Line and slice selection for moving spins

Abstract

The Bloch equations are solved numerically for spins moving at constant velocity for the case of 90.degree. slice selection, and for subsequent 180.degree. spin-echo line selection. The calculations use the comoving, rotating reference frame which is shown to be valid for general accelerated motions. Magnetization profiles and phase shifts are presented for slice selection for both plug and laminar flow. Line selection is studied for the case of plug flow, where a "local vector sum" is introduced to demonstrate effective localization of activated spins. In addition to local magnetization densities, the spatially integrated signals seen by a nonselective antenna are calculated. The relation between net phase shift and flow rate is examined and deviations from the predictions of simple analytic models are discussed.