Fig. 1
Timing diagram of the hASL MRA protocol. The “labeled cycle” (top panel) and the “control cycle” (bottom panel) were acquired in an interleaved manner. Using a parallel acceleration factor of 2, 140 phase-encoding steps were acquired in each cycle. The sequence repeated until all slice-encoding steps were collected. Complex subtraction of data acquired in the two readouts produced the angiogram. Pseudo-continuous (PC) RF labeling (1 cm thickness), pulsed RF labeling (10 cm thickness) and an inversion RF pulse for background suppression (BSIR) (20 cm thickness) were applied 5 cm below, 10 cm below and 5 cm above the center of the coronal imaging slab, respectively. Parameters for pseudo-continuous labeling were: 1.5 ms repetition time, 25° flip angle, 750 μs RF duration, 3mT/m maximum gradient strength, 0.5mT/m average gradient strength. The axial 10 cm-thick pulsed RF inversion was applied 60 ms before the fast low-angle shot (FLASH) readout. An abbreviated pseudo-continuous control phase (PCC) indicated by the asterisk (*) was used during the “labeled cycle” to lessen RF power deposition and neutralize magnetization transfer effects. PCL = pseudo-continuous labeling phase; TR = repetition time; ky = 0 denotes central phase-encoding line