Method and apparatus for MRI using selectively shaped image volume of homogeneous NMR polarizing field
AssigneeThe Regents of the University of California
InventorsJoseph W. Carlson, Leon Kaufman
Life of this patent
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Abstract
An electromagnet shim coil is utilized for temporarily altering the shape of a volume in which there is provided a substantially homogeneous NMR polarizing field. By temporarily energizing the electromagnet shim coil and thus altering the shape of the volume, magnetic resonance imaging can take place in other than a substantially spherical volume (e.g., in an elongated ellipsoidal-like volume extending axially along a patient so as to encompass a longer section of the spinal column). In the exemplary embodiment, the electromagnet shim coil takes the form of a pancake-like coil with windings positioned so as to create fourth power spherical harmonic in a transverse magnet-type of MRI system.
Claims20
A method of performing MRI, said method comprising the steps of:
- (a) temporarily altering a dimension of a volume in which there is provided a substantially homogeneous NMR polarizing field so as to reduce the volume of the homogeneous NMR polarizing field in a first direction while simultaneously increasing it in a second different direction;
- and
- (b) performing MRI to acquire image data for objects located within said volume of altered dimension.
In an MRI system, the improvement comprising:
- (a) at least one electromagnet shim coil which, when energized, alters a dimension of a volume in which there is provided a substantially homogeneous NMR polarizing field;
- and
- (b) MRI data acquisition apparatus electromagnetically coupled to said volume and configured for (i) acquiring image data for objects located within said volume of altered dimension while said electromagnet shim coil is energized and (ii) acquiring image data for objects located within a volume of unaltered shape while said electromagnet shim coil is unenergized.
An MRI system comprising:
- a main magnet including magnetic shimming for providing a predetermined volume of a substantially homogeneous polarizing magnetic field B.sub.o ;
- at least one magnetic gradient coil magnetically coupled to said volume for providing switched magnetic gradients in said polarizing field at selected times in an MRI data acquisition sequence;
- at least one RF coil electromagnetically coupled to said volume and coupling NMR RF signals into and/or out of imaged patient tissue located in said homogeneous polarizing field;
- and
- at least one selectively activated electromagnet shim coil also magnetically coupled to said volume and which alters a dimension of the volume of the homogeneous polarizing field when energized so as to reduce the volume of the homogeneous polarizing field in a first direction while simultaneously increasing it in a second different direction.
An MRI system comprising:
- a main magnet providing a predetermined volume of a substantially homogeneous polarizing magnetic field B.sub.o ;
- at least one magnetic gradient coil magnetically coupled to said volume and providing switched magnetic gradients in said polarizing field at selected times in an MRI data acquisition sequence;
- at least one RF coil electromagnetically coupled to said volume and coupling NMR RF signals into and/or out of imaged patient tissue located in said homogeneous polarizing field;
- and
- at least one selectively activatable magnetic shim coil also magnetically coupled to said volume and which alters a dimension of the volume of the homogeneous polarizing field when activated;
- wherein said magnetic shim coil includes a plurality of conductive turns with right angles disposed symmetrically in each of four quadrants.
An MRI system having a main magnet producing a substantially homogeneous polarizing field B.sub.o within an approximately spherical MR image volume, said system comprising:
- MRI RF signal transmitting and receiving circuits electromagnetically coupled to said image volume;
- a shim electromagnet which alters the polarizing field B.sub.o when energized to produce a substantially non-spherical MR image volume;
- and
- a switchable power supply connected to said shim electromagnet for selectively activating said shim electromagnet and thus providing said non-spherical MR image volume for selected MRI processes.
An MRI system having a main magnet producing a substantially homogeneous polarizing field B.sub.o within an approximately spherical MR image volume, said system comprising:
- MRI RF signal transmitting and receiving circuits electromagnetically coupled to said image volume;
- a shim electromagnet which alters the polarizing field B.sub.o when energized to produce a substantially non-spherical MR image volume;
- and
- a switchable power supply connected to said shim electromagnet for selectively activating said shim electromagnet and thus providing said non-spherical MR image volume for selected MRI processes;
- wherein said shim electromagnet includes windings shaped to produce fields having a spatial distribution proportional to a fourth power spherical harmonic.
An MRI system having a main magnet producing a substantially homogeneous polarizing field B.sub.o within an approximately spherical MR image volume, said system comprising:
- MRI RF signal transmitting and receiving circuits electromagnetically coupled to said image volume;
- a shim electromagnet which alters the polarizing field B.sub.o when energized to produce a substantially non-spherical MR image volume;
- and
- a switchable power supply connected to said shim electromagnet for selectively activating it and thus providing said non-spherical MR image volume for selected MRI processes;
- wherein said main magnet comprises transverse magnet poles and said shim electromagnet includes a substantially fiat circular pancake-like structure disposed near at least one said pole and having four symmetrical quadrants of windings therewithin.
Dependencies are inferred from the claim text.