Validation of noninvasive quantification of bone marrow fat volume with microCT in aging rats.

Marrow fat infiltration is one of the hallmarks of age-related bone loss. This fat infiltration has been quantified by invasive and noninvasive methods. However, the validity of the noninvasive methods has not been correlated with a gold standard. In this study we aim to validate the usefulness of marrow fat quantification by correlating microCT (μCT) images with histology analysis. Fat volume (FV) and bone volume (BV) of distal femora of young (4 months) and old (27 months) Louvain/c (LOU) rats (n=22) were quantified by histology and compared with μCT images analyzed by an image analysis software (SliceOMatic). We found that for SliceOMatic/μCT the intra-rater reliability for duplicate measurements was 0.94 (p<0.001) and the inter-rater reliability for FV/BV ratio in young and old rats was 98% and 99% respectively. Both methods showed a significant increase (~2 fold) in the FV/BV ratio in the old rats as compared with their young counterparts (p<0.001). A significantly higher correlation (r2=0.85) in the old rats was found between our noninvasive method and histology. Furthermore, our noninvasive method showed good agreement with histology. In conclusion, noninvasive quantification of FV/BV ratio using an image analysis software is as reliable as histology for identifying age related marrow fat changes with high inter and intra-rater reliability. These findings provide a new noninvasive method for quantifying marrow fat, which is useful and can be tested not only in animals but also in human studies.