JPH0542123A - Rf coil for mri - Google Patents

Abstract

PURPOSE:To effectively execute decoupling of a birdcage resonator being an RF coil for an MRI. CONSTITUTION:Diodes D1-D32 are inserted in series to each other to plural pieces of capacity elements C1-C3 for constituting a birdcage resonator, and also, induction elements L1-L32 are connected in parallel to each other to the capacity elements C1-C32, and by a control voltage applied to between terminals E1, E2, and between E1 and E3, these diodes are subjected simultaneously to turnon/off control. In such a way, diode groups placed in each conductive loop in order to open and close plural conductive loops can be batch-controlled without using a complicated private line.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to an RF coil for MRI. In particular, the present invention relates to an RF coil suitable for an MRI apparatus in which a transmission RF coil and a reception RF coil are present close to each other and it is necessary to decouple them.

[0002]

2. Description of the Related Art A birdcage resonator (hereinafter abbreviated as BCR) having 4-pole symmetry as an RF coil for MRI.
Those provided with JP-A-61-95234 and JP-A-60-
It is disclosed in -132547. The coil has a pair of ring elements spaced apart along a common longitudinal axis and a plurality of axially conductive segments electrically interconnecting the ring elements, and the ring elements or the axially conductive segments. The segment has a BCR in which a plurality of capacitive elements are arranged. An example of this coil is shown in the schematic view of FIG. 1, where 20 is a BCR and 24 is a pickup coil. In case of high-pass type BCR, two rings 21, 2
Each of 2 is divided by 16 capacitors to form a ring element. However, the capacitive element is omitted in this figure. Each ring is parallel to the vertical 16
BCRs 20 are formed by connecting together with a conductor (called an axial conductive segment or rung) 23 of a book. However, only one part of the rung is shown in the figure. The power supply is, for example, an inductive coupling method using the pickup coil 24. The diameter of the ring is, for example, 490 mm, and the length of the coil in the vertical direction is 400 mm. The ring and 16 conductors are 4 mm diameter copper pipes. When each of the 32 capacitors is set to 48 pF, the resonance frequency for generating a uniform magnetic field in the coil is 61.22 MHz.

FIG. 2 shows an equivalent circuit of a high-pass type BCR. However, the inductance of the conductor forming the coil is omitted in this figure. In this figure, A1 and B1 and A2 and B2 are connected. It can be seen that one current loop is formed by the capacitors C1 and C17 and the adjacent rung, which in this example has 16 current loops, but in the figure the first to third and 15, 1
Only the sixth current loop is shown (the fourth to fourteenth current loops are omitted). The 16th current loop and the 1st current loop are connected. Power is supplied from the terminals S1 and S2 of the pickup coil 24.
The pickup coil 24 and the BCR 20 are inductively coupled.

Various decoupling techniques are required to utilize such a BCR as a transmitting coil or a receiving coil. As an example of the decoupling technique, there is JP-A-63-175403. However, the conventional decoupling is often applied to the case where the current path of the coil is simple, such as a surface coil or saddle type coil, and is not immediately applicable to a coil having a complicated shape that forms many current loops such as BCR. .. On the other hand, as an example of BCR decoupling, there is JP-A-63-171548. In this example, as shown in FIG. 3, diodes are inserted in series in a plurality of rungs, and these are on / off controlled using a dedicated control line. In this example, since a complicated dedicated line is required to collectively control the diode group, the coil shape is complicated. Further, these electric wires have a problem of disturbing the homogeneity of the magnetic field generated by the coil.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to overcome these drawbacks and provide an RF coil suitable for an MRI apparatus that requires decoupling between a plurality of RF coils.

[0006]

Adjacent to a pair of ring elements, each including a plurality of series-connected capacitive elements spaced apart along a common longitudinal axis, each spaced along its perimeter. At least a plurality of capacitors in an MRI RF coil having a plurality of axial conductive segments electrically interconnecting the ring elements at points between series-connected capacitive elements and having at least one feed point Each element has a diode in series with the element and an inductive element in parallel with the element, and the diode group is controlled to be turned on and off by a voltage externally applied to the RF coil.

