JP3107529B2 - Multiple tuning type high frequency coil for magnetic resonance imaging equipment. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multiple tuning type high frequency coil for a magnetic resonance imaging apparatus which can be tuned to a plurality of frequencies.

[0002]

2. Description of the Related Art As is well known, a magnetic resonance imaging apparatus is a high-frequency magnetic field that rotates at a specific frequency when a group of nuclei having a unique magnetic moment is placed in a uniform static magnetic field. It is a device that visualizes chemical and physical microscopic information of a substance or observes a chemical shift spectrum by utilizing the phenomenon of resonantly absorbing the energy of a substance.

In such a magnetic resonance imaging apparatus, a high-frequency coil is indispensable for exciting a specific nucleus in a region of interest of a subject or detecting an MR signal from the specific nucleus.

In recent years, MRS which detects signals of compounds containing ³¹ P (phosphorus), ¹⁹ F (fluorine), ¹³ C (carbon), ²³ N (sodium) and the like to observe the metabolic state in a living body has been developed. (Magnetic resonance spectroscopy) and MRSI
(Magnetic resonance spectroscopic imaging) is drawing attention.

[0005] However, since the concentrations of these elements in the body are extremely low and the relative sensitivity to ¹ H (proton) is very low, countermeasures such as performing signal observation at ¹ H using a technique such as polarization transfer have been taken. ing.

In such a case, ³¹ P
Dual tuneability, tuned to both equal and ¹ H, is required. Furthermore, when observing the bonding state of multiple nuclides,
Three or more synchronicities are required.

FIG. 11 shows a structural diagram and FIG. 12 shows an equivalent circuit of such a double-tuned high-frequency coil. As shown in FIG.
Double-tuned high-frequency coils are known.

However, in such a conventional double-tuned high-frequency coil, since the path of the high-frequency current flowing through the coil varies depending on the frequency, there is a problem that the transmission / reception sensitivity varies depending on the frequency. This problem is particularly inconvenient when the above-mentioned ¹ H is used for the signal observation method or when the bonding state of a plurality of nuclides is observed.

More specifically, taking the double-tuned high-frequency coil of FIG. 11 as an example, the low-frequency current mainly flows through the two inner ring-shaped conductors 101 and the linear conductor 102 parallel to the axis. High-frequency current is applied to the two outer ring-shaped conductors 1
03, and flows mainly to the linear conductor 102 parallel to the axis.

[0010] Therefore, when comparing areas where the transmission and reception sensitivities are uniform, the high frequency is wider than the low frequency.
In order to make these regions the same size, the inner and outer ring-shaped conductors 101 and 103 need to be close to each other, but this degrades particularly high-frequency sensitivity. In order to prevent the sensitivity deterioration, the distance between the inner ring-shaped conductors 101 is generally set to about 約 of the distance between the outer ring-shaped conductors 103.

Therefore, the low-frequency uniformity region is narrowed by about half in the axial direction with respect to the high frequency. For this reason, when photographing a human head, since the axial length of the high-frequency coil is limited by the interference between the two shoulders, there arises a problem that the entire head cannot be contained in the low-frequency sensitivity uniformity region.

In order to eliminate the frequency dependence of the sensitivity region size and make the sensitivity uniform regions of both frequencies the same, for example, as shown in FIG. 13, the inductance is brought close to both ends of the capacitance element. However, in that case, most of the energy of the coil is stored in the parallel resonance circuit composed of the inductance and the capacitance element, so that the high-frequency sensitivity is reduced.

[0013]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a multi-tuning type high frequency radio frequency resonance apparatus for a magnetic resonance imaging apparatus in which the transmission / reception sensitivity is not reduced and the axial length of a region where the sensitivity is uniform does not change with frequency. It is intended to provide a coil.

