JP2531879B2 - Magnetic resonance imaging device - Google Patents

Description

Detailed Description of the Invention

 [Object of the Invention]

[0001]

BACKGROUND OF THE INVENTION The present invention relates to magnetic resonance (MR: ma).
Information based on the density distribution of specific nuclear spins in a specific cross section of a subject is calculated by using a so-called computed tomography (CT) method using a phenomenon of “granular resonance” (hereinafter referred to as “MR”).
The present invention relates to a magnetic resonance imaging apparatus which forms a CT image with a hy).

[0002]

2. Description of the Related Art In this type of magnetic resonance imaging apparatus, in order to obtain a tomographic image at a specific position of a subject, as shown in FIG. A magnetic field H ₀ is applied, and a linear magnetic field gradient G _z is added to the static magnetic field H ₀ by the pair of gradient magnetic field coils 1A and 1B. The specific nuclei resonate with the static magnetic field H ₀ at an angular frequency ω ₀ represented by the following equation.

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 the pair of transmitting coils 2A and 2B. By doing so, the specific slice portion S for obtaining a tomographic image selectively acts only on the illustrated xy plane portion which is selectively set in the z-axis direction by the linear magnetic field gradient G _z.
The MR phenomenon occurs only in (a flat portion, but actually has a certain thickness). This MR phenomenon is caused by free induction attenuation (FID: free) via a pair of receiving coils 3A and 3B.
induction decay) signal (hereinafter "FID
Signal)) and Fourier transforming this signal yields a single spectrum at the rotation frequency of a particular nuclear spin. In order to obtain a tomographic image as a CT image, projected images of the slice portion S in multiple directions in the xy plane are required. Therefore, after the slice portion S is excited to cause the MR phenomenon, as shown in FIG. 2, a linear gradient having a linear gradient in the magnetic field H ₀ in the x ′ axis direction (the coordinate system rotated by an angle θ from the x axis). When the magnetic field gradient G _xy is applied (by a coil (not shown) or the like), the constant magnetic field line E in the slice portion S of the subject P becomes a straight line, and the rotation frequency of the specific nuclear spin on the constant magnetic field line E is as described above ( It is expressed by equation (1). Here for convenience isomagnetic E description and E ₁ to E _n, each of these such as magnetic field lines E ₁ to E
Signals D _{1 to} D that are a kind of FID signal due to the magnetic field on _n
Consider that each yields _n . Signals D ₁ amplitude to D _n is proportional to the specific nuclear spin density on such field lines E ₁ to E _n each penetrating the slice portion S. However, the actually observed FID signal is a combined FID signal in which all the signals D _{1 to} D _n are added. Therefore, the composite information FID signal FID is Fourier transformed to obtain the 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 G _xy is performed by, for example, two pairs of gradient magnetic field coils x,
By changing the composition ratio of the magnetic field gradients G _x and G _{y in} the y direction), projection information in each direction in the xy plane can be obtained in the same manner as described above, and these pieces of information can be obtained. Based on this, the CT image can be synthesized.

[0004]

By the way, in this type of magnetic resonance imaging apparatus, the S / N (signal-to-noise ratio) of the composite FID signal obtained for the slice region S of the subject P is the slice region of the subject P. S receiving coil 3
It is proportional to the ratio occupied in the loop of A and 3B, that is, the filling rate. However, in the conventional magnetic resonance imaging apparatus for diagnosis, only one pair of the receiving coils 3A and 3B is used as described above. Therefore, for example, it is detected depending on whether the slice site S is the head or the abdomen of the human body. FI
The S / N ratio of the D signal is greatly different. Therefore,
In imaging the head, the S / N is worse than in the case of the abdomen, and the image quality of the obtained image information is deteriorated.

It is an object of the present invention to obtain magnetic information without deteriorating the image quality even when the size of the measurement target region for imaging the subject is changed to a plurality of types. It is to provide a resonance imaging apparatus.

[Constitution of Invention]

[0007]

In order to solve the above-mentioned problems, a magnetic resonance imaging apparatus according to the present invention applies a high frequency pulse to a subject and obtains image information based on a magnetic resonance signal generated from the subject. the magnetic resonance imaging apparatus for obtaining a reception coil for detecting the application and the magnetic resonance signal of the high frequency pulse, a receiver coil arranged in imaging target site near the subject to detect the magnetic resonance signal, the transceiver coil a first receiving means including a connection to the reception system circuit, a transmission unit including a connection to the transmission system circuit to the reception coil, a second receiving means including a receiving circuit connected to said receiving coil, before Symbol And a control means for controlling the first receiving means to be out of synchronization when the magnetic resonance signal is collected by the second receiving means.

