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

Area of Science:

• Magnetic Resonance Imaging
• Radiofrequency Engineering
• Biomedical Physics

Background:

• B1+ field inhomogeneity is a significant challenge in ultra-high field (UHF) MRI, particularly for body imaging.
• Existing methods for B1+ inhomogeneity mitigation often require subject-specific calibration, which is time-consuming and impractical.
• Developing calibration-free solutions is crucial for efficient and accessible UHF MRI.

Purpose of the Study:

• To introduce universal modes derived from the universal pulse concept for time-interleaved acquisition of modes (TIAMO).
• To achieve calibration-free B1+ inhomogeneity mitigation in body imaging at ultra-high fields.
• To evaluate the performance of universal modes compared to default and subject-specific modes.

Main Methods:

• Universal modes were computed using two RF coil array databases: 31 cardiac 7T in vivo datasets and 6 simulated 10.5T pelvic datasets.
• Subject-specific and universal modes were evaluated against predefined default modes.
• Optimization was performed using least-squares (LS) TIAMO and acquisition modes optimized for refocused echoes (AMORE).
• Subpopulation analysis was conducted for the cardiac database.

Main Results:

• Universal modes demonstrated significant improvements over default modes, reducing median excitation error by up to 51% (cardiac) and 30% (pelvic) with two modes.
• Subpopulation-specific modes offered further gains but increased errors when applied outside their designated group.
• Direct in vivo application of simulation-based universal modes reduced excitation error by up to 14% and peak local specific absorption rate (SAR) by up to 34% compared to subject-specific solutions.

Conclusions:

• Universal modes are a feasible approach for calibration-free B1+ inhomogeneity mitigation at ultra-high fields.
• Simulation-based universal modes can be directly applied in vivo, eliminating the need for extensive in vivo calibration datasets.
• This approach enhances the practicality and efficiency of UHF MRI body imaging.