JPH0263010B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to magnetic resonance (MR) technology.
Information based on the density distribution of specific nuclear spins in a characteristic cross section of the object is obtained using the so-called computed tomography (CT) phenomenon.
This relates to a magnetic resonance imaging device that produces images as CT images (computed tomograms).

[Technical Background of the Invention] 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 the figure, a very uniform static magnetic field H ₀ along the illustrated Z-axis direction is applied to the subject P, and a linear magnetic field gradient Gz is applied to the static magnetic field H ₀ by a pair of gradient magnetic field coils 1A and 1B. Add. For a static magnetic field H ₀ , a specific atomic nucleus has an angular frequency ωo expressed by the following equation
It resonates with me.

ωo=γH ₀ ...(1) In this equation (1), γ is the gyromagnetic ratio, which is specific to the type of atomic nucleus. Therefore, a rotating magnetic field _H1 having an angular frequency ωo that causes only specific atomic nuclei to resonate is applied to the subject P via the pair of transmitting coils 2A and 2B. In this way, the linear magnetic field gradient Gz selectively acts only on the illustrated xy plane portion selectively set in the Z-axis direction, and a specific slice portion S (a planar portion) from which a tomographic image is obtained. In reality, the MR phenomenon occurs only when the material has a certain thickness. This MR phenomenon is transmitted via a pair of receiving coils 3A and 3B to a free induction decay (FID) signal (hereinafter referred to as "FID").
By Fourier transforming this signal, a single spectrum for the rotational frequency of a specific nuclear spin can be obtained. In order to obtain a tomographic image as a CT image, projection images of the slice portion S in multiple directions within the xy plane are required. Therefore, after exciting the slice portion S to cause the MR phenomenon, the magnetic field H ₀ has a linear slope in the x'-axis direction (coordinate system rotated by an angle θ from the x-axis) as shown in Figure 2. linear magnetic field gradient
When Gxy is applied (by a coil, etc. not shown), the isomagnetic field line E in the sliced portion S of the subject P becomes a straight line, and the rotation frequency of a specific nuclear spin on this isomagnetic field line E is calculated by the above equation (1). It is expressed as Here, for convenience of explanation, the isomagnetic field lines E are assumed to be E ₁ to En, and it is assumed that the magnetic fields on these isomagnetic field lines E ₁ to En generate signals D ₁ to Dn, which are a type of FID signal, respectively. The amplitudes of the signals D ₁ to Dn are proportional to the spin densities of specific atomic nuclei on the isomagnetic field lines E ₁ to En passing through the slice portion S, respectively. However,
The FID signal that is actually observed is a composite FID signal FID that is the sum of all the signals D ₁ to Dn. Therefore, by Fourier transforming this composite FID signal FID, the projection information (1
Dimensional image) Obtain PD. This x' axis is rotated in the x-y plane (this rotation of the magnetic field gradient Gxy creates a magnetic field gradient Gxy as a composite magnetic field of the magnetic field gradients Gx and Gy in the x and y directions by two pairs of gradient magnetic field coils, for example, By changing the composite ratio of the magnetic field gradients Gx and Gy, projection information in each direction in the x-y plane can be obtained in the same way as described above, and based on this information, CT Images can be combined.

By the way, in this type of magnetic resonance imaging apparatus, the S/N (signal-to-noise ratio) of the composite FID signal FID obtained for the sliced site S of the subject P is
is the receiving coil 3A of the slice site S of the subject P,
It is proportional to the proportion occupied in the loop of 3B, that is, the filling rate. However, in the conventional diagnostic magnetic resonance imaging apparatus, only one pair of receiving coils 3A and 3B is used as described above, and therefore, for example, whether the slice region S is the head or the abdomen of the human body is used. Therefore, the detected FID signal FID S/
N There will be a big difference. Therefore, when imaging the head, the S/N is worse than when imaging the abdomen.
This results in deterioration in the quality of the image information obtained.

[Purpose of the invention]

An object of the present invention is to provide a magnetic resonance imaging apparatus that can obtain image information without deteriorating image quality even when the size of a measurement target region for imaging a subject changes to multiple types. Our goal is to provide the following.

[Summary of the invention]

The present invention is characterized in that a plurality of receiving coils of different sizes are provided as receiving coils, and these are selectively used depending on the size of the part to be measured.

[Embodiments of the invention]

First, the principle of an embodiment of the present invention will be explained.

