Three-dimensional gradient and spin-echo readout for time-encoded pseudo-continuous arterial spin labeling: Influence of segmentation factor and flow compensation.

PURPOSE: To monitor the complete passage of the labeled blood through the vascular tree into tissue and improve the quantification of ASL maps, we evaluated the effect of 3D gradient and spin-echo (GRASE) readout segments on temporal SNR (tSNR) and image blurriness for time-encoded pseudo-continuous arterial spin labeling and the effect of flow-compensation gradients on the presence of intravascular signal. METHODS: Fifteen volunteers were scanned using time-encoded pCASL with 2D EPI and single-segment, two-segments, and three-segments 3D-GRASE readouts with first-order flow compensation (FC) gradients. Two-segments 3D-GRASE scans were acquired with 25%, 50%, 75%, and 100% of full first-order FC. Temporal SNR was assessed, and cerebral blood flow and arterial blood volume were quantified for all readout strategies. RESULTS: For single-segment 3D GRASE, tSNR was comparable to 2D EPI for perfusion signal but worse for the arterial signal. Two-segments and three-segments 3D GRASE resulted in higher tSNR than 2D EPI for perfusion and arterial signal. The arterial signal was not well visualized for 3D-GRASE data without FC. Visualization of the intravascular signal at postlabeling delays of 660 ms and 1060 ms was restored with FC. Adequate visualization of the intravascular signal was achieved from 75% of FC gradient strength at a postlabeling delay of 660 ms. For a postlabeling delay of 1060 ms, full-FC gradients were the best option to depict intravascular signal. CONCLUSION: Segmented GRASE provided higher effective tSNR compared with 2D-EPI and single-segment GRASE. Flow compensation with GRASE readout should be carefully controlled when applying for time-encoded pCASL to visualize intravascular signal.