Limitations of clinical CT in assessing cortical thickness and density.

The peak CT number (CT) and full width at half maximum (FWHM) were obtained from the image profiles of aluminium of thickness ranging 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 and FWHM provide measures of the density and thickness which are largely independent of FOV (i.e. pixel size) and algorithm. Below 3 mm, CT falls progressively whilst FWHM remains relatively constant. At these small thicknesses the effect of FOV on CT is more pronounced when the bone algorithm is used, whilst FWHM remains relatively constant and independent of both FOV and algorithm. The results are discussed in terms of thickness relative to pixel size and spatial resolution as characterized by the point spread function. A linear relationship was found between the product CT x FWHM and thickness that is independent of both FOV and algorithm. This product may be useful in studies of cortical bone and changes due to osteoporosis.