JP3415781B2 - RF coil and MRI device - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an RF coil and an M coil.
Regarding the RI apparatus, more specifically, an RF coil and its R that can eliminate mutual interference between adjacent coils even if they are closely arranged without overlapping adjacent coils.
The present invention relates to an MRI apparatus equipped with an F coil.

[0002]

2. Description of the Related Art A conventional MRI apparatus uses an RF coil in which three or more small coils are combined.
In such a conventional RF coil, in order to prevent mutual interference between the coils, the coil surfaces of adjacent coils are arranged to overlap each other by about 10%.

[0003]

However, it is inconvenient to limit the coil surfaces of adjacent coils to each other by overlapping by about 10%. Therefore, an object of the present invention is to provide an RF coil and an MRI apparatus including the RF coil that can eliminate mutual interference between adjacent coils even if they are closely arranged without overlapping adjacent coils. is there.

[0004]

SUMMARY OF THE INVENTION In a first aspect, the present invention provides three or more coils arranged in close proximity to each other, an inter-coil connection capacitor provided in each coil, and a coil of each coil. A parallel connection line connecting the inter-connection capacitors in parallel and a neutralization capacitor interposed in the parallel connection lines, wherein the parallel connection line and the neutralization line are connected from both ends of the inter-coil connection capacitor of one coil. The transfer voltage transmitted to both ends of the inter-coil connection capacitor of the other coil via the capacitor cancels the induced voltage generated at the both ends of the inter-coil connection capacitor of the other coil via the mutual inductance. An RF coil is provided. In the RF coil according to the first aspect, a voltage that cancels a voltage induced by one coil in another coil due to mutual inductance is transmitted from one coil to another coil via a parallel connection line and a neutralizing capacitor. To do. Therefore, mutual interference between the coils can be eliminated. It is also possible to arrange adjacent coils in close proximity without overlapping.

According to a second aspect, the present invention provides an MRI apparatus characterized by including the RF coil according to the first aspect. In the MRI apparatus of the first aspect, three or more coils can be used without being limited to the arrangement in which the coil surfaces of adjacent coils overlap by about 10%.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below with reference to the embodiments of the invention shown in the drawings. The present invention is not limited to this.

First Embodiment FIG. 1 is a perspective view of an RF coil according to the first embodiment of the present invention. The RF coil 100 includes a first coil 1, a second coil 2 and a third coil 3, and inter-coil connection capacitors 4, 5 and 6 and resonance capacitors 7 and 8 provided in the coils 1, 2 and 3, respectively. , 9 and parallel connection lines 10 to 13 for connecting the inter-coil connection capacitors 4, 5 and 6 of the respective coils 1, 2 and 3 in parallel, and a neutralization capacitor interposed in the parallel connection lines 10 to 13. 14-1
7 and 7. Then, the resonance capacitor 7,
Coaxial cables 24, 25 and 26 are connected to both ends of 8 and 9 via baluns 21, 22 and 23, respectively.

The first coil 1, the second coil 2 and the third coil 3 are track-type of athletics (for example, a linear portion of about 330 mm, a semicircular portion of a radius of about 110 mm, a conductor width of about 50 mm, and a conductor thickness of about 50 mm). 1mm), with the coil surfaces facing each other and close to each other (for example, a gap of about 150m)
m), lined up.

FIG. 2 is an equivalent circuit diagram of the RF coil 100. L1, L2, L3 are the first coil 1, the second
It is the inductance of the coil 2 and the third coil 3.
R1, R2 and R3 are resistances of the first coil 1, the second coil 2 and the third coil 3. V1, V2, and V3 are electromotive forces of the first coil 1, the second coil 2, and the third coil 3 (for example, induced by an NMR signal). I
1, I2 and I3 are loop currents flowing through the first coil 1, the second coil 2 and the third coil 3, respectively. M1 is a mutual inductance between the first coil 1 and the second coil 2. M2 is a mutual inductance between the second coil 2 and the third coil 3. M3 is a mutual inductance between the first coil 1 and the third coil 3.

