetic flux density is 0.15 T (tesla)
, the uniformity of the magnetic field is approximately 50 ppm/30 cyyr+d
sy (sphere). The ridge current is supplied from a static magnetic field power supply 2. The subject lies on the patient table 5 and is sent into the center of the air-core magnet. For static magnetic fields.

A gradient magnetic field for acquiring spatial position information is used. A high frequency wave that generates an NMR phenomenon is sent from a transmitter 7 to an irradiation coil 3. What is the NMR signal generated from the subject or the substance being tested?

It is detected by the receiving coil 4 and sent to the receiver 8.

Since the phase information of the NMR signal is also important in the NMR phenomenon, the phase relationship in the transmitter and receiver is accurately synchronized via the receiver gate signal 12.

The gradient magnetic field power supply 9 is composed of a three-channel constant current power supply in order to independently generate gradient magnetic fields in the three axis directions of X, Y, and Z. Since the gradient magnetic field is applied in the form of pulses, a high-speed response is required. The pulse is generated.

It is controlled by a gradient magnetic field control section 20.

The system is operated using an operator console 10. The console is equipped with 12 CRTs in addition to various keys. One is an interactive method, which is used to set various parameters and operate the entire system. The other is for displaying the obtained video.

The computer 21 controls the entire system and performs high-speed calculations for image construction. Communication between the computer 21 and each control unit is via a path 17. Control of various pulse sequences.

The main sequence, which is performed by the sequence control unit 16, is related to the combination of high frequency pulses and gradient magnetic field pulses.

The basics of the imaging method using the NMR phenomenon will be explained with reference to FIGS. 2 and 3.

FIG. 2 shows a cross section of the measurement section of the NMR imaging device. The electromagnet in FIG. 2 is composed of four static magnetic field coils 101, inside which are a gradient magnetic field coil 102 and an irradiation coil 1.
03. A receiving coil 104 is installed. Static magnetic field direction 1
06 is shown in Figure 1, but 4 Usually, the direction of the static magnetic field is Z.
Define it as the axis.

It is necessary to apply gradient magnetic fields in three directions, X, Y, and Z, which are completely independent of each other.

Three types of coils are installed for Y and Z.

FIG. 3 shows an example of a pulse sequence. 1 high frequency 201 from the top. That is, it shows the pulse waveform of high-frequency power irradiated from the irradiation coil to the subject.

Signal 202 is an amplified electromotive force in the receiving coil. The gradient magnetic field Z203 is a gradient magnetic field applied in the direction of the static magnetic field. The gradient magnetic field Y2O4 encodes the phase in the Y-axis direction. The gradient magnetic field X2O3 is for creating a one-to-one correspondence between coordinates and frequencies in the X direction, and is generally used to generate spin echoes, so it may be interpreted as a readout gradient magnetic field. Time axis 206 reveals the time relationships for all pulse sequences thereon.

Next, the roles of these various pulses will be explained in more detail, and the principle of an image construction method called the two-dimensional 7-lier method will be described.

In the example shown in FIG. 3, a zinc function is used for the waveform of the high-frequency pulse. When the zinc function is Fourier transformed, it becomes a rectangular waveform. That is, since the zinc function between one time window becomes a rectangular wave between one frequency quantity, it has only a frequency in a certain limited section.

In FIG. 3, a gradient magnetic field pulse 210 is applied to the gradient magnetic field Z202 at the same time as a 90° pulse (a pulse that tilts the nuclear spins by 90°). Since the resonance condition in the NMR phenomenon is expressed by the following equation, only a specific tomographic plane in the Z direction is selectively excited.

ωo = rCHo +Ho (Z)) −”
・−(1) Here, ω. is the angular velocity at the resonance point, r is the gyromagnetic ratio, Ho is the magnetic flux density of the static magnetic field, and HG(Z) is.

This is the magnetic flux density of the gradient magnetic field at position Z.

In normal NMR imaging, the thickness of the tomographic plane is 1 to 2
The frequency of selective irradiation is set in the range of 0 m. In this example, a 180° pulse 8 is applied after a 90° pulse 7 to obtain a spin echo signal 209. (In the original two-dimensional Fourier method, spin echoes are generated by gradient magnetic fields and do not use 180° pulses.) The spin echo technique uses a nonuniform magnetic field to generate rapid spin echoes with an apparent transverse relaxation time T2''. This is a technique that aligns the dispersed phases again after a certain period of time.

The gradient magnetic field is also a type of non-uniform magnetic field, and in order to obtain signals with one phase, it is necessary to reverse the gradient magnetic field or apply a 180° pulse simultaneously with the gradient magnetic field.

When actually starting up a gradient magnetic field, the rise and fall times are finite.

In reality, about 1 ms is required. Therefore, the phase is disturbed during this transient period. In order to compensate for this, by applying a compensation pulse 211 after the gradient magnetic field pulse 210, the rising and falling edges are canceled out, making it equivalent to the case where an apparently perfect rectangular wave is applied.

Next, let's talk about phase encoding.

The basic properties of nuclear spin behavior in NMR phenomena include (1) the direction of the magnetic moment, (2) the magnitude of the magnetic moment, (3) the number of magnetic moments, (4) the perturbation frequency of the magnetic moment, (5)! There is a phase of perturbation of Qi moment. The behavior of macroscopic sensitization can be described as a statistical result of these single parameters. In particular, frequency and phase are independent parameters, and by encoding one phase, they can be associated with spatial coordinates.

The gradient magnetic field that encodes one phase is the gradient magnetic field Y2O4 shown in FIG. Since the amount of phase encoding is determined by the integral value of the encoding gradient magnetic field pulse, it is sufficient to change the pulse amplitude or the pulse width. In FIG. 3, the amplitude is changed.

