MRI gradient coil with variable field of view and apparatus and methods employing the same

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 base gradient coil set disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates the substantially linear magnetic field gradients which are substantially linear over a first imaging volume and nonlinear outside of the first imaging volume;
• and
• a correction gradient coil set disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates a magnetic field gradient having third and higher order terms and substantially no first order term, said third and higher order terms combining with higher-order terms of the base gradient coil set to produce magnetic field gradients which supplement the nonlinear gradient surrounding the first imaging volume to increase the imaging volume over which the gradient is linear from the first imaging volume to a second imaging volume.

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:

• configuring a base gradient coil to produce a first magnetic field gradient in response to supplying a first current to the base gradient coil, the first magnetic field gradient being generally linear over a first useful imaging volume, the first magnetic field gradient increasing proportionally with the supplied current;
• configuring a correction coil to produce a second magnetic field gradient in response to supplying a second current to the correction coil, the second magnetic field gradient having substantially no first order moment, the second magnetic field gradient combining with the first magnetic field gradient to generate a combined magnetic field gradient which is generally linear over a second useful imaging volume.

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

• designing a base coil that produces a first magnetic field gradient that is generally linear over a first imaging volume;
• designing a correction coil that produces a second magnetic field gradient having substantially zero first order moment, the first and second magnetic field gradients combining to produce a third magnetic field gradient that is generally linear over a second imaging volume that is different from the first imaging volume.