[0007]

In the diode group, although the RF coil is divided by a large number of capacitive elements, the voltage signal applied from the outside to the RF coil passes through the inductive element arranged in parallel with the capacitive element. Since it is simultaneously applied to the large number of capacitance elements, the ON / OFF control of the plurality of diodes can be simultaneously performed arbitrarily even if there is no dedicated line for controlling the diodes.

[0008]

EXAMPLE One example of the present invention will be described with reference to FIG.
FIG. 4 is a circuit diagram showing an expanded probe coil of the embodiment. That is, as in the case of FIG. 2, the node A1 and the node B1 are connected, and the node A2 and the node B2 are connected to form a cylindrical resonator as shown in FIG. Note that this point is exactly the same for each of the embodiments of FIGS. 5 to 9.

In the embodiment of FIG. 4, node A1 and node B
1 is connected to the first ring element 21, and the second ring 22 is formed by connecting the nodes A2 and B2.
And 16 are connected by 16 axial conductive segments (rungs) 23 to form a bird cage resonator (BCR). That is, the BCR is formed by combining 16 adjacent current loops formed by two adjacent rungs and the segments of the first and second ring elements. Capacitors C1 to C16 are inserted in the respective circumferential segments that are separated at each connection point between the ring element 21 and the rung 23.
Further, diodes D1 to D16 are connected in series with the respective capacitors.
However, inductive elements L1 to L16 are connected in parallel with each capacitor. Similarly, the ring element 22 has a rung 23.
Capacitors C1 to C16 are inserted in the respective segments divided for each connection point, and diodes D17 to D32 are connected in series with the respective capacitors, and inductive elements L17 to L32 are connected in parallel with the respective capacitors. In addition,
In the figure, among the 16 current loops, the first to third current loops, the 15th and 16th current loops are displayed, and the rest are omitted. All the diodes are aligned in the same direction, that is, from the first current loop to the second, third, ... Sixteenth current loops. An inductive element L33 is connected further downstream of the parallel circuit of the capacitor C16 and the inductive element L16 when viewed from the diode D16 of the sixteenth current loop,
Further, a capacitor C33 is inserted on the downstream side.
Further, as seen from the diode D32, the inductive element L is provided further downstream of the parallel circuit of the capacitor C32 and the inductive element L32.
34 is connected, and a capacitor C34 is inserted further downstream. On the other hand, the inductive element L35 is connected to the upstream side of the diode D1 or D17 (whichever is acceptable) of the first current loop. The diode is preferably a PIN diode, for example. The inductance of the inductive element is BCR
Set it sufficiently high for the resonance frequency of. Also C3
The capacitances of 3 and C34 are made sufficiently larger than those of C1 to C32. As a result, the characteristics of the BCR are not affected by the inductive element and the capacitors C33 and C34.

When all the diodes D1 to D32 are on, the resonator operates at the initial resonant frequency, but when they are off all current loops are cut off by the diodes. The diodes are controlled by inductive elements L33 and L3.
4 and the voltage signal supplied using terminals E1, E2, and E3 connected to L35. E2 and E3 are connected to ground.
If a positive potential is applied to E1, D1 and L1 of the first element
And via D17, L17 to the second element,
Similarly, a forward voltage is applied to the diodes of the third element and all the elements in sequence, and all the diodes are turned on. Therefore, the BCR operates normally. Applying a negative potential to E1 applies a reverse voltage to all diodes, breaking all current loops in the BCR. Therefore, it becomes a decoupling state. More specifically, FIG.
When this coil is used as a coil for transmitting a high frequency magnetic field, a positive potential is applied to the terminal E1 and a high frequency signal is applied to the terminals S1 and S2 of the pickup coil 23 during the transmission period.
In order to prevent the load of the receiving coil provided side by side during the reception period, a negative potential is applied to the terminal E1 to disconnect each element of the BCR, and decoupling between the receiving coil and the transmitting coil along the line is performed. On the contrary, when the coil shown in FIG. 4 is used as the receiving coil, a negative potential is applied to the terminal E1 to disconnect each current loop of the BCR during the transmission period to perform decoupling with the transmitting coil. During the reception period, a positive potential is applied to the terminal E1 and the terminals S1 and S2 of the pickup coil 23 are applied.
Extract the received signal from. In this way, the capacitor C3
3 and C34 are inserted in order to disconnect the ring elements 21 and 22 in a direct current manner and to apply a control voltage to each diode. The control signal is given from one place, and no special signal line is required.

