JP3519818B2 - Flat coil for MRI - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention is applied to MRI (Magnetic).
More specifically, the present invention relates to a planar coil for MRI, which is suitable for imaging near the surface of a subject and which can improve sensitivity uniformity and efficiency.

[0002]

2. Description of the Related Art FIG. 12 shows a conventional high-pass (High Pas)
It is a block diagram which shows an example of the s) type birdcage coil for MRI. The basic structure of such a bird cage coil for MRI is disclosed in, for example, Japanese Patent Application Laid-Open No. 60-132547, Japanese Patent Application No. 3-16321 and Japanese Utility Model Application No. 6-17703. This bird cage coil 50 for MRI
No. 0 stretches a large number of elements E, E, ... Between the first ring R51 and the second ring R52, and the first ring R5
1 and each of the elements E, E, ... Between the plurality of connection points and between the plurality of connection points of the second ring R52 and each of the elements E, E ,. . And quadrature phase reception (Quadature Detec)
), the balun (B) is applied to the derivation points P and Q whose geometrical positions viewed from the center of the first ring R51 differ by π / 2.
ALance to UNbalance transformers 11a and 11b are connected, and coaxial cables SP and SQ are led out from the baluns 11a and 11b, respectively. Specifically, the transmission lines from the baluns 11a and 11b are connected to both ends of the capacitors C and C located at the derivation points P and Q, respectively.
When the directions of the elements E, E, ... Are defined as the y-axis and the directions of the two axes orthogonal to the y-axis are defined as the x-axis and the z-axis, normally, the y-axis,
The x-axis is in the horizontal plane and the z-axis is the vertical direction. And
In the case of a vertical magnetic field type MRI apparatus, a static magnetic field is applied in the z-axis direction.

FIG. 13 shows a conventional low pass.
It is a block diagram which shows an example of the birdcage coil for type MRI. The basic structure of such a birdcage coil for MRI is disclosed in Japanese Patent Application No. 6-270001. This MRI birdcage coil 700 includes a first ring R
61 and a plurality of elements E between the second ring R62,
.. are stretched, and capacitors Co, Co ,. Then, in the case of quadrature phase reception, the geometrical position viewed from the center of the first ring R61 or the second ring R62 (correctly, the center of the hollow section circle passing through the capacitor Co) is π / 2.
Baluns 11a and 11b are connected to different derivation points P and Q, and coaxial cables SP and SQ are led out from the baluns 11a and 11b, respectively. That is, the transmission lines from the baluns 11a and 11b are connected to both ends of the capacitors Co and Co located at the derivation points P and Q, respectively.
When the directions of the elements E, E, ... Are defined as the y-axis and the directions of the two axes orthogonal to the y-axis are defined as the x-axis and the z-axis, normally, the y-axis,
The x-axis is in the horizontal plane and the z-axis is the vertical direction. And
In the case of a vertical magnetic field type MRI apparatus, a static magnetic field is applied in the z-axis direction.

The birdcage coil 500 for MRI,
The 600 has a relatively wide effective sensitivity region and is suitable for imaging an internal organ of a patient. A saddle coil and a solenoid coil are known as other examples of the MRI RF coil having a relatively wide effective sensitivity region.

FIG. 14 is a block diagram showing an example of a conventional planar coil for MRI. This MRI flat coil 7
00 is also called an 8-shaped coil because the coil shape is similar to the numeral "8". X in the vertical direction of the figure 8
Axis, the horizontal direction is the y-axis, and the direction orthogonal to them is z
When the axes are used, normally, the y-axis and the x-axis are in the horizontal plane, and z
The axis is vertical. Then, in the case of the vertical magnetic field type MRI apparatus, a static magnetic field is applied in the z-axis direction.

FIG. 15 shows the flat coil 70 for MRI.
It is explanatory drawing which shows the state of the magnetic flux distribution of the zx surface produced when the electric current i is made to flow into 0. Sensitive regions effective for generating (or receiving) an oscillating magnetic field for MRI are shaded regions Z1 and Z2. This MRI flat coil 700
Are used to generate (or receive) an oscillating magnetic field Bx (FIG. 16) in the x-axis direction.

