High resolution cortical thickness measurement from clinical CT data

The distribution of cortical bone in the proximal femur is believed to be critical in determining fracture resistance. Current CT technology is limited in its ability to measure cortical thickness, especially in the sub-millimetre range which lies within the point spread function of today's clinical scanners. In this paper, we present a novel technique that is capable of producing unbiased thickness estimates down to 0.3 mm. The technique relies on a mathematical model of the anatomy and the imaging system, which is fitted to the data at a large number of sites around the proximal femur, producing around 17,000 independent thickness estimates per specimen. In a series of experiments on sixteen cadaveric femurs, estimation errors were measured as −0.01±0.58 mm (mean ± 1 std. dev.) for cortical thicknesses in the range 0.3 mm to 4 mm. This compares with 0.25±0.69 mm for simple thresholding and 0.90±0.92 mm for a variant of the 50% relative threshold method. In the clinically relevant sub-millimetre range, thresholding increasingly fails to detect the cortex at all, whereas the new technique continues to perform well. The many cortical thickness estimates can be displayed as a colour map painted onto the femoral surface. Computation of the surfaces and colour maps is largely automatic, requiring around 15 minutes on a modest laptop computer.