Shielded Dual-Loop Resonator for Arterial Spin Labeling at the Neck

Purpose: To construct a dual-loop coil for continuous arterial spin labeling (CASL) at the human neck and characterize it using computer simulations and magnetic resonance experiments. Materials and Methods: The labeling coil was designed as a perpendicular pair of shielded-loop resonators made from coaxial cable to obtain balanced circular loops with minimal electrical interaction with the lossy tissue. Three different excitation modes depending on the phase shift, Delta psi, of the currents driving the two circular loops were investigated including a Maxwell mode (Delta psi=0 degrees: ie, opposite current directions in both loops), a quadrature mode (Delta psi = 90 degrees), and a Helmholtz mode (Delta psi = 180 degrees; ie, identical current directions in both loops). Results: Simulations of the radiofrequency field distribution indicated a high inversion efficiency at the locations of the carotid and vertebral arteries. With a 7-mm-thick polypropylene insulation. a sufficient distance from tissue was achieved to guarantee robust performance at a local specific absorption rate (SAR) well below legal safety limits. Application in healthy volunteers at 3 T yielded quantitative maps of gray matter perfusion with low intersubject variability. Conclusion: The coil permits robust labeling with low SAR and minimal sensitivity to different loading conditions.