FIG. 3 shows a configuration for obtaining a tomographic image of the abdomen of a subject P, which in this case is a human body. This shows the state where is set. In this case, since the receiving coils 4A and 4B are used not only for transmission but also for transmitting and receiving, there is no need to provide a separate receiving coil.

Furthermore, FIG. 4 shows a configuration for obtaining a tomographic image of the head of the subject P, in which a pair of transmitting coils 4 having a size larger than that corresponding to the head are used.
This figure shows a state in which the head of the subject P is set inside a pair of receiving coils 5A and 5B whose size approximately corresponds to the size of A'4B' and the head. In this case, the transmitting and receiving coils 4A' and 4B' can be used as long as they are sufficiently large compared to the size of the head, and the transmitting and receiving coils 4A and 4B can be used only for transmission. Bye. In order to minimize mutual electromagnetic coupling, it is desirable that the transmitting/receiving coils 4A, 4B and the receiving coils 5A, 5B are mechanically orthogonally arranged as shown in the figure.

Therefore, the transmitting/receiving coils 4A, 4B and the receiving coils 5A, 5B are arranged orthogonally as shown in FIG. 4 to constitute a probe head, and when the measurement target area is the abdomen, the transmitting/receiving coils 4A, 4b are arranged for both transmitting and receiving. When the measurement target part is the head, the transmitting and receiving coils 4A and 4B are used only for transmission, and the receiving coils 5A and 4B are used only for transmission.
If 5B is used for reception, both the head and abdomen can be imaged with good S/N. This is the principle of one embodiment of the present invention.

The structure of an embodiment of the present invention based on such a principle is shown in FIGS. 5 and 6. In this case, the transmitting/receiving coil 4 and the receiving coil 5 respectively represent two pairs of coils 4A, 4B and 5A, 5B arranged orthogonally as shown in FIG. 5B are connected in series (or in parallel) and function in the same way as a single coil in terms of electronic circuitry.) FIG. 4 shows a circuit configuration of a transmitting/receiving coil 4 made up of 4B.

In FIG. 5, a tuning capacitor 6 consisting of a variable capacitor is connected in series to the transmitting/receiving coil 4. The series circuit of the transmitting/receiving coil 4 and the tuning capacitor 6 transmits data via an anti-parallel diode (also called a "crossing diode") 7, which is made up of a pair of diodes connected in anti-parallel at the end on the tuning capacitor 6 side. It is connected to the output end of the system circuit 8, and the end on the transmitting/receiving coil 4 side is grounded.
Further, the connection point between the tuning coil 6 and the anti-parallel diode 7 is grounded through a series circuit of an anti-parallel diode 9, which is made up of a pair of diodes connected in anti-parallel to the coil 25.
is a tuning capacitor 10 consisting of a variable capacitor.
are connected in parallel. This tuning capacitor 1
The non-grounded end of 0, that is, the connection point between the coil 25 and the anti-parallel diode 9 is connected to the reception system circuit 12 via the preamplifier 11.

Further, FIG. 6 shows the circuit configuration of the receiving coil 5, which is composed of coils 5A and 5B arranged orthogonally to the transmitting and receiving coil 4 (4A, 4B) as shown in FIGS. 3 and 4. It is something.

In FIG. 6, a receiving coil 5, an anti-parallel diode B consisting of a pair of diodes connected in anti-parallel to each other, and a tuning capacitor 14 consisting of a variable capacitor constitute a parallel circuit whose one end is grounded. The non-grounded end of this parallel circuit is connected to the receiving circuit 12 via the preamplifier 15.

Next, the operation of such a configuration will be explained.

First, a case will be described in which a tomographic image of the abdomen of the subject P is obtained. In this case, the specific atomic nucleus whose tomographic image is to be obtained is, for example, ¹ H.

A high frequency pulse voltage for exciting the specific atomic nucleus ¹ H spin system is applied from the transmission system circuit 8 to the transmission/reception coil section 4 via the reverse column diode 7. At this time, the anti-parallel diodes 7 and 9 are in a low impedance state because the transmitted high-frequency pulse has a large amplitude, so one end of the coil 25 is grounded, and a tank circuit consisting of a series circuit of the tuning capacitor 6 and the transmitting/receiving coil 4 is connected. and the coil 25 are connected in parallel. Since the tank circuit is tuned to the frequency of the input high-frequency pulse, it has a low impedance, which is much lower than the impedance of the coil 25, so most of the high-frequency current flows through the tank circuit.
A high frequency magnetic field that excites the spin system is generated from the transmitter/receiver coil 4 .