[0008]

When the magnetic resonance signals are collected by the second receiving means, the first receiving means is controlled so as to be detuned, so that good image quality can always be obtained even in various imaging methods.

[0009]

First, the principle of the first embodiment of the present invention will be described.

FIG. 3 shows an example of the configuration for obtaining a tomographic image of the abdomen of the subject P, which is a human body. A subject P is set in the pair of transmission / reception coils 4A and 4B.
The transmission / reception coils 4A and 4B have a size substantially corresponding to the size of the abdomen. When imaging the abdomen, the transmission / reception coils 4A and 4B are used for both transmission and reception, and therefore it is not necessary to separately provide a reception coil.

FIG. 4 shows a configuration example for obtaining a tomographic image of the head of the subject P. A pair of transmitting coils 4A ', 4
The head of the subject P is set inside B ′ and the pair of receiving coils 5A and 5B. Transmitting coils 4A ', 4B'
Since it suffices that the size is equal to or larger than the size corresponding to the head, the transmission / reception coils 4A and 4B can be used, and the transmission / reception coil can be used only for transmission. Receiver coils 5A, 5B
Has a size substantially corresponding to the size of the head of the subject P.

As shown in FIG. 4, transmitting / receiving coils 4A, 4
B and the receiving coils 5A and 5B are arranged to form a probe head. Here, by mechanically arranging the transmitting / receiving coils 4A, 4B and the receiving coils 5A, 5B at right angles as shown in the figure, mutual electromagnetic coupling can be minimized. When the part to be measured is the abdomen, the transmitting and receiving coils 4A, 4
B is used for both transmission and reception. When the measurement site is the head, the transmission / reception coils 4A and 4B are used only for transmission, and the reception coil 5
A and 5B are used for reception. By controlling in this way, both the head and the abdomen can be imaged with good S / N. This is the principle of the first embodiment.

The detailed circuit configuration of the first embodiment of the present invention based on such a principle is shown in FIGS. In this case, the transmission / reception coil 4 and the reception coil 5 are two pairs of coils 4A, 4B and 5A, 5 arranged as shown in FIG.
B respectively (each pair of coils 4A and 4A
B, or 5A and 5B are connected in series or in parallel, and function equivalent to a single coil in terms of electronic circuit).

FIG. 5 shows a circuit configuration of the transmission / reception coil 4 including the coils 4A and 4B having the arrangements shown in FIGS. 3 and 4. In FIG. 5, a tuning capacitor 6 composed of a variable capacitor is connected in series to the transmission / reception coil 4. The series circuit of the transmission / reception coil 4 and the tuning capacitor 6 is an anti-parallel diode (also referred to as a "crossover diode") 7 composed of a pair of diodes anti-parallel connected at the end of the tuning capacitor 6 side.
Is connected to the output end of the transmission system circuit 8 via, and the end of the transmission / reception coil 4 side is grounded. Further, the connection point between the tuning capacitor 6 and the anti-parallel diode 7 is grounded via the series circuit of the anti-parallel diode 9 composed of a pair of diodes that are also anti-parallel connected to the coil 25. A tuning capacitor 10 made of a capacitor is connected in parallel. This tuning capacitor 10
The non-grounded side end portion of, that is, the connection point of the coil 25 and the antiparallel diode 9 is connected to the reception system circuit 12 via the preamplifier 11.

Further, FIG. 6 shows a circuit configuration of the receiving coil 5 including the coils 5A and 5B arranged orthogonally to the transmitting and receiving coils 4 (4A and 4B) as shown in FIGS. It is a thing.

In FIG. 6, a receiving coil 5, an anti-parallel diode 13 composed of a pair of diodes connected in anti-parallel to each other, and a tuning capacitor 1 composed of a variable capacitor.
And 4 form a parallel circuit whose one end is grounded. The end of the parallel circuit on the non-grounded side is connected to the reception system circuit 12 via the preamplifier 15.

Next, the operation of this structure will be described.

First, the case of obtaining a tomographic image of the abdomen of the subject P will be described. In this case, the specific atomic nucleus for which a tomographic image is to be obtained is, for example, ¹ H. High frequency pulse voltage for exciting the specific atomic nuclei ¹ H spin system applied is <br/> are in transmitting and receiving coil unit 4 through the antiparallel diode 7 from the transmission system circuit 8. At this time, the anti-parallel diodes 7 and 9 are in a low impedance state because the transmission high frequency pulse has a large amplitude, so that one end of the coil 25 is grounded, and a tank circuit including a series circuit of the tuning capacitor 6 and the transmission / reception coil 4 is formed. And the coil 25 are connected in parallel. Since the tank circuit has a low impedance because it is tuned to the frequency of the input high frequency pulse, and the impedance is much lower than the impedance of the coil 25, most of the high frequency current flows through the tank circuit and spins from the transmitting / receiving coil 4. A high frequency magnetic field is generated that excites the system.

