JP3422556B2 - RF coil for MRI - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface coil used in a magnetic resonance imaging (MRI) apparatus, and in particular, an MR used for imaging in a direction interlinking with a main magnetic field.
The present invention relates to an RF coil for I.

[0002]

2. Description of the Related Art An MRI apparatus measures the density distribution, relaxation time distribution, etc. of nuclear spins at a desired examination site in a subject by utilizing the nuclear magnetic resonance phenomenon, and the cross section of the subject is determined from the measured data. The image is displayed.

Nuclear spins of a subject placed in a uniform and strong static magnetic field generator perform precession around the direction of the static magnetic field at a frequency (Larmor frequency) determined by the strength of the static magnetic field. Therefore, when a high frequency pulse having a frequency equal to this Larmor frequency is irradiated from the outside, spins are excited and transition to a high energy state. This is called a nuclear magnetic resonance phenomenon. When the irradiation of this high-frequency pulse is stopped, the spin returns to the original low energy state with a time constant corresponding to each state, and at this time, the electromagnetic wave is emitted to the outside. A high-frequency receiver coil (R
F coil) to detect. At this time, a triaxial gradient magnetic field is applied to the static magnetic field space for the purpose of adding position information to the space. As a result, position information in the space can be captured as frequency information.

Such an MRI apparatus is equipped with a surface coil having a high sensitivity particularly in imaging the spine of a subject, and this surface coil is used as the RF coil for MRI. .

FIG. 5 is a configuration diagram showing a conventional configuration of this type of surface coil. This coil 101 is vertical (Y
It is used for an MRI apparatus having a static magnetic field in the (axis) direction, and is arranged below a mounted subject.

Here, the coil 101 has a current line in which two loops are connected and arranged in a figure eight shape. The coil 101 is provided with first and second conductor portions 101a, 10a arranged parallel to the spinal direction of the subject.
1b, and the first and second conductor portions 101
It is formed as an 8-shaped coil so that the currents flowing through a and 101b are in the same direction. In addition, this conductor portion 10
1a and 101b are called a main path unit. Then, the signal flux (flux) from the X-direction magnetic field component 103 created by the spins in the spine on the main path portions 101a and 101b is detected. The conductor parts 101c and 101d are called return path parts.

[0007]

If the main path portions 101a and 101b thus configured have an infinite length, the signal flux from the X-axis direction magnetic field component 103 can be detected with a constant sensitivity. However, the main path unit 101
Since a and 101b actually have a finite length, the sensitivity of the end portion is lowered due to the influence of the end portion (end effect).
Due to such an influence of the end portion, in reality, a portion near the end portion of the area (imaging area) surrounded by the main path portions 101a and 101b causes a defect due to sensitivity decrease.

FIG. 6 shows that the coil longitudinal direction Z0 is 30 cm, 2X.
Taking Y as an example of a coil with a main path of 8 cm, Y
(Sensitivity in the vertical direction) 4,6,8 cm sensitivity characteristics,
FIG. 7 is a characteristic diagram showing a region from the center point (Z0 / 2) in the Z direction as the origin to the end. In this FIG. 6, Z = 0,
Using the sensitivity of Y = 4 cm as the reference value (1), relative sensitivity Bx ′ (= Bx (z, y) at other points of Z (0 to 15 cm) and Y (4,6,8 cm)) / Bx (0,4))
Is shown. As is apparent from FIG. 6, it can be read that the sensitivity gradually decreases in the coil longitudinal direction Z from the center toward the end. For this reason, there is a problem that the imaging region having practical sensitivity is narrowed in the Z direction.

The present invention has been made in view of the above points, and an object thereof is to reduce the sensitivity (uneven sensitivity) at the end of the main path portion of the coil.
It is to realize the F coil.

[0010]

A first means for solving the above-mentioned problems has a current line in which two loops are connected and arranged in a figure-eight shape. The current lines at the portions where the two loops of the line oppose each other are arranged side by side as two main path parts with a predetermined interval in the same current direction, and the interval between the central parts of these two main path parts is close. Is an RF coil for MRI characterized in that it is configured to be larger than the distance between the end portions.

Further, the second means for solving the above-mentioned problem has a current line in which two loops are connected and arranged in an 8-shape, and two of the 8-shaped current lines are provided. The current lines in the portions where the loops face each other are arranged side by side as two main path portions having a predetermined distance in the same current direction, and the distance between the central portions of these two main path portions is the maximum, The RF coil for MRI is characterized in that the interval is gradually reduced toward the distal end.

[0012]

The RF for MRI which is the first means for solving the problems
In the coil, the distance between the two main path portions arranged in parallel is smaller at the terminal end than in the vicinity of the central part, and the sensitivity improves as this distance becomes narrower, so the sensitivity decreases as it approaches the terminal end. Offset. As a result,
The decrease in sensitivity near the end is suppressed, and the area where the sensitivity is decreased becomes smaller.