[0014]

SUMMARY OF THE INVENTION A multi-tuning high-frequency coil for a magnetic resonance imaging apparatus according to the present invention is provided with two ring-shaped conductors arranged opposite to each other, and arranged in a cylindrical shape so as to be connected to the ring-shaped conductors. A plurality of first linear conductors, and a plurality of second linear conductors arranged in a cylindrical shape outside the first linear conductor, wherein the first linear conductor and the second linear conductor are provided. A conductor is electrically connected to two opposed ring-shaped conductors by sharing a connection point, and a first capacitance element is inserted between at least one of the two ring-shaped conductors between adjacent connection points. A second capacitance element is inserted into the first linear conductor or the second linear conductor. (Operation) With this configuration, the first linear conductor, the second linear conductor sharing a connection point with the first linear conductor, and the first linear conductor or the second linear conductor are inserted into the first linear conductor. A parallel resonance circuit is formed from the second capacitance element and a series resonance circuit is formed from a part of the ring-shaped conductor sandwiched between adjacent connection points and the first capacitance element inserted into the ring-shaped conductor. Be composed.

At a high frequency, the current mainly flows through the lower one of the impedances of the first linear conductor and the second linear conductor, that is, the inner first linear conductor in which the second capacitance element is inserted. Outer second with relatively high impedance
Since almost no current flows through the linear conductor, the parallel resonant circuit exhibits capacitive characteristics. On the other hand, a series resonance circuit equivalently shows inductive properties. Therefore, at high frequencies, it can be regarded as a low-pass birdcage coil.

On the other hand, at a low frequency, current hardly flows through the inner first linear conductor having a relatively high impedance, and mainly flows through the outer second linear conductor having a relatively low impedance. The parallel resonance circuit shows inductive properties. On the other hand, a series resonance circuit equivalently shows capacitive. Therefore,
It can be considered as a high-pass birdcage coil at low frequencies.

As described above, the double tuning can be realized, and the current flows through the same ring-shaped conductor irrespective of the high frequency and the low frequency, so that the high-frequency uniformity region and the low-frequency uniformity region are the same in the axial direction. Length.

[0018]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a multi-tuning high-frequency coil for a magnetic resonance imaging apparatus according to the present invention. This high-frequency coil is of a so-called multiple tuning type that can tune to a plurality of frequencies, but is described as a double tuning for convenience of explanation. (First Embodiment) FIG. 1 shows the structure of a high-frequency coil according to a first embodiment. This high-frequency coil is of a so-called "bird cage" in which conductors are assembled in a cylindrical shape. The central axis of this cylinder is indicated by reference numeral 20. A plurality of linear conductors (hereinafter, referred to as first linear conductors) 3 having a certain length are arranged in a cylindrical shape around the central axis 20. That is, a plurality of first
The linear conductor 3 is parallel to the central axis 20 and the central axis 20
Are arranged discretely and regularly at regular intervals at positions equidistant from.

Similarly, outside the first linear conductor 3, the same length and the same number of linear conductors (hereinafter referred to as “the first linear conductor 3”)
7) are arranged in a cylindrical shape. That is, the plurality of second linear conductors 7 are parallel to the central axis 20,
Moreover, they are arranged discretely and regularly at regular intervals at positions equidistant from the central axis 20.

The inner first linear conductor 3 is electrically connected to the two ring-shaped conductors 2 facing each other at a connection point 8. Similarly, the outer second linear conductor 7 also
Two ring-shaped conductors 2 sharing the connection point 8 with the linear conductor 3
Is electrically connected to

Each of these two ring-shaped conductors 2 has one capacitance element 5 inserted between adjacent connection points 8. Capacitance elements 4 are inserted one by one also near the center of the first linear conductor 3 (or the second linear conductor 7).

FIG. 2 shows the structure of a part of FIG. 1 in detail. The cylindrical arrangement of the first and second linear conductors 3 and 7 is the first and second linear conductors on the inner side 1 and the outer side 6 of a nonmagnetic plastic or a double cylindrical structure made of resin such as acrylic, ABS, Teflon or the like. The conductors 3 and 7 are held by forming them.

