Photon-Counting Micro-CT for High-Resolution Bone Morphometry in Murine Models

Background/Objectives: This study evaluates photon-counting CT (PCCT) for high-resolution imaging of mouse femurs and investigates how APOE genotype, sex, and humanized nitric oxide synthase (HN) expression influence bone morphology during aging. Methods: A custom-built micro-CT system with a photon-counting detector (PCD) was used to acquire dual-energy scans of mouse femur samples. PCCT projections were corrected for tile gain differences, iteratively reconstructed with 20 µm isotropic resolution, and decomposed into calcium and water maps. PCD performance was benchmarked against an energy-integrating detector (EID) using modulation transfer functions and line profiles. Contrast-to-noise ratio quantified effects of iterative reconstruction and material decomposition. Femur features such as mean cortical thickness, mean trabecular spacing (TbSp_mean), and trabecular bone volume fraction (BV/TV) were extracted from calcium maps using BoneJ. Statistical analysis used 57 aged mice representing APOE22, APOE33, and APOE44 genotypes, including 27 expressing HN. We used generalized linear models (GLMs) to evaluate main and interaction effects of age, sex, genotype, and HN status on femur features and Mann-Whitney U tests for stratified analyses. Results: PCCT outperformed EID-CT in spatial resolution and enabled effective separation of calcium and water. GLMs revealed significant interactions between sex and HN status affecting trabecular features. Female HN mice exhibited reduced BV/TV and increased TbSp_mean compared to both male HN and female non-HN mice. While genotype effects were modest, genotype by sex stratified analysis found significant effects of HN status only in female APOE22 and APOE44 mice. Conclusions: These results demonstrate PCCT’s utility for femur analysis in mice, supporting its application in skeletal disease research.