Specimen size and porosity can introduce error into microCT-based tissue mineral density measurements

The accurate measurement of tissue mineral density, rho(m), in specimens of unequal size or quantities of bone mineral using polychromatic microCT systems is important, since studies often compare samples with a range of sizes and bone densities. We assessed the influence of object size on microCT measurements of rho(m) using (1) hydroxyapatite rods (HA), (2) precision-manufactured aluminum foams (AL) simulating trabecular bone structure, and (3) bovine cortical bone cubes (BCt). Two beam-hardening correction (BHC) algorithms, determined using a 200 and 1200 mg/cm(3) HA wedge phantom, were used to calculate rho(m) of the HA and BCt. The 200 mg/cm(3) and an aluminum BHC algorithm were used to calculate the linear attenuation coefficients of the AL foams. Equivalent rho(m) measurements of 500, 1000, and 1500 mg HA/cm(3) rods decreased (r(2)>0.96, p<0.05 for all) as HA rod diameter increased in the 200 mg/cm(3) BHC data. Errors averaged 8.2% across these samples and reached as high as 29.5%. Regression analyses suggested no size effects in the 1200 mg/cm(3) BHC data but differences between successive sizes still reached as high as 13%. The linear attenuation coefficients of the AL foams increased up to approximately 6% with increasing volume fractions (r(2)>0.81, p<0.05 for all) but the strength of the size-related error was also BHC dependent. Equivalent rho(m) values were inversely correlated with BCt cube size (r(2)>0.92, p<0.05). Use of the 1200 mg/cm(3) BHC ameliorated the size-related artifact compared to the 200 mg/cm(3) BHC but errors with this BHC were still significant and ranged between 5% and 12%. These results demonstrate that object size, structure, and BHC algorithm can influence microCT measurements of rho(m). Measurements of rho(m) of specimens of unequal size or quantities of bone mineral must be interpreted with caution unless appropriate steps are taken to minimize these potential artifacts.