FIG. 16 is a block diagram showing another example of a conventional planar coil for MRI. This MRI flat coil 8
00 is a structure in which an 8-shaped coil 81 and an 8-shaped coil 82 are orthogonally overlapped with each other. 8-shaped coil 8
1 generates (or receives) an oscillating magnetic field Bx in the x-axis direction,
Since the oscillating magnetic field By in the y-axis direction can be generated (or received) by the 8-shaped coil 82, the rotating magnetic field B1 can be generated (or received). The planar coils 700 and 800 for MRI have the horizontal plane (x
Since it has an effective sensitivity region in the (y-plane), it is suitable for imaging the spine near the surface of the patient.

[0008]

The above-mentioned conventional birdcage coils 500 and 600 for MRI are suitable for imaging the organs of a patient, but are suitable for imaging a region near the surface of the patient. Absent. That is, since the effective sensitivity region is wide, the ratio of unnecessary sensitivity regions is large, and the SN
There is a problem that R (Signal to Noise Ratio) becomes low. This problem also applies to saddle coils and solenoid coils.

On the other hand, the conventional planar coil 7 for MRI described above.
00 and 800 are suitable for imaging a region near the surface of the patient because the effective sensitivity regions Z1 and Z2 are relatively narrow, but a coil size approximately twice that of the effective sensitivity regions Z1 and Z2 is required. However, there is a problem that the effective ratio of the sensitivity region to the coil size is low. Furthermore, the magnetic field distribution (Fig. 15)
As will be understood from the reference), since it has sensitivity to a magnetic field ineffective in MRI, there is a problem that transmission power is wasted during transmission or noise is received during reception. Further, in the above-described conventional planar coil 800 for MRI, the orthogonality of the 8-shaped coils 81 and 82, that is, the oscillating magnetic field Bx is generated due to the limitation in accuracy of the geometrical positions of the 8-shaped coils 81 and 82 ( (Or reception) mode and oscillating magnetic field By
However, there is a problem that it is difficult to completely isolate the mode for generating (or receiving). In addition, since there is anisotropy in sensitivity (a region having uniform sensitivity becomes smaller in a direction other than the x-axis direction or the y-axis direction), it is necessary to match the coil direction according to the region to be imaged, which is not convenient. There is a point.

Therefore, an object of the present invention is to provide a planar coil for MRI suitable for imaging an area near the surface of a patient. Another object of the present invention is to provide a planar coil for MRI, which has a high effective ratio of the sensitivity region to the coil size. Another object is to provide a planar coil for MRI that can improve orthogonality. Another object of the present invention is to provide a planar coil for MRI which has no sensitivity anisotropy.

[0011]

In a first aspect, the present invention provides a first ring and a second ring having a diameter smaller than that of the first ring and arranged inside the first ring, A plurality of elements radially stretched between the first ring and the second ring, between a plurality of connection points of the first ring and the element, and between a plurality of connection points of the second ring and the element Provided is a planar coil for MRI, comprising a plurality of capacitors respectively interposed.

In a second aspect, the present invention comprises a ring,
A plane for MRI, comprising a plurality of elements extending radially from substantially the center of the ring, and a plurality of capacitors respectively interposed between a plurality of connection points of the ring and the element. Provide a mold coil.

According to a third aspect of the present invention, in the planar coil for MRI of the above construction, instead of interposing the capacitor between the connection point of the ring and the element, the capacitor is provided substantially at the center of the element. Provided is a planar coil for MRI, which is characterized by being interposed.

[0014]

In the planar coil for MRI according to the first aspect, a plurality of elements are radially stretched between the first ring and the second ring arranged inside the first ring, and the plurality of elements of the first ring and each element are radially extended. Capacitors are respectively provided between the connection points of and the second ring and a plurality of connection points of each element. Since this structure can be regarded as a structure in which the high-pass birdcage coil for MRI (FIG. 12) is developed in a plane, it has an effective sensitivity region without sensitivity anisotropy in the horizontal plane (xy plane) near the coil. Therefore, it is suitable for imaging the spine near the surface of the patient (high SNR). In addition, the effective rate of the sensitivity region with respect to the coil size becomes high. Moreover, orthogonality can be improved. Furthermore, since there is no anisotropy of sensitivity, usability is improved.

