Mitigating Inhomogeneity in High-Field MRI Excitations: Arbitrary Waveform Optimization and Multiphoton Parallel Transmission (MP-pTx)

Abstract

High-field magnetic resonance imaging (MRI) using a standard volume coil results in a spatially varying flip angle across the body, which renders images difficult to clinically interpret. This arises from the complex interactions of electromagnetic fields from current-carrying elements surrounding the imaging region. Parallel transmission (pTx) mitigates this issue by employing multiple high-power, independently controlled transmit elements for more precise excitation control. However, since the wavelength of the applied radio waves is shortened in tissue, the effect becomes highly dependent on the patient’s anatomy. As a result, optimization must be performed on a patient-by-patient basis, and methods that attempt full control of these independent waveforms are too computationally intensive to execute during the limited examination time. Additionally, the high-field excitations create complex electric field distributions that require control and careful monitoring to avoid excessive tissue power deposition (and ultimately heating), quantified as the specific absorption rate (SAR). To address these challenges, we introduce a method for optimizing patient-specific pulses using a global waveform (Ritz) approach, enabling rapid, in-scanner optimization. While pTx effectively addresses flip angle inhomogeneity, it remains costly and introduces challenges in SAR management. We address the SAR management and cost problems of pTx by introducing and characterizing the MP-pTx method, which leverages the multiphoton phenomenon to improve homogeneity using a standard volume coil supplemented with low-frequency (kilohertz) parallel channels. MP-pTx reduces costs and simplifies SAR management by shifting the parallel irradiation to low-cost, lowSAR shim array channels. These channels supplement an off-resonant excitation from a conventional birdcage coil with an oscillating, z-directed field that satisfies the resonance condition for spin state transitions.