Optimized z-Direction RF Coil for Multiphoton Excitation with Improved Homogeneity

Motivation: Generating spatially uniform multiphoton excitation requires a z-directed transmit coil to produce a homogeneous B1,z field. Goal(s): To improve the spatial homogeneity of the B1,z field with a numerically optimized coil. Approach: We compared two coil winding patterns: solenoid vs. numerically optimized. We computed the magnetic field spatial distribution and experimentally measured the B1,z field by adapting the double flip angle method to multiphoton excitation. Results: Our optimized coil demonstrated a 28% improvement in spatial homogeneity, characterized by the diameter with a coefficient of variation (CV) of 20% or less. In simulation, we observed a 20-fold decrease in CV for the optimized coil. Impact: Our numerically optimized z-directed transmit coil provides improved spatial homogeneity and field strength for multiphoton MRI applications. This advancement supports our future experimental requirements as we further explore the applications of multiphoton MRI.