Per-Channel Complex B 1 + Mapping for Parallel Transmit Ultra-High Field MRI

Ultra-high field (UHF, B₀ ≥ 7 T) MRI provides substantial gains in signal-to-noise ratio, spatial resolution and contrast. Routine clinical deployment has been limited by challenges of transmit field ( $B_1^{+}$ ) inhomogeneity and specific absorption rate (SAR). Parallel transmission (pTx) can mitigate these challenges but it requires per-channel complex $B_1^{+}$ maps to design effective pTx pulses. We review established and emerging approaches to obtain these maps. We begin by summarising the underlying physical principles and theory. Then we describe methods for magnitude $B_1^{+}$ mapping such as actual flip-angle imaging (AFI), saturation-prepared $B_1^{+}$ mapping (e.g., satTFL) and dual refocusing echo acquisition mode (DREAM) that are the basis of widely-applied per-channel complex $B_1^{+}$ mapping methods, as well as methods of pedagogical and/or historical relevance. Next, we describe how relative $B_1^{+}$ mapping with low-flip angle gradient-recalled echo (GRE) scans can determine the relative phases (and relative magnitudes) of $B_1^{+}$ for each channel. We recap the important interferometric approach that is now widely used. We explain how magnitude and phase maps are combined into complex per-channel $B_1^{+}$ maps. We discuss remaining arbitrary factors such as the position-dependent transmit reference phase. We discuss recent practical implementations such as Sandwich TFL, 3DREAM, compressed-sensing SA2RAGE and respiratory-resolved $B_1^{+}$ mapping. We explore the workflow improvements that could accrue if per-channel $B_1^{+}$ maps do not need to be acquired for every subject. This includes deep-learning-based approaches to synthesize $B_1^{+}$ maps from the normal localiser scans, the universal pulses (UPs) approach to precalibrate pulses and variants such as standardised universal pulses (SUPs), Fast-Online Customised (FOCUS) and machine-learning online customisation. We hope to encourage greater openness and harmonisation of these critical calibration steps for parallel transmit pulse design. This will become increasingly important as 7-T pTx MRI is adopted for multisite cross-vendor clinical research studies and to deliver the full scientific potential of new extremely high field human MRIs.