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

The accurate measurement of tissue mineral density, ρm, in specimens of unequal size or quantities of bone mineral using polychromatic μCT systems is important, since studies often compare samples with a range of sizes and bone densities. We assessed the influence of object size on μCT measurements of ρm 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/cm3 HA wedge phantom, were used to calculate ρm of the HA and BCt. The 200 mg/cm3 and an aluminum BHC algorithm were used to calculate the linear attenuation coefficients of the AL foams. Equivalent ρm measurements of 500, 1000, and 1500 mg HA/cm3 rods decreased (r2 > 0.96, p < 0.05 for all) as HA rod diameter increased in the 200 mg/cm3 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/cm3 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 (r2 > 0.81, p < 0.05 for all) but the strength of the size-related error was also BHC dependent. Equivalent ρm values were inversely correlated with BCt cube size (r2 > 0.92, p < 0.05). Use of the 1200 mg/cm3 BHC ameliorated the size-related artifact compared to the 200 mg/cm3 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 μCT measurements of ρm. Measurements of ρm 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.