How is bone mineral density related to ultra-short echo time MRI-derived bone porosity index in the thoroughbred metacarpal condyle?

Introduction / Purpose: Bone porosity index (PI) has been correlated to bone composition and mechanical stiffness. The relationship between bone porosity and bone mineral density (BMD) has not been studied in equine bones and may be relevant for fracture propagation. Purpose of the study is to validate ultra-short echo-time MRI-derived PI measurements in the equine distal Mc/MtIII condyle and evaluate the relationship between PI and BMD.
Methods:
Five post-mortem equine Mc/MtIII (McIII; n=1/MtIII; n=4) were collected. All specimens underwent high-resolution peripheral quantitative computed tomography in a 64-slice CT scanner (Siemens, Somatom Definition AS). MR images were acquired using a 3T clinical scanner (MAGNETOM Skyra 3T, Siemens Healthcare, Erlangen, Germany). BMD and PI were calculated and corresponding images generated. BMD and PI images were co-registered in Elastix using a mutual information metric and a B-spline transform. Co-registered images were loaded into an open-source 3D imaging software and the volume of the distal condyle was defined manually (3D-ROI). Subsequently, BMD and PI values were plotted against each other for each pixel in the 3D-ROI.
Results:
BMD and PI of the equine distal Mc/MtIII are inversely correlated with an average slope of -0.0163 in the scatterplot and an average correlation coefficient of 0.312.
Discussion / Conclusion:
BMD and PI are negatively correlated in the equine distal Mc/MtIII. Further work is needed to assess how correlation patterns behave in different areas of the bone and to evaluate PI in the presence of microcracks in horses with and without clinically relevant stress fractures of the parasagittal groove.