Dynamic Changes in Blood Transit Time and Flow during Somatosensory Stimulation Measured by Dynamic ASL with High Temporal Resolution

Introduction The blood transit time from arterial spin labeling plane to capillaries in the imaging slice (referred to as “tissue transit time”) decreased during neuronal activation in humans (1), while it appeared no changes during somatosensory stimulation in rats (2). Since a tissue transit time is shorter in rats compared to humans, shortening blood transit time may not be detectable with arterial spin labeling (ASL) data with low temporal resolution. Thus, it is important to carefully measure dynamic transit time changes during stimulation. In this work, the dynamics of both CBF and the blood transit time during rat forepaw stimulation were simultaneously measured with a modified dynamic ASL (DASL) method with improved temporal resolution. Materials and Methods The original DASL method (3) was modified to increase temporal resolution. The magnetizations of spins in the imaging slice were repeatedly excited with Look-Locker excitations and interval TI. Data were collected in a cycle of N acquisitions for one cycle, in which the spin labeling pulse was applied for first half, while control images were obtained for the second half (Fig. 1). For measuring tissue transit time, bi-polar diffusion gradients were applied in all three directions with the b value of 50 s/mm to eliminate the arterial blood signal (4). With ASL, magnetization M(τ) at τ (τ =nTI, 0≤n10 s). Our data indicate that dynamic changes in CBF and velocity (i.e., inverse of transit time) are mismatched; the transit time decrease dominates at right after stimulation, while arterial blood vessel dilation dominates at a later time period. Thus, velocity and diameter changes measured with a short stimulation period may not directly applicable to long stimulation periods which often use in fMRI community.