Opportunistic Bone Mineral Density Measurement Using Photon-Counting Detector CT Spectral Localizer Images: A Prospective Study.

Background: Spectral localizer images from photon-counting detector (PCD) CT can be used for bone mineral density (BMD) evaluation given their 2D-projectional nature and material decomposition capability. As all CT examinations include localizer images, this approach could allow opportunistic osteoporosis screening in patients undergoing clinically indicated imaging by PCD CT. Objective: To assess the utility of PCD-CT spectral localizer images for opportunistic derivation of area BMD (aBMD) values and T-scores, using dual-energy X-ray absorptiometry (DXA) as the reference standard. Methods: This prospective study included patients ≥18 years old scheduled for clinically indicated lumbar spine CT between October 2023 and February 2024 and who underwent DXA within the 13 prior months or were scheduled for DXA within the subsequent 13 months. Participants underwent lumbar spine CT by PCD CT including spectral localizer images. Lumbar spine aBMD was extracted from clinical DXA reports. ROIs were placed on lumbar vertebral bodies and background soft tissues to extract areal densities from spectral localizer images using material decomposition; areal densities were used to derive lumbar spine aBMD values. The aBMD values were used to derive T-scores, which were classified as representing normal (≥-1) or abnormal (<-1) bone mass. DXA-derived and PCD-CT derived measurements were compared. Results: The study included 51 participants (mean age: 62 years [range, 28-83 years]; 31 female, 20 male). Mean DXA-derived T-score was 0.39±1.64; mean PCD-CT derived T-score was 0.28±1.77 (p=.29). Lin's concordance correlation coefficient between DXA-derived and PCD-CT T-scores was 0.90. The difference between DXA-derived and PCD-CT derived T-scores showed a small correlation with patient age (r=-0.13), absolute interval between DXA and PCD CT (r=.15), and BMI (r=0.28); this difference in scores did not show a significant difference between male and female patients (0.08 vs 0.13, respectively; p=.81). PCD-CT T-scores had sensitivity of 97%, specificity of 71%, PPV of 90%, and NPV of 91% for detecting abnormal bone mass using DXA-derived T-scores as the reference standard. Conclusion: PCD-CT spectral localizers showed clinical utility for deriving aBMD values and, consequently, T-scores. Clinical Impact: The T-score derived from PCD-CT spectral localizers may serve as an opportunistic screening tool for low bone mass and osteoporosis.