High frequency coil and magnetic resonance imaging device
In a multi-channel array coil used as an RF coil of an MRI apparatus, even when magnetic coupling occurs between the respective sub-coils, it is possible to suppress the influence of a current flowing through the sub-coil of a coupling counterpart to maintain the desired sensitivity and suppress deterioration of image quality. Therefor, even when magnetic coupling occurs between sub-coils constituting the multi-channel array coil used as the RF coil of the MRI apparatus, the sub-coils are connected to a signal processing circuit so that a phase difference between a rotating magnetic field generated by the influence of a current flowing through the sub-coil of the coupling counterpart and a rotating magnetic field generated by the sub-coil is less than 90 degrees.
1. A radio frequency coil comprising:
a first sub-coil and a second sub-coil, each of which includes a loop coil part that is formed of a conductor and detects a radio frequency signal and a signal processing circuit to which the radio frequency signal detected by the loop coil part is input,
wherein the loop coil part of the first sub-coil and the loop coil part of the second sub-coil are adjusted to be magnetically coupled to each other and are respectively arranged at positions at which a radio frequency magnetic field generated in a space between the first sub-coil and the second sub-coil is detectable as the radio frequency signal, and
the loop coil part and the signal processing circuit are connected such that a phase difference between a rotating magnetic field generated by a first radio frequency magnetic field generated in the space by the loop coil part alone and a rotating magnetic field formed by a second radio frequency magnetic field generated in the space by magnetic coupling of the loop coil part is less than 90 degrees, in each of the first sub-coil and the second sub-coil.
2. The radio frequency coil according to claim 1 , wherein one terminal of an output side of the signal processing circuit is grounded, and
the loop coil part is a parallel resonance circuit.
3. The radio frequency coil according to claim 1 , wherein a resonance frequency of the first sub-coil alone is different from a reception frequency which is a frequency of the radio frequency signal to be received, and the first sub-coil is arranged and adjusted such that a circulating current path is formed in each of a loop of the loop coil part of the first sub-coil and a loop of the loop coil part of the second sub-coil by being magnetically coupled to the second sub-coil, and the first sub-coil resonates at the reception frequency.
4. The radio frequency coil according to claim 3 , wherein the resonance frequency of the second sub-coil alone is different from the reception frequency, and the second sub-coil is adjusted such that a circulating current path is formed in each of the loop of the loop coil part of the first sub-coil and the loop of the loop coil part of the second sub-coil by being magnetically coupled to the first sub-coil, and the second sub-coil resonates at the reception frequency.
5. The radio frequency coil according to claim 3 , wherein each of the first sub-coil and the second sub-coil includes a magnetic coupling adjusting part that connects the loop coil part and the signal processing circuit,
the loop coil part includes:
a series capacitor inserted in series with an inductor component of the loop; and
a parallel capacitor inserted in parallel with the inductor component to make the loop coil part a parallel resonance circuit,
the magnetic coupling adjusting part includes at least one of a capacitor and an inductor as an adjustment circuit element, and
the first sub-coil and the second sub-coil are tuned by the adjustment circuit element, the series capacitor and the parallel capacitor.
6. The radio frequency coil according to claim 3 , wherein each of the first sub-coil and the second sub-coil further includes a coupling inductor, and
the first sub-coil and the second sub-coil are magnetically coupled by the respective coupling inductors.
7. The radio frequency coil according to claim 1 , wherein the first sub-coil and the second sub-coil are arranged in substantially a same plane.
8. The radio frequency coil according to claim 1 , further comprising a third sub-coil including the loop coil part and the signal processing circuit,
wherein the loop coil part of the second sub-coil and the loop coil part of the third sub-coil are adjusted so as to be magnetically coupled to each other and are respectively arranged at positions at which a radio frequency magnetic field generated in a space between the second sub-coil and the third sub-coil is detectable as the radio frequency signal, and
the loop coil part and the signal processing circuit are connected such that a phase difference between a rotating magnetic field generated by a second radio frequency magnetic field generated in the space by the loop coil part alone and a rotating magnetic field formed by a third radio frequency magnetic field generated in the space by magnetic coupling of the loop coil part is less than 90 degrees, in each of the third sub-coil and the second sub-coil.
9. The radio frequency coil according to claim 8 , wherein a resonance frequency of the third sub-coil alone is different from a frequency of the radio frequency signal, and the third sub-coil is arranged and adjusted such that a circulating current path is formed in each of a loop of the loop coil part of the third sub-coil and a loop of the loop coil part of the second sub-coil by being magnetically coupled to the second sub-coil, and the third sub-coil resonates at the frequency of the radio frequency signal.
10. The radio frequency coil according to claim 8 , further comprising a fourth sub-coil including the loop coil part and the signal processing circuit,
wherein the fourth sub-coil is arranged such that the loop coil part of the fourth sub-coil has an overlap region with each of the loop coil part of the first sub-coil and the loop coil part of the second sub-coil, and
the area of the overlap region is determined such that the fourth sub-coil is not magnetically coupled to the first sub-coil and the second sub-coil.
11. A magnetic resonance imaging apparatus comprising:
a static magnetic field forming unit that forms a static magnetic field;
a gradient magnetic field forming unit that forms a gradient magnetic field;
a radio frequency magnetic field generating unit that generates a radio frequency magnetic field;
a transmitting coil that irradiates a subject with the radio frequency magnetic field;
a receiving coil that detects a nuclear magnetic resonance signal, which is a radio frequency signal, from the subject; and
an image reconstructing unit that reconstructs an image from the detected nuclear magnetic resonance signal,
wherein the receiving coil is a radio frequency coil according to claim 1 , and
each of the transmitting coil and the receiving coil includes a magnetic coupling prevention circuit that prevents magnetic coupling between the transmitting coil and the receiving coil.