Issues of threshold selection when determining the fractal dimension in HRCT slices of lumbar vertebrae

The box counting dimension is a frequently applied tool for the classificat ion of trabecular bone structure. The algorithm requires a binarization of the gray value data, for example that acquired by high resolution CT (HRCT) . We recently proposed a method to eliminate bone mineral density (BMD) by applying a linear normalization scheme. Further consideration has shown tha t full BMD independence has not been achieved, and the structural parameter proposed was therefore difficult to interpret. In this study we present an alternative approach to obtain a structural parameter that is independent of BMD. HRCT volume data was acquired on 21 lumbar vertebrae from five cada vers. In the segmented spongiosa, thresholding was based on different quant iles of the gray value histogram, yielding invariance over linear and non-l inear transformations. Thresholding at high gray value levels (80% quantile ) shows the highest level of significance when discriminating between osteo porotic and non-osteoporotic cases. As an addition to the measurement of BM D alone, the determination of structural properties allows an improvement o f the assessment of the individual fracture risk.