Preparation and characterization of calibration standards for bone density determination by micro-computed tomography

Phantoms for the calibration of local bone mineral densities by micro-computed tomography (microCT), consisting of lithium tetraborate (Li(2)B(4)O(7)) with increasing concentrations of hydroxyapatite [HAp, Ca(10)(PO(4))6(OH)2] have been prepared and characterized for homogeneity. Large-scale homogeneity and concentration of HAp in the phantom materials was determined using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), while homogeneity on the micrometer scale was assessed through microCT. A series of standards was prepared by fusion of pure HAp with Li(2)B(4)O(7) in a concentration range between 0.12 and 0.74 g cm(-3). Furthermore, pressed and sintered pellets of pure HAp were prepared to extend the calibration range towards densities of up to 3.05 g cm(-3). A linear calibration curve was constructed using all individual standard materials and the slope of the curve was in good agreement with calculated absorption coefficients at the effective energy of the microCT scanner.