The Utility of Photon Counting CT Localizer Radiograph in Bone Densitometry

Abstract

Purpose: Osteoporosis is a major public health concern, yet early detection remains limited due to low utilization of dual-energy X-ray absorptiometry (DEXA), the current gold standard for bone mineral density (BMD) assessment. This study investigates the feasibility of using photon-counting CT (PCCT) localizer radiographs for opportunistic osteoporosis screening by assessing their accuracy in measuring areal BMD (aBMD) in comparison to DEXA.Methods: A combination of physical phantom experiments and virtual imaging trials (VITs) was conducted. A lumbar spine anthropomorphic phantom was scanned using a clinical PCCT system (NAEOTOM Alpha, Siemens Healthineers) and DEXA to directly compare aBMD values. Virtual imaging trials simulated various mAs levels (35-160 mAs) and bone density conditions (normal, osteopenia, and osteoporosis) to evaluate measurement accuracy under different acquisition settings. A custom segmentation and material decomposition algorithm was implemented to extract hydroxyapatite and water maps, allowing for aBMD calculation with soft tissue corrections. Results: PCCT-derived aBMD measurements demonstrated strong agreement with DEXA, with percent differences ranging from -4.95% to 5.05% across vertebrae. Virtual imaging results showed that increasing mAs generally improved accuracy, with mean absolute error (MAE) decreasing significantly up to 80-100 mAs, beyond which improvements plateaued. Accuracy varied with bone density, with the lowest MAE observed in osteoporotic bone (0.0239) and the highest in normal bone (0.1521), suggesting greater variability in denser bone measurements. Radiation dose estimates confirmed that PCCT localizer radiographs deliver substantially lower doses than full CT scans, with a dose-area product (DAP) of 39 mGy·cm² at 35 mAs, comparable to fan-beam DEXA. Conclusion: PCCT localizer radiographs offer a promising low-dose alternative for opportunistic osteoporosis screening, capable of accurate aBMD measurement with seamless integration into routine CT workflows. The results support further clinical validation of this technique to improve early osteoporosis detection and reduce fracture risk in at-risk populations.