Robust intensity-based 3D-2D registration of ct and X-ray images for precise estimation of cup alignment after total hip arthroplasty

The widely used procedure of evaluation of cup orientation following THA using single standard anteroposterior radiograph is known inaccurate, largely due to the wide variability in individual pelvic position relative to X-ray plate. 3D-2D image registration methods have been introduced to estimate the transformation between a CT volume and the radiograph for an accurate estimation of the cup orientation relative to an anatomical reference extracted from the CT data. However, the robustness of these methods is questionable. This paper presents a robust image similarity measure which is derived from a variational approximation to mutual information and allows for incorporation of spatial information via energy minimization. Experimental results on estimating cup alignment from single X-ray radiograph with gonadal shielding demonstrate the robustness and the accuracy of the present approach.