When receiving the MR signal of the spin system excited in this way, since the received signal is weak, the antiparallel diodes 7 and 9 are in a high impedance state and the coil 25
The tuning capacitor 10 functions as a tuning circuit, and the transmission / reception coil 4 constitutes a tuning circuit together with the tuning capacitor 6 as a reception coil. Therefore, the MR signal received and detected by the transmission / reception coil 4 is amplified by the preamplifier 11 via the above two tuning circuits and input to the reception system circuit 12.

Next, the case of obtaining a tomographic image of the head of the subject P will be described.

In this case, in order to improve the S / N ratio of the MR signal, the receiving coil 4 dedicated to the head is used. Therefore, during reception, the tuning capacitor 10 on the reception side of the transmission / reception coil 4 is adjusted to shift the tuning of the tuning circuit to reduce electromagnetic coupling with the reception coil 5. At this time, since the transmission / reception coils 4 or 4A, 4B and the reception coils 5 or 5A, 5B are mechanically orthogonally arranged as described above, mutual electromagnetic coupling is further reduced. The receiving coil 5 constitutes a tuning circuit together with the tuning capacitor 14, and the MR signal detected by the receiving coil 5 is input to the receiving system circuit 12 amplified by the preamplifier 15.

When the abdomen is imaged as described above, if the tuning frequency of the receiving coil 5 is adjusted by adjusting the tuning capacitor 14 on the receiving coil 5 side during reception, the receiving coil 5 side not used for reception by the transmitting and receiving coil 4 is detected. The influence of circuits and the like becomes even less likely to occur.

In this way, the receiving coils 4 and 5 suitable for both the abdomen and the head can be selectively used, MR signals with good S / N can be collected, and high-quality image information can be obtained. To be Further, at this time, the two receiving coils 4 and 5 (one of which is also used for transmitting and receiving) are arranged so that their mechanical arrangements are orthogonal to each other, and the tuning circuit on the side not used for reception is adjusted to shift the tuning so that the receiving system on the non-use side. Since the adverse effect on the receiving system by the is minimized, the M / N ratio is further improved.
R signal detection can be performed.

The present invention is not limited to the embodiments described above and shown in the drawings, and various modifications can be made without departing from the scope of the invention.

The second embodiment of the present invention will be described below.

7 to 12 show examples of the configuration of the second embodiment of the present invention. The magnetic resonance imaging apparatus according to the second embodiment includes a pair of transmission coils 16A, 1
6B, multiple pairs of receiving coils 17A, 17B and 18
Equipped with A and 18B. The receiving coils 17A, 17B are used for the abdomen, and the receiving coils 18A, 1B
8B is used for the head.

In the above structure, only the transmission coils 16A and 16B are used when transmitting a high frequency pulse. Subject P
When capturing a tomographic image of the abdomen of the receiving coil 17A, 1
Only 7B is used. When capturing a tomographic image of the head of the subject P, only the receiving coils 18A and 18B are used. By controlling in this way, both the head and the abdomen can be imaged under optimum conditions.

Here, the transmission coil and the reception coil are a pair of transmission coils 16A and 16B as shown in FIG.
The abdominal receiving coils 17A and 17B are placed inside the abdomen, and as shown in FIG. 8, the abdominal receiving coils 17A and 17B are provided.
The head receiving coils 18A and 18B can be arranged inside. FIG. 9 is a cross-sectional view of an arrangement example of the coils shown in FIG. At this time, as shown in FIG. 9, the abdominal receiving coils 17A and 17B and the head receiving coil 18
By mechanically arranging A and 18B orthogonal to the transmission coils 16A and 16B, the electromagnetic coupling between the transmission coil and each reception coil can be minimized.

FIG. 10 shows a transmitter coil 16 (16A, 16A).
The circuit structure of the transmission means connected to B) is shown. The tuning circuit formed of the series circuit of the transmission coil 16 and the tuning capacitor 19 has one end, that is, the coil 16 side, grounded,
A high frequency pulse voltage is applied to the non-grounded end from the transmission system circuit 8 via an antiparallel diode (cross diode) 20, and a high frequency pulse magnetic field for excitation is generated from the transmission coil 16.

FIG. 11 shows the receiving coil 17 (17A, 17A).
The circuit structure of the receiving means connected to B) is shown. Here, the circuit configuration is similar to that shown in FIG.

FIG. 12 shows a receiver coil 18 (18A, 18A).
The circuit structure of the receiving means connected to B) is shown. Here, the circuit configuration is similar to that shown in FIG. 6 or 11.