When receiving the spin-based MR signal excited in this way, the received signal is weak, so the anti-parallel diodes 7 and 9 enter a high impedance state.
The coil 25 and the tuning capacitor 10 function as a tuning circuit, and the transmitting/receiving coil 4 forms a tuning circuit with the tuning capacitor 6 as a receiving coil. Therefore, the MR signal received and detected by the transmitting/receiving coil 4 is amplified by the preamplifier 11 via the two tuning circuits mentioned above, and is input to the receiving system circuit 12.

Next, a case will be described in which a tomographic image of the head of the subject P is obtained.

In this case, a receiving coil 5 dedicated to the head is used to improve the S/N ratio of the MR signal, but a transmitting/receiving coil 4 is used to excite the spin system of the slice section S. Therefore, when receiving, the transmitting/receiving coil 4
The tuning capacitor 10 on the receiving side of the receiver is adjusted to shift the tuning of the tuning circuit to reduce the electromagnetic coupling with the receiving coil 5. At this time, the transmitting/receiving coil 4, that is, 4A, 4B, and the receiving coil 5, that is, 5A, 5B.
As mentioned above, since they are mechanically orthogonally arranged, mutual electromagnetic coupling is further reduced. The above-mentioned receiving coil 5 constitutes a tuning circuit together with a tuning capacitor 14, and the signal detected by the receiving coil 5
The MR signal is input to the receiving circuit 12 where it is amplified by the preamplifier 15.

In addition, when imaging the abdomen described above, if the tuning capacitor 14 on the receiving coil 5 side is adjusted during reception to shift the tuning frequency, the circuits on the receiving coil 5 side that are not used for reception by the transmitting/receiving coil 4 can be Effects are less likely to occur.

In this way, receiving coils 4 and 5 suitable for the respective sizes of the abdomen and head can be selectively used, and MR signal collection with good S/N can be performed and high-quality image information can be obtained. . Also at this time 2
The two receiving coils 4 and 5 (one is used for both transmitting and receiving) are arranged so that their mechanical arrangement is 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 side not used is connected to the receiving system. Since the adverse effects are minimized, MR signal detection with even better S/N ratio can be performed.

It should be noted that the present invention is not limited to the embodiments described above and shown in the drawings, but can be implemented with various modifications without changing the gist thereof.

For example, FIGS. 7 to 12 show configurations of other embodiments of the present invention.

FIG. 7 shows the arrangement of transmitting and receiving coils for obtaining tomographic images of the abdomen. Transmitting coils 16A, 16B and receiving coils 17A, 17B, both of which are sized to correspond to the abdomen, are arranged orthogonally to each other. Electromagnetic coupling is minimized.

Fig. 8 shows the arrangement of the transmitting and receiving coils for obtaining a tomographic image of the head, and Fig. 9 shows a schematic cross-section of the head-dedicated receiving coils 18A and 18B, which are sized to correspond to the head. As shown in the top view, it is arranged inside the abdominal receiving coils 17A and 17B described above. At this time, the head dedicated receiving coil 18A,
Similar to the receiving coils 17A and 17B for the abdomen, the coils 18B are mechanically orthogonally arranged to minimize electromagnetic coupling with the transmitting coils 16A and 16B (commonly used for the abdomen and the head).

Such a transmitting coil 16 (16A, 16B)
Receiving coils 17 (17A, 17B) and 18
(18A, 18B) The circuit configurations for each are shown in FIGS. 10, 11, and 12.

As shown in FIG. 10, the tuning circuit consisting of a series circuit of a transmitting coil 16 and a tuning capacitor 19 has one end, that is, the coil 16 side, grounded, and an anti-parallel diode (crossing diode) connected to the non-grounded end from the transmitting system circuit 8. ) 20, and a high-frequency pulsed magnetic field for excitation is generated from the transmitting coil 16.