A second means for solving the problem, M
In the RI RF coil, the distance between the two main paths arranged in parallel is the largest in the central portion and gradually decreases toward the end, so the distance between the main paths is narrowed. Accordingly, the sensitivity is improved, and the decrease in sensitivity that occurs as the distance to the end portion is canceled out. As a result, the decrease in sensitivity near the end is suppressed, and the area in which the sensitivity is decreased becomes small.

[0014]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a configuration diagram showing a configuration of an MRI RF coil (hereinafter, simply referred to as a coil) and an MRI apparatus according to an embodiment of the present invention. Further, FIG. 2 is an explanatory diagram showing in detail the shape of the coil.

In these drawings, the coil 1 is a kind of surface coil called a conventional figure-eight coil. Here, the coil 1 has two loops connected to each other.
It has a current line arranged in a V shape. Then, the current lines (1a, 1b) at the portions where the two loops of the figure-eight current line are opposed to each other will be described below as two main path parts 1a, 1b having a predetermined interval in the same current direction. They are arranged side by side in the shape described. That is, the main path portions 1a and 1b to be arranged along the longitudinal direction of the measurement object (for example, the spine of the subject) and the return path portions 1c and 1d to be arranged at positions distant from the measurement object. I have it. Here, the currents are designed to flow in the directions 1c, 1a, 1d, 1b, and in the main path portions 1a, 1b arranged in parallel, the currents flow in the same direction, and the main path portions 1a, 1b. The area surrounded by is the imaging area.

The weak received signal at the coil 1 is amplified to a predetermined level by the preamplifier 3 via a protection means such as a λ / 4 protection section 2 for protecting the preamplifier from the transmission RF power. After that, the amplified received signal is subjected to various signal processing required for a predetermined MRI in the signal processing device 4. Since the signal processing in the signal processing device 8 is various known processing, it will not be described here.

Here, the sensitivity of the coil 1 when it is assumed to have an infinite length is inversely proportional to the distance between the main path portions 1a and 1b. On the other hand, since the actual coil 1 has a finite length, the sensitivity is lowered at the end by the end effect. Therefore, as shown in FIG. 2, by increasing the distance (Xc) near the center and decreasing the distance (Xe) toward the end, the influence of the end effect can be reduced. . As a result, the area in which the sensitivity is lowered is reduced and the practical imaging area is expanded.

FIG. 3 shows a coil having a longitudinal direction Z0 of 30 cm, a main path center interval Xc of 8.4 cm, and a main path end interval Xe of 7.6 cm. It is a sensitivity characteristic figure which shows the result at the time of actually measuring the sensitivity characteristic of 4,6,8 cm of the depth direction of a vertical direction with respect to the area | region to an end with the center point (Z0 / 2) of Z direction as an origin.

In FIG. 3, the sensitivity of Z = 0 and Y = 4 cm is set as the reference value (1), and Z (0 to 15c) is set for other points.
m), Y (4, 6, 8 cm) Relative sensitivity at each point Bx ′
(Bx '= Bx (z, y) / Bx (0,4)).

As is apparent from FIG. 3, by widening the interval Xc at the center of the main path and narrowing the interval at the end Xe of the main path, the sensitivity is increased between the center and the end. You can read that.

That is, paying attention to the sensitivity at Y = 4 cm, the sensitivity reaches a peak around Z = 5 cm away from the central portion (Z = 0), and the sensitivity gradually decreases from there. ing.

Here, the central portion in the Z direction (Z = 0)
When the region in which the sensitivity decrease from 10 to 10% is within the coil sensitivity uniform region is read from FIG. 3 and FIG. 6 showing the characteristics of the conventional coil, the following results are obtained.

Y Uniform area of prior art Uniform area of the present invention Improvement rate 4 cm 17.2 cm 18.8 cm 9% 6 cm 16.8 cm 18.2 cm 8% 8 cm 17.2 cm 18.0 cm 5% When the area of 10% is a uniform area, the effect of expanding the uniform area by about 5% to 10% is obtained as compared with the conventional coil of the parallel main path portions.

Here, the improvement rate decreases as the value of Y (measurement depth in the vertical direction) increases. The reason is that the signal flux shown in FIG. This is because the range that can be detected is affected.

Since the distance between the main path portions is changed relative to the central portion and the terminal portion, the central portion is widened at the same time as the central portion is widened as compared with the conventional parallel main path portions. Three types of shapes are conceivable: narrowing, widening the central part and leaving the end part at the same interval as before, and narrowing the end part while leaving the center part at the same interval as before. The effect of expanding the uniform region is the same in these three modes.