Another embodiment will be described with reference to FIG. In the probe of this embodiment, a series circuit (D1 and L1, D2 and L2, ... D32 and L32) of a diode and an inductive element is provided for each of the capacitors C1 to C32 inserted in each segment of the ring elements 21 and 22. ) Are connected in parallel. Control voltage distribution capacitors C33, C
34 and the inductive elements L33, L34, L35 are connected as shown in FIG.
The example is exactly the same. Here, the circuit consisting of the capacitor C1, the inductive element L1, the diode D1 and other similar circuits form a trap circuit which resonates at the resonance frequency of the BCR when the diode is on. Therefore, in this embodiment, the trap circuit operates when the diode is turned on, and the entire probe is in the decoupling state. When the diode is off, the trap circuit opens and BC
R works normally. That is, the probes are decoupled by connecting the terminals E2 and E3 to the ground and applying a positive potential control signal to the terminal E1. Also, terminal E
The probe is normally operated by applying a negative potential control signal to 1. The present embodiment is also characterized in that all current loops are disconnected during decoupling, a control signal is given from one location, and no special signal line is required. In addition, this method of externally controlling the diode can use a PIN diode, which has excellent on / off characteristics of the diode, as compared with the passive decoupling method in which the diode is not externally controlled, and the S / N of the probe is high.

Still another embodiment will be described with reference to FIG. The probe of this example is similar to the BCR shown in FIG.
A decoupling mechanism is added for using the BCR as a transmitter coil or a transmitter / receiver coil. The difference from FIG. 4 is that the diodes D1 to D8 and D1 are
The direction from 7 to D24 is the opposite. Inductive elements L33 and L34 are respectively connected to the connection points of the first segment and the sixteenth segment and the connection points of the eighth segment and the ninth segment in which the directions of the diodes are inverted, and the other ends E1 and E2 of the respective induction elements are connected. Use as control terminal. No control voltage distribution capacitor is required. When the terminal E2 is connected to the ground and a negative potential control signal is applied to the terminal E1, each diode is turned off and the probe is in the decoupling state. When a positive potential control signal is applied to E1, each diode is turned on and the probe operates normally. In this case E1
The control signal current path given by the above is divided into a total of four directions toward the segments on both sides of the first and second ring elements, and finally reaches E2. As in the present embodiment, it is preferable to determine the positions of E1 and E2 so that the lengths of the current paths divided in four directions are equal. As a result, an equal current flows through all the diodes, and the diodes act equally.
Both the probe-on characteristic and the probe-off characteristic are spatially uniform, which is suitable as an MRI probe.

FIG. 7 shows still another embodiment. Similar to the BCR shown in FIG. 4, the probe of this embodiment is additionally provided with a decoupling mechanism for using the BCR as a transmitting coil or a transmitting / receiving coil. In this embodiment, capacitors C35 to C49 for disconnecting the circuit in a direct current manner are inserted in each rung except one rung 23-1. In the first ring element 21, the diode is inserted in the direction from the rung 23-1 to the rung 23-9, and in the second conductive loop element 22, the diode is inserted in the direction from the rung 23-9 to the rung 23-1. ing. The diode control signal is applied to terminals E1 and E2 which are connected to both ends of the rung 23-9 via inductive elements L33 and L34, respectively. That is, when the terminal E2 is connected to the ground and a positive potential signal is applied to the terminal E1, each diode is turned on and the probe operates normally. When a negative potential signal is applied to the terminal E1, each diode is turned off and enters a decoupling state. It is preferable that the capacitances of the capacitors C35 to C49 inserted in the respective rungs be sufficiently large with respect to the resonance frequency of the BCR so that the BCR resonance characteristics are not substantially affected. In particular, as in the embodiment, it is preferable that the rung serving as a passage for the control signal and the rung to which the control terminal is connected are not symmetrical with respect to the central axis of the BCR, without inserting the capacitor. In this case, the same current flows through all the diodes, and the diodes act uniformly, so that the probe characteristics also become uniform.