C1, C2 and C3 are capacitances of the resonance capacitors 7, 8 and 9, respectively. C1 ', C2', C
3'is the capacitance of the inter-coil connection capacitors 4, 5, and 6. Cm1, Cm1, Cm2 and Cm2 are capacitances of the neutralizing capacitors 14, 15, 16 and 17, respectively. Im1 is a loop current flowing in a loop formed by the inter-coil connection capacitor 4, the neutralization capacitor 14, the inter-coil connection capacitor 5 and the neutralization capacitor 15. Im2 is a loop current flowing in a loop formed by the inter-coil connection capacitor 5, the neutralization capacitor 16, the inter-coil connection capacitor 6 and the neutralization capacitor 17.

In the equivalent circuit of FIG. 2, when the target angular frequency is ω, the following circuit equation holds.

Solving for Im1 and Im2 from equations (4) and (5),

Using equations (6) and (7), the equations (1), (2) and (3) are summarized as follows:

In order to eliminate mutual interference between the coils,
(1) '(2)' (3) 'The crosstalk term in the equation must be "0", so

If ω is eliminated from the equations (8) and (9),

If ω is eliminated from the equations (9) and (10),

On the other hand, each of the coils 1 to 3 has a target angular frequency ω
Therefore, the underlined term in equations (1) '(2)' (3) 'must be "0".

Therefore, after all, the value of the capacitor may be set as follows. (Step 1) Create coils 1 to 3 and measure inductances L1 to L3. (Step 2) Place the coils 1 to 3 in a desired arrangement and measure the mutual inductances M1 to M3. (Step 3) Determine C1 'appropriately. Then, Cm1 is determined from the equation (19). (Step 4) C3 'is determined appropriately. Then, Cm2 is determined from the equation (18). (Step 5) C2 'is determined so that the expressions (8), (9) and (10) are satisfied at the target angular frequency ω. By the above steps 1 to 5, the three coils 1 to
Mutual interference between the three can be eliminated.

(Step 6) C1 is determined so that the underlined term in the equation (1) 'becomes "0". (Step 7) C2 is determined so that the underlined term in the equation (2) 'becomes "0". (Step 8) C3 is determined so that the underlined term in the expression (3) 'becomes "0". By the above steps 6 to 8, the three coils 1 to
The resonance frequency of 3 can be adjusted to ω.

Here, in order to simplify the condition, C1 '=
C3 '. Substituting equations (18) and (19) into equation (10) and solving for C2 ',

Since C2 'must be a positive value,

On the other hand, by substituting the expressions (18), (19), and (20) into the underlined terms in the expressions (1) '(2)' (3) ', the condition (the solution is Condition) with

Therefore, the value of the capacitor may be set as follows. (Step a) Creating the coils 1 to 3 and measuring the inductances L1 to L3. (Step b) Put the coils 1 to 3 in a desired arrangement and measure the mutual inductances M1 to M3. At this time, (23)
Arrange to satisfy the formula. (Step c) C1 'is determined so as to satisfy the equations (21), (22) and (24). Then, Cm1 is determined from the equation (19). (Step d) C3 '= C1' and Cm2 = Cm1. (Step e) C2 'is determined from the equation (20). (Step f) C1 is determined so that the underlined term in the equation (1) 'becomes "0". (Step g) C2 is determined so that the underlined term in the equation (2) 'becomes "0". (Step h) C3 = C1. As described above, it is possible to eliminate mutual interference between the three coils 1 to 3 and to resonate at the frequency ω.

Next, the above steps a to h will be described with numerical examples. (Step a) Each of the first coil 1, the second coil 2 and the third coil 3 is a track-type track track, and has a linear portion of about 330 mm, a semicircular portion of a radius of about 110 mm, a conductor width of about 50 mm, and a conductor thickness. It is made of about 1 mm copper. At this time, L
= 830 nH. (Step b) The first coil 1, the second coil 2 and the third coil 3 are arranged in close proximity with their respective coil surfaces facing each other, and the first coil 1 and the second coil 2 are arranged.
And the distance between the second coil 2 and the third coil 3 are each about 150 mm. At this time, M1 = M2 =
It was 186.5 nH and M3 = 69.5 nH. (Step c) From equations (21), (22) and (23), 300pF <C
Since 1 '<620 pF, C1' = 600 pF. If ω / 2π = 8.54 MHz, from equation (19),
Cm1 = 712.8 pF was obtained. (Step d) C2 '= C1' = 600 pF, Cm2 = Cm1
= 712.8 pF. (Step e) From equation (20), C2 '= 246.8 pF was obtained. (Step f) C1 = 874 pF was obtained from the condition that the underlined item in the equation (1) 'was "0". (Step g) C2 = 1054 pF was obtained from the condition that the underlined item in the equation (2) 'was "0". (Step h) C3 = C1 = 874 pF. As described above, it was possible to realize the RF coil 100 in which the three coils 1 to 3 are arranged close to each other without mutual interference.

