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

The Limited spatial resolution of clinical CT systems causes difficulties i n the measurement of the density and thickness of thin structures such as t he vertebral cortical shell. We simulated the imaging process by convolving experimentally determined point spread functions with rectangular and Gaus sian profiles, for various fields of view or pixel sizes and reconstruction kernels. The simulations successfully explained the reported overestimatio n of thickness and underestimation of density when imaging thin structures. Both effects are larger for Gaussian profiles. For the rectangular profile s, experimental estimates of thickness and density will only be accurate wh en the true thickness is greater than about 1.5 times (for the bone reconst ruction kernel) or 2.0 times (for the standard kernel) the full width at ha lf maximum of the point spread function (PSF) of the imaging system. For Ga ussian profiles imaged by a system with a Gaussian PSF, there are straightf orward analytical expressions for the overestimation of thickness and under estimation of density: and these are useful approximations to the simulatio ns of Gaussian profiles with experimental (pseudo-Gaussian) PSFs. We have d emonstrated that thresholding of the vertebral image cannot provide accurat e estimates of cortical thickness and density because the appropriate thres hold 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 an d the thickness of the shell. (C) 1999 American Association of Physicists i n Medicine. [S0094-2405(99)00507-6].