3D bone mineral density distribution and shape reconstruction of the proximal femur from a single simulated DXA image: an in vitro study

Area Bone Mineral Density (aBMD) measured by Dual energy X-ray Absorptiometry (DXA) is an established criterion in the evaluation of hip fracture risk. The evaluation from these planar images however is limited to 2D while it has been shown that proper 3D assessment of both the shape and the BMD distribution improves the fracture risk estimation. In this work we present a method to reconstruct both the 3D bone shape and 3D Bone Mineral Density (BMD) distribution of the proximal femur from a single DXA image. A statistical model of shape and a separate statistical model of the BMD was automatically constructed from a set of quantitative computed tomography (QCT) scans. The reconstruction method incorporates a fully automatic intensity based 3D-2D registration process, maximizing the similarity between the DXA and a digitally reconstructed radiograph of the combined model. For the experiments an in vitro dataset of QCT scans of 90 anatomical specimens was used. Out of these, 60 were used to construct both the shape and the density model. To evaluate the reconstruction accuracy, experiments were performed on simulated DXA images from the remaining 30 QCT scans. Comparisons between the reconstructions from DXA with the same subject QCT scans showed a mean shape accuracy of 1.2mm whereby 95% of the error is below 3.2mm, and a mean density error of 81mg/cm corresponding to 4.6% of the whole range of bone density values. The results show that this method is capable of accurately reconstructing both the 3D shape and 3D BMD distribution of the proximal femur from DXA images used in clinical routine, potentially improving the diagnosis of osteoporosis and fracture risk assessments at a low radiation dose and low cost.