JPH0722572B2 - Magnetic resonance imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention is based on the density distribution of each spin of a specific atom in a specific cross section of an object using a magnetic resonance (MR) phenomenon. Information is called so-called computer tomography (CT: c
CT image (computed tomogra) by omputed tomography
The present invention relates to a magnetic resonance imaging apparatus for imaging as m), and particularly to a magnetic resonance imaging apparatus having an improved probe head configuration.

(Prior Art) For example, in a magnetic resonance imaging apparatus for biomedical diagnosis, in order to obtain a tomographic image of a specific part of a subject, as shown in FIG. A static magnetic field H ₀ is applied, and a pair of gradient magnetic field coils 1
A linear magnetic field gradient Gx is added to the static magnetic field H ₀ by A and 1B.
Specific nuclei resonate at an angular frequency ω ₀ represented by the following equation with respect to the static magnetic field H ₀ .

ω ₀ = γH ₀ (1) In this equation (1), γ is a gyromagnetic ratio and is unique to the type of atomic nucleus. Therefore, a rotating magnetic field H ₁ having an angular frequency ω ₀ that causes only specific nuclei to resonate is applied to the subject P via a pair of transmitting coils 2A and 2B provided in the probe head. By doing so, the linear magnetic field distribution Gx is selected and set in the Z-axis direction in the illustrated xy direction.
The magnetic resonance phenomenon occurs only in a specific slice portion S (a portion on the plane but actually having a certain thickness) that selectively acts on only the planar portion and obtains a tomographic image. This magnetic resonance phenomenon is generated by a free induction decay signal (FID: free in-flight) via a pair of receiving coils 3A and 3B provided in the probe head.
duction decacy, hereinafter referred to as "FID signal"), and a Fourier transform of this FID signal gives a single spectrum at the rotation frequency of a specific nuclear spin.

To obtain a tomographic image as a CT image, x of the slice portion S
-Projections for multiple directions in the y plane are required. Therefore, the slice part after causing the magnetic resonance phenomenon is excited to S, the 8 x 'axial magnetic field H ₀ as shown in FIG. (X
When a linear magnetic field distribution Gxy having a linear inclination is applied by a coil or the like (not shown) to a coordinate meter rotated by an angle θ from the axis, the equal magnetic field line E in the slice portion S of the subject P becomes a straight line, and this equal magnetic field The rotation frequency of the specific nuclear spin on the line E is represented by the above formula (1).

Here, for convenience of explanation, it is considered that the equal magnetic field lines E are E _{1 to} En, and the magnetic fields on the respective equal magnetic field lines E _{1 to} En generate signals D _{1 to} Dn, which are a kind of FID signals. The amplitudes of the signals D _{1 to} Dn are proportional to the specific nuclear spin densities on the isomagnetic field lines E _{1 to} En penetrating the slice portion S, respectively. However,
The FID signal actually observed is a combined FID signal in which all the signals D _{1 to} Dn are added. Then, the composite FID signal is subjected to Fourier transform to obtain projection information (one-dimensional image) PD of the slice portion S on the x ′ axis. This x'axis is rotated in the xy plane (the rotation of this magnetic field gradient Gxy is performed by, for example, two pairs of gradient magnetic field coils in the magnetic field gradients Gx, Gy in the x, y directions.
A magnetic field gradient Gxy is created as a synthetic magnetic field of
By changing the composition ratio of Gy), projection information in each direction in the xy plane can be obtained in the same manner as above, and a CT image can be composed based on these information. .

In this type of magnetic resonance imaging apparatus, the probe head is a transmitter (transmitter coil 2) that generates an exciting rotating magnetic field.
A, 2B) and a receiver (reception coil 3A, 3B) that detects a magnetic resonance signal based on the FID signal. The coil form has a relationship between the static magnetic field and the exciting rotating magnetic field (the magnetic resonance phenomenon There is a surface coil, a saddle coil and a solenoid coil. In addition, it is needless to say that the excitation rotating magnetic field generated in the specific portion of the subject is efficiently generated, and the FID signal detected from the specific portion of the subject is weak, so that it must be efficiently detected.

(Problems to be Solved by the Invention) In a magnetic resonance imaging apparatus of this type, since a magnetic resonance signal detected by a probe head is very weak, this signal is generally amplified through an amplifier, It is being processed. Here, the amplifier is normally provided in the probe head, so that the signal cable and the power supply cable for supplying power to the amplifier are derived from the probe head.

In order to improve the image quality, one of the conditions is to improve the S / N ratio of the magnetic resonance signal, which is realized by bringing the probe head close to the subject. However, bringing the probe head close to the subject causes the patient (subject) to feel uncomfortable due to the presence of the amplifier and the cable, which is a problem for this type of diagnostic layer.

