Cross calibration of a chalk phantom with standard bone phantom materials for Quantitative Computed Tomography

Background. Quantitative computed tomography (CT) is used to assess bone mineral density (BMD). This method uses a direct CT number calibration to correlate a phantom of known density with the CT numbers measured in the bone. While dual-energy x-ray absorptiometry has largely replaced this method, direct CT number calibration maintains its accuracy for dry bone studies. Calibration phantoms are commonly composed of calcium hydroxyapatite (CaHA); while this material is effective, it can be expensive to purchase for infrequent measurements. Purpose. This study aims to develop a low-cost alternative BMD phantom based on a mixture of chalk that is cross-calibrated against commercial CaHA and calcium phantoms. Methods. A BMD computed tomography (CT) study was conducted with a 2-mm thick cadmium zinc telluride (CZT) photon counting detector (PCD) with 330-um pixels and six energy thresholds. PCD-CT images with five energy bins were reconstructed for a 120-kVp imaging beam. PCD-CT images were independently used to measure CT number trends for the newly-developed calibration phantom. The phantom was constructed from mixtures of chalk and baking powder to represent various levels of bone density in a manner similar to the commercial Gammex CaHA phantoms. The chalk phantoms performance was further validated against Gammex calcium phantoms. Results. Our results showed that our constructed chalk phantoms had linear trends within 5% in CT number as a function of calcium density to the commercial Gammex calcium phantoms, which was expected since the primary attenuator of chalk is calcium. Compared to CaHA phantoms, the chalk phantoms demonstrated similar trends in attenuation while overestimating the calcium content in these phantoms by approximately 8%. Conclusions. This study concludes that chalk phantoms can reliably translate to the trends expected by CaHA phantoms and can thus be used to measure bone mineral density accurately provided that an 8% correction between calcium density and bone mineral density is accounted for.