The double-tuned high-frequency coil configured as described above is represented by an equivalent circuit shown in FIG. This high-frequency coil is formed by combining a plurality of parallel resonance circuits 30 and a plurality of series resonance circuits 40 on the circuit. Parallel resonance circuit 30
Are the first linear conductors 3 and the first linear conductors 3 and the connection points 8
And a capacitance element 4 inserted in the first linear conductor 3. On the other hand, the series resonance circuit 40 includes a part (hereinafter, this part is referred to as a ring element) 9 of the ring-shaped conductor 2 sandwiched between the adjacent connection points 8 and a capacitance element inserted into the ring-shaped conductor 2. It is made up of five.

With this configuration, the two tunable resonance frequencies are defined as ω _{H for} the higher one and ω _L for the lower one.
It is given as: L1 is the series resonance circuit 40
C3 is the capacitance of the capacitance element 5 of the series resonance circuit 40, L2 is the inductance of the second linear conductor 7 of the parallel resonance circuit 30, C4
Is the capacitance of the capacitance element 4 of the parallel resonance circuit 30. N is the number of elements formed of one parallel resonance circuit 30 and one series resonance circuit 40.

[0025]

(Equation 1)

At a high frequency, the current mainly flows through the lower one of the impedance of the first linear conductor 3 and the second linear conductor 7, that is, the inner first linear conductor 3 in which the capacitance element 4 is inserted. Since the current flows and hardly flows to the outer second linear conductor 7 having a relatively high impedance, the parallel resonance circuit 30 exhibits a capacitance. On the other hand, the series resonance circuit 40 shows inductive properties equivalently. Therefore, at high frequencies, it can be regarded as a low-pass birdcage coil.

On the other hand, at a low frequency, the current hardly flows through the inner first linear conductor 3 having a relatively high impedance, and mainly flows through the outer second linear conductor 7 having a relatively low impedance. Therefore, the parallel resonance circuit 30 exhibits inductive properties. On the other hand, the series resonance circuit 40 is equivalently capacitive. Therefore, it can be regarded as a high-pass birdcage coil at a low frequency.

As described above, in this embodiment, double tuning can be realized, and the current flows through the same ring-shaped conductor irrespective of the high frequency and the low frequency. Can be formed to the same length in the axial direction.

In the above description, the capacitance element 4 is inserted into the inner first linear conductor 3, but the outer second linear conductor 3 is inserted.
The capacitance element 4 may be inserted into the linear conductor 7. (Second Embodiment) FIG. 4 shows the structure of a high-frequency coil according to a second embodiment. The same parts as those in FIG.

The second embodiment is different from the first embodiment in that the capacitance element 4 is inserted into the first linear conductor 3 inside in the first embodiment, whereas the second embodiment differs from the first embodiment. Is that the capacitance elements 4 are alternately inserted into the inner first linear conductors 3 and the outer second linear conductors 7.

As described above, in the first embodiment, the current path in the axial direction is different between the high frequency and the low frequency. That is, the high-frequency current flows through the inner first linear conductor 3, while the low-frequency current flows through the outer second linear conductor 7. Therefore,
The transmission / reception sensitivity with respect to the center axis 20 is slightly different depending on a slight difference in the radial distance between the two paths.

The high-frequency current flows through the inner first linear conductor 3 and the outer second linear conductor 7 with the capacitance element 4 inserted, while the low-frequency current flows through the inner first linear conductor 3. And the second linear conductor 7 on the outside where the capacitance element 4 is not inserted.

As described above, as in the present embodiment, the capacitance element 4 is connected to the inner first linear conductor 3 and the outer second linear conductor 3.
By alternately inserting the high-frequency current and the low-frequency current into the linear conductor 7, the bending path has the inner first path.
It is not biased to one of the linear conductor 3 and the outer second linear conductor 7.

