JP3492009B2 - MRI coil device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention is applied to MRI (Magnetic R
The present invention relates to a coil device for esonance imaging, and more particularly, to a coil device for MRI capable of improving both RF (Radio Frequency) shield performance and reducing eddy current.

[0002]

2. Description of the Related Art FIG. 7 is a cross-sectional view of essential parts showing an example of a conventional MRI coil device. This MRI coil device 6
00 is a birdcage type RF for photographing the head, for example.
The coil 1, a gradient coil 2 provided on the outer side of the coil 1, and an RF shield 603 formed by adhering copper foil to the inner surface of the gradient coil 2 without interruption are provided.

FIG. 8 is a cross-sectional view of essential parts showing another example of a conventional MRI coil device. This MRI coil device 70
0 is the RF coil 1, the gradient coil 2 provided outside the RF coil 1, and the copper foils 31 to 38 with eight cuts 51 to 58 on the inner surface of the gradient coil 2 for reducing eddy currents.
RF shield 103 formed by adhering and copper foils 31 to 3
8 to connect capacitors 741 to 748.

[0004]

In the above conventional MRI coil device 600, since the continuous copper foil is used as the RF shield 603, the RF shield performance is excellent. However, since the eddy current due to the gradient magnetic field easily flows, there is a problem that the response of the gradient magnetic field is lowered (slew rate is lowered).

On the other hand, the conventional MRI coil device 70 described above.
In No. 0, since the divided copper foil is used as the RF shield 103, the eddy current due to the gradient magnetic field can be reduced. However, since the divided copper foils are connected to each other in an RF manner by capacitors, the RF shield performance can be obtained to some extent, but the RF shield 60 uses a continuous copper foil.
Compared with No. 3, there is a problem that the RF shield performance is inferior. Especially, MRI of low magnetic field (0.5T or less) with low resonance frequency
In the device, if the capacitance of the capacitor is not increased, R
There is a problem that the F shield performance becomes insufficient. That is, the conventional MRI coil device has a problem that it is difficult to achieve both the improvement of the RF shield performance and the reduction of the eddy current. Therefore, an object of the present invention is to improve the RF shield performance and reduce the eddy current at the same time.
It is to provide a coil device for RI.

[0006]

According to the present invention, an RF coil, a gradient magnetic field coil provided outside the RF coil, and a coupling between the RF coil and the gradient magnetic field coil are provided. In an MRI coil device provided with an RF shield for preventing the RF shield, the RF shield is divided into two or more divided portions, and a switch means for connecting or disconnecting the divided portions and at the time of RF transmission and reception of an NMR signal. There is provided a coil device for MRI, comprising: a switch control means for connecting the divided portions and for controlling the switch means so as to cut off the divided portions when the gradient magnetic field changes. MR with the above configuration
In the coil device for I, the switch means is, for example, a diode switch. Further, the bias current of the diode switch is configured to flow in opposite directions in the adjacent divided portions. Note that capacitors may be mixed up to about 50% of the connection points of the divided portions.

Further, according to the present invention, the RF shield has a divided structure, and a switch means is provided between each divided portion, and the switch means is actively controlled by the switch control means to perform RF transmission and NMR signal reception. There is provided an RF shield and an eddy current reduction method in MRI, which is characterized in that each divided portion is connected when the gradient magnetic field changes and each divided portion is cut off when the gradient magnetic field changes.

[0008]

In the MRI coil device according to the present invention, the RF shield has a divided structure, the switch means is provided between the divided portions, and the switch means is actively controlled by the switch control means to perform RF transmission and Each divided portion is connected when the NMR signal is received, and each divided portion is cut off when the gradient magnetic field changes. Since the divided parts are connected by the switch means at the time of RF transmission and the reception of the NMR signal, the RF shield performance can be improved as compared with the conventional technique in which the divided parts are connected by the capacitor. Further, when the gradient magnetic field changes, each of the divided portions is cut off by the switch means, so that the divided portions are more reliably separated than in the conventional technique in which the divided portions are connected by the capacitor, and the eddy current can be reduced. That is, it is possible to improve the RF shield performance and reduce the eddy current at the same time.

