MAGNETIC RESONANCE IMAGING APPARATUS
A magnetic resonance imaging apparatus includes a bore configured to accommodate a subject therein, an RF coil positioned about the bore, and an RF shield positioned about the RF coil. The RF coil includes a first portion positioned adjacent a lower surface side of the bore and spaced a distance from the RF shield that is larger than a distance between a second portion of the RF coil and an upper surface side of the bore.
This application claims the benefit of Japanese Patent Application No. 2010-169901 filed Jul. 29, 2010, which is hereby incorporated by reference in its entirety.
The embodiments described herein relate to a magnetic resonance imaging apparatus having an RF coil.
An RF coil for sending transmission pulses is installed within a magnetic resonance imaging apparatus. The diameter of the RF coil is related to the size of a bore into which a subject is carried. Therefore, the value of the RF coil diameter is very important. The bore can be made large by making the RF coil diameter large, thus making it possible to diminish the sense of oppression of a subject when carried into the bore. However, if the RF coil diameter is made large, it is required to increase the electric power to be supplied to the RF coil, thus giving rise to the problem that electric power consumption increases. Moreover, since an RF shield is disposed around the RF coil, if the RF coil diameter is made large, the spacing between the RF coil and the RF shield becomes narrower. The RF shield acts to cancel a magnetic field generated by the RF coil, and the narrower the spacing between the RF coil and the RF shield, the more remarkable the action of the RF shield. Thus, there arises the problem that the narrower the spacing between the RF coil and the RF shield, the larger the electric power to be supplied to the RF coil, resulting in a further increase of electric power consumption.
The use of an elliptic RF coil has been proposed as a method for solving the above problem in, for example, Japanese Unexamined Patent Publication No. Hei 7 (1995)-222729.
However, since the RF coil described in Japanese Unexamined Patent Publication No. Hei 7 (1995)-222729 is elliptic, the coil diameter in the minor axis direction of the ellipse cannot be made large. Accordingly, there is the problem that the bore cannot be made large in the minor axis direction of the ellipse and that therefore a subject who has been carried into the bore is apt to have a sense of oppression.
BREIF DESCRIPTION OF THE INVENTIONThe embodiments described herein provide a magnetic resonance imaging apparatus including: a bore for accommodating a subject; an RF coil disposed around the bore; and an RF shield disposed around the RF coil, the RF coil being constructed such that a portion of the RF coil disposed on a lower surface side of the bore is spaced more distant from the RF shield than a portion of the RF coil disposed on an upper surface side of the bore.
By constructing the RF coil as above it is possible to decrease electric power consumption of the RF coil while ensuring a required size of the bore.
Further objects and advantages of the embodiments described herein will be apparent from the following description of embodiments of the invention as illustrated in the accompanying drawings.
An embodiment of the invention will be described below, but the invention is not limited to the following embodiment.
The magnetic resonance imaging apparatus (hereinafter referred to as “MRI apparatus,” MRI: Magnetic Resonance Imaging) indicated at 100 has a magnetic field generator 2 and a table 3.
The magnetic field generator 2 is provided with a bore 21 for accommodating a subject. Within the magnetic field generator 2, a birdcage coil 22 for transmission of RF pulses and reception of magnetic resonance signals from the subject and an RF shield 23 for decreasing RF power radiated outside the MRI apparatus 100 are installed.
The magnetic field generator 2 has a coil support 220 for supporting the birdcage coil 22. The coil support 220 is cylindrical, and inside the coil support 220 is mounted a cradle support base 221 for supporting a cradle 31 (see
The birdcage coil 22 has two rings D1, D2 and n number of legs Li (i=1 to n) for connecting the two rings D1 and 2 with each other. In this embodiment, n is set at 16. Therefore, the birdcage coil 22 has sixteen legs L1 to L16.
The birdcage coil 22 has loop circuits Ci,j. Each loop circuit Ci,j is formed using adjacent legs Li, Lj and the rings D1, D2. For example, a loop circuit C1,2 is formed using two adjacent legs L1, L2 and the two rings D1, D2, and a loop circuit C16, 1 is formed using two legs L16, L1 and the two rings D1, D2. In
Next, a description will be given below about the shape of the birdcage coil 22 and that of the RF shield 23.
As shown in
On the other hand, the ring D1 of the birdcage coil 22 is constructed so that an upper half Da (a portion positioned on an upper surface 21a side of the bore 21) of the ring D1 and a lower half Db (a portion positioned on a lower surface 21b side of the bore 21) of the ring D1 provide an asymmetric shape.
As shown in
Although the ring D1 is illustrated in
Therefore, as shown in
In the case of a birdcage coil 22′ shown in
On the other hand, in
Since the lower half 22b of the birdcage coil 22 and the upper half 22a of the birdcage coil 22 are asymmetric in shape, there sometimes is a case where the uniformity of the magnetic field is disordered. Once the uniformity of the magnetic field is disordered, a bad influence is exerted on the image quality. Therefore, it is desirable that the magnetic field be as uniform as possible. The magnetic field can be made as uniform as possible by adjusting the impedance distribution of the birdcage coil 22 so that the impedance of the upper half 22a of the birdcage coil 22 becomes high, while the impedance of the lower half 22b of the birdcage coil 22 becomes low. For example, in connection with the loop circuits Ci,j (see
In the graph of
In
In the graph of
In the birdcage coil 22 according to this embodiment, the upper half Da of the ring D1 has a semi-circular shape (see
An upper half Da of the ring D1 has a semi-circular shape like the ring D1 shown in
Although in the above embodiment there is shown an example of using the birdcage coil 22 as the RF coil, the RF coil used in the invention may be an RF coil other than the birdcage coil.
