Arterial spin labeling (ASL) provides quantitative and reproducible measurements of regional cerebral blood flow, and is therefore an attractive method for functional MRI. However, most existing ASL functional MRI protocols are based on either two-dimensional (2D) multislice or 3D spin-echo and suffer from very low image signal-to-noise ratio or through-plane blurring. 3D ASL with multishot (segmented) readouts can improve the signal-to-noise ratio efficiency relative to 2D multislice and does not suffer from T2-blurring. However, segmented readouts require lower imaging flip-angles and may increase the susceptibility to temporal signal fluctuations (e.g., due to physiology) relative to 2D multislice. In this article, we characterize the temporal signal-to-noise ratio of a segmented 3D spiral ASL sequence, and investigate the effects of radiofrequency phase cycling scheme and flip-angle schedule on image properties. We show that radiofrequency-spoiling is essential in segmented 3D spiral ASL, and that 3D ASL can improve temporal signal-to-noise ratio 2-fold relative to 2D multislice when using a simple polynomial (cubic) flip-angle schedule. Functional MRI results using the proposed optimized segmented 3D spiral ASL protocol show excellent activation in the visual cortex. Magn Reson Med, 2013. (C) 2012 Wiley Periodicals, Inc.
