Projection flow imaging by bolus tracking using stimulated echoes.

Abstract

Previous investigators have employed the concept of bolus tracking using either spin echoes or gradient echoes. In this paper we introduce two methods of bolus tracking using planar- and volume-selective stimulated echoes. The planar method employs a selective 90 degrees rf pulse which tags all spins in a particular plane. At a time tau 1 later, a nonselective 90 degrees rf pulse is employed, followed after a time tau 2, by another nonselective rf pulse. Only spins which experience all three rf pulses form a stimulated echo at time tau 1 after the third rf pulse. A balanced pair of flow-compensated dephasing (crusher) gradients further ensures that the stimulated echo is due only to the effect of all three rf pulses while minimizing flow dephasing. The first part of this gradient pair is applied after the initial rf pulse in the first tau 1 period to dephase the tagged spins. The second part of this gradient pair is applied after the third rf pulse to rephase the spins. Since the plane of the excited slice is orthogonal to the readout direction, flowing spins are imaged in an angiographic manner as they move away from the excited slice. A modification to this basic sequence excites only a small volume. In this manner, the suppression of stationary spins is effected by volume-selective excitation. In both the planar- and the volume-selective techniques, the excited spins undergo T1 and T2 relaxation during the tau 1 period but only T1 relaxation in the tau 2 period. In blood, where T1 is much greater than T2, keeping tau 1 as short as possible minimizes signal loss due to T2 dephasing. These methods demonstrate increased sensitivity compared to similar bolus tracking methods using either spin echoes or gradient echoes.