RESUMO
PURPOSE: The transmit field B1+ at 3 T in brain affects the spatial uniformity and contrast of most image acquisitions. Here, B1+ spatial variation in brain at 3 T is characterized in a large healthy population. METHODS: Bloch-Siegert B1+ maps were acquired at 3 T from 385 healthy subjects aged 5-90 years on a single MRI system. After transforming all B1+ maps to a standard brain atlas space, region-of-interest analysis was performed, and intersubject voxel-wise coefficient of variation was calculated across the whole brain. The B1+ variability due to age and brain size was studied separately in males and females, along with B1+ variability due to nonideal transmit calibration. RESULTS: The voxel-based mean coefficient of variation was 4.0% across all subjects, and the difference in B1+ between central (left thalamus) and outer regions (left frontal gray matter) was 24.2% ± 2.3%. The least intersubject variability occurred in central regions, whereas regions toward brain edges increased markedly in variation. The B1+ variability with age was mostly attributed to lifespan changes in CSF volume (which alters brain conductivity) and head orientation. Larger brain size correlated with more B1+ inhomogeneity (p < .001). Varying head position and anatomy resulted in an inaccurate transmit calibration. CONCLUSION: In standard atlas space, intersubject B1+ variability at 3 T was relatively small in a large population aged 5-90 years. The B1+ varied with age-related changes of CSF volume and head orientation, as well as differences in brain size and transmit calibration.
