Comparative testing of calibration phantoms for quantitative CT vertebral mineral determinations

Three different calibration phantoms for quantitative CT bone densitometry employing liquid dipotassium hydrogen phosphate (conventional [K2HPO4] and Picker International [PI]) and solid calcium hydroxyapatite (Pyramid Medical [PM]) were compared in patient and in vitro experiments. Twelve premenopausal women underwent quantitative CT of the L-2 through L-4 vertebral bodies. with mineral equivalent values calculated from K2HPO4 and PI data. Thirty postmenopausal women underwent evaluation from T-11 through L-3, with data correction by the K2HPO4 and PM phantoms. High correlation (r = .96) was observed between results derived using the K2HPO4 and PI phantoms. Mineral equivalent values were generally higher for the PM than the K2HPO4 phantom. The PM phantom has the advantage of affording beam hardening correction and extraction of densitometric information from CT examinations performed for other reasons. Different phantoms should not be used to correct longitudinal data in a given patient, since the magnitude of methodologic discrepancy is equivalent to or greater than the precision of quantitative CT