In the above structure, in order to feed the RF pulse for MRI or extract the NMR signal, the signal transmission line from the both ends of the capacitor whose geometrical position viewed from the center of the ring is different by about π / 2 to the outside. When deriving, the quadrature phase transmission and the quadrature phase reception are possible.

In the above structure, if at least one of the capacitors is a variable capacitor, good orthogonality can be ensured even if there is some variation in the capacitor capacitance or element length (inductance). Like

In the MRI planar coil according to the second aspect, a plurality of elements are radially stretched between the ring and substantially the center of the ring, and between the ring and a plurality of connection points between the elements. Each is equipped with a capacitor.
This is a modification of the planar coil for MRI according to the first aspect, and in addition to achieving the same effect, the number of parts can be reduced.

In the above structure, when the ring has an elliptical shape, the aspect of the sensitivity region can be changed by changing the flatness of the elliptical shape. Therefore, the mode of the sensitivity region can be made suitable for the imaging region.

In the MRI planar coil according to the third aspect, in the above structure, the capacitor is provided in each element instead of being provided in the ring. This is a low-pass MRI birdcage coil (Fig. 13)
Can be regarded as a two-dimensional structure. Therefore, the same action as the planar coil for MRI of the first and second aspects is achieved.

[0020]

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 is a block diagram of a planar coil for MRI according to a first embodiment of the present invention. This planar coil 10 for MRI has a large number (number n; n ≧ 3) of elements E, E, ... Between a first ring R1 and a concentric second ring R2 having a smaller diameter. (Adjust the angular difference between adjacent elements by approximately 2π so that each element has an approximately equal angular interval.
/ N) between the plurality of connection points of the first ring R1 and each of the elements E, E, ... And between the plurality of connection points of the second ring R2 and each of the elements E, E ,. ., And capacitors Ci, Ci, ... In the case of quadrature phase reception, the first ring R1
The baluns 11a and 11b are connected to the derivation points P and Q whose geometrical positions viewed from the center are different by π / 2, and the coaxial cables SP and SQ are derived from the baluns 11a and 11b, respectively. That is, the transmission lines from the baluns 11a and 11b are connected to both ends of the capacitors C and C located at the derivation points P and Q among the capacitors C provided in the first ring R1. Hereinafter, for convenience of explanation, the derivation point P is on the φ = 0 side, and the derivation point Q is on the φ = π / 2 side.

The center of the first ring R1 and the derivation point Q
When the direction passing through is the y-axis, the direction passing through the center of the first ring R1 and the derivation point P is the x-axis, and the direction orthogonal to them is the z-axis, the y-axis and the x-axis are usually the horizontal plane. And the z-axis is the vertical direction. And vertical magnetic field type MRI
In the case of the device, a static magnetic field is applied in the z-axis direction.

Now, the planar coil 10 for MRI is
A high-pass bird cage coil 500 for MRI (see FIG. 1).
2) can be regarded as a two-dimensional development. Therefore, the current distribution of each part when driving only through the coaxial cable SP (this is called a single drive) is shown in FIG. That is, when the angle from the x-axis is θ, the current i1 on the first ring R1 and the second
The magnitude of the current i2 on the ring R2 is proportional to cos θ,
The magnitude of the current ie on the element E is proportional to sin θ (since a high-frequency standing wave is generated on each ring, such a current distribution is obtained). Therefore, the oscillating magnetic field B in the x-axis direction
Focusing on x, on the WW ′ line, it is relatively large inside the second ring R2, but rapidly decreases outside the second ring R2. This is due to the current ie on element E and the second
This is because the oscillating magnetic field Bx generated by the current i2 of the ring R2 is canceled by the magnetic field of the current i1 of the first ring R1. On the other hand, on the V-V 'line, the oscillating magnetic field Bx generated by the current ie on the element E and the current i2 on the second ring R2 is not canceled by the magnetic field by the current i1 on the first ring R1. It does not decrease rapidly outside the 2 ring R2. Rather, the contribution of the current ie on the element E is large, so that it is larger on the element E than in the second ring R2. Then, it becomes smaller outside the first ring R1. On the other hand, the current distribution of each part when a single drive is assumed through the coaxial cable SQ is shown in FIG.
Is rotated 90 degrees clockwise. Therefore, by superposing the driving through the coaxial cable SP and the driving through the coaxial cable SQ, a magnetic field distribution (sensitivity distribution) having excellent uniformity can be obtained.

