Functional imaging with Turbo-CASL: transit time and multislice imaging considerations.

The optimal use of turbo continuous arterial spin labeling (Turbo-CASL) for functional imaging in the presence of activation-induced transit time (TT) changes was investigated. Functional imaging of a bilateral finger-tapping task showed improved sensitivity for Turbo-CASL as compared to traditional CASL techniques for four of six subjects when scanned at an appropriate repetition time (TR). Both experimental and simulation results suggest that for optimal functional sensitivity with Turbo-CASL, the pulse TR should be set to a value that is 100-200 ms less than the resting-state TT. Simulations were also run to demonstrate the differences in TT sensitivity of different slices within a multislice acquisition, and the signal loss that is expected as the number of slices is increased. Despite the lower baseline ASL signal provided by the Turbo-CASL acquisition, one can achieve equal or improved functional sensitivity due in part to the signal enhancement that accompanies the decrease in TT upon activation. Turbo-CASL is thus a promising technique for functional ASL at higher temporal resolution.