LIMITATIONS OF CLINICAL CT IN ASSESSING CORTICAL THICKNESS AND DENSITY

The peak CT number (<(CT)over cap>) and full width at half maximum (FW HM) were obtained from the image profiles of aluminium of thickness ra nging from 0.1-9.5 mm. The scans were performed at different fields of view (FOVs) and with different reconstruction algorithms ('bone' and 'standard'). Above 3 mm, <(CT)over cap> and FWHM provide measures of t he density and thickness which are largely independent of FOV (i.e. pi xel size) and algorithm. Below 3 mm, <(CT)over cap> falls progressivel y whilst FWHM remains relatively constant. At these small thicknesses the effect of FOV on <(CT)over cap> is more pronounced when the bone a lgorithm is used, whilst FWHM remains relatively constant and independ ent of both FOV and algorithm. The results are discussed in terms of t hickness relative to pixel size and spatial resolution as characterize d by the point spread function. A linear relationship was found betwee n the product <(CT)over cap> x FWHM and thickness that is independent of both FOV and algorithm. This product may be useful in studies of co rtical bone and changes due to osteoporosis.