JPH0261252B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to nuclear magnetic resonance
The NMR signal (FID; Free Induction
RF coil device for detecting Decay).

NMR imaging devices that use NMR techniques to obtain tomographic images of a subject focusing on specific atomic nuclei have been known for a long time. First, an overview of the principle of this NMR imaging device will be briefly explained.

Static magnetic field in the z-axis direction for specific substances such as hydrogen
When H ₀ is applied, the nucleus precesses around the z-axis at an angular velocity ω as shown in the following equation.

ω = γH ₀ (Larmor angular velocity), where γ: gyromagnetic ratio When an electromagnetic wave (usually radio wave: RF signal) with a frequency corresponding to the angular velocity ω is applied to a system in this state, resonance occurs, and the atomic nucleus is moved by the spin quantum number. transition to the higher energy level of the determined energy level. Even if several types of atomic nuclei with nuclear spin angular momentum coexist, each nucleus has a different gyromagnetic ratio γ, so the resonant frequencies differ, and it is therefore possible to extract only the resonance of a specific atomic nucleus. Furthermore, by measuring the strength of the resonance, it is possible to determine the amount of atomic nuclei present. Further, after resonance, the atomic nucleus excited to a higher level returns to a lower level during a time determined by a time constant called relaxation time. Among these relaxation times, the spin-interstitial relaxation time called _T1 is a time constant that depends on the way each compound binds, and it is known that the value differs greatly between normal tissues and malignant tumors. It is being

Therefore, using this NMR, we excite protons in a virtual cross section of the subject using the same principle as X-ray CT, and obtain NMR signals corresponding to each projection in many directions of the subject. By determining the NMR signal intensity at each position of the specimen using a reconstruction method, it is possible to obtain a tomographic image of the specimen focusing on a specific atomic nucleus.

FIG. 1 is a waveform diagram for explaining an example of an inspection method in such a conventional device. First, a z-gradient magnetic field is applied to the subject as shown in Figure 1B.
Gz ⁺ and the narrow frequency spectrum f as shown in A.
Apply RF pulse (90° pulse). in this case,
Protons are excited only on the plane where the Larmor angular velocity ω = γ (H ₀ + ΔGz), and if the magnetization M is expressed on a rotating coordinate system rotating at ω as shown in Figure 2A, then it will be 90° in the y'-axis direction. The direction will be changed. Subsequently, as shown in FIG. 1C and D, an x gradient magnetic field Gx and a y gradient magnetic field Gy are applied, thereby creating a two-dimensional gradient magnetic field, and an NMR signal as shown in E is detected. Here, as shown in Figure 2 (b), the magnetization M gradually disperses in the direction of the arrow in the x', y' plane due to the non-uniformity of the magnetic field, so the NMR signal eventually decreases.
As shown in FIG. 1E, it disappears after a period of τ.
When the NMR signal obtained in this way is subjected to Fourier transformation, it becomes a projection in the direction perpendicular to the gradient magnetic field synthesized by the x gradient magnetic field Gx and the y gradient magnetic field Gy.

Thereafter, in the same way, wait for a predetermined time τ' and repeat the next sequence. In each sequence, Gx and Gy are changed little by little. Thereby, NMR signals corresponding to each projection can be obtained in many directions of the object.

The magnetic field coil portion in this case is constructed as shown in FIG. That is, a static magnetic field coil 2 that provides a uniform static magnetic field H ₀ (in the z direction) around a cylinder 1 in which the subject is placed, and a z gradient magnetic field.
A gradient magnetic field coil 3 consisting of a z gradient magnetic field coil, a y gradient magnetic field coil, and an x gradient magnetic field coil for generating Gz ⁺ and Gz ^- , and excitation to give an RF pulse with a narrow frequency spectrum f to the subject as an electromagnetic field. A coil 4 and a detection coil 5 for detecting NMR signals from the subject are installed.

Incidentally, in such a conventional device, even if the uniformity of the static magnetic field H ₀ and the precision of perturbation components such as the gradient magnetic field are good, there is a problem in that the RF coil of the detection coil 5 is too large. 3~5M
Even if it resonates at Hz (at 800G to 1.2KG), approx.
It is wound several times with a diameter of 0.7 to 1.0 mφ, and the wire is formed using a thick copper band or pipe-shaped wire. Generally speaking, when magnetic energy enters the space surrounding a coil, the energy density is small. Therefore, the larger the size of the coil, the rougher the exchange of energy between the spin and the spin.
Detection sensitivity decreases. However, if the size is large, Q ₀ can be increased accordingly, so the decrease in detection sensitivity can actually be suppressed to a small amount. However, large-sized detection coils are very inconvenient and cumbersome to manufacture and install.

The present invention has been made in view of these points, and its purpose is to provide an RF coil for an NMR imaging device that avoids large-sized detection coils, has good signal capture efficiency, and can improve S/N. The goal is to provide equipment.