FIG. 3A shows the distribution state of the oscillating magnetic fields Bx and Bz in the zx plane passing through the line WW 'in FIG.
The bell-shaped distribution shown by the solid line in FIG. 3B is the sensitivity distribution on the line AA ′ in FIG. 3A during driving through the coaxial cable SP (φ = 0) (FIG. 2). W-
Corresponds to the distribution on the W'line). In addition, the twin mountain distribution shown by the broken line in FIG. 3 (b) shows that the coaxial cable SQ (φ =
A− in FIG. 3A in driving through π / 2)
It is the sensitivity distribution on the line A ′ (corresponding to the distribution on the line VV ′ in FIG. 2 rotated clockwise by 90 °).

Since the uniformity of the direction of the element E, that is, the direction radially extending from the coil center depends on the ratio of the radius of the first ring R1 and the radius of the second ring R2, it is desired to change this ratio. The sensitivity distribution of can be obtained. Further, the uniformity in the angle θ direction, that is, the uniformity in the circumferential direction along each ring depends on the symmetry of the coil shape, and therefore increases as the number of the elements E increases. SN
From the viewpoint of improving R and reducing excitation power, the widths of the first ring R1 and the second ring R2 are maximized and the width of the element E is optimized (SNR).
It is preferable to empirically find a width that is suitable for improvement and low pumping power (Japanese Patent Application No. 7-93908).

The planar coil 1 for MRI of the first embodiment described above.
According to 0, the horizontal plane (xy plane) near the coil has an effective sensitivity region without sensitivity anisotropy. Therefore, it is suitable for imaging the spine near the surface of the patient (high SNR). In addition, the effective rate of the sensitivity region with respect to the coil size increases. Moreover, orthogonality can be improved. Furthermore, since there is no anisotropy of sensitivity, usability is improved.

-Second Embodiment- FIG. 4 is a block diagram of a planar coil for MRI according to a second embodiment of the present invention. In this MRI flat coil 20, one of the capacitors C provided in the first ring R1 of the first embodiment is used as a variable capacitor Cv for adjusting orthogonality.
Are connected in parallel. However, the connection position of the orthogonality adjusting variable capacitor Cv is the feeding point (θ = 0,
-Π / 2) and a point 180 ° opposite to the feeding point (θ =
Other than π, π / 2). This variable capacitor Cv
Can adjust the orthogonality. According to the planar coil 20 for MRI of the second embodiment, in addition to the effect of the first embodiment, there are some variations in the capacitance of the capacitors C and Ci, the length (inductance) of the element E, and the like. Also, it becomes possible to secure good orthogonality.

-Third Embodiment- FIG. 5 is a block diagram of a planar coil for MRI according to a third embodiment of the present invention. In this MRI planar coil 30, the shapes of the first ring R1 and the second ring R2 of the first embodiment are elliptical. The aspect of the sensitivity region can be changed by changing the flatness of the elliptical shape.
When the elliptical shape is used, the inductance of the element E varies depending on the location, so that the capacitors C, Ci
It is necessary to change the capacity depending on the location. According to the planar coil 30 for MRI of the third embodiment, the mode of the sensitivity region can be made suitable for the imaging region.