Therefore, the difference between the transmission / reception sensitivity with respect to the central axis 20 at a high frequency and the transmission / reception sensitivity with respect to the central axis 20 at a low frequency can be minimized to zero or negligible. Third Embodiment FIG. 5 shows the structure of a high-frequency coil according to a third embodiment. The third embodiment is different from the first embodiment in that one of the two ring-shaped conductors 2, specifically, the ring-shaped conductor on the opposite side of the insertion opening (A) for inserting the subject into the cylinder. 2 is replaced with a disc-shaped conductor 11.

A so-called mirror image phenomenon is exerted by the disc-shaped conductor 11, and a region having uniform transmission / reception sensitivity is substantially formed as if a high-frequency coil of the same shape exists on the opposite side of the disc-shaped conductor 11. Will be expanded.

This is based on the assumption that the subject is inserted from the direction (A). However, as shown in FIG. 6, an annular conductor 11 'having an open central portion is adopted, and The subject may be inserted even from the direction (B).

In the above description, the capacitance element 4 is inserted into the inner first linear conductor 3, but the outer second conductor 3 is inserted.
The capacitance element 4 may be inserted into the linear conductor 7. (Fourth Embodiment) Although the first embodiment has been described with double tuning, it can be actually extended to multiple tuning of triple or more. The fourth embodiment relates to this extension method.

FIG. 7 shows the structure of the high-frequency coil according to the fourth embodiment. FIG. 8 shows the structure of a part of FIG. 7 in detail. The fourth embodiment is different from the first embodiment in that two capacitance elements 13 and 14 are inserted into each of the inner first linear conductors 3 and the inner first linear conductor 3 is inserted between the capacitance elements 13 and 14. The first linear conductor 3 and the outer second linear conductor 7 are connected by a connecting conductor 12.

FIG. 9 shows an equivalent circuit of the high-frequency coil according to the present embodiment. With the configuration described above, 2
The two parallel resonance circuits 50 and 60 are connected in series. Although there is inductive coupling between the resonance circuits, in order to make the principle easier to understand, ignoring this and simplifying it, the relationship between the electrical equivalent circuit (FIG. 9) and FIGS. 7 and 8 is considered as follows. Can be

That is, the parallel resonance circuit 50 is formed by a part 1 of the inner first linear conductor 3 substantially separated by the connecting conductor 12.
7, a portion 15 of the outer second linear conductor 7 in a parallel relationship with the portion 17, and a capacitance element 13 inserted into the portion 17 of the first linear conductor 3. .

On the other hand, the parallel resonance circuit 60 includes a portion 18 of the inner first linear conductor 3 substantially separated by the connecting conductor 12.
It is conceivable that the second linear conductor 7 is arranged in parallel with the portion 18 and the capacitance element 14 inserted into the portion 18 of the first linear conductor 3.

FIG. 10A shows the frequency characteristic of the impedance of the series resonance circuit 40 of the ring element 9, and FIG. 10B shows two parallel resonance circuits 50 and 60 connected in series.
3 shows the frequency characteristics of the impedance of FIG. The resonance frequency of the series resonance circuit 40 is represented by ω ₁₁ , the resonance frequency of the parallel resonance circuit 50 is represented by ω _1P , and the resonance frequency of the parallel resonance circuit 60 is represented by ω _2P .

The frequencies ω ₁ ′, ω indicated by the dotted lines in FIG.
₂ ′, ω ₃ ′, the series resonance circuit 40 and the parallel resonance circuit 50
And the parallel resonance circuit 60 are in a series resonance state, and are associated with the resonance state of the entire coil. The three actually tunable resonance frequencies ω ₁ , ω ₂ , ω ₃ are
Assuming that the total number of basic elements each including 0, one parallel resonance circuit 50, and one parallel resonance circuit 60 is N, the number is given as follows.

