Efficiently shielded MRI gradient coil with discretely or continuously variable field of view

A gradient coil assembly for inducing magnetic field gradients across an examination region in a magnetic resonance imaging apparatus of the type including a magnet for generating a temporally constant magnetic field through the examination region, the gradient coil assembly comprising:

• a primary gradient coil set disposed about the examination region including an array of conductive coil loops which are switchable between a first configuration in which a current flowing thereon generates magnetic field gradients which are substantially linear over a first useful imaging volume and a second configuration in which a current flowing therein generates magnetic field gradients which are substantially linear over a second useful imaging volume which is smaller than the first;
• a first shielding coil set disposed about the primary coil set including an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the first configuration;
• and
• a second shielding coil set disposed about the primary coil set including an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the second configuration.

A method of magnetic resonance imaging, comprising:

• generating a temporally constant magnetic field through an examination region of a magnetic resonance imaging apparatus;
• exciting and manipulating magnetic resonance in selected dipoles in the examination region;
• demodulating magnetic resonance signals received from the examination region;
• reconstructing the demodulated resonance signals into an image;
• and
• inducing gradient magnetic fields across the temporally constant magnetic field with a gradient coil assembly comprising:
• a primary gradient coil set disposed about the examination region including an array of conductive coil loops which are switchable between a first configuration in which a current flowing thereon generates magnetic field gradients which are substantially linear over a first useful imaging volume and a second configuration in which a current flowing therein generates magnetic field gradients which are substantially linear over a second useful imaging volume which is smaller than the first;
• a first shielding coil set disposed about the primary coil set including an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the first configuration;
• and
• a second shielding coil set disposed about the primary coil set including an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the second configuration.

In a method of magnetic resonance imaging, a method of producing a magnetic field gradient which is generally linear over a selected imaging volume, comprising:

• providing a primary gradient coil having a first configuration to produce a first magnetic field gradient in response to supplying current thereto, the first magnetic field gradient being generally linear over a first useful imaging volume;
• identifying one or more turns of the primary gradient coil which, when electrically decoupled from the primary coil, reconfigures the primary gradient coil to a second configuration to produce a second magnetic field gradient in response to supplying the first current thereto, the second magnetic field gradient being generally linear over a second useful imaging volume;
• configuring a first shield coil to substantially shield a fringe field from the primary coil when the primary coil set is operating in the first configuration;
• and
• configuring a second shield coil to substantially shield a fringe field from the primary coil when the primary coil set is operating in the second configuration.

A method of designing a gradient coil system for a magnetic resonance imaging system having a variable useful imaging volume, comprising:

• designing a primary gradient coil having a first configuration that produces a first magnetic field gradient that is generally linear over a first imaging volume;
• identifying one or more turns of the primary gradient coil which, when electrically decoupled from the primary gradient coil, reconfigures the primary gradient coil to a second configuration which produces a second magnetic field gradient that is generally linear over a second imaging volume;
• designing a first shield coil that produces a magnetic field which substantially cancels in an area outside a region defined by the shielding coil a first fringe magnetic field generated by the primary gradient coil in one of the first and second configurations;
• designing a second shield coil that produces a magnetic field, either alone or in combination with the first shield coil, which substantially cancels in an area outside a region defined by the shielding coil a second fringe magnetic field generated by the primary gradient coil in the other of the first and second configurations.