An actively decoupled dual transceiver coil system for continuous ASL at 7 T

Abstract

7 T arterial spin labeling (ASL) faces major challenges including the increased specific absorption rate (SAR) and increased B-0 and B-1 inhomogeneity. This work describes the design and implementation of a dual-coil system that allows for continuous ASL (CASL) at 7 T. This system consisted of an actively detunable eight-channel transceiver head coil, and a three-channel transceiver labeling coil. Four experiments were performed in 5 healthy subjects: (i) to demonstrate that active detuning during ASL labeling reduces magnetization transfer; (ii) to measure the B-1 profile at the labeling plane; (iii) to quantify B-0 off-resonance at the labeling plane; and (iv) to collect in vivo CASL data. The magnetization transfer ratio in the head coil was reduced to 0.0 +/- 0.2% by active detuning during labeling. The measured B-1 profiles in all 5 subjects were sufficient to satisfy the flow-driven adiabatic inversion necessary for CASL, however the actual labeling efficiency was significantly impacted by B-0 off-resonance at the labeling plane. The measured CASL percent signal change in gray matter (0.94%+/- 0.10%) corresponds with the low labeling efficiency predicted by the B-0 off-resonance. This work demonstrates progress in the technical implementation of 7 T CASL, and reinforces the need for improved B-0 homogeneity at the labeling plane.