[0044]

(Equation 2)

To further increase the tuning frequency, the number of parallel resonant circuits connected in series may be increased. The present invention is not limited to the embodiments described above, and can be implemented with various modifications. For example, in the above description, it is described as being assembled in a cylindrical shape, but it may be assembled in an elliptical cylindrical shape.

[0046]

According to the present invention, the high-frequency uniformity region and the low-frequency uniformity region can be formed to have the same length in the axial direction without lowering the transmission efficiency and the signal detection sensitivity.

[Brief description of the drawings]

FIG. 1 is a structural view of a multi-tuning high-frequency coil for a magnetic resonance imaging apparatus according to a first embodiment of the present invention.

FIG. 2 is a detailed structural diagram of a peripheral portion of the linear conductor of FIG. 1;

FIG. 3 is an equivalent circuit diagram of the multiple tuning type high frequency coil for the magnetic resonance imaging apparatus of FIG. 1;

FIG. 4 is a structural diagram of a multiple tuning type high frequency coil for a magnetic resonance imaging apparatus according to a second embodiment.

FIG. 5 is a structural diagram of a multi-tuning high-frequency coil for a magnetic resonance imaging apparatus according to a third embodiment.

FIG. 6 is a diagram showing a modification of the third embodiment.

FIG. 7 is a structural diagram of a multi-tuning high-frequency coil for a magnetic resonance imaging apparatus according to a fourth embodiment.

FIG. 8 is a detailed structural view of the periphery of the linear conductor of FIG. 7;

FIG. 9 is an equivalent circuit diagram of the multiple tuning type high frequency coil for the magnetic resonance imaging apparatus of FIG. 7;

FIG. 10 is a diagram illustrating frequency characteristics of impedance of the series resonance circuit and the parallel resonance circuit of FIG. 9;

FIG. 11 is a structural diagram of a conventional typical double-tuned high-frequency coil.

FIG. 12 is an equivalent circuit diagram of FIG. 11;

FIG. 13 is a structural view of another conventional double-tuned high-frequency coil.

[Explanation of symbols]

 Reference numeral 2 denotes a ring conductor, 3 denotes a first linear conductor, 4 denotes a capacitance element for parallel resonance, 5 denotes a capacitance element for series resonance, 7 denotes a second linear conductor, 8 denotes a connection point, and 9 denotes a ring element.

Claims (5)

(57) [Claims] 1. Two ring-shaped conductors arranged to face each other, a plurality of first linear conductors arranged in a cylindrical shape so as to be connected to the ring-shaped conductors, and an outer side of the first linear conductors A plurality of second linear conductors arranged in a cylindrical shape, and the first linear conductor and the second linear conductor
The two ring-shaped conductors are electrically connected by sharing a connection point, and at least one of the two ring-shaped conductors is inserted with a first capacitance element between adjacent connection points,
A multi-tuning high-frequency coil for a magnetic resonance imaging apparatus, wherein a second capacitance element is inserted into the first linear conductor or the second linear conductor. 2. The device according to claim 1, wherein the second capacitance element is alternately inserted into the first linear conductor and the second linear conductor along a circumferential direction of the ring conductor. 2. The multiple tuned high frequency coil for a magnetic resonance imaging apparatus according to claim 1. 3. The multiple tuned high-frequency coil for a magnetic resonance imaging apparatus according to claim 1, wherein one of said two ring-shaped conductors is a substantially disc-shaped conductor. 4. The multiple tuned high-frequency coil for a magnetic resonance imaging apparatus according to claim 1, wherein one of said two ring-shaped conductors is a substantially annular plate-shaped conductor. 5. A third linear conductor inserted into the first linear conductor or the second linear conductor in series with the second capacitance element.
It further includes a capacitance element, and a communication conductor that electrically connects the first linear conductor and the second linear conductor between the second capacitance element and the third capacitance element. A multiple tuned high frequency coil for a magnetic resonance imaging apparatus according to claim 1.