Many widely different embodiments of the invention may be configured without departing from the spirit and the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.
Claims
1. A magnetic resonance imaging apparatus comprising:
- a bore configured to accommodate a subject therein;
- an RF coil positioned about the bore; and
- an RF shield positioned about the RF coil,
- wherein the RF coil comprises a first portion positioned adjacent a lower surface side of the bore and spaced a distance from the RF shield that is larger than a distance between a second portion of the RF coil and an upper surface side of the bore.
2. A magnetic resonance imaging apparatus according to claim 1, wherein the first portion of the RF coil has a smaller impedance than an impedance of the second portion of the RF coil.
3. A magnetic resonance imaging apparatus according to claim 1, wherein the RF coil comprises:
- a first ring and a second ring both positioned about the bore; and
- a plurality of legs connecting the first and second rings with each other.
4. A magnetic resonance imaging apparatus according to claim 2, wherein the RF coil comprises:
- a first ring and a second ring both positioned about the bore; and
- a plurality of legs connecting the first and second rings with each other.
5. A magnetic resonance imaging apparatus according to claim 3, wherein:
- an upper half of the first ring and an upper half of the second ring are semi-circular in shape; and
- a lower half of the first ring and a lower half of the second ring are semi-elliptic in shape.
6. A magnetic resonance imaging apparatus according to claim 4, wherein:
- an upper half of the first ring and an upper half of the second ring are semi-circular in shape; and
- a lower half of the first ring and a lower half of the second ring are semi-elliptic in shape.
7. A magnetic resonance imaging apparatus according to claim 3, wherein the RF coil comprises a plurality of loop circuits including the first and second rings and a plurality of legs, a first portion of loop circuits of the plurality of loop circuits that is positioned adjacent the lower surface side of the bore having a smaller impedance than an impedance of a second portion of loop circuits of the plurality of loop circuits that is positioned adjacent the upper surface side of the bore.
8. A magnetic resonance imaging apparatus according to claim 4, wherein the RF coil comprises a plurality of loop circuits including the first and second rings and a plurality of legs, a first portion of loop circuits of the plurality of loop circuits that is positioned adjacent the lower surface side of the bore having a smaller impedance than an impedance of a second portion of loop circuits of the plurality of loop circuits that is positioned adjacent the upper surface side of the bore.
9. A magnetic resonance imaging apparatus according to claim 5, wherein the RF coil comprises a plurality of loop circuits including the first and second rings and a plurality of legs, a first portion of loop circuits of the plurality of loop circuits that is positioned adjacent the lower surface side of the bore having a smaller impedance than an impedance of a second portion of loop circuits of the plurality of loop circuits that is positioned adjacent the upper surface side of the bore.
10. A magnetic resonance imaging apparatus according to claim 6, wherein the RF coil comprises a plurality of loop circuits including the first and second rings and a plurality of legs, a first portion of loop circuits of the plurality of loop circuits that is positioned adjacent the lower surface side of the bore having a smaller impedance than an impedance of a second portion of loop circuits of the plurality of loop circuits that is positioned adjacent the upper surface side of the bore.
11. A magnetic resonance imaging apparatus according to claim 7, wherein an upper loop circuit of the plurality of loop circuits that is located at a highest position has a largest impedance and a lower loop circuit of the plurality of loop circuits that is located at a lowest position has a smallest impedance.
12. A magnetic resonance imaging apparatus according to claim 8, wherein an upper loop circuit of the plurality of loop circuits that is located at a highest position has a largest impedance and a lower loop circuit of the plurality of loop circuits that is located at a lowest position has a smallest impedance.
13. A magnetic resonance imaging apparatus according to claim 9, wherein an upper loop circuit of the plurality of loop circuits that is located at a highest position has a largest impedance and a lower loop circuit of the plurality of loop circuits that is located at a lowest position has a smallest impedance.
14. A magnetic resonance imaging apparatus according to claim 10, wherein an upper loop circuit of the plurality of loop circuits that is located at a highest position has a largest impedance and a lower loop circuit of the plurality of loop circuits that is located at a lowest position has a smallest impedance.
15. A magnetic resonance imaging apparatus according to claim 1, wherein the RF coil comprises a birdcage coil.
16. A magnetic resonance imaging apparatus according to claim 2, wherein the RF coil comprises a birdcage coil.
17. A magnetic resonance imaging apparatus according to claim 3, wherein the RF coil comprises a birdcage coil.
18. A magnetic resonance imaging apparatus according to claim 5, wherein the RF coil comprises a birdcage coil.
19. A magnetic resonance imaging apparatus according to claim 7, wherein the RF coil comprises a birdcage coil.
20. A magnetic resonance imaging apparatus according to claim 11, wherein the RF coil comprises a birdcage coil.
Type: Application
Filed: Jul 29, 2011
Publication Date: Aug 23, 2012
Inventors: Mina Iwama (Tokyo), Yusuke Asaba (Tokyo), Takashi Ishiguro (Tokyo)
Application Number: 13/194,235
International Classification: G01R 33/32 (20060101);