A complication in prescan strategy when using surface coils.

Abstract

The "prescan" process for setting the gain of an MRI scanner can be confounded by an effect arising from the spatial variation of the angle between the vector reception field of a local or surface coil and the excitation field of a whole-body coil. It is most pronounced when the phase-encoding direction is parallel to the plane of the coil but not along the magnetic field. The largest signal is shown to reside in a data row that is shifted from the one that corresponds to zero phase-encoding gradient. The effect depends on the field-of-view and the dimensions of the receiving coil. Strategies are suggested by which manufacturers of scanners can cope with this intrinsic problem. In the meantime, users of surface coils should be aware of the difficulty.