Cortical thickness estimation of the proximal femur from multi-view Dual X-ray Absorptiometry
My PhD research targets a very significant burden of our society: hip fractures. They are the leading cause of acute orthopaedic hospital admission amongst the elderly, with one out of five women over fifty years old enduring one. One year mortality rates are as high as 33%, with survivors suffering pain, reduced quality of life and disability. Although various preventative therapies are available, patient selection is difficult, and the current state-of-the-art risk assessment tool ignores focal structural defects, such as cortical bone thinning, a critical component in characterizing hip fragility. The cortex is a layer of mineral-rich tissue, just under the surface of the bone (see Figure 1), which is thought to contribute up to 90% to the strength. Moreover, various studies provide strong empirical evidence that fractures may initiate at locations where the cortical layer is eggshell thin and maximum stresses are observed.
We first developed a tool capable of producing detailed cortical thickness estimates using information from multi-view DXA scans (see Figure 3) and a deformable template model of a femur. Figures 4 and 5 briefly explain the workflow of our approach: we first estimate the cortical thickness at many cross-sections of the bone, and then we present the results as colour-maps for ease of visualisation.
We then examined an alternative approach: we extended our model by incorporating more prior information about the shape and density distribution of the femur using a large training cohort of more than 700 femurs. The statistical models we built capture mathematically common anatomical variations and are used to compensate for the sparseness of information in the DXA datasets. Figures 6-9 show the two most important shape and thickness modes of variation we obtained from our statistical study. Subsequently, these models are fitted to patient's DXA scans using an iterative optimisation algorithm. Similarly to our previous approach, the final result is a colour-mapped femoral surface, where the colour represents the cortical thickness at each location – the results can be seen in Figure 10.
Our results for the clinically relevant experiments are encouraging and it remains to be seen whether the current performance meets clinical requirements; this will depend on the location and size of fracture-predicting regions of interest that are only now beginning to emerge from cohort studies. We hope that this technique will allow early detection of fragile bones and assist in treatment monitoring.