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 of patients undergoing clinically indicated imaging with PCD CT. OBJECTIVE. The purpose of this study was to assess the use of PCD CT spectral localizer images for opportunistic derivation of areal BMD (aBMD) values and T-scores, with dual-energy x-ray absorptiometry (DEXA) used as the reference standard. METHODS. This prospective study included patients 18 years old or older who were scheduled for a clinically indicated lumbar spine CT examination between October 2023 and February 2024 and who underwent DEXA within the previous 13 months or were scheduled for DEXA within the subsequent 13 months. Participants underwent lumbar spine CT by PCD CT including spectral localizer images. The lumbar spine aBMD was extracted from clinical DEXA 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 (≥ -1.0) or abnormal (< -1.0) bone mass. DEXA-derived and PCD CT-derived measurements were compared. RESULTS. The study included 51 participants (31 women and 20 men; mean age, 62 years [range, 28-83 years]). The mean DEXA-derived T-score was 0.4 ± 1.6 [SD], and the mean PCD CT-derived T-score was 0.3 ± 1.8 [SD] (p = .29). The Lin concordance correlation coefficient between DEXA-derived and PCD CT-derived T-scores was 0.90. The difference between DEXA-derived and PCD CT-derived T-scores showed a small correlation with patient age (r = -0.13), absolute interval between DEXA and PCD CT (r = 0.15), and BMI (r = 0.28); this difference in scores did not show a significant difference between men and women (0.08 vs 0.13, respectively; p = .81). PCD CT-derived T-scores had a sensitivity of 97%, specificity of 71%, PPV of 90%, and NPV of 91% for detecting abnormal bone mass, with DEXA-derived T-scores used as the reference standard. CONCLUSION. PCD CT spectral localizer images showed clinical utility in deriving aBMD values and, consequently, T-scores. CLINICAL IMPACT. The T-score derived from PCD CT spectral localizer images may serve as an opportunistic screening tool for low bone mass and osteoporosis.