1. A method of determining data defining an estimate of the thickness of a tissue structure from tomographic imaging data, the method comprising: inputting a tomographic imaging data set defining at least one slice through a region of tissue including said tissue structure; modelling said tissue structure as a variation of an imaging parameter of said imaging data along a line crossing said tissue structure, a tissue structure model defined by said variation having at least a first tissue modelling parameter defining a value of said imaging parameter along said line and within said tissue structure and having at least one parameter defining said thickness of said tissue structure; modelling a variation of said tomographic imaging data along said line as a blurred version of said variation of said imaging parameter modelling said tissue structure to provide a blurred tomographic imaging model, said blurred version of said variation of said imaging parameter being defined by a blurring function applied to said tissue structure model, wherein said blurred tomographic imaging model has a blurring parameter representing a degree of blurring applied by said blurring function; determining a constraining value of said first tissue modelling parameter; and fitting said blurred tomographic imaging model defining said blurred version of said variation of said imaging parameