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The recently introduced Photon Counting CT (PCCT) offers major advances in spatial resolution and material discrimination compared to conventional multi-detector CT. We investigate whether these new capabilities may enable accurate in vivo quantification of the trabecular microstructure of human bone. Human femoral bone was imaged using reference HR-pQCT (isotropic 60 μm voxels) and PCCT operated in a High Resolution mode (HR, 80 μm in-plane voxel size. 200 μm slice thickness) and in a Calcium-selective mode (CA, isotropic 390 μm voxels). 468 spherical Regions-of-Interest (ROIs) of 5 mm diameter were placed at corresponding locations in the HR-pQCT and PCCT volumes. The bone voxels of HR-pQCT and CA PCCT ROIs were segmented (binarized) using global Otsu thresholding; local Bernsen segmentation was used for HR PCCT. Trabecular thickness (TbTh), spacing (TbSp), number (TbN), and bone volume fraction (BV/TV) were measured in the binarized ROIs. The performance of PCCT morphometrics was evaluated in terms of correlation coefficient and numerical agreement with HR-pQCT. For ROIs with mean TbTh⪆250 μm (approaching the nominal resolution of HR PCCT), the average trabecular measurements obtained from HR PCCT achieved excellent correlations with the reference HR-pQCT: 0.88 for BvTv, 0.89 for TbTh, 0.81 for TbSp and 0.78 for TbN. For ROIs with mean TbTh of 200 μm – 250 μm, the correlations were slightly worse, ranging from 0.61 for TbTh to 0.84 for BvTv. The spatial resolution of CA PCCT in its current implementation is insufficient for microarchitectural measurements, but the material discrimination capability appears to enable accurate estimation of BvTv (correlation of 0.89 to HR-pQCT). The results suggest that the introduction of PCCT may enable microstructural evaluation of the trabecular bone of the lumbar spine and hip, which are inaccessible to current in vivo high-resolution bone imaging technologies. The findings of this work will inform the development of clinical indications for PCCT trabecular bone assessment.
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