EVALUATION OF THE CORTICAL STRUCTURE IN HIGH-RESOLUTION CT IMAGES OF LUMBAR VERTEBRAE BY ANALYZING LOW BONE-MINERAL DENSITY CLUSTERS AND CORTICAL PROFILES

The structural classification of trabecular bone is of considerable cl inical importance for the diagnosis of osteoporosis. Assessment of the cortical bone mineral density (BMD) and analysis of cortical structur e 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 cort ical ridge, and the other one evaluates the distribution of connected regions (clusters) of low BMD, i.e. areas that lie below a certain thr eshold value. In HRCT images from 22 lumbar vertebrae, the number of i ntersections of the cortical intensity profile with a horizontal line was determined at various positions of this threshold line. At a certa in position, there was a local maximum in the number of intersections which was 38% higher in the osteoporotic cases. The maximum shows a mo derate correlation with the cortical BMD of r(ni) = -0.72 (p < 0.0001) . The number n(c) of connected regions (clusters) with pixel values be low a certain threshold value was determined in relation to the thresh old value T. The resulting function n(c)(T) shows a relative maximum s lightly below the average grey scale value of the respective image. Th e curve depends on the degree of osteoporosis: the height of the maxim um (i.e. the maximal number of clusters n(cmax)) allows distinction to be made between different degrees of osteoporosis, and n(cmax) shows a correlation with the cortical BMD of r(nc) = -0.84 (p < 0.0001).