FIG. 8 shows still another embodiment. Similar to the BCR shown in FIG. 4, the probe of this embodiment is additionally provided with a decoupling mechanism for using the BCR as a transmitting coil or a transmitting / receiving coil. The difference from FIG. 4 is that a diode for decoupling and an inductive element for transmitting a control signal are arranged in each of 16 current loops, one set each. However, in the first current loop, the diode D17 and the inductive element L17 are connected to the second ring 22.
However, in the second current loop, the diode D2 and the inductive element L2 are provided in the first ring 21, and the decoupling elements are alternately provided between the first and second rings. The control signal terminals E1 and E2 are connected to the rungs 23-9 and 23-1 via the inductive mail elements L34 and L33, respectively, and the direction of each diode is such that it becomes the forward direction when the terminal E1 is at a positive potential. is there. Therefore, when a negative potential control signal is applied to the terminal E1, the probe is in the decoupling state. The feature of this embodiment is that the number of diodes is reduced, and therefore the deterioration of the probe characteristics due to the ON resistance of the diodes is small.

FIG. 9 shows another embodiment. The probe of this embodiment has a diode D33 inserted in the rung 23-9 of the probe shown in FIG. 8, and other configurations are the same as those of FIG. The direction of the diode D33 is the direction in which the diode becomes the forward direction when a positive potential is applied to the terminal E1. The compound current loop passing through C1, C18, C3 ... C31, C16, which is left when the probe of FIG. 8 is in the decoupling state, is cut at D33 in this embodiment. That is, according to this embodiment, all the conductive loops of the resonator, including the complex current paths, can be opened / closed by one set of control signals. The position of the diode D33 may be another rung.

For the decoupling of the pickup coil used for power supply or detection in the above embodiments, a known surface coil decoupling technique can be applied if necessary. Further, in the above embodiments, the power feeding method is the inductive coupling method using the pickup coil, but it goes without saying that the decoupling technique satisfying the gist of the present invention can be applied to the capacitive coupling method.

In the above embodiment, an example in which a decoupling mechanism for mainly using the BCR as a transmitting coil or a transmitting / receiving coil is added has been described.
It is also possible to add a decoupling circuit for the reception-only probe shown in.

Although the number of BCR elements is 16 in the above embodiment, other values such as 8 or 12 may be used.

Further, although the embodiment has been disclosed by taking the high-pass type BCR as an example, the present invention can be applied to the low-pass type BCR, the band-pass type BCR, the linear type and the QD type slotted tube resonator according to their respective characteristics. ..

Next, an example of the MRI probe system will be shown.
With the structure shown in FIG. 4, a BCR having a large diameter (for example, a diameter of 550 mm and a length of 550 mm) is used as a transmitting probe,
A BCR having a small diameter (for example, a diameter of 300 mm and a length of 300 mm) with the structure shown in FIG. 5 is used as a receiving probe. With this configuration, it can be used as an RF probe for cross head imaging. Further, in this system, the RF probe system of this system has high decoupling performance and spatially uniform probe transmission / reception characteristics. If the small-diameter probe is removed and the large-diameter probe is always turned on, the large-diameter probe can be used for transmission and reception and can be used as a whole-body probe. It is also possible to use the other coil described above in this embodiment.

[0021]

As described above, according to the present invention, the diodes arranged in each current loop for opening and closing a plurality of current loops can be collectively controlled without using a complicated dedicated line, and the original characteristics of the RF coil are obtained. The coil can be decoupled when necessary without damaging the above.

[Brief description of drawings]

FIG. 1 is a general schematic diagram of a BCR according to the present invention.

FIG. 2 is an equivalent circuit diagram of a BCR.

FIG. 3 is a circuit diagram showing a known example of a BCR having a decoupling means.

FIG. 4 is a circuit diagram showing an embodiment of the present invention.

FIG. 5 is a circuit diagram of another embodiment of the present invention.

FIG. 6 is a circuit diagram of still another embodiment of the present invention.

FIG. 7 is a circuit diagram of still another embodiment of the present invention.

FIG. 8 is a circuit diagram of still another embodiment of the present invention.

FIG. 9 is a circuit diagram of still another embodiment of the present invention.

[Explanation of symbols]

21, 22 ... Ring element, 23, 23-1, 23-9 ...
Axial conductive segment, 24 ... Pickup coil, C
1-C32 ... Capacitance element, D1-D32 ... Diode, L
1-L32 ... Inductive element, E1-E3 ... Control terminals.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ryokuni Matsunaga 1-280, Higashi Koikekubo, Kokubunji, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd.

