Universal modes: Calibration-free time-interleaved acquisition of modes

Abstract

Purpose: To introduce universal modes by applying the universal pulse concept to time-interleaved acquisition of modes (TIAMO), thereby achieving calibration-free $B_1^{+}$ inhomogeneity mitigation for body imaging at ultra-high fields.

Methods: Two databases of different RF arrays were used to demonstrate the feasibility of universal modes. The first comprised 31 cardiac in vivo data sets acquired at 7T while the second consisted of 6 simulated 10.5T pelvic data sets. Subject-specific solutions and universal modes were computed and subsequently evaluated alongside predefined default modes. For the cardiac database, subdivision into subpopulations was investigated. The optimization was performed using least-squares (LS) TIAMO and acquisition modes optimized for refocused echoes (AMORE). Finally, universal modes based on simulated pelvis data were applied in vivo at 10.5T.

Results: In all studied cases, the universal modes yield improvements over the predefined default modes of up to 51% (cardiac) and 30% (pelvic) in terms of median excitation error when using two modes. The subpopulation-specific cardiac solutions revealed a further improvement of universal modes at the expense of increased errors when applied outside the appropriate subpopulation. Direct application of simulation-based universal modes in vivo resulted in up to a 14% reduction in excitation error compared to default modes and up to a 34% reduction in peak 10 g local specific absorption rate (SAR) compared to subject-specific solutions.

Conclusions: Universal modes are feasible for calibration-free $B_1^{+}$ inhomogeneity mitigation at ultra-high fields. In addition, simulation-based solutions can be applied directly in vivo, eliminating the need for large in vivo databases.

Keywords: AMORE; MRI; RF shimming; TIAMO; body; parallel transmission; ultra‐high field.