Assessment of bone tissue mineralization by conventional x-ray microcomputed tomography: comparison with synchrotron radiation microcomputed tomography and ash measurements.

Assessment of bone tissue mineral density (TMD) may provide information critical to the understanding of mineralization processes and bone biomechanics. High-resolution three-dimensional assessment of TMD has recently been demonstrated using synchrotron radiation microcomputed tomography (SRmuCT); however, this imaging modality is relatively inaccessible due to the scarcity of SR facilities. Conventional desktop muCT systems are widely available and have been used extensively to assess bone microarchitecture. However, the polychromatic source and cone-shaped beam geometry complicate assessment of TMD by conventional muCT. The goal of this study was to evaluate muCT-based measurement of degree and distribution of tissue mineralization in a quantitative, spatially resolved manner. Specifically, muCT measures of bone mineral content (BMC) and TMD were compared to those obtained by SRmuCT and gravimetric methods. Cylinders of trabecular bone were machined from human femoral heads (n = 5), vertebrae (n = 5), and proximal tibiae (n = 4). Cylinders were imaged in saline on a polychromatic muCT system at an isotropic voxel size of 8 microm. Volumes were reconstructed using beam hardening correction algorithms based on hydroxyapatite (HA)-resin wedge phantoms of 200 and 1200 mg HA/cm3. SRmuCT imaging was performed at an isotropic voxel size of 7.50 microm at the National Synchrotron Light Source. Attenuation values were converted to HA concentration using a linear regression derived by imaging a calibration phantom. Architecture and mineralization parameters were calculated from the image data. Specimens were processed using gravimetric methods to determine ash mass and density, muCT-based BMC values were not affected by altering the beam hardening correction. Volume-averaged TMD values calculated by the two corrections were significantly different (p = 0.008) in high volume fraction specimens only, with the 1200 mg HA/cm3 correction resulting in a 4.7% higher TMD value. MuCT and SRmuCT provided significantly different measurements of both BMC and TMD (p < 0.05). In high volume fraction specimens, muCT with 1200 mg HA/cm3 correctionteg resulted in BMC and TMD values 16.7% and 15.0% lower, respectively, than SRmuCT values. In low volume fraction specimens, muCT with 1200 mg HA/cm3 correction resulted in BMC and TMD values 12.8% and 12.9% lower, respectively, than SRmuCT values. MuCT and SRmuCT values were well-correlated when volume fraction groups were considered individually (BMC R2 = 0.97-1.00; TMD R2 = 0.78-0.99). Ash mass and density were higher than the SRmuCT equivalents by 8.6% in high volume fraction specimens and 10.9% in low volume fraction specimens (p < 0.05). BMC values calculated by tomography were highly correlated with ash mass (ash versus muCT R2 = 0.96-1.00; ash versus SRmuCT R2 = 0.99-1.00). TMD values calculated by tomography were moderately correlated with ash density (ash versus muCT R2 = 0.64-0.72; ash versus SRmuCT R2 = 0.64). Spatially resolved comparisons highlighted substantial geometric nonuniformity in the muCT data, which were reduced (but not eliminated) using the 1200 mg HA/cm3 beam hardening correction, and did not exist in the SRmuCT data. This study represents the first quantitative comparison of muCT mineralization evaluation against SRnuCT and gravimetry. Our results indicate that muCT mineralization measures are underestimated but well-correlated with SRmuCT and gravimetric data, particularly when volume fraction groups are considered individually.