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

Furthermore, during reception when obtaining a tomographic image of the head, the anti-parallel diode 23 has a high impedance, and the head-dedicated receiving coil 18 connected in parallel with this diode 23 has a high impedance.
The receiving coil 1 is connected to the receiving coil 1 by a tuning circuit (one end of which is grounded) consisting of a parallel circuit of a tuning capacitor 24 and a tuning capacitor 24.
At 8, an MR signal is detected, which is derived from the non-grounded end of the tuning circuit, amplified by the preamplifier 15, and input to the receiving circuit 12. At this time, in order to minimize the electromagnetic coupling between the receiving coils 17 and 18, the tuning of the tuning circuit on the abdominal receiving coil 17 side is shifted by the tuning capacitor 22 when imaging the head.

Of course, when imaging the abdomen, the receiver coil 1 dedicated to the head is
Even better results can be obtained by shifting the tuning using the tuning capacitor 24 on the 8th side.

In addition, the tuning capacitor for tuning control may be either a mechanically controlled variable capacitor such as a so-called variable capacitor or an electrically controlled variable capacitor such as a so-called variable cap, and the tuning capacitor can also be adjusted manually. Even if it is performed, it may be automatically performed in conjunction with the region selection or coil selection operation.

In addition, although it was not explained in detail above, the selection of the receiving coil is also possible by the tuning frequency control of the above-mentioned tuning circuit. The signal system may be selected by switching between the circuits, or the signal system may be selected within the receiving system circuit.

〔Effect of the invention〕

According to the present invention, for example, various parts of the human body (head,
Even if the size of the measurement target area changes to multiple types, such as the abdomen (abdomen, etc.), a good S/N ratio can be achieved across each area.
It is possible to provide a magnetic resonance imaging apparatus that can obtain MR signals and obtain high quality MR image information.

[Brief explanation of drawings]

FIGS. 1 and 2 are schematic diagrams for explaining the principle configuration of a conventional magnetic resonance imaging apparatus, and FIGS. 3 and 4 are schematic diagrams for explaining the arrangement of transmitting and receiving coils in an embodiment of the present invention. Schematic diagrams, FIGS. 5 and 6 are schematic circuit diagrams showing the circuit configuration of the transmitter/receiver section in the same embodiment, and FIGS.
FIG. 12 is a diagram for explaining another embodiment of the present invention. 4 (4A, 4B)... Transmitting/receiving coil, 5 (5
A, 5B), 17 (17A, 17B), 18 (18
A, 18B)... Receiving coil, 6, 10, 14,
19, 22, 24... Tuning capacitor, 7, 9,
13, 20, 21, 23... anti-parallel diode,
8... Transmission system circuit, 11... Preamplifier, 12...
...Reception system circuit, 16 (16A, 16B)...Transmission coil, 25...Coil.

Claims (1)

[Claims] 1. Projection information in multiple directions of a specific atomic nuclear spin density on a specific cross section of an object is obtained by detecting a magnetic resonance signal, and image reconstruction processing using this projection information is performed to obtain the above-mentioned information on the specific cross section. A magnetic resonance imaging apparatus that obtains image information based on the density distribution of nuclear spins includes a probe head having a transmitter coil and a receiver coil arranged in an orthogonal relationship to each other;
A transmitting means consisting of a tuning capacitor, a crossing diode, and a transmitting system circuit connected in series, and a receiving means consisting of a crossing diode, a tuning capacitor, and a preamplifier and a receiving system circuit connected in series to the receiving coil. and a control means for controlling the transmitting means to detun the receiving means when the transmitting means is in operation, and controlling the receiving means to detune the transmitting means when the receiving means is in operation. 2 By detecting magnetic resonance signals, projection information about the specific nuclear spin density in multiple directions on a specific cross section of the object is obtained, and through image reconstruction processing using this projection information, the density distribution of the specific nuclear spin in the relevant cross section is determined. A magnetic resonance imaging apparatus that obtains image information based on a probe head includes a probe head having a transmitting coil and a plurality of receiving coils of different sizes arranged so as to be orthogonal to each other, and a tuning capacitor connected in series to the transmitting coil. , a transmitting means consisting of a crossed diode and a transmission system circuit, a plurality of crossed diodes connected in parallel to each of the plurality of reception coils, a plurality of tuning capacitors, a plurality of preamplifiers connected in series, and a plurality of reception systems. a plurality of receiving means comprising a circuit; and the transmitting means, when in operation, all of the plurality of receiving means are detuned, and one of the plurality of receiving means, when in operation, untunes the other receiving means and the transmitting means. 1. A magnetic resonance imaging apparatus comprising: control means for controlling the removal of the magnetic resonance imaging apparatus.