Image registration of proximal femur with substantial bone changes : application in 3D visualization of bone loss of astronauts after long-duration spaceflight

We recently studied bone loss in crewmembers making 4 to 6 months flights on the International Space Station. We employed Quantitative Computed Tomography (QCT) technology (Lang et. al., J Bone Miner Res. 2004; v. 19, p. 1006), which made measurements of both cortical and trabecular bone loss that could not be obtained by using 2-dimensional dual x-ray absorptiometry (DXA) imaging technology. To further investigate the bone loss after spaceflight, we have developed image registration technologies to align serial scans so that bone changes can be directly visualized in a sub-regional level, which can provide more detailed information for understanding bone physiology during long-term spaceflight. To achieve effective and robust registration when large bone changes exist, we have developed technical adaptations to standard registration methods. Our automated image registration is mutual-information based. We have applied an automatically adaptive binning method in calculating the mutual information. After the pre- and post-flight scans are geometrically aligned, the interior bone changes can be clearly visualized. Image registration can also be applied to Finite Element Modeling (FEM) to compare bone strength change, where consistent loading conditions must be applied to serial scans.