For example, the same effect can be obtained by not narrowing the end portion too much and widening the center portion as much as possible. In this case, the main path interval tends to be widened as a whole. The range in which the signal flux can be detected becomes wider, and the effect of widening the uniform region at the position where the measurement depth is large tends to become greater. Therefore,
The distance between the central portion and the end portion may be arbitrarily set so that the effect of expanding the uniform region at the desired measurement depth is increased.

Further, in the coil 1 shown in FIGS. 1 and 2, the intervals are widened in the central portions (Z0 / 2) of both main pass portions so that the enclosed area becomes a diamond shape (two main pass portions). The distance between the central portions of the portions is the maximum, and the distance gradually decreases toward the end portion), but other shapes are also possible. For example, it is possible to make the main path portion into an arc, parabola, or polygonal shape so that the distance between the central portions is wider than the distance between the end portions. In this case, in addition to the circular arc shape as shown in FIG. 4A and the polygonal shape as shown in FIG. 4B, a main path portion which is parallel and a main path portion which is not parallel as shown in FIG. 4C are combined. It is also possible. In this case, the position of the sensitivity peak of the sensitivity characteristic shown in FIG. 3 and the characteristic of the sensitivity decrease at the end portion change, so that it may be arbitrarily selected so as to obtain the desired uniformity.

Further, it is possible to widen the positions other than the central portion (Z0 / 2) to the maximum, and it is also possible to widen the position deviated from the central portion to either one. In this case, it is also possible to combine with the shapes of FIGS. 4 (a) to 4 (c) described above. In this case as well, the position of the sensitivity peak changes, so that it can be arbitrarily selected so as to obtain desired characteristics. That is, various deformations are possible as long as the shape is such that the distance between the central portions of the two main path portions (including the central portion and the surrounding area) is larger than the distance between the end portions.

As described in detail above, the two main parts arranged side by side for a predetermined distance are arranged side by side so that the central portion of the figure-eight current line is in the same direction as the current direction along the object to be measured. The pass portion is sandwiched between these two main pass portions, and the interval is configured such that the vicinity of the central portion is larger than the end portion, so that the sensitivity decrease that occurs toward the end portion is suppressed, and the sensitivity is substantially uniform. The effect of expanding the area is obtained.

Further, since the distance between the central portions sandwiched between these two main path portions is the maximum and the distance is gradually reduced toward the terminal end, the distance is generated toward the terminal end. The decrease in sensitivity is suppressed evenly toward both end portions, and the effect that the region where the sensitivity is substantially uniform spreads toward both end portions is obtained.

[0031]

As described in detail above, by being configured so that the distance between the central portion is sandwiched between the main path portions and is larger than that at the end portion, the decrease in sensitivity occurring toward the end portion is suppressed, The effect that the region where the sensitivity is substantially uniform is expanded can be obtained.

Further, since the distance between the central portions sandwiched by the main path portions is the largest and the distance is gradually reduced toward the terminal end, the sensitivity decrease toward the terminal end is reduced. The effect is suppressed evenly toward the end portions, and an effect is obtained in which a region having substantially uniform sensitivity spreads toward both end portions. Therefore, it is possible to realize the MRI RF coil capable of improving the nonuniformity of the sensitivity in the measurement region.

[Brief description of drawings]

FIG. 1 is an RF coil for MRI and M according to an embodiment of the present invention.
It is a block diagram which shows the structural example of RI apparatus.

FIG. 2 is a configuration diagram showing details of a configuration example of an MRI RF coil according to an embodiment of the present invention.

FIG. 3 is a characteristic diagram showing sensitivity characteristics of the MRI RF coil according to the embodiment of the present invention.

FIG. 4 is a configuration diagram showing a configuration example of an MRI RF coil according to another embodiment of the present invention.

FIG. 5 is an explanatory diagram showing a configuration and a sensitivity distribution of a conventional 8-shaped coil.

FIG. 6 is a characteristic diagram showing sensitivity characteristics when a conventional 8-shaped coil is used.

[Explanation of symbols]

1 coil 1a, 1b Current path of main path 1c, 1d Current path of return path 2 λ / 4 protection section 3 preamplifier 4 Signal processing unit

Claims (2)

(57) [Claims] 1. A current line is provided in which two loops are connected to each other and are arranged in a figure eight shape. Two of the figure eight current lines are provided.
The current lines in the part where the two loops face each other are arranged side by side as two main path parts with a predetermined interval in the same current direction, and the interval near the center of these two main path parts is the end part. An RF coil for MRI, wherein the RF coil is configured to be larger than the interval. 2. A current line is provided in which two loops are connected and arranged in a figure eight shape, and two of the figure eight current lines are provided.
The current lines at the portions where the two loops face each other are arranged side by side as two main path parts with a predetermined interval in the same current direction, and the interval between the central parts of these two main path parts is maximum. The RF coil for MRI, wherein the distance is gradually reduced toward the end.