The present invention achieves this objective by arranging a large number of detection coils that detect signals for nuclides having the same resonance frequency in a ring around a subject, connecting each of them to a receiving amplifier, The present invention is characterized in that it is configured to receive the magnetic response of the subject while combining the outputs of the receiving amplifier into a single signal, either uniformly or using only a portion as necessary.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 4 is a schematic diagram showing an embodiment of the present invention. Although the perturbation coil H ₀ is omitted in the figure, it is assumed that H ₀ is in the horizontal direction and the RF magnetic field Hrf is in the vertical direction with respect to the sample slice 41 of the subject (the thickness of which is determined by selective excitation). In such a state, a large number of small detection coils 4 detecting signals for nuclides with the same resonance frequency are activated.
2 are arranged in a ring around the subject, and their outputs are respectively taken out via receiving amplifiers 43, and all of the outputs are used uniformly or selectively. Each detection coil 42 is arranged near the subject slice 41 and has a connection with a shallow part of the slice 41, so each detection coil 42 may perform a specific excitation using a large excitation coil 44. coil 42
You may also do this using

FIG. 5 is a diagram of another embodiment. In this case, H ₀ is applied to the subject slice 41 in a vertical direction, and Hrf is applied in a radial direction centered on the axis of H ₀ . Since the FID signal can be observed from any direction perpendicular to H ₀ ,
Excitation can be received from any direction. However, excitation with only 90° pulses creates blind spots, so
If 90° pulse and 180° pulse are used together as appropriate, or if imaging is performed only at 90°, two sets of excitation-only coils 51a, 5 orthogonal as shown in the figure are used.
1b, 52a, and 52b are arranged and used appropriately. In the case of this figure as well, the individual detection coils 53 are more strongly coupled to the shallow slice portion immediately below, so that it is suitable for performing precise imaging of a specific region. Note that the output of each detection coil 53 is transmitted to the receiving amplifier 5.
4, all of them may be used uniformly or any number of locations may be selectively used. When used selectively, this is done by varying the weight given to each output by the weighting coefficient unit 55. If all are to be used uniformly, each weight of the weighting coefficient unit 55 may be made equal.
All outputs of the weighting coefficient unit 55 are uniformly added together by an adding amplifier 56 and outputted as one signal.

Adding in this manner creates the need to ensure proper phase matching. If proper phase matching is ensured, the noise figure in the overall detection will be improved. It can be seen that the fact that the size (volume) of the sensitive area is significantly smaller (approximately 1/10 or less) compared to the conventional one leads to an improvement in the S/N ratio.

The method of using the FID signal thus obtained and the method of imaging reconstruction are the same as conventional methods.

In addition, although each detection coil is shown in a fixed arrangement in FIG. 5, as shown in FIG. 6, each detection coil can be moved as a probe coil in the radial direction shown using a plunger 61 such as an air cylinder. It is also possible to configure it freely so that each detection coil is pressed against the surface of the subject. According to such a configuration, the area directly below the body surface can be well imaged without being too limited by the thickness and shape of the subject. Further, a probe coil may also be arranged on the subject mounting table 62.

As explained above, according to the present invention, in which a large number of detection coils for detecting signals of nuclides having the same resonance frequency are arranged in a ring around the subject, large-sized detection coils can be arranged around the subject. Not only is it easier to manufacture and install the detection coil than in the case of conventional large-sized detection coils, but the large number of detection coils can individually adjust the sensitivity. We will observe while securing the
The detection coil as a whole observes nuclides having the same resonance frequency over a wide range, making it possible to realize an RF coil device for an NMR imaging device that has good signal capture efficiency and can improve S/N.

[Brief explanation of the drawing]

Figure 1 is a waveform diagram for explaining an example of an inspection method in a conventional NMR device, Figure 2 is a diagram for explaining the direction of magnetization M by the method in Figure 1, and Figure 3 is a diagram for explaining the magnetic field in a conventional device. FIG. 4 is a block diagram showing one embodiment of the present invention, and FIGS. 5 and 6 are block diagrams showing other embodiments of the present invention. 1... Cylinder, 2... Coil for static magnetic field, 3... Coil for gradient magnetic field, 4... Excitation coil, 5, 42,
53...Detection coil.

Claims (1)

[Claims] 1. A large number of detection coils for detecting signals of nuclides having the same resonance frequency are arranged in a ring around the subject, a reception amplifier is connected to each of them, and a receiving amplifier is connected to each of the detection coils. RF in a nuclear magnetic resonance imaging apparatus characterized in that it is configured to receive a magnetic response of a subject while combining all the outputs of the amplifiers uniformly or using only a part as necessary into one signal. coil device. 2. The nuclear magnetic resonance imaging apparatus according to claim 1, wherein the detection coil is also capable of exciting the subject.
RF coil device. 3. The nuclear magnetism according to claim 1, wherein the detection coil is configured to be movable and can be placed in contact with the surface of the subject or moved at a close distance. RF coil device in resonance imaging equipment.