Radiofrequency transmit calibration: A multi-center evaluation of vendor-provided radiofrequency transmit mapping methods

Purpose: To determine the accuracy and test-retest repeatability of fast radiofrequency (RF) transmit measurement approaches used in Dynamic Contrast Enhanced Magnetic Resonance Imaging (DCE-MRI). Spatial variation in the transmitted RF field introduces bias and increased variance in quantitative DCE-MRI metrics including tracer kinetic parameter maps. If unaccounted for, these errors can dominate all other sources of bias and variance. The amount and pattern of variation depend on scanner-specific hardware and software. Methods: Human tissue mimicking torso and brain phantoms were constructed. RF transmit maps were measured and compared across eight different commercial scanners, from three major vendors, and three clinical sites. Vendor-recommended rapid methods for RF mapping were compared to a slower reference method. Imaging was repeated at all sites after 2 months. Ranges and magnitude of RF inhomogeneity were compared scanner-wise at two time points. Limits of Agreement of vendor-recommended methods and double-angle reference method were assessed. Results: At 3 T, B₁⁺ inhomogeneity spans across 35% in the head and 120% in the torso. Fast vendor provided methods are within 30% agreement with the reference double angle method for both the head and the torso phantom. Conclusions: If unaccounted for, B₁⁺ inhomogeneity can severely impact tracer-kinetic parameter estimation. Depending on the scanner, fast vendor provided B₁⁺ mapping sequences allow unbiased and reproducible measurements of B₁⁺ inhomogeneity to correct for this source of bias.