Objective. To assess the accuracy and stability of areal bone-mineral-density (aBMD) measurements from multi-energy CT localizer radiographs acquired using photon-counting detector (PCD) CT. Approach. A European Spine Phantom (ESP) with hydroxyapatite (HA 0.5, 1.0 and 1.5 g cm(-2)) was scanned using clinical PCD-CT and a dual-energy x-ray absorptiometry (DXA) to compare aBMD values. To assess aBMD stability and reproducibility, PCD-localizers were acquired twice a day for one week, and once per week for five weeks. Multiple ESP anteroposterior thicknesses (18, 26, 34, and 40 cm) were achieved using a synthetic gel layer and scanned across eight tube current values for both 120 kV (15-120 mA) and 140 kV (10-80 mA). One-way analysis of variance was performed for statistical significance (p < 0.05 considered significant). Quantitative HA and water maps were reconstructed using a prototype material-decomposition software, and aBMD was calculated after background correction. In vivo performance of PCD-based aBMD was illustrated using a patient scan acquired at 140 kV and 40 mA, and lumbar aBMD values were compared with DXA. Main results. The ESP aBMD values from PCD-localizers demonstrated excellent day-to-day stability with a coefficient-of-variation ranging from 0.42% to 0.53%, with mean absolute percentage errors (MAPE) of less than 5% for all three vertebral bodies. The coefficient-of-variation for weekly scans ranged from 0.17% to 0.60%, again with MAPE below 5% for all three vertebral bodies. Across phantom sizes and tube currents, the MAPE values varied ranging from 1.84% to 13.78% for 120 kV, and 1.38%-9.11% for 140 kV. No significant difference was found between different tube currents. For the standard phantom size, DXA showed 11.21% MAPE whereas PCD-CT showed 3.04% MAPE. For the patient scan, deviation between PCD-based aBMD values and those obtained from DXA ranged from 0.07% to 9.82% for different lumbar vertebra. Significance. This study highlights the accuracy and stability of PCD-CT localizers for measuring aBMD. We demonstrated aBMD accuracy across different sizes and showed that higher radiation doses did not inherently increase aBMD accuracy.
