Myocardial perfusion mapping with intra-arterial spin labeling: Optimization of the labeling efficiency

Intra-arterial spin labeling (iASL) applies a labeling pulse from a transmit coil at the tip of the catheter for regional myocardial perfusion measurements during MR-guided catheterizations. This study investigates the labeling efficiency of catheter-mounted coils under varying conditions to enable robust iASL measurements.

The labeling efficiency was assessed by analytical calculations, numerical simulations, and in vitro measurements at 3 T. Analytical solutions were provided for various simplified $B_1^{+}$ field distributions, and a numerical simulation was used to calculate $B_1^{+}$ for different catheter coil designs using the Biot-Savart Law. Using these $B_1^{+}$ fields, the RF excitation of the blood magnetization flowing around the catheter was calculated using the Bloch equation. Finally, in vitro measurements were performed in a flow phantom using loop and solenoid catheter coils.

$B_1^{+}$ fields produced by the catheter coils result in a saturation of the mean magnetization when the applied pulse exceeds a threshold power. This threshold power varies between 1 and 25 mW for different coil designs, flow velocities, and geometric parameters.

iASL creates a saturation that is robust against variations in flow and coil positioning. The simulations could determine the threshold pulse power for robust iASL excitation, which needs to be adapted to applications in different vessels. Thus, iASL may provide an efficient alternative to perfusion measurements with exogenous contrast agents during MR-guided interventions.