Continuous arterial spin labeling (CASL) setup for the primate brain at 7T using a three-coil approach

Arterial spin labeling is commonly used to measure cerebral blood flow (CBF). Since CBF signal is intrinsically low, optimization of the signal-to-noise ratio (SNR) is critical. CASL with a separate labeling coil has the advantages of increased SNR, multi-slice capability, and absence of magnetization transfer. Here, we report the development of CASL for use on monkeys. Increased sensitivity was achieved by utilization of a custom-made three-coil setup at high magnetic field (7T) with its favorable longer T1 values. The feasibility of this approach is demonstrated with a flow phantom and with initial experiments in the monkey. Measurements were performed on a vertical 7T/60cm system dedicated for monkeys, which was equipped with a second 1H transmit channel. A saddle-shaped volume coil was applied for RF transmission and surface coil was used for RF reception. For spin tagging at carotid- and vertebrate arteries a concavely-shaped dual loop coil set was built. Its design was based on angiographic scans and space restriction in the neck. All three RF coils were actively-decoupled and switched with a self-built logic unit and current driver. The flow phantom consisted of a saline-filled bottle with two flow tubes. Flow was adjustable from 20cm/s to 100cm/s. Single-shot, multi-slice GE-EPI was acquired at 1x1x2mm3 resolution (128x64x5, TR/TE=3500/12ms). A 2s labeling period was followed by a variable post-labeling delay prior to the imaging module. For CASL, the labeling frequency was switched in interleaved scans. The CASL three-coil setup was validated on the flow phantom. Maximum label efficiencies of 0.85 were reached with a labeling power of 0.3 and 2W on the phantom and in the monkey, respectively. CBF maps were aquired in the monkey with variable labeling periods and postlabeling delays. These first in vivo data demonstrate feasibility and potential for future functional studies to measure CBF, BOLD and CMRO2 changes