JP2603921B2 - Receiving coil for magnetic resonance diagnostic equipment - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a receiving coil of a magnetic resonance diagnostic apparatus.

[Technical Background of the Invention and its Problems] In a conventional magnetic resonance diagnostic apparatus, a uniform static magnetic field is applied to a required imaging region of a subject, and an RF magnetic field is applied in a direction perpendicular to the static magnetic field by a transmission coil. The application causes a magnetic resonance (MR) phenomenon only in a specific slice portion for obtaining a tomographic image.
A magnetic resonance signal (hereinafter, referred to as an FID signal) generated from the nucleus after the release of the magnetic field is detected by a receiving coil,
By performing Fourier transform on this FID signal, a single spectrum for a specific nuclear spin rotation frequency is obtained. Then, after exciting the slice portion to cause an MR phenomenon, a linear magnetic field gradient having a linear gradient with respect to the X′-axis direction (a coordinate system rotated θ ゜ from the X-axis) is further applied to the static magnetic field. To obtain a composite FID signal,
The projection information of the slice portion is obtained by Fourier transforming the ID signal. Then, by rotating the X ′ axis in the XY plane and repeating the above-described operation, X
Projection information in each direction in the −Y plane is obtained, and a tomographic image of the subject can be synthesized based on the information.

Here, a conventional example of the above-described receiving coil will be described below taking a saddle type as an example.

As shown in FIG. 5, the conventional receiving coil 20 is, for example, N
A wound (N is a positive integer) electrically connected in series is used.

In such a receiving coil 20, the received signal can be increased by increasing the number of turns in order to change the inductance, but at the same time, the resistance value also increases, so that the Q (sharpness) of the receiving coil decreases.

In addition, since the inductance increases, the resonance frequency also decreases, which is not suitable as a high-frequency receiving coil.

[Object of the Invention] The present invention has been made in view of the above circumstances, and has a high resonance frequency, and a high Q value due to a skin effect in a high frequency band.
It is an object of the present invention to provide a receiving coil of a magnetic resonance diagnostic apparatus, which can prevent the decrease in noise by increasing the equivalent sectional area.

[Summary of the Invention] In order to achieve the above object, a receiving coil of a magnetic resonance diagnostic apparatus according to the present invention is arranged near an imaging area of the subject in a state where a plurality of coils connected in parallel with each other are stacked, The magnetic resonance signal synthesized by the parallel connection of the plurality of coils alone is used for the construction of a magnetic resonance tomographic image.

[Examples of the Invention] The principles and examples of the present invention will be described in detail below.

FIG. 3 is an explanatory view showing the principle of the present invention.
2 shows a state where two coils 1a and 1b are arranged to face each other.

The inductance of these coils 1a, 1b is L1, L2, and both coils
The mutual inductance between 1a and 1b is M, the current flowing through both coils 1a and 1b is I1 and I2, the strength of the magnetic field is H, the number of interlinkage magnetic flux is φ,
Assuming that the effective radius of both coils L1 and L2 is r, both coils 1a and 1b
Can be expressed by the following equation (1).

NI = _S Idφ = Hdl = 2πrH (1) where I = I1 + I2.

The number of interlinkage magnetic fluxes φ can be represented by the following equation (2).

φ = BS = μHS (2) where μ is the magnetic permeability and B is the magnetic flux density.

The induced voltage V generated by both coils 1a and 1b is given by (3)
It can be represented by an equation.

V = −∂φ / ∂t == − μSN1N2 / 2πr∂I / ∂t (3) Further, the induced voltages V1 and V2 of the coils 1a and 1b are as follows (4),
It can be expressed by equation (5).

V1 = L1∂I1 / ∂t + M∂I2 / ∂t (4) V2 = M∂I1 / ∂t + L∂I2 / ∂t (5) Therefore, total induction when the coils 1a and 1b are connected in series The voltage Vs is represented by the following equation (6).

Vs = V1 + V2 = (L1 + M) ∂I1 / ∂t + (L2 + M) ∂I2 / ∂t (6) On the other hand, the total induced voltage Vp when the coils 1a and 1b are connected in parallel
Is given by the following equation (7).

Vp = V1 = V2 = L1∂I1 / ∂t + M (∂I / ∂t-∂I1 / ∂t) = M∂I1 / ∂t + L2 (∂I / ∂t-∂I1 / ∂t) (7) However, I2 = I−I1.

From the above equation (7), ∂I1 = (L2−M) ∂I / (L1 + L2−2M) (8) By substituting equation (8) into equation (7) and rearranging, the following equation (9) is obtained. Become.

When ^{Vp = (L1L2-M 2)} / (L1 + L2-2M) · ∂I / ∂t ... (9) Considering the degree of coupling k ^{where, Vpα (N 2 1N 2 2} -kN 2 1N 2 2) / ( N ² 1 + N ² 2−2kN1N2) · ∂I / ∂t (10) And the parallel inductance Lp of both coils 1a and 1b is expressed by the following equation (11).

