Multiple tap gradient field coil for magnetic resonance imaging

In an NMR imaging system, an apparatus for producing a magnetic field gradient comprising:

• a source of a gradient signal;
• a plurality of saddle coils positioned on a cylindrical form, wherein each saddle coil has a spiral shaped conductive pattern, a primary termination point on the conductive pattern, and a pair of secondary termination points on the conductive pattern;
• and
• a switch mechanism for connecting the plurality of saddle coils in series with the source of a gradient signal, so that voltage from the gradient signal is applied between the primary termination point and a selected secondary termination point of each saddle coil, with the selected secondary termination point being determined by a control signal applied to the switch mechanism.

In an NMR system for imaging a body, a magnet field generation assembly comprising:

• a main magnet which produces a polarizing magnetic field B.sub.0 within an imaging volume;
• two coils for creating magnetic field gradients along two orthogonal axes within the imaging volume, each of said coils having a primary termination and a pair of secondary terminations;
• means for selecting one of the pair of secondary terminations for each coil in response to a size of a region of interest within the body;
• a switch responds to said means for selecting by applying electric current between the primary termination and the selected secondary termination of each gradient coil;
• and
• a radio frequency coil for exciting nuclei of the body and for receiving NMR signals emitted by excited nuclei.