Quantification and reproducibility assessment of the regional brain T2 relaxation in naïve rats at 7T.
To measure the reproducibility of T2 relaxation and to determine the statistical power of T2 mapping in the rat brain as a characteristic of the baseline performance of the T2 relaxation as a potential biomarker of neurotoxicity.  Multislice multiecho spin-echo imaging was utilized to obtain the quantitative T2 maps in 138 naïve rats at 7T. Images were skull-stripped and coregistered to the common anatomical reference.  A full anatomical segmentation mask, which included all major brain structures, was created using the same reference T2 map.  The overall variability map was also calculated from all T2 maps and the areas with arbitrarily high variability (coefficient of variation >25%) were excluded from the full segmentation mask to produce a trimmed mask.  T2 maps were segmented using both these masks and statistical power analysis was conducted in all segmented areas.  The coefficient of variation of T2 relaxation in different brain areas varied from 5.4% (cerebrospinal fluid) to 1.2% (cortex) when using a full segmentation mask.  The use of a trimmed segmentation mask decreased the coefficient of variation in many areas, which ranged between 3.2% (inferior colliculi) and 1.2% (cortex) in this case.  As revealed by statistical power analysis to detect 5% change with power of 0.8, the minimum number of observations needed for different areas ranged from 3 (cortex) to 8 (inferior colliculi) in the case of use of a trimmed segmentation mask.  T2 relaxation is a very reproducible MRI parameter of the rat brain with high statistical power, which allows detecting very small changes in groups consisting of a minimal number of experimental animals.  1 J. Magn.  Reson.  Imaging 2017;45:700-709.