When receiving a tomographic image of the abdomen, the anti-parallel diode 21 has a high impedance and is composed of a parallel circuit of the abdominal receiving coil 17 and the tuning capacitor 22 connected in parallel with this (one end is grounded). The MR signal is detected in the receiving coil 17 by the tuning circuit, which is derived from the non-grounded end of the tuning circuit and is fed to the preamplifier 11
The signal is amplified by and input to the receiving circuit 12.

When receiving a tomographic image of the head,
The anti-parallel diode 23 has a high impedance, and an MR signal is detected by the receiving coil 18 by a tuning circuit composed of a parallel circuit of the receiving coil 18 for the abdomen and the tuning capacitor 24 connected in parallel to the receiving coil 18 (one end is grounded). This is derived from the non-grounded end of the tuning circuit, amplified by the preamplifier 15 and input to the reception system circuit 12. At this time, in order to minimize electromagnetic coupling between the receiving coils 17 and 18, tuning of the tuning circuit on the abdominal receiving coil 17 side is shifted by the tuning capacitor 22 during head imaging. Of course, when the tuning capacitor 24 on the head-only receiving coil 18 side is used to shift the tuning during imaging of the abdomen, a better result can be obtained.

In addition, the tuning capacitor for tuning control may be either a variable capacitor such as a so-called variable capacitor which is mechanically controlled or a variable capacitor such as a so-called varicap which is electrically controlled, and the tuning capacitor is adjusted. Alternatively, the operation may be performed manually or automatically in conjunction with the site selection or the coil selection operation.

Although not specifically described above, the receiving coil can be selected by the tuning frequency control of the tuning circuit. The signal system may be switched and selected between the system circuits, or may be performed in the receiving system circuit.

[0036]

According to the present invention, various kinds of photographing according to the photographing target such as each part of the human body (head, abdomen, etc.)
MR with a good S / N across each part even if the form is taken
It is possible to provide a magnetic resonance imaging apparatus that can obtain a signal and can obtain high-quality MR image information.

[Brief description of drawings]

FIG. 1 is a schematic diagram for explaining the configuration of a conventional magnetic resonance imaging apparatus.

FIG. 2 is a schematic diagram for explaining the principle of a conventional magnetic resonance imaging apparatus.

FIG. 3 is a schematic diagram for explaining an arrangement configuration of transmission / reception coils in the first embodiment of the present invention.

FIG. 4 is a schematic diagram for explaining an arrangement configuration of a transmission / reception coil and a reception coil in the first embodiment of the present invention.

FIG. 5 is a circuit diagram showing a circuit configuration of a transmission / reception unit in the first embodiment of the present invention.

FIG. 6 is a circuit diagram showing a circuit configuration of a receiving unit according to the first embodiment of the present invention.

FIG. 7 is a schematic diagram for explaining an arrangement configuration of a transmission coil and a reception coil according to a second embodiment of the present invention.

FIG. 8 is a schematic diagram for explaining an arrangement configuration of a transmission coil and a plurality of reception coils according to a second embodiment of the present invention.

9 is a cross-sectional view of the coil shown in FIG.

FIG. 10 is a circuit diagram showing a circuit configuration of a transmission unit according to a second embodiment of the present invention.

FIG. 11 is a circuit diagram showing a circuit configuration of a receiving unit according to a second embodiment of the present invention.

FIG. 12 is a circuit diagram showing a circuit configuration of a receiving unit according to a second embodiment of the present invention.

[Explanation of symbols]

4 (4A, 4B) ... Transmitting and receiving coils, 5 (5A, 5B),
17 (17A, 17B), 18 (18A, 18B) ... Receiving coil, 6, 10, 14, 19, 22, 21, 21, ...
Tuning capacitor, 7, 9, 13, 20, 21, 23 ... Anti-parallel diode, 8 ... Transmission system circuit, 11 ... Preamplification circuit, 12 ... Reception system circuit, 16 (16A, 16B) ... Transmission coil, 25 ... coil

Claims (1)

(57) [Claims] 1. A magnetic resonance imaging apparatus that applies a high-frequency pulse to a subject and obtains image information based on a magnetic resonance signal generated from the subject, applies the high-frequency pulse and detects the magnetic resonance signal. A transmission / reception coil, a reception coil arranged near the imaging target region of the subject to detect the magnetic resonance signal, a first reception unit connected to the transmission / reception coil and including a reception system circuit, and connected to the transmission / reception coil. and transmitting means including a transmitting circuit, a second receiving means including a receiving circuit connected to said receiving coil, at the time of magnetic resonance signals collected by the previous SL second receiving means removing the tuning of the first receiving means Magnetic resonance imaging apparatus comprising: