A Universal Bz Coil for Uniform Multiphoton Excitation in High-Field MRI

Motivation: Contrast in high-field MRI is obscured by the spatially non-uniform excitation flip angle profile of conventional birdcage transmit coils. Goal(s): We demonstrate that a single $$$B_z$$$ coil operated in the kHz range can supplement a birdcage to create a spatially-uniform flip angle profile using multiphoton excitation. Approach: We use a stream function boundary element method to optimize the $$$B_z$$$ coil windings to produce homogeneous nonselective multiphoton excitations across a universal pulse database and validate with Bloch simulations. Results: The single $$$B_z$$$ channel achieved a mean flip angle NRMSE of 13.9% for a 90º target MP-pTx pulse in test subjects. Impact: The design method provides a simplified hardware configuration and reduced local SAR concerns compared to either conventional pTx or our previous work using a full shim array in conjunction with multiphoton parallel transmission.