Characterization of B1+ field variation in brain at 3 T using 385 healthy individuals across the lifespan.

Abstract

The transmit field at 3 T in brain affects the spatial uniformity and contrast of most image acquisitions. Here, spatial variation in brain at 3 T is characterized in a large healthy population. Bloch-Siegert maps were acquired at 3 T from 385 healthy subjects aged 5-90years on a single MRI system. After transforming all 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 variability due to age and brain size was studied separately in males and females, along with variability due to nonideal transmit calibration. The voxel-based mean coefficient of variation was 4.0% across all subjects, and the difference in 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 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 inhomogeneity (p<.001). Varying head position and anatomy resulted in an inaccurate transmit calibration. In standard atlas space, intersubject variability at 3 T was relatively small in a large population aged 5-90years. The varied with age-related changes of CSF volume and head orientation, as well as differences in brain size and transmit calibration.