Age- and sex-specific values of regional bone mineral density from opportunistic CT of the lower lumbar spine, pelvis, sacrum, and proximal femur

Abstract Objective Standard osteoporosis assessment using dual-energy X-ray absorptiometry (DXA) provides limited 2D measures (BMD, T-score), potentially missing localized bone density differences relevant to fracture risk. Opportunistic CT allows comprehensive 3D volumetric bone density analysis. This study presents a detailed 3D statistical model of CT-derived apparent volumetric bone mineral density (CT-vBMD) covering the L4 and L5 vertebrae, pelvic bones, sacrum, and proximal femur and examines spatial distributions and associations with age and sex. Materials and methods Retrospective analysis included 330 routine abdominal CT scans (ages 20–91; approximately balanced by sex). Bones were segmented with a semi-automated workflow, internally calibrated, registered to bone-specific population templates, and projected onto corresponding tetrahedral meshes. Whole-bone averages were compared across age groups and between sexes. For each element, we calculated mean vBMD, variability, correlations with age and sex, and cluster-defined low-density and age-sensitive subregions. Results The global spatial pattern was similar in females and males. The highest CT-vBMD values were localized mainly to cortical load-bearing zones, whereas the lowest values were concentrated in trabecular spongiosa. Older age was associated with lower whole-bone CT-vBMD in all bones and with stronger negative associations in trabecular-rich regions, such as the subchondral acetabulum, femoral neck, sacral ala, and vertebral bodies. Cortical-dominant load-bearing regions showed weaker negative associations with age. Conclusion This framework maps anatomically localized associations between age and CT-vBMD beyond the regions routinely assessed by DXA. Opportunistic CT mapping may complement standard osteoporosis assessment by providing spatial information for the pelvis, sacrum, lower lumbar spine, and proximal femur.