^{Lp = (L1L2-M 2)} / (L1 + L2-2M) α (N 2 1N 2 2-kN 2 1N 2 2) / (N 2 1 + N 2 2-2kN1N2) ...... (1
1) From the above equation (11), when the degree of coupling k between both coils 1a and 1b is zero (M = 0), the parallel inductance Lp of both coils 1a and 1b is expressed by the following equation (12).

Lp = L1L2 / (L1 + L2) (12) Especially when L1 = L2, Lp = L1 / 2. Also, L1 =
If L2 and N1 = N2, the parallel inductance Lp at this time is given by the following equation (13).

Lp = (1−k ² ) / 2 (1−k) = (1 + k) / 2 (13) The coupling k and the parallel inductance Lp shown in the equation (13)
Is shown in FIG.

As described above, if the coils 1a and 1b are sufficiently separated so that the mutual inductance M is not related, Lp = L1 / 2 (L1
= L2), and the value of the parallel inductance Lp can be kept low in the range of 1/2 <Lp <1 even if the mutual inductance M exists to some extent.

Next, an embodiment for realizing the above principle will be described with reference to FIG.

The receiving coils 10 shown in FIG. 1 are arranged to face each other so as to sandwich the subject P, and one terminal of each is connected to one electrode 15a, and the other terminal is connected to the other electrode 15b. First coil unit 11 composed of coils 11a and 11b
Like the first coils 11a and 11b, the first coil 11a and the first coil 11b are arranged so as to sandwich the subject P, and one terminal is connected to one electrode 15a and the other terminal is connected to the other electrode 15b. And a second coil unit 12 composed of the coils 12a and 12b, which are connected in parallel.

The first and second coil units 11 and 12 each have an arbitrary number of turns (N1, N2), and the coil 11a and the coil 11b have an arbitrary interval α (for example, several cm). The coils 11b and 12b are also arranged so as to have an interval α, and the mutual positional relationship between the two coils 11 and 12 can be adjusted by a conductive ring having a double structure (not shown).

Next, the operation of the receiving coil 10 having the above configuration will be described with reference to FIGS.
A description will be given also with reference to an explanatory diagram showing the arrangement relationship of the individual coils 11 and 12 with respect to the subject M shown in FIG.

When the first coil unit 11 and the second coil unit 12 are vertically symmetrical with respect to the subject P as shown in FIG. 2A, the magnetic resonance signal from the subject P Simple substance
Detected with almost the same phase by 11,12. At this time, the inductance of the coil 11 is L1, the coil 12 is
Assuming that the inductance of the detection coil 10 is L2 and the mutual inductance between the two is M,
Said (11) Lp = from equation ^{(L1L2-M 2) / (} L1 + L2-2M)
Becomes

Here, it is assumed that the mutual inductance M can be adjusted by the distance α between the coils 11 and 12.

On the other hand, if the positional relationship between the two coils 11 and 12 is shifted,
That is, as shown in FIG.
When only 12 is rotated clockwise in the figure, the mutual inductance between them becomes M '. As a result, the parallel inductance Lp' at this time becomes Lp '=
Become ^{(L1L2-M 2 ') /} (L1 + L2-2M').

In this case, it is necessary to correct the phase of the received signal according to the positional relationship between the two coils 11 and 12 alone. Here, M ′ can be adjusted by the angle θ between the single coils.

As described above, by adjusting the positional relationship between the two coils 11 and 12 as necessary, the value of the parallel inductance Lp can be reduced and the resonance frequency can be set to a high value.

In addition, since the two coils 11 and 12 are connected in parallel, the equivalent sectional area increases and the resistance value decreases compared to the case of the conventional receiving coil 20, so that the value of Q can be increased.

The present invention is not limited to the embodiments described above,
Various modifications are possible within the scope of the gist. For example,
In the above-described embodiment, the case where the receiving coil is configured by connecting two coils alone in parallel has been described. However, the present invention can be similarly implemented when the number of the coils is three or more.

[Effects of the Invention] According to the present invention described above, the resonance frequency and Q can be set to higher values, respectively, as compared with the conventional receiving coil.
Further, it is possible to provide a receiving coil of the magnetic resonance diagnostic apparatus which is not affected by the skin effect.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing an embodiment of the present invention, FIGS. 2 (a) and (b) are explanatory views each showing an arrangement relationship of a single coil in this embodiment, and FIG. 3 is a view showing the principle of the present invention. FIG. 4 is a graph showing the relationship between the coupling degree of the coil and the inductance, and FIG. 5 is an explanatory diagram showing a conventional receiving coil. 10: Receive coil, 11: First coil, 12: Second coil

Claims (2)

(57) [Claims] 1. A receiving coil of a magnetic resonance diagnostic apparatus for detecting a magnetic resonance signal generated from a subject, wherein the receiving coil is located near an imaging region of the subject in a state where a plurality of coils connected in parallel with each other are stacked. A receiving coil of a magnetic resonance diagnostic apparatus that is arranged and that provides a magnetic resonance tomographic image with a magnetic resonance signal synthesized by connecting a plurality of coils in parallel. 2. A coil according to claim 1, wherein said coils are arranged such that their mutual positional relationship is adjustable.
The receiving coil of the magnetic resonance diagnostic apparatus according to claim.