FIG. 3 is an equivalent circuit diagram of an essential part assuming a case where only the loop current I1 of the first coil 1 flows. First
Loop current I1 of coil 1 and interaction between coils M1, M2
Thus, the inter-coil connection capacitor 5 (C
2 ') induced voltage of -jωM1I1 across both ends
An induced voltage of −jωM3I1 is generated across the inter-coil connecting capacitor 6 (C3 ′) of the coil 3. On the other hand, the voltage across the inter-coil connecting capacitor 4 (C1 ') of the first coil 1 is passed through the neutralizing capacitors 14 (Cm1) and 15 (Cm1) to the inter-coil connecting capacitor 5 (C1 of the second coil 2).
2 ') is transmitted to both ends of the neutralization condenser 16
It is transmitted to both ends of the inter-coil connecting capacitor 6 (C3 ') of the third coil 3 via (Cm2) and 17 (Cm1).

Inter-coil connection capacitor 5 of second coil 2
If the induced voltage -jωM1I1 generated at both ends of (C2 ') is canceled by the voltage transmitted to both ends of the inter-coil connection capacitor 5 (C2') via the neutralizing capacitors 14 (Cm1) and 15 (Cm1). , The first coil 1 is the second coil 2
Does not affect the. That is, if the following equation is established, the first coil 1 does not affect the second coil 2.

The induced voltage -jωM3I1 generated across the inter-coil connecting capacitor 6 (C3 ') of the third coil 3
But neutralizing capacitors 14 (Cm1), 15 (Cm1), 16
If the voltage transmitted to both ends of the inter-coil connecting capacitor 6 (C3 ') of the third coil 3 through (Cm2), 17 (Cm2) cancels out, the first coil 1 affects the third coil 3. There is no.

Next, in the loop circuit in which the loop current Im1 flows, the following equation is established.

Next, in the loop circuit in which the loop current Im2 flows, the following equation is established.

The equations (8) and (10) are derived from the equations (11) to (14).

Similarly to the above, assuming that only the loop current I2 of the second coil 2 flows (or only the loop current I3 of the third coil 3 flows), the above equation (9) is derived. Get burned.

Therefore, the expressions (8), (9) and (10) mean that the inter-coil connection capacitors of one coil are connected to the other end of the inter-coil connection capacitors of the other coil through the parallel connection line and the neutralization capacitor. It can be understood that the transfer voltage transmitted to both ends cancels out the induced voltage generated by the one coil across the inter-coil connection capacitor of the other coil through the mutual inductance.

According to the above RF coil 100, since there is no interaction between the coils, the three coils 1, 2, and 3 are independent from each other in the NMR signals of the sharing regions without being affected by the noise components of the other coils. Can be received. Therefore, the S / N ratio can be improved as compared with the case where there is an interaction between the coils. The RF coil 100 can be used as both a transmitting coil and a receiving coil.

-Second Embodiment- FIG. 4 is a perspective view of an RF coil according to a second embodiment of the present invention. The RF coil 200 is similar to the RF coil 100 of the first embodiment in that the passive decoupling circuit 3 is used.
This is a configuration in which 1, 32 are added. By adding the passive decoupling circuits 31, 32, and 33 in this way, when high frequency transmission is performed from another transmission coil, R
It is possible to prevent a current from flowing through the F coil 200. Therefore, the RF coil 200 is useful as a receive-only coil.

-Third Embodiment-In the first and second embodiments, the neutralization capacitors 14 and 15 having the same capacitance Cm1 are provided in the parallel connection lines 10 and 11, respectively, but they are different. It may be capacity. Further, although the neutralization capacitors 16 and 17 having the same capacitance Cm2 are provided on the parallel connection lines 12 and 13, respectively, they may have different capacitances. Further, one neutralizing capacitor may be provided only in one of the parallel connection lines 10 or 11 and one neutralizing capacitor may be provided in only one of the parallel connection lines 12 or 13. Connection lines 10, 11,
A plurality of neutralizing capacitors may be provided in each of 12 and 13. The point is that the voltage transmitted to both ends of the inter-coil connecting capacitor by the neutralizing capacitor should cancel the induced electromotive force due to the interaction between the coils. In addition,
When the target angular frequency ω is high, it is preferable to provide a plurality of neutralizing capacitors on each of the parallel connection lines 10, 11, 12, and 13 in order to eliminate the influence of the stray capacitance.