It is preferable to use a diode switch as the switching means because high speed switching can be easily performed. In addition, since the bias currents of the diode switches flow in opposite directions in the adjacent divided portions, the magnetic fields due to the bias currents cancel each other out, and the influence of the magnetic field due to the bias currents on MRI is reduced.
You can do it.

[0010]

The present invention will be described in more detail with reference to the embodiments shown in the drawings. The present invention is not limited to this.

First Embodiment FIG. 1 shows a simplified perspective view of an MRI apparatus. The MRI apparatus M includes an MRI coil apparatus 100 including an RF coil and a gradient coil. FIG. 2 is a structural explanatory view showing an MRI coil device of the first embodiment of the present invention. In this MRI coil device 100, for example, a birdcage type RF coil 1 for imaging a head or the like, a gradient coil 2 provided on the outside thereof, and a coupling coil on the inner side surface of the gradient coil 2 in a direction in which coupling is reduced. Break 51-
RF formed by adhering eight copper foils 31 to 38 with 58 open
Shield 103 and each copper foil 31, 32, 33, 34, 3
Diodes 41, 42, which connect 5 in this order in the forward direction
43, 44 and diodes 48, 47, 4 for connecting the copper foils 31, 38, 37, 36, 35 in this order in the forward direction.
6 and 45, and a switch control circuit 6 which is connected to the copper foils 31 and 35 by a blocking circuit 71, 72 in a direct current manner and is separated in an RF manner. The switch control circuit 6 includes a DC power supply 60 and polarity changeover switches 61 and 62 that are switched by a changeover control signal S from a control unit (not shown) of the MRI apparatus.

As shown in FIG. 3, a plurality of diodes 41 are provided in parallel. The parallel pitch of these diodes 41 is sufficiently smaller than the wavelength of RF.
For example, when the RF frequency is 21 MHz (wavelength 14.3 m), it is about 10 cm. Other diodes 42-48
Is also the same.

FIG. 4 is an explanatory diagram showing the MRI pulse sequence, the switching control signal S, and the polarity of the DC voltage applied to the copper foils 31, 35. The pulse sequence for MRI is a pulse sequence of SE (Spin Echo) method here.

First, in the section P1 before the rise of the slice selection gradient magnetic field Gs, neither RF shield nor eddy current prevention is necessary, so the copper foils 31 to 38 may be connected or cut off. That is, the polarity of the DC voltage applied to the copper foils 31 and 35 is arbitrary (the polarity x in the figure represents arbitrary). Therefore, the switching control signal S may be a "forward bias" command or a "reverse bias" command. In FIG. 4, the command is "reverse bias".

Next, in the rising section A of the slice selective gradient magnetic field Gs, the RF shield is not necessary, but eddy current prevention is necessary, so the copper foils 31 to 38 are cut off. That is, a DC voltage is applied with a polarity in which the copper foil 31 becomes negative and the copper foil 35 becomes positive, and the diodes 41 to 48 are reversely biased. Therefore, the switching control signal S becomes a "reverse bias" command.

Next, in the 90 ° pulse transmission section B, RF
Since the shield is required, the copper foils 31 to 38 are connected. That is, the copper foil 31 becomes positive and the copper foil 35
DC voltage is applied with a negative polarity,
Forward bias 48. Therefore, the switching control signal S becomes a "forward bias" command.

Next, in the trailing edge of the slice selection gradient magnetic field Gs and the application section C of the encode gradient magnetic field Ge, an RF shield is not necessary, but eddy currents must be prevented, so the copper foils 31 to 38 are cut off. . That is, the copper foil 31
Becomes negative and the copper foil 35 becomes positive, and a DC voltage is applied to reverse bias the diodes 41 to 48. Therefore, the switching control signal S becomes a "reverse bias" command.

In the next section P2, since neither RF shield nor eddy current prevention is necessary, the copper foils 31 to 38 may be connected or cut off. That is, the copper foils 31, 35
The polarity of the DC voltage applied to is arbitrary. Therefore, the switching control signal S may be a "forward bias" command or a "reverse bias" command. Incidentally, in FIG. 3, the command is "reverse bias".

