Quantification of width and density of bone structures by computed tomography

In computed tomography (CT), the representation of edges between objects of different densities is influenced by the limited frequency response of the scanner. This results in the misrepresentation of density of narrow objects, leading to errors of up to 70% and more. Our interest is in the measurement of cortical bone density in infants. Some of the problems encountered in these images are: varying cortical thickness and density around the circumference of individual bones; large noise due to a low-dose scanner (OsteoQuant®); limited resolution of the scanner (1 lp/mm). Mathematical models, phantoms and tests with patient data led to the following procedures: i) extract density profiles at one-degree increments from the CT images at right angles to the cortex boundary; ii) consider the outer and inner edge of each profile separately due to different adjacent soft tissues; iii) measure the width of each profile based on a threshold at fixed percentage of the difference between the soft-tissue value and a first approximated bone value; iv) correct the peak density of each profile based on the measured width with the help of the basic shape of the density-versus-width curve obtained from computer simulations and phantom measurements. This latter curve is specific to a certain scanner and is not dependent on the densities of the tissues within the range seen in patients. This procedure allows the calculation of the average density of the cortical bone. Based on phantom measurements, we estimate the error to be below 3% relative to the normal density of 2.0 g/cm³.