Receiving coil, magnetic resonance imaging apparatus, and modified dipole
In one embodiment, a receiving coil includes a plurality of coil elements, wherein: at least one of the coil elements includes a loop coil and a modified dipole disposed inside the loop coil; and the modified dipole includes a main dipole configured to receive a radio-frequency “RF” signal and output a reception signal and a parasitic element that includes a split ring having a gap in part of a ring shape.
1 . A receiving coil, comprising:
a plurality of coil elements, wherein:
at least one of the plurality of coil elements comprises a loop coil and a modified dipole disposed inside the loop coil;
the modified dipole comprises:
a main dipole configured to receive a radio-frequency (RF) signal and output a reception signal, and
a parasitic element that includes a split ring having a gap in part of a ring shape:
a length of a radiating element of the main dipole is shorter than half wavelength; and
the parasitic element is configured to function as a matching circuit for the main dipole.
2 . The receiving coil according to claim 1 , wherein
the main dipole has a ring-shaped portion, and
the ring-shaped portion and the split ring are coupled to each other by electromagnetic induction.
3 . The receiving coil according to claim 1 , wherein
the main dipole includes a ring-shaped portion, and
the main dipole and the parasitic element are arranged in such a manner that the ring-shaped portion and the split ring are coaxially stacked with a predetermined gap therebetween.
4 . The receiving coil according to claim 1 , wherein
the main dipole includes a ring-shaped portion, and
a read-out point of the ring-shaped portion of the main dipole is provided at a position where the gap of the split ring is rotated by approximately 180° around a central axis of the ring-shaped portion of the main dipole and the split ring.
5 . The receiving coil according to claim 1 , wherein the parasitic element and the main dipole are perpendicular to a same plane and have symmetrical structures with respect to respective center positions.
6 . The receiving coil according to claim 1 , wherein the main dipole and the parasitic element are perpendicular to a same plane and orthogonal to a same straight line on the same plane,
the main dipole is point-symmetric with respect to a first point on the same straight line, and
the parasitic element is point-symmetric with respect to a second point different from the first point on the same straight line.
7 . The receiving coil according to claim 1 , wherein
the main dipole is formed on one surface of a substrate, and
the parasitic element is formed on another surface of the substrate opposite to the one surface such that the parasitic element is insulated from a conductor of the main dipole.
8 . The receiving coil according to claim 1 , wherein the radiating element, which extends from a ring-shaped portion of the main dipole, is at least partially formed in a meander line in which a thin conductor is folded at a plurality of positions into a crank shape.
9 . The receiving coil according to claim 1 , wherein
the parasitic element includes two linear elements that extend in opposite directions from the split ring and are arranged in parallel with the radiating element of the main dipole,
the parasitic element functions as a matching circuit for the main dipole,
a capacitance component of the matching circuit is adjusted by a width of the linear elements and a size of the gap of the split ring, and
an inductance component of the matching circuit is adjusted by a length of the linear elements.
10 . The receiving coil according to claim 1 , wherein:
the main dipole includes a ring-shaped portion from which the reception signal is outputted, and a plurality of radiating elements extending in both directions from the ring-shaped portion, and
the main dipole is configured as a folded dipole antenna in which the radiating elements are folded, and both ends of the radiating elements face each other near the ring-shaped portion.
11 . The receiving coil according to claim 10 , wherein
the parasitic element comprises a first parasitic element and a second parasitic element,
a first split ring included in the first parasitic element is disposed to face the ring-shaped portion, and
a second split ring included in the second parasitic element is disposed to face a position where both ends of the radiating elements face each other.
12 . The receiving coil according to claim 10 , wherein the radiating elements of the main dipole are formed as meander lines from the ring-shaped portion to both fold-back positions and from the both fold-back positions to the both ends facing each other, each of the meander lines being having a crank shape in which a thin conductor is folded at a plurality of positions.
13 . The receiving coil according to claim 10 , wherein
the main dipole is formed on one surface of a substrate,
the parasitic element is formed on another surface of the substrate opposite to the one surface such that the parasitic element is insulated from conductors of the main dipole,
a particular radiating element of the radiating elements of the main dipole is formed as first linear elements with a predetermined length from the ring-shaped portion to first intermediate positions toward both fold-back positions,
the particular radiating element of the main dipole is formed as meander lines from the first intermediate positions to the both fold-back positions and from the both fold-back positions to both ends facing each other, each of the meander lines having a shape in which a thin conductor is folded at a plurality of positions into a crank shape,
the parasitic element extends from the split ring in opposite directions, is folded back at positions corresponding to the both fold-back positions of the particular radiating element, and is formed in a shape approximately similar to the radiating element such that both ends of the parasitic element face each other near the split ring, and
the parasitic element is formed as second linear elements with a predetermined length from the split ring to both second intermediate positions toward both fold-back positions, and is formed as the meander lines from the second intermediate positions to both fold-back positions and from the both fold-back positions to both ends facing each other.
14 . The receiving coil according to claim 13 , wherein a length of the second linear element is adjusted depending on a frequency to be used.
15 . The receiving coil according to claim 1 , further comprising:
an output circuit that outputs a signal obtained by combining an output of the loop coil and an output of the modified dipole, and causes the receiving coil to function as a combined diversity coil,
an output circuit that separately outputs an output of the loop coil and an output of the modified dipole, and causes the receiving coil to function as a region diversity coil having different sensitivity between a far field and a near field, or
an output circuit that outputs an output of the loop coil and an output of the modified dipole as signals with different center frequencies, and causes the receiving coil to function as a frequency diversity coil.
16 . An MRI apparatus comprising the receiving coil according to claim 1 .
17 . A receiving coil, comprising:
a plurality of coil elements, wherein
at least one of the plurality of coil elements comprises a loop coil and a modified dipole disposed inside the loop coil;
the modified dipole comprises:
a main dipole configured to receive a radio-frequency (RF) signal and output a reception signal, and
a parasitic element that includes a split ring having a gap in part of a ring shape,
wherein the parasitic element includes two linear elements that extend in opposite directions from the split ring and are arranged in parallel with a radiating element of the main dipole.
18 . The receiving coil according to claim 17 , wherein
a length of an entirety of the linear elements is shorter than a length of an entirety of the radiating element,
the parasitic element functions as a wave director for the main dipole, and
the modified dipole is enhanced in directivity in a direction from the main dipole toward the parasitic element.
19 . A modified dipole, comprising:
a main dipole configured to receive a radio-frequency (RF) signal and output a reception signal, and
a parasitic element that includes a split ring having a gap in part of a ring shape of the parasitic element, wherein
a length of a radiating element of the main dipole is shorter than half wavelength; and
the parasitic element is configured to function as a matching circuit for the main dipole.