Evaluation of the cortical structure in high resolution CT images of lumbar vertebrae by analysing low bone mineral density clusters and cortical profiles.

The structural classification of trabecular bone is of considerable clinical importance for the diagnosis of osteoporosis. Assessment of the cortical bone mineral density (BMD) and analysis of cortical structure is necessary because the cortex is also affected by osteoporosis. We describe a method for the automatic classification of the cortex from its structure on high resolution (HR) CT images. The method is based on the distribution of areas with low BMD in the cortex. Two different approaches are presented; one uses the grey scale profile of the cortical ridge, and the other one evaluates the distribution of connected regions (clusters) of low BMD. i.e. areas that lie below a certain threshold value. In HRCT images from 22 lumbar vertebrae, the number of intersections of the cortical intensity profile with a horizontal line was determined at various positions of this threshold line. At a certain position, there was a local maximum in the number of intersections which was 38% higher in the osteoporotic cases. The maximum shows a moderate correlation with the cortical BMD of rni = -0.72 (p < 0.0001). The number nc of connected regions (clusters) with pixel values below a certain threshold value was determined in relation to the threshold value T. The resulting function nc(T) shows a relative maximum slightly below the average grey scale value of the respective image. The curve depends on the degree of osteoporosis: the height of the maximum (i.e. the maximal number of clusters ncmax) allows distinction to be made between different degrees of osteoporosis, and ncmax shows a correlation with the cortical BMD of rnc = -0.84 (p < 0.0001).