Next, in the rising section D of the slice selective gradient magnetic field Gs, the RF shield is not necessary, but the eddy current is required to be prevented, so the copper foils 31 to 38 are cut off. That is, a DC voltage is applied with a polarity in which the copper foil 31 becomes negative and the copper foil 35 becomes positive, and the diodes 41 to 48 are reversely biased. Therefore, the switching control signal S becomes a "reverse bias" command.

Next, in the 180 ° pulse transmission section E, R
Since the F shield is required, the copper foils 31 to 38 are connected. That is, the copper foil 31 becomes positive and the copper foil 3
DC voltage is applied with the polarity that 5 becomes negative, and diode 41
Forward bias ~ 48. Therefore, the switching control signal S
Is a "forward bias" command.

Next, in the falling section F of the slice selective gradient magnetic field Gs, the RF shield is not necessary, but the eddy current is required to be prevented, so the copper foils 31 to 38 are cut off. That is, a DC voltage is applied with a polarity in which the copper foil 31 becomes negative and the copper foil 35 becomes positive, and the diodes 41 to 48 are reversely biased. Therefore, the switching control signal S becomes a "reverse bias" command.

Next, in the read gradient magnetic field application section G,
Since the RF shield is required, the copper foils 31 to 38 are connected. That is, a DC voltage is applied with a polarity in which the copper foil 31 becomes positive and the copper foil 35 becomes negative, and the diode 4
Forward bias 1 to 48. Therefore, the switching control signal S becomes a "forward bias" command. After the application of the read gradient magnetic field, the process returns to the pre-rise section P1 of the slice selection gradient magnetic field Gs.

According to the above-described MRI coil device 100, since the copper foils 31 to 38 are connected by turning on the diodes 41 to 48 at the time of RF transmission and reception of an NMR signal, it is more RF than the conventional technique of connecting with a capacitor. Shielding performance can be improved, and RF can be sufficiently shielded even in a low magnetic field MRI apparatus having a low resonance frequency. Further, when the gradient magnetic field changes, the copper foils 31 to 38 are cut off by turning off the diodes 41 to 48, so that the eddy current can be reduced.

Instead of the diodes 41 to 48, FETs, photocouplers, etc. may be used.

-Second Embodiment- FIG. 5 is a structural explanatory view showing an MRI coil device of a second embodiment of the present invention. This MRI coil device 200
Is provided in the MRI apparatus M as shown in FIG. 1, and is, for example, a birdcage type RF coil 1 for imaging the head and the like, a gradient coil 2 provided outside thereof, and this gradient coil 2
A cut 51 on the inner surface of the
~ 58 with 8 copper foils 31 to 38 attached
The F shield 103, the capacitor 201 that connects the copper foils 31, 32, and the copper foils 32, 33, 34, 35, 36,
Diodes 42, 43, 44, 45, 46, 47, 48 that connect 37, 38, 31 in this order in the forward direction, and DC circuits are connected to the copper foils 31, 32 by blocking circuits 71, 72. And a switch control circuit 6 separated from each other in terms of RF. The switch control circuit 6
Is a DC power supply 60 and a control unit (not shown) of the MRI apparatus.
Polarity changeover switches 61 and 62 which are switched by the changeover control signal S from. The operation of the MRI coil device 200 is similar to that of the first embodiment.

According to the MRI coil device 200 described above, the capacitor 201 is used to connect the copper foils 31 and 32, but otherwise, the adjacent copper foils are connected by the diodes 42 to 48. An effect similar to that of 1 can be obtained.