[Fourth Embodiment] FIG. 6 is a diagram showing the configuration of a flat coil for MRI according to a fourth embodiment of the present invention. The planar coil 40 for MRI includes a number of elements E, E, E between the ring R1 and its center point.
Are radially extended, and capacitors C, C are respectively provided between the ring R1 and a plurality of connection points of the elements E, E ,.
It is a structure in which C, ... Then, in the case of quadrature phase reception, the geometrical position viewed from the center of the ring R1 is π.
Baluns 11a and 11b are connected to derivation points P and Q that differ by / 2, and coaxial cables SP and SQ are derived from these baluns 11a and 11b, respectively. That is, the lead-out points P and Q of the capacitor C interposed in the ring R1.
The baluns 11a, C
The transmission lines from 11b are respectively connected. The direction passing through the center of the first ring R1 and the derivation point Q is the y-axis, and the direction passing through the center of the first ring R1 and the derivation point P is x.
When the z-axis is the axis and the direction orthogonal to them is the z-axis, the y-axis and the x-axis are usually in the horizontal plane, and the z-axis is the vertical direction. Then, in the case of the vertical magnetic field type MRI apparatus, a static magnetic field is applied in the z-axis direction.

The MRI flat coil 40 is equivalent to the MRI flat coil 10 (see FIG. 1) of the first embodiment in which the second ring R2 is minimized. Second ring R2 at 10
Therefore, the capacitor C provided on the second ring R2 can be eliminated. MRI flat coil 40 of the fourth embodiment
According to the above, the number of parts is reduced as compared with the first embodiment,
The configuration can be simplified.

-Fifth Embodiment- FIG. 7 is a block diagram of a flat coil for MRI according to a fifth embodiment of the present invention. The planar coil for MRI 50 includes a large number of elements E, E, between a first ring R11 and a concentric second ring R12 having a diameter smaller than that of the first ring R11.
Are radially extended, and capacitors Ch, Ch, ... Are respectively interposed between a plurality of connection points between the first ring R11 and each of the elements E, E ,. No capacitor is provided). The capacitance of the capacitor Ch is determined such that the resonance frequency between the capacitor Ch and the element E substantially matches the frequency of the MRI RF pulse or the NMR signal. And in the case of quadrature reception,
The geometrical position viewed from the center of the first ring R11 is π
Coaxial cables SP and SQ at the derivation points P and Q that differ by only / 2
(The core of the coaxial cable and the outer conductor are connected to both ends of the capacitor C at that position), and the coaxial cables SP and SQ are guided to the second ring R12 along the neighboring elements Ea and Eb, respectively. (Coaxial cable S
The outer conductors of P and SQ may or may not be connected to the elements Ea and Eb), and the outer conductors of the coaxial cables SP and SQ are respectively connected to the points PP and PQ on the second ring R12 (first). It may be a point other than the points PP and PQ on the two-ring R12) and the coaxial cables SP and SQ are led out from the second ring R12 to the outside. Balun 11a,
The reason why it is not necessary to use 11b (see FIG. 1) is that the second ring R12 has a substantially zero potential (if the inductance is sufficiently small) so that it can be directly connected to the outer conductor of the coaxial cables SP and SQ having a zero potential. This is because As described above, a technique for supplying power without using a balun is disclosed in Japanese Patent Application No. 6-2010.
No. 29.

Contrary to the above, the first ring R11 is not provided with a capacitor, the second ring R12 is provided with a capacitor, the second ring R12 is provided with the derivation points P and Q, and the coaxial cable SP, Connect SQ, coaxial cable SP, SQ
May be guided to the first ring R11 along the element E, an external conductor may be connected, and the coaxial cables SP and SQ may be guided to the outside from the first ring R11.

-Sixth Embodiment- FIG. 8 is a block diagram of a planar coil for MRI according to a sixth embodiment of the present invention. The planar coil 60 for MRI includes a large number of elements E, E, ... Between a first ring R1 ′ and a concentric second ring R2 having a smaller diameter.
Are stretched radially. In the case of quadrature phase reception, the geometrical position seen from the center of the first ring R1 ′ among the plurality of connection points of the first ring R1 ′ and each element E, E, ... And capacitors C1 and C
2 series circuits are connected, and capacitors C, C, ... Are provided at other positions respectively. Further, the capacitor C1
Then, the series circuit of the inductor L1 and the diode D1 is connected in parallel. Further, a series circuit of an inductor L and a diode D2 is connected in parallel to the capacitor C. Further, the coaxial cables SP and SQ are connected to both ends of the capacitors C2 and C2.
Connect the outer conductor and the core wire of. The parallel resonant circuit of the capacitor C1 and the inductor L1 and the parallel resonant circuit of the capacitor C and the inductor L are MR
It resonates with the frequency of the RF pulse for I or the NMR signal and becomes a high impedance. The capacitor C
The combined capacitance of 1 and C2 is equal to the capacitance of the capacitor C (1 / C1 + 1 / C2 = 1 / C). Regarding the second ring R2, capacitors Ci, Ci, ... Are provided between a plurality of connection points with the respective elements E, E ,.

