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.

Limitations of stimulated echo acquisition mode (steam) techniques in cardiac applications

“…Via a cylindrical axis (position 4), the rotation is transmitted to the movable part (position 6) in front of the agarose gel cylinder. A control curve (position 5) generates the translation motion also transmitted to position 6. The maximum amplitude of this translational motion is 3 cm.…”

“…There is an increasing interest in the application of magnetic resonance imaging (MRI) sequences using the stimulated echo acquisition mode (STEAM). Stimulated echo imaging sequences (1) have a large potential for imaging classical parameters, such as spin density, relaxation times (2), and chemical shift displacement (3), as well as for the assessment of dynamic values, such as quantitative flow (4)(5)(6), diffusion coefficients (7)(8)(9)(10)(11)(12)(13), and microcirculation rate (9,14). The flow sensitivity of stimulated echoes is also used to obtain magnetic resonance angiograms (15,16).…”

Virtual dye angiography: Flow visualization for MRI‐guided interventions

“…Edelman et al (9) have used a 90° slice-selective excitation, i.e., a 1D RF pulse, followed by a dephasing gradient to tag blood in a slice intersecting a vein in order to evaluate flow in the portal venous system. A combination of slice-selective and nonselective excitations have been used to image flow using inversion recovery (8) or stimulated echos (10). In arterial spin labeling (11) blood in a slab proximal to the region of interest is tagged by saturation or inversion, and perfusion is quantified by computing the difference with a baseline non-labeled image.…”

MR angiography using spin‐lock flow tagging

“…However, by altering the interval between the exciting pulses, signal loss due to T 2 dephasing can be minimized. Nevertheless, the signal from a stimulated echo is only one-half of that obtained by spin or gradient echo (4).…”