The gradient magnetic field X2O3 is a gradient magnetic field applied in the X direction. When a gradient magnetic field in the X direction is applied to the spins excited by the 90° pulse 7 and precessing coherently, the frequency of one precession changes linearly in the X direction. 180°
By applying the same gradient magnetic field after pulse 208,
A spin echo signal 209 can be generated. X
The coordinates and resonance frequency are in a linear relationship, and the spin echo signal 209 is subjected to Fourier transformation.

A relationship of signal strength with respect to the X coordinate can be obtained. When this is Fourier-transformed again in the phase encoding direction (Y axis), the relationship of signal intensity with respect to the X coordinate is obtained this time. In this way, a signal distribution is obtained on the XY plane, and a tomographic image can be obtained by displaying the signal intensity on a CRT.

FIG. 4 shows the principle of the two-dimensional 7-lier method. Sample 301
has a cylindrical shape, and a projection projected by a gradient magnetic field in the X direction exhibits a curve shown by projection number 1. That is, when a gradient magnetic field in the X-axis direction is applied,
The axial resonance frequency shifts in proportion to the strength of the gradient magnetic field. By Fourier transforming the time domain signal obtained at this time, one frequency domain signal, that is,
A projection projected in the axial direction is obtained.

Furthermore, for each projection from numbers 1 to N obtained by sequentially applying a gradient magnetic field in the Y direction with varying pulse amplitude or pulse width,
If we perform a second Fourier transform.

An image is formed. The point where the gradient magnetic field strength in the X direction is zero,
That is, the line connecting the centers of each projection is like the dotted line in FIG. For ease of understanding, the grosgeection number is displayed on the horizontal axis as shown in Figure 5 (A).
become that way. Since the actual data contains noise components, it is as shown in the fifth drawing. By the way, the curve in Figure 2 is in the form of a Ziffug function, and when it is subjected to 7-lier transformation, it becomes a rectangular function. This image corresponds to a cross section of the cylindrical sample 301 shown in Figure 4, taken along a plane that is parallel to the Y axis and includes the center line of the cylinder, so the shape is rectangular. There is.

The curve of m2 before Fourier transformation as shown in Figure 5 is 1
It corresponds to the frequency teaching precepts. That is, a projection number of zero corresponds to a DC signal, and as the projection number increases, it corresponds to a high frequency component. Therefore, the fine graininess of the formed image is caused by the noise of the projection with a large projection number, and the large change that degrades the resolution of the image is caused by the noise of the projection with a small projection number.

FIG. 6 shows an embodiment of the one-line method.

Here, as shown in FIG. 6(B), the number of integrations has a linear relationship with the projection number. That is, the number of integrations is large near θ where the projection number is small, and the number of integrations is set to become smaller as the projection number becomes larger.

Further, the integrated signal is normalized by dividing it by the product X number.

in this case.

The curve before the second Fourier transform shown in Figure 5 CB) is
It changes as shown in Figure 6 (A). That is, the noise in the high frequency range increases and the noise in the low frequency range becomes medium. Note that since the integral value of the number of integrations does not change, the time required for measurement does not change. Regarding the signal before performing the second Fourier transform, since more information is carried in the low frequency component than in the high frequency component, it is advantageous in terms of image quality if the low frequency component has less noise. Furthermore, even if noise components in the high frequency range are largely removed by high-frequency cut filtering, since the amount of information is originally small, the overall amount of information will not be significantly reduced.Therefore, high-frequency cut filtering, Alternatively, better image quality can be obtained by replacing the high frequency components with zero values.

One type of method can be considered for weighting the projection. For example, the human eye is particularly sensitive to low-frequency granular noise that appears in one image, but by increasing the number of projections corresponding to this frequency component, conventional methods can produce images with noticeable granular noise. It is possible to create a smooth and good-looking image without reducing resolution.

〔Effect of the invention〕

As explained above, according to the present invention, although the method is based on a completely different principle from the conventional filtering method, since the S/N can be controlled for any frequency component, an effect similar to that of filtering can be obtained.

Further, according to one aspect of the present invention, it is possible to selectively increase the S/N ratio of frequency components that contribute highly to the formation of high-quality images.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the NMR imaging device, Fig. 2 is a cross-sectional view of the measurement part of the NMR imaging device, Fig. 3 is a diagram showing the pulse sequence, and Fig. 4 is a diagram showing the principle of the two-dimensional 7-lier method. Figure 5 shows the actual data, Figure 6 shows the actual data.
The figure is a data diagram showing an example. DESCRIPTION OF SYMBOLS 1... Magnet, 2... Static magnetic field power supply, 3... Irradiation coil, 4... Receiving coil, 5... Patient table, 6...
・Gradient magnetic field coil, 7... transmitter, 8... receiver,
9...Gradient magnetic field 1 source, 10...Operation console, 11...
Amplitude data, 12...Receiver gate signal, 13...
RF amplitude control section. 14...RF time control unit, 15...Data acquisition unit. 16... Sequence system m, 17... Pass, 18...
console control unit, 19... CRT control unit, 20... gradient magnetic field control unit, 21... computer.

Claims (1)

[Scope of Claims] 1. An NMR imaging apparatus using weighted projection, which uses a two-dimensional Fourier method, and is characterized in that the number of signal integrations is changed along with the projection. 2. An NMR imaging apparatus using weighted projection according to the invention as set forth in claim 1, wherein the change in the number of integrations is such that a projection having a high contribution rate on the image is increased.