The direction passing through the center of the first ring R1 'and the derivation point Q is the y-axis, the direction passing through the center of the first ring R1' and the derivation point P is the x-axis, and the direction orthogonal to them is Is the z-axis, the y-axis and the x-axis are usually in the horizontal plane, and the z-axis is the vertical direction. And the vertical magnetic field type M
In the case of an RI device, a static magnetic field is applied in the z-axis direction.

Now, the RF for MRI from another transmitting coil
At the time of transmitting the pulse, a positive DC voltage is superimposed on the coaxial cables SP and SQ (in this case, since there is no original transmission signal, it is sufficient to simply supply the DC voltage). Then, the diodes D1 and D2 are turned on to form a parallel resonance circuit of the capacitor C1 and the inductor L1 and a parallel resonance circuit of the capacitor C and the inductor L. As a result, all of the connection points between the first ring R1 'and the element E are in a high impedance state with respect to the RF pulse for MRI, the function as a coil is turned off, and it is possible to decouple with another transmission coil. . When receiving the NMR signal, a negative DC voltage is superimposed on the coaxial cables SP and SQ. Then, the diode D1 and the diode D
Since 2 is turned off, the parallel resonance circuit of the capacitor C1 and the inductor L1 and the parallel resonance circuit of the capacitor C and the inductor L are not formed. As a result, it becomes equivalent to the planar coil 10 for MRI of the first embodiment. The decoupling circuit as described above is disclosed in Japanese Patent Application No. 7-1825.
No.

The plane type coil 6 for MRI of the sixth embodiment
According to 0, in addition to the effect of the first embodiment, decoupling with another coil can be performed at the time of transmitting the RF pulse for MRI or at the time of receiving the NMR signal.

-Seventh Embodiment- FIG. 9 is a diagram showing the configuration of a planar coil for MRI according to a seventh embodiment of the present invention. This MRI flat coil 70 includes a large number of elements E, E, ... Between a first ring R21 and a concentric second ring R22 having a smaller diameter than the first ring R21.
Are radially extended, and capacitors Co are respectively provided at substantially the centers of the elements E, E, .... Then, in the case of quadrature phase reception, the derivation points P and Q whose geometric positions viewed from the center of the first ring R21 differ by π / 2.
Connect the baluns 11a and 11b to
Coaxial cables SP and SQ are derived from a and 11b, respectively. That is, among the capacitors Co provided in each element E, the capacitors C located at the derivation points P and Q.
Transmission lines from the baluns 11a and 11b are connected to both ends of o, respectively. The direction passing through the center of the first ring R21 and the derivation point Q is the y-axis, the direction passing through the center of the first ring R21 and the derivation point P is the x-axis direction, and the direction orthogonal thereto is the z-axis. In this case, usually, the y-axis and the x-axis are in the horizontal plane, and the z-axis is the vertical direction. Then, in the case of the vertical magnetic field type MRI apparatus, a static magnetic field is applied in the z-axis direction.

Now, the plane coil 70 for MRI is
Low-pass MRI birdcage coil 600 (see FIG. 1)
(See 3) can be regarded as a two-dimensional development. Therefore, in the planar coil 70 for MRI, as described above with reference to FIG. 3, it is possible to obtain the sensitivity distribution having excellent uniformity. According to the MRI planar coil 70 of the seventh embodiment, the horizontal plane (xy plane) near the coil has an effective sensitivity region without sensitivity anisotropy. Therefore, it is suitable for imaging the spine near the surface of the patient (high SNR). In addition, the effective rate of the sensitivity region with respect to the coil size becomes high. Moreover, orthogonality can be improved. Furthermore, since there is no anisotropy of sensitivity, usability is improved.

