Calibration-free parallel-transmit RF pulse design for 7 T-MRI of the brain

Abstract

Magnetic resonance imaging (MRI) is a versatile imaging modality that has substantially advanced clinical diagnostics by enabling early and accurate detection of disease. However, the inherently low signal-to-noise ratio (SNR) of MRI remains a fundamental limitation and continues to motivate methodological developments. <br/>

An established strategy to increase SNR is the use of higher static magnetic field strengths, which enhance thermal equilibrium spin polarization and thereby improve signal levels. Nevertheless, operation at ultra-high field (UHF) strengths (≥ 7T) introduces additional challenges, most notably spatial inhomogeneities of the radiofrequency (RF) excitation field caused by wave interference effects at high frequencies. These effects result in spatially varying flip angles, signal non-uniformity, and local signal dropouts. <br/>

Parallel transmission (pTx) has emerged as an effective technique to mitigate RF field inhomogeneities by enabling spatially tailored RF excitation through multiple independently driven transmit channels. Despite its technical advantages, the routine use of pTx remains limited due to the requirement for time-consuming subject-specific calibration measurements prior to each imaging session. Such calibrations increase workflow complexity, require expert supervision, and are not guaranteed to yield robust results. <br/>

This thesis advances the concept of universal pulses (UPs), which are calibration-free ("plug-and-play") pTx RF pulses designed to operate reliably across subjects without subject-specific calibration. By eliminating subject-specific calibration, UPs improve the robustness, efficiency, and accessibility of UHF MRI. Application-specific UPs were developed for clinically relevant sequences, including chemical exchange saturation transfer (CEST) imaging and 3D single-slab variable flip angle turbo spin echo (TSE) imaging. In addition, UPs were designed for more general application enabling homogeneous slice- or slab-selective excitation or simultaneous water excitation with fat suppression. <br/>

Furthermore, the Gradient Ascent Pulse Engineering (GRAPE) algorithm was systematically adapted for applications that previously suffered from limited homogenization performance, sensitivity to static field inhomogeneities, or long durations due to limited flexibility in the design of parameterized RF pulses. Short, unparameterized GRAPE pulses were developed to improve excitation homogeneity, robustness to field inhomogeneities, and time-efficiency for both spatially and spectrally selective RF excitation.