We describe a method of estimating the thickness of a tissue structure, in particular cortical bone thickness from tomographic imaging data such as CT data. The method models the tissue structure as a variation of an imaging parameter along a line; models a variation of the tomographic imaging data along the line as a blurred version of the variation of the imaging parameter modelling the; and fits the blurred tomographic imaging model to data from the tomographic imaging by holding a tissue modelling parameter at a constraining value and allowing variation of a blurring parameter and at least one parameter defining the thickness of the tissue structure to determine an estimate of the thickness of the structure.