-Eighth Embodiment- FIG. 10 is a block diagram showing an MRI planar coil according to an eighth embodiment of the present invention. This MRI flat coil 80
Has a large number of elements E, E, ... Radially stretched between the ring R21 and its center point, and each element E, E ,.
Is a structure in which capacitors Co, Co, ... In the case of quadrature phase reception, the ring R21
The baluns 11a and 11b are connected to the derivation points P and Q whose geometrical positions viewed from the center are different by π / 2, and the coaxial cables SP and SQ are derived from the baluns 11a and 11b, respectively. That is, the derivation points P, Q of the capacitors Co, Co, ... Between the elements E, E ,.
A balun 11 is placed between both ends of the capacitors Co located at
The transmission lines from a and 11b are respectively connected. The direction passing through the center of the ring R21 and the derivation point Q is the y-axis,
The direction passing through the center of the ring R21 and the derivation point P is x
When the z-axis is the axis and the direction orthogonal to them is the z-axis, the y-axis and the x-axis are usually in the horizontal plane, and the z-axis is the vertical direction. Then, in the case of the vertical magnetic field type MRI apparatus, a static magnetic field is applied in the z-axis direction.

The MRI flat coil 80 is equivalent to the MRI flat coil 70 (see FIG. 9) of the seventh embodiment in which the second ring R22 is minimized.
The second ring R2 in the planar coil 70 for MRI
2 can be eliminated. That is, the MRI of the eighth embodiment described above.
According to the planar coil 80 for use, the number of parts can be reduced and the structure can be simplified as compared with the seventh embodiment.

-Ninth Embodiment- FIG. 11 is a structural view showing a planar coil for MRI of a ninth embodiment of the present invention. This planar coil 90 for MRI
Has a large number of elements E, E, ... Radially stretched between a first ring R31 and a concentric second ring R32 having a diameter smaller than that of the first ring R31. A plurality of first DC-cutting capacitors (generally a large capacity for reducing high-frequency impedance) Cx are provided every other plurality of connection points, and a plurality of connections of the second ring R32 and the element E are provided. A second DC-cutting capacitor Cx is provided alternately with the first DC-cutting capacitor Cx at every other point.
Then, in the case of quadrature phase reception, each element E, E, ...
Among them, a series circuit of capacitors Ca and Cb is connected to approximately the center of elements Ea and Eb whose geometrical positions viewed from the center of the first ring R31 differ by π / 2, and a capacitor Co is connected to other elements. Set up. Further, a series circuit of an inductor La and a diode D1 is connected in parallel to the capacitor Ca. Further, a series circuit of an inductor Lb and a diode D2 is connected in parallel to the capacitor Co. The parallel resonant circuit of the capacitor Ca and the inductor La and the parallel resonant circuit of the capacitor Co and the inductor Lb resonate with respect to the frequency of the RF pulse for MRI or the NMR signal, and have high impedance. The combined capacitance of the capacitors Ca and Cb is equal to the capacitance of the capacitor Co (1 / Ca + 1 / Cb = 1 / Co).
Is). The center of the first ring R31 and the derivation point Q
When the direction passing through is the y axis, the direction passing through the center of the first ring R31 and the derivation point P is the x axis, and the direction orthogonal to them is the z axis, the y axis and the x axis are usually the horizontal plane. And the z-axis is the vertical direction. And the vertical magnetic field type MR
In the case of device I, a static magnetic field is applied in the z-axis direction.

Now, RF for MRI from another transmitting coil
At the time of transmitting the pulse, a positive DC voltage is superimposed on the coaxial cables SP and SQ (in this case, since there is no original transmission signal, it is sufficient to simply supply the DC voltage). Then, the diodes D1 and D2 are turned on to form a parallel resonance circuit of the capacitor Ca and the inductor La and a parallel resonance circuit of the capacitor Co and the inductor Lb. As a result, all of the elements E, E, ... Approximately the center (where the capacitor is interposed) are in a high impedance state with respect to the RF pulse for MRI, the function as a coil is turned off, and decoupling with another transmission coil is performed. You can do it. When receiving the NMR signal, the coaxial cables SP, SQ
Superimpose a negative DC voltage on. Then, since the diode D1 and the diode D2 are turned off, the parallel resonance circuit of the capacitor Ca and the inductor La and the parallel resonance circuit of the capacitor Co and the inductor Lb are not formed. As a result, it becomes equivalent to the planar coil 70 for MRI of the seventh embodiment. The above decoupling circuit is disclosed in Japanese Patent Application No. 7-1825.