Therefore, an object of the present invention is to provide a magnetic resonance imaging apparatus capable of improving the image quality without giving a discomfort to a patient (subject).

[Structure of the Invention] (Means for Solving Problems) The present invention is characterized by taking the following means in order to solve the above problems and achieve the object. That is,
An emphasizing means including at least a coil that realizes at least one of emphasizing an excitation rotating magnetic field with respect to a subject specific portion and enhancing a magnetic resonance signal from the subject specific portion is magnetically orthogonal to the probe head coil. It is characterized in that it is arranged to do. Here, magnetically orthogonal means that the magnetic fields generated from the coils are orthogonal to each other.

(Operation) By taking such means, the emphasizing means acts so as to reduce the coupling value of the probe head, thereby enhancing the excitation rotating magnetic field with respect to the object specific part and the magnetic field from the object specific part. At least one of enhancement of the resonance signal is realized, and at least one of efficient generation of the magnetic field and high S / N ratio of the received signal is realized.

(Example) The principle of the present invention will be described prior to the description of the examples of the present invention. That is, the inventor of the present application has filed a patent application for an invention capable of improving the receiving sensitivity prior to this application. The specification and the drawings of that application disclose a configuration in which a director composed of a closed loop coil is arranged between a probe head (reception coil) and a subject. According to this configuration, it can be expected that the reception sensitivity can be improved, but it was verified that the coupling characteristic changes depending on the arrangement of the probe head and the director, and the reception sensitivity also changes.

Therefore, it is necessary to reduce the coupling value between the probe head and the waveguide in order to improve the sensitivity, and specifically, the following three conditions are raised.

Method 1: Adjust the positional relationship between the receiving coil and the director, especially arrange them orthogonally.

Method 2: Adjust the resonance (tuning) frequency between the receiving coil and the director with a capacitor or an inductance (coil length).

Method 3: Change the distance between the receiving coil and the director.

The present invention has been made based on the principle as described above.

FIG. 1 shows a first embodiment of the present invention, which is an example applied to a saddle type coil as a probe head and to a receiving coil.

FIG. 1 (a) is a perspective view showing the overall structure, and FIG. 1 (b).
FIG. 4 is an axial sectional view. In this embodiment, between the receiving saddle coil 4A, which is the preceding stage of the receiving saddle coil 4A, and the subject P, a director 5A made of a saddle coil made of copper wire or the like is arranged. The arrangement relationship between the saddle coil for reception 4A and the director 5A is such that when the generated magnetic field from the saddle coil for reception 4A and the generated magnetic field from the director 5A are assumed, both magnetic fields are orthogonal to each other. The positional relationship is set.

Further, a variable-capacitance capacitor 6A is inserted in the saddle coil for reception 4A and an amplifier 8 is connected to the saddle coil for reception 4A, and an excitation rotating magnetic field is applied to the subject P by a high frequency output applied to a transmission coil (not shown) is doing. In addition, director 5A
A variable capacitance capacitor 7A is inserted in to form an LC resonance circuit.

As described above, according to the receiving system of the first embodiment, the following actions are obtained. That is, the magnetic resonance signal from a specific portion of the subject P, that is, the vicinity of the director 5A is first received by the director 5A, and the induced current i ₁ flows in the LC resonance circuit of the director 5A. The magnetic flux generated by this induced current i ₁ and the object P
The induced current i ₂ flows through the receiving saddle coil 4A by the magnetic resonance signal from the.

In this case, the positional relationship between the receiving saddle coil 4A and the director 5A is such that when the generated magnetic field from the receiving saddle coil 4A and the generated magnetic field from the director 5A are assumed, both magnetic fields are orthogonal. Saddle coil 4A for reception because the positional relationship is set to
The degree of coupling between the resonant circuit of (3) and the LC resonant circuit of the director 5A is small (coupling value is small).

Therefore, the magnetic resonance signal from the specific portion of the subject P is
The magnetic flux generated by the induced current i ₁ is emphasized and is received by the saddle coil for reception 4A. At this time, director 5A
The adverse effect of the coupling is small. Therefore, the director
The S / N ratio of the magnetic resonance signal is improved from around 5 A, and high image quality is achieved. This is based on the implementation of method 1 above. In addition, by adjusting the capacitors 6A and 7A, the resonance frequency of the resonance circuit of the saddle coil for reception 4A and the LC resonance circuit of the director 5A can be adjusted. Of the magnetic resonance signal from near the director 5A.
The S / N ratio is improved and high image quality is achieved.

The receiving system of this embodiment is also configured as in the second and third embodiments shown in FIGS. 2 and 3 based on the same principle as that of the first embodiment. That is, the second embodiment shown in FIG. 2 has a configuration in which the receiving solenoid coil 9A is used as the receiving coil of the probe head instead of the receiving saddle coil 4A.

The third embodiment shown in FIG. 3 has a structure in which a solenoid coil 10A is used as a director of a saddle type coil.

