Evaluating Bone Mass with Opportunistic Chest CT: T8 as the Optimal Vertebral Level

Opportunistic chest CT (OC-CT) presents significant potential for early osteoporosis screening. However, its application remains limited to specific patient populations, and the optimal vertebral level for osteoporosis detection has yet to be determined. This study aimed to determine the optimal vertebral level and assess the feasibility of OC-CT, with dual-energy X-ray absorptiometry (DXA) being the gold standard. We retrospectively included 1236 participants aged 50 years or above and who completed both DXA and routine chest CT scans within 6 months. Receiver operation characteristic (ROC) analysis was used to evaluate the diagnostic performance in detecting low bone mass and osteoporosis. Among 15,007 vertebrae analyzed, CT_AV at the 1st-12th thoracic (T1-T12) and the 1st lumbar (L1) vertebrae exhibited good discrimination for low bone loss and osteoporosis with the area under the ROC curve exceeding 0.80. Notably, T8 was the only vertebra to achieve sensitivity above 0.80 for both conditions: a cut-off value of 163.5 Hu yielded 81% sensitivity and 84% specificity for low bone mass, while 132.3 Hu yielded 82% sensitivity and 75% specificity for osteoporosis. These findings indicated that OC-CT demonstrated comparable diagnostic performance to DXA for bone mass evaluation in individuals aged 50 years or above. T8 was recommended as the optimal vertebral level for opportunistic bone mass evaluation, and T7 and T10 vertebrae were the preferred alternatives when T8 was not available. We also provided a user-friendly self-check table for rapid bone mass evaluation to facilitate clinical utility.