Evaluation of Cortical Porosity from HR-pQCT at the Tibia: Comparison with Synchrotron Radiation Micro-computed Tomograph

International audienceThe HR-pQCT (XtremeCT Scanco®) with a voxel size at 82 (µm3) is carried out routinely in clinical research protocols. Micro-computed tomography (micro-CT) has become a standard tool for examination of trabecular and cortical bone in 3 dimensions (3D).The purpose of this study is to evaluate the accuracy and precision of cortical measurements derived from HR-pQCT images using a local comparison with morphological measurements from synchrotron radiation (SR) micro-CT.Thirty tibias specimens (mean age 82.2±9.7) were scanned at 3.5 cm from the tibial pilon. The acquisition and analysis protocols provided by the manufacturer were used. Fourteen samples were scanned twice to calculate the Root Mean Square Coefficient of Variation (RMSCV,%). The usual outcomes include: volumetric bone density (gHA/cm3) for entire (Dtot) and cortical (Dcomp) regions, cortical thickness (Ct.Th,mm). New features as cortical porosity (Ct.Po) and mean pore diameter (mPo.Dm) were measured with manual correction of the endosteal contour. In a site matched region of HR-pQCT images, bone samples were taken at the posterior part of the tibia and imaged on beamline ID19 (ESRF,Grenoble). The acquisition parameters used were 30 keV, 3500 views over 360°, the exposure time for each view was 0.3s. The voxel size is 7.5 (µm3). Ct.Th_SR was manually measured. A unique threshold was used for the whole dataset. Using the CTAn Skyscan® software, the following parameters were obtained: Pore volume/Bone volume (PoV/TV_SR %), pore diameter (Po.Dm_SR,mm), pore spacing (Po.Sp_SR,mm), pore number (Po.N_SR,mm-1) and the degree of anisotropy (DA,no unit). Maps of linear attenuation coefficient were converted into degrees of mineralization of bone (DMB) with data expressed in g/cm3.Pearson correlation coef. (r) between Ct.Th vs Ct.Th_SR was r=0.58**. Correlation coef. were not significant for Dcomp vs DMB and for m.Po.Dm vs Po.Dm_SR and PoV/TV_SR. Significant correlation coef. between PoV/TV_SR and cortical parameters from HR-pQCT are shown in Table 1.To explain Dcomp in multivariate analysis, the determination coef (r2) was 0.80 with a combination of PoV/TV_SR, Po.N_SR and Po.Sp_SR instead of 0.77 with PoV/TV_SR alone.In conclusion, Dcomp from HR-pQCT images is the most pertinent parameter reflecting with a good precision cortical porosity measured from SR micro-CT images. Dcomp was not better explained by a combination of morphological parameters from SR micro-CT images