Quantitative calcium-based assessment of osteoporosis in dual-layer spectral CT

Objectives: To evaluate a novel calcium-only imaging technique (VCa) with subtracted bone marrow in osteoporosis in dual-layer CT (DLCT) compared to conventional CT images (CI) and dual-energy X-ray absorptiometry (DXA). Material and methods: Images of a multi-energy CT phantom with calcium inserts, quantitative CT calibration phantom, and of 55 patients (mean age: 64.6 +/- 11.5 years) were acquired on a DLCT to evaluate bone mineral density (BMD). CI, calcium-suppressed images, and VCa were calculated. For investigating the association of VCa and CI with DXA a subsample of 30 patients (<90 days between DXA and CT) was used. Multiple regression analysis was performed to identify further factors improving the prediction of DXA BMD. Results: The calcium concentrations of the CT phantom inserts were significantly associated with CT numbers from VCa (R-2 = 0.94) and from CI (R-2 = 0.89-0.92). VCa showed significantly higher CT numbers than CI in the phantom (p <= 0.001) and clinical setting (p < 0.001). CT numbers from VCa were significantly associated with CI (R-2 = 0.95, p < 0.001) and with DXA (R-2 = 0.31, p = 0.007), whereas no significant association between DXA and CI was found. Prediction of DXA BMD based on CT numbers derived from VCa yielded R-2 = 0.76 in multiple regression analysis. ROC for the differentiation of normal from pathologic BMD in VCa yielded an AUC of 0.7, and a cut-off value of 126HU (sensitivity: 0.90; specificity: 0.47). Conclusion: VCa images showed better agreement with DXA and known calcium concentrations than CI, and could be used to estimate BMD. A VCa cut-off of 126 HU could be used to identify abnormal bone mineral density.