Measurement of thickness and density of thin structures by computed tomography: a simulation study

The limited spatial resolution of clinical CT systems causes difficulties in the measurement of the density and thickness of thin structures such as the vertebral cortical shell. We simulated the imaging process by convolving experimentally determined point spread functions with rectangular and Gaussian profiles, for various fields of view or pixel sizes and reconstruction kernels. The simulations successfully explained the reported overestimation of thickness and underestimation of density when imaging thin structures. Both effects are larger for Gaussian profiles. For the rectangular profiles, experimental estimates of thickness and density will only be accurate when the true thickness is greater than about 1.5 times (for the bone reconstruction kernel) or 2.0 times (for the standard kernel) the full width at half maximum of the point spread function (PSF) of the imaging system. For Gaussian profiles imaged by a system with a Gaussian PSF, there are straightforward analytical expressions for the overestimation of thickness and underestimation of density: and these are useful approximations to the simulations of Gaussian profiles with experimental (pseudo-Gaussian) PSFs. We have demonstrated that thresholding of the vertebral image cannot provide accurate estimates of cortical thickness and density because the appropriate threshold level requires foreknowledge of the cortical thickness. To circumvent such difficulties we suggest that the average value of the peak CT numbers measured along the medial axis of the cortical shell be adopted as an index of cortical shell strength, since its value depends on both the density and the thickness of the shell.