MRI flat coil 9 of the ninth embodiment
According to 0, in addition to the effect of the seventh embodiment, the MR
It is possible to perform decoupling with another coil when transmitting an I RF pulse or when receiving an NMR signal.

[0044]

According to the planar coil for MRI of the present invention, since it has an effective sensitivity region on a plane parallel to the coil surface near the coil, it is suitable for imaging the spine and the like near the surface of the patient. (High SNR). In addition, the effective rate of the sensitivity region with respect to the coil size becomes high. Moreover, orthogonality can be improved. Furthermore, since there is no anisotropy of sensitivity, usability is improved. Therefore, it is useful as a surface coil.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a planar coil for MRI according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a current distribution in each part of the planar coil for MRI of FIG.

FIG. 3 is an explanatory diagram showing a magnetic flux distribution and a sensitivity distribution based on the current distribution of FIG.

FIG. 4 is a configuration diagram showing a planar coil for MRI according to a second embodiment of the present invention.

FIG. 5 is a configuration diagram showing a planar coil for MRI according to a third embodiment of the present invention.

FIG. 6 is a configuration diagram showing a planar coil for MRI according to a fourth embodiment of the present invention.

FIG. 7 is a configuration diagram showing a planar coil for MRI according to a fifth embodiment of the present invention.

FIG. 8 is a configuration diagram showing a planar coil for MRI according to a sixth embodiment of the present invention.

FIG. 9 is a configuration diagram showing a planar coil for MRI according to a seventh embodiment of the present invention.

FIG. 10 is a configuration diagram showing a planar coil for MRI according to an eighth embodiment of the present invention.

FIG. 11 is a configuration diagram showing a planar coil for MRI according to a ninth embodiment of the present invention.

FIG. 12 is a configuration diagram of an example of a conventional high-pass birdcage coil for MRI.

FIG. 13 is a configuration diagram of an example of a conventional low-pass birdcage coil for MRI.

FIG. 14 is a configuration diagram showing an example of a conventional planar coil for MRI.

15 is an explanatory diagram showing a magnetic flux distribution of the planar coil for MRI of FIG.

FIG. 16 is a configuration diagram showing another example of a conventional planar coil for MRI.

[Explanation of symbols]

10, 20, 30, 40, 50, 60, 70, 80, 9
0 ... Planar coil for MRI, 11a, 11b ... Balun, R1, R1, R11, R1 ', R21, R31 ... 1st
Ring, R2, R12, R22, R32 ... Second
Ring, E ... Element, C, Ch, C1, C2, Co, Cx, Ca, Cb ...
Capacitors, Cv ... Variable capacitors L, L1, La, Lb ... Inductors, D1, D2 ... Diodes, SP, SQ ... Coaxial cables.

Claims (3)

(57) [Claims] 1. A first ring, a second ring having a diameter smaller than that of the first ring and arranged inside the first ring, and radially between the first ring and the second ring. A plurality of stretched elements; and a plurality of capacitors respectively interposed between a plurality of connection points between the first ring and the element and between a plurality of connection points between the second ring and the element. A planar coil for MRI characterized by the above. 2. A ring, eight elements radially extended from the substantial center of the ring, and eight capacitors respectively interposed between the eight connection points of the ring and the element. , The geometrical positions viewed from the center of the ring are π / 2 with respect to each other.
An MRI characterized in that it has two baluns that are respectively connected to both ends of two distant capacitors.
Type flat coil. 3. The MRI according to claim 1 or 2.
A planar coil for use in MRI, wherein the capacitor is provided at substantially the center of the element instead of being provided between the connection points of the ring and the element.