Magnetic resonance imaging apparatus and magnet for magnetic resonance imaging apparatus
According to one embodiment, a magnetic resonance imaging apparatus includes an imaging unit and a shield. The imaging unit is configured to perform magnetic resonance imaging of an object by transmitting a radio frequency signal from a radio frequency coil while magnetic fields are formed by a gradient coil and a superconducting magnet respectively. The shield is configured to form a gradient magnetic field for the magnetic resonance imaging with the gradient coil and to prevent ingress of heat into the superconducting magnet.
1. A magnetic resonance imaging apparatus comprising:
an imaging unit configured to perform a magnetic resonance imaging of an object by transmitting a radio frequency signal from a radio frequency coil when magnetic fields are formed by a gradient coil and a superconducting magnet respectively; and
a shield configured to conduct eddy currents which generate an eddy magnetic field and to prevent ingress of heat into the superconducting magnet, the eddy magnetic field being superposed with a magnetic field generated by the gradient coil having a pattern of windings determined such that a gradient magnetic field for magnetic resonance imaging is formed including the eddy magnetic field.
2. A magnetic resonance imaging apparatus comprising:
a superconducting magnet configured to form a static magnetic field, for magnetic resonance imaging of an object, in an imaging area where the object is set;
a gradient coil;
a radio frequency coil configured to transmit a radio frequency signal to the imaging area; and
a shield configured to conduct eddy currents which generate an eddy magnetic field and to prevent ingress of heat into the superconducting magnet, the eddy magnetic field being superposed with a magnetic field generated by the gradient coil having a pattern of windings determined such that a gradient magnetic field for magnetic resonance imaging is formed including the eddy magnetic field.
3. A magnetic resonance imaging apparatus of claim 1 ,
wherein said shield is configured to form the gradient magnetic field with the gradient coil by canceling at least a magnetic field generated, from the gradient coil, at a side of the superconducting magnet.
4. A magnetic resonance imaging apparatus of claim 1 ,
wherein said shield is made of a metal plate whose eddy magnetic field time constant is not less than 500 ms.
5. A magnetic resonance imaging apparatus of claim 4 ,
wherein said shield is made of a tubal aluminum plate having a thickness not less than 10 mm or a tubal copper plate having a thickness not less than 3 mm.
6. A magnetic resonance imaging apparatus of claim 1 ,
wherein said shield is made of a metal plate whose eddy magnetic field time constant is not less than a time constant required for magnetic resonance imaging.
7. A magnetic resonance imaging apparatus of claim 1 ,
wherein said shield is set as a passive coil which does not need an input and output current.
8. A magnetic resonance imaging apparatus of claim 1 ,
wherein said shield is set inside a casing constituting the superconducting magnet.
9. A magnetic resonance imaging apparatus of claim 1 ,
wherein said shield is configured to form a part of a casing constituting the superconducting magnet.
10. A magnetic resonance imaging apparatus of claim 1 ,
wherein said shield has a structure determined with a condition of the windings of the gradient coil so as to form an intended gradient magnetic field appropriate for magnetic resonance imaging.
11. A magnetic resonance imaging apparatus of claim 10 ,
wherein said shield has a diameter or a radius determined with a number of the windings of the gradient coil so as to form the intended gradient magnetic field.
12. A magnetic resonance imaging apparatus of claim 1 ,
wherein said shield has a pipe for conducting a cooling media on a side of the gradient coil.
13. A magnetic resonance imaging apparatus of claim 1 ,
wherein said shield has a channel for conducting a cooling media inside the shield.
14. A magnetic resonance imaging apparatus of claim 1 ,
wherein said shield is a heat shield to prevent ingress of heat into the superconducting magnet.
15. A magnet for a magnetic resonance imaging apparatus comprising:
a magnet body having liquid helium and a superconducting coil in a casing; and
a shield set inside said magnet body and having an eddy magnetic field time constant that is not less than 500 ms, the eddy magnetic field being superposed with a magnetic field generated by the gradient coil having a pattern of windings determined such that a gradient magnetic field for magnetic resonance imaging is formed including the eddy magnetic field.