-Third Embodiment- FIG. 6 is a structural explanatory view showing an MRI coil device of a third embodiment of the present invention. This MRI coil device 300
Is provided in the MRI apparatus M as shown in FIG.
For example, a birdcage type RF coil 1 for photographing the head and the like
And an RF shield formed by adhering eight copper foils 31 to 38 on the inner side surface of the gradient coil 2 with gaps 51 to 58 in the direction in which the coupling becomes smaller. 103, a diode 41 connecting the copper foils 31 and 32 in this order in the forward direction, a diode 42 connecting the copper foils 33 and 32 in this order in the forward direction, and a copper foil 3
Diode 43 connecting 3, 34 in this order in the forward direction
, A diode 44 connecting the copper foils 35 and 34 in the forward direction in this order, a diode 45 connecting the copper foils 35 and 36 in the forward direction, and the copper foils 37 and 36 in the forward direction. A diode 46 for connecting, a diode 47 for connecting the copper foils 37, 38 in this order in the forward direction, and copper foils 31, 38
And the blocking circuit 3 for the diode 48 which connects in this order in the forward direction and the copper foils 31, 33, 35 and 37.
71, 373, 375, 377 has one end side a connected in a direct current manner and separated in an RF manner, and the copper foil 3
Blocking circuit 37 for 2, 34, 36 and 38
2, 374, 376, and 378, and the other end side b is connected in a direct current and is separated in RF. The switch control circuit 6 includes a DC power source 6
0 and the polarity changeover switches 61 and 62 which are changed over by a changeover control signal S from a control unit (not shown) of the MRI apparatus.
Includes and. The operation of the MRI coil device 300 is similar to that of the first embodiment.

According to the MRI coil device 300 described above, substantially the same effects as those of the above-described first embodiment can be obtained. further,
Since the bias currents flowing through the copper foils 31, 33, 35, 37 and the bias currents flowing through the copper foils 32, 34, 36, 38 are in opposite directions, the magnetic fields due to the bias currents cancel each other out, and the MRI of the magnetic field due to the bias currents occurs. Can be less affected.

[0029]

According to the MRI coil device of the present invention, the RF shield has a divided structure, the switch means is provided between the divided portions, and the switch means is actively controlled by the switch control means, During RF transmission and N
Since each divided portion is connected when the MR signal is received and each divided portion is cut off when the gradient magnetic field changes, the RF shield performance can be improved as compared with the conventional technique in which each divided portion is connected by a capacitor. In addition, the eddy current can be reduced as compared with the conventional technology that uses a continuous copper foil and the conventional technology that connects each divided portion with a capacitor.

[Brief description of drawings]

FIG. 1 is a simplified perspective view of an MRI apparatus.

FIG. 2 is a structural explanatory view of an MRI coil device according to the first embodiment of the present invention.

3 is a development view of the RF shield of FIG. 1. FIG.

FIG. 4 is an explanatory diagram showing the MRI pulse sequence, the switching control signal, and the polarity of the DC voltage applied to the copper foil.

FIG. 5 is a structural explanatory view of an MRI coil device according to a second embodiment of the present invention.

FIG. 6 is a structural explanatory view of an MRI coil device according to a third embodiment of the present invention.

FIG. 7 is a configuration explanatory view showing an example of a conventional MRI coil device.

FIG. 8 is a structural explanatory view showing another example of a conventional MRI coil device.

[Explanation of symbols]

1 RF coil 2 gradient coil 6 switch control circuit 31, 32, ..., 38 Copper foil 41, 42, ..., 48 Diodes 51, 52, ..., 58 breaks 60 DC power supply 61, 62 polarity switch 71, 72, 371 to 378 Blocking circuit 100 MRI coil device 103,603 RF shield 201,741 to 748 capacitors

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) A61B 5/055

Claims (3)

(57) [Claims] 1. An RF coil, a gradient magnetic field coil provided outside the RF coil, and an RF shield interposed between the RF coil and the gradient magnetic field coil to prevent coupling between the RF coil and the gradient magnetic field coil. In an MRI coil device, the RF shield is divided into two or more divided portions, and a switch means for connecting or disconnecting the divided portions, and the divided portions are connected and a gradient at the time of RF transmission and NMR signal reception. An MRI coil device, comprising: a switch control means for controlling the switch means so as to cut off the divided portion when the magnetic field changes. 2. The coil apparatus for MRI according to claim 1, wherein the switch means includes a diode switch. 3. The coil device for MRI according to claim 2, wherein the bias currents of the diode switches flow in opposite directions in the adjacent divided portions.