Claims (11)

[Claims] 1. A first and a second ring element, each comprising a plurality of circumferential segments connected in a ring shape and spaced apart along a common longitudinal axis, and a plurality of the circumferential segments. A plurality of axial conductive segments for electrically interconnecting the first and second ring elements at the positions of the connection points, the axial conductive segments being adjacent to each other, and two circumferential directions connected by these. In an MRI RF coil having a plurality of current loops each including a segment and having at least one feeding point, a capacitive element is inserted in each of the plurality of circumferential segments, and a diode is connected in series with the capacitive element. An inductive element is connected in parallel with the capacitive element, and each diode in the circumferential segment is collectively turned on and off by a control voltage applied from the outside. MRI is being controlled
RF coil for use. 2. The RF coil for MRI according to claim 1, wherein the inductive element has a sufficiently large inductance with respect to the resonance frequency of the RF coil. 3. The diode is inserted in the same direction on the circumference of the first and second ring elements, and the first and second ring elements are inserted.
2. The RF coil for MRI according to claim 1, wherein said ring element is DC-disconnected at each one position across the terminal for applying the control voltage. 4. The diode is inserted on the circumference of the first and second ring elements in a direction from a connection point of a specific one of the plurality of axial conductive segments toward a connection point of the other. The RF coil for MRI according to claim 1, wherein: 5. A capacitive element for direct-current disconnecting each axial conductive segment is inserted into the remaining axial conductive segments except for a specific one of the plurality of axial conductive segments, and the diode is connected to the diode. Is inserted on the circumference of the first ring element in a direction from the specific one of the axial conductive segments to the other, and on the circumference of the second ring element the first ring. 2. The MRI device according to claim 1, wherein the MRI device is inserted in a direction opposite to that on the circumference of the device.
RF coil for use. 6. A first and a second ring element, each of which is formed by connecting a plurality of circumferential segments in a ring shape and is spaced apart along a common longitudinal axis, and the plurality of circumferential segments. A plurality of axial conductive segments for electrically interconnecting the first and second ring elements at the positions of the connection points, the axial conductive segments being adjacent to each other, and two circumferential directions connected by these. In an MRI RF coil having a plurality of current loops each including a segment and having at least one feeding point, a capacitive element is inserted in each of the plurality of circumferential segments, and each of the plurality of current loops is formed. A diode is connected in series with one of the capacitive elements of each of the two axial segments constituting the element, and an inductive element is connected in parallel with the capacitive element, and the diode is an external element. On collectively by a control voltage which is more applied, R for MRI, characterized in that the OFF control
F coil. 7. The diode and the inductive element are alternately connected to a circumferential segment of the first ring element and a circumferential segment of the second ring element along a circumferential direction. The RF coil for MRI according to claim 6. 8. A first and a second ring element, each comprising a plurality of circumferential segments connected in a ring shape and spaced apart along a common longitudinal axis, and a plurality of said circumferential segments. A plurality of axial conductive segments for electrically interconnecting the first and second ring elements at the positions of the connection points, the axial conductive segments being adjacent to each other, and two circumferential directions connected by these. In an MRI RF coil having a plurality of current loops each including a segment and having at least one feeding point, a capacitive element is inserted in each of the plurality of circumferential segments, and a diode is provided in parallel with the capacitive element. A series circuit of inductive elements is connected, and each of the diodes is collectively turned on / off by a control voltage applied from the outside. RF coil for RI. 9. An MRI RF coil including a high-frequency resonator, wherein the high-frequency resonator is arranged such that a plurality of current loops electrically coupled to each other are arranged to generate a uniform high-frequency magnetic field region. Current loop includes at least one diode, and each diode of the plurality of current loops is collectively controlled by a control voltage applied between first and second control terminals connected to the high frequency resonator through an inductive element. An RF coil for MRI, which is controlled to be turned on and off after that. 10. The control voltage propagates from the first control terminal through a plurality of parallel current paths in the high-frequency resonator to reach the second terminal, and the plurality of current paths electrically connect to each other. RF for MRI characterized by being equivalent to
coil. 11. The RF coil according to claim 1 and the RF coil according to claim 8.
Cross-coil RF coil system for MRI.