FIG. 4 shows a fourth embodiment of the present invention, which is an example applied to a saddle type coil as a probe head and also applied to a transmission coil.

FIG. 4 (a) is a perspective view showing the overall structure, and FIG. 4 (b).
FIG. 4 is an axial sectional view. In this embodiment, a director 5B made of a saddle-shaped coil made of copper wire or the like is arranged between the subject P and the transmission saddle-shaped coil 4B, which is the preceding stage of the transmission saddle-shaped coil 4B. The arrangement relationship between the transmission saddle coil 4B and the director 5B is such that when the generated magnetic field from the transmission saddle coil 4B and the generated magnetic field from the director 5B are assumed, both magnetic fields are orthogonal to each other. The positional relationship is set.

Further, a variable capacitor 6B is inserted in the saddle coil 4B for transmission and a high frequency power source 11 is connected thereto, and an excitation rotating magnetic field is applied to the subject P by the high frequency output. Further, the variable capacitor 7B is inserted in the director 5B to form an LC resonance circuit.

According to the transmission method of the fourth embodiment as described above, since there is a reversible relationship between transmission and reception, the magnetic field characteristics similar to those of the above-described reception method are exhibited, and highly efficient magnetic field characteristics are obtained.
Therefore, high image quality can be achieved.

The transmission method of this embodiment is also configured as in the fifth and sixth embodiments shown in FIGS. 5 and 6 based on the same principle as that of the fourth embodiment. That is, the fifth embodiment shown in FIG. 5 has a configuration in which a transmitting solenoid coil 9B is used as the transmitting coil of the probe head in place of the transmitting saddle coil 4B.

The sixth embodiment shown in FIG. 6 has a structure in which a solenoid coil 10B is used as a director of a saddle type coil.

The present invention is not limited to the above-described embodiment. For example, even if the probe head is configured to have both the transmitting unit and the receiving unit, the same as above by providing the directors orthogonally. The effect of is obtained. Besides this, various modifications can be made without departing from the scope of the present invention.

[Effects of the Invention] As described in detail above, the magnetic resonance imaging apparatus according to the present invention realizes at least one of the enhancement of the excitation rotating magnetic field with respect to the subject specific region and the enhancement of the magnetic resonance signal from the subject specific region. It is configured such that the emphasis means including at least a coil is arranged with respect to the probe head so that the two magnetic fields are orthogonal to each other when the generated magnetic field from the emphasis means and the generated magnetic field from the probe head are assumed.

According to such a configuration, the emphasizing means acts so as to reduce the coupling value of the probe head, and thereby enhances the excitation rotating magnetic field with respect to the subject specific portion and the magnetic resonance signal from the subject specific portion. It is possible to provide a magnetic resonance imaging apparatus in which at least one of them is realized, at least one of efficient magnetic field generation and high S / N ratio of received signals is realized, and transmission / reception efficiency is improved.

[Brief description of drawings]

1 to 6 are configuration diagrams showing first to sixth embodiments of the present invention, and FIGS. 7 and 8 are diagrams for explaining the principle of magnetic resonance imaging. 4A …… Saddle coil for reception, 4B …… Saddle coil for transmission, 5A, 5
B: Waveguide consisting of saddle type coil, 6A, 6B, 7A, 7B ... Variable capacitor, 8 ... Amplifier, 9A ... Reception solenoid coil, 9B ... Transmission solenoid coil, 10A, 10B ...
… Director consisting of solenoid coil, 11 ………… High frequency power supply.

Claims (4)

[Claims] 1. A magnetic resonance phenomenon is applied to a subject by arranging the subject in a uniform static magnetic field, superimposing a gradient magnetic field on the uniform static magnetic field, and applying an exciting rotating magnetic field by a probe head composed of a coil. In the magnetic resonance imaging apparatus that obtains the image information of the subject by performing image reconstruction processing by detecting the induced magnetic resonance signal with the probe head, and enhancing the excitation rotating magnetic field with respect to the subject specific portion. And an enhancement means including at least a coil for realizing at least one of enhancement of the magnetic resonance signal from the subject specific portion, when the generated magnetic field from this enhancement means and the generated magnetic field from the probe head are assumed. A magnetic resonance imaging apparatus characterized in that the two magnetic fields are arranged orthogonal to each other with respect to the probe head. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the emphasizing means is a closed loop coil, a saddle type coil or a solenoid coil. 3. The magnetic head according to claim 1, wherein the probe head is configured so that a transmitter for generating an exciting rotating magnetic field and a receiver for detecting a magnetic resonance signal are also used. Resonance imaging device. 4. The magnetic head according to claim 1, wherein the probe head has a transmitter for generating an exciting rotating magnetic field and a receiver for detecting a magnetic resonance signal, which are separately configured. Resonance imaging device.