Analysis of sample power loss in MRI gradient fields

A derivation of the power dissipation in a conducting spherical sample in a time-dependent gradient magnetic field is presented and the predicted values are compared with those observed in an experimental apparatus consisting of a four-element saddle coil and a Maxwell coil pair. The derived expressions for power dissipation are shown to agree with values obtained experimentally via Q measurements of the loaded coils. It is also shown the large rapidly fluctuating gradient fields encountered in echo planar imaging produce relatively small levels of absorbed power.