-Fourth Embodiment- In the first and second embodiments, the RF coils 100 and 200 using the three coils 1, 2 and 3 have been described, but four or more coils are used. Also in the RF coil using, the conditional expressions are derived in the same manner as above, and the values of the inter-coil connection capacitor, the neutralization capacitor, and the resonance capacitor may be determined.

-Fifth Embodiment- FIG. 5 is a perspective view showing a main part of an MRI apparatus according to a fifth embodiment of the present invention. The MRI apparatus 300 is a vertical magnetic field type MRI apparatus that generates a main magnetic field in the vertical direction, and includes a lower magnet 140 and an upper magnet 150 facing each other in the vertical direction, and a cradle 120 that goes in and out of a space between the magnets 140 and 150. That cradle 1
20 and the RF coil 100 installed on the upper part 20. The RF coil 100 is the RF coil according to the present invention described in the above embodiment, and is installed with the coil surface facing the longitudinal direction of the cradle 120. The subject (broken line) is placed on the cradle 120 so that the body portion thereof penetrates the coil surface of each coil of the RF coil 100.

According to the MRI apparatus 300 described above, since there is no interaction between the three coils (1, 2, 3) forming the RF coil 100, the MRI apparatus 300 is not affected by noise components of other coils. The three coils 1, 2, and 3 can independently receive the NMR signals in the sharing regions. Therefore,
The S / N ratio can be improved as compared with the case where there is an interaction between the coils.

[0039]

According to the RF coil and MRI apparatus of the present invention, a coil array using three or more coils can eliminate mutual interference between the coils. Therefore, S / N
The ratio can be improved. Further, it is possible to arrange adjacent coils close to each other without overlapping each other, and it is possible to improve the degree of freedom.

[Brief description of drawings]

FIG. 1 is a perspective view of an RF coil according to a first embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram of the RF coil according to the first embodiment of the present invention.

FIG. 3 is an essential part equivalent circuit diagram focusing on an influence of a first coil.

FIG. 4 is an equivalent circuit diagram of an RF coil according to a second embodiment of the present invention.

FIG. 5 is a perspective view of a main part of an MRI apparatus according to a fifth embodiment of the present invention.

[Explanation of symbols]

1, 2, 3 coils 4, 5, 6 coil connection capacitors 7,8,9 Resonant capacitor 10 to 13 parallel connection lines 14-17 Neutralizing condenser 21-23 coaxial cable 100,200 RF coil 300 MRI device

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takashi Ishiguro 127 GE Yokogawa Medical Systems Co., Ltd., 4-7 Asahigaoka, Hino City, Tokyo In-house (56) Reference JP-A-9-201346 (JP, A) Special Features Kaihei 3-236829 (JP, A) JP-A-6-86769 (JP, A) JP-A-6-343618 (JP, A) JP-A-3-297447 (JP, A) (58) Fields investigated ( Int.Cl. ⁷ , DB name) A61B 5/055

Claims (3)

(57) [Claims] 1. Three or more coils arranged in close proximity, an inter-coil connection capacitor provided in each coil, and a parallel connection line connecting the inter-coil connection capacitors of each coil in parallel, And a neutralization capacitor interposed in a parallel connection line, wherein the two parallel connection lines are connected to both ends of the inter-coil connection capacitor in one coil located at an end of the three or more coils. The parallel connection lines are connected to both ends of the inter-coil connection capacitors in the adjacent coils via the neutralizing capacitors, and the connection of the parallel connection lines is repeated toward the adjacent coils to locate at the end. The two parallel connection lines are connected to both ends of the inter-coil connection capacitor in the other coil. The transfer voltage transmitted from both ends of the continuous capacitor to both ends of the inter-coil connection capacitor of the other coil through the parallel connection line and the neutralization capacitor causes the one coil to pass through the mutual inductance to the coils of all the other coils. An RF coil characterized by canceling all induced voltages generated at both ends of an inter-connection capacitor. 2. A passive decoupling circuit is added to both ends of the neutralizing capacitor.
The RF coil according to 1. 3. An MRI apparatus comprising the RF coil according to claim 1 or 2.