Conversion relations for quantitative CT bone mineral densities measured with solid and liquid calibration standards.

A vast data base exists for QCT measurements of bone mineral density (BMD) referenced to K2HPO4 in water (liquid) standards. To effectively utilize the more stable hydroxyapatite in water-equivalent plastic (solid) standards that have recently been introduced, it will be necessary to derive conversion relations. A study was performed to investigate the dependence of these relations upon x-ray tube voltage, marrow composition, and patient body size. Test objects included five diverse composition vertebral marrow inserts within three different size lumbar simulators and an L1 vertebra within a Humanoid phantom. The calibration standards were manufactured by Image Analysis, and all data were acquired with a GE 9800 CT scanner operated at 80 kVp and 140 kVp. Least square fits to corresponding liquid versus solid referenced BMD measurements of the inserts all had r's > 0.999. SEEs, < 2 mg/ml, and intercepts of approximately 0. The slopes (BMDK2HPO4/BMDhydroxyapatite) for the various body sizes were all about the same with values of 0.86, 0.81, and 0.96 to 1.02 for the single-energy@80 kVp, single-energy@140 kVp, and dual-energy measurements, respectively. Corresponding ratios for the Humanoid vertebra were 0.86, 0.82, and 0.96. The conversion relations were essentially independent of marrow composition and body size but did depend upon kVp. Finally, although the solid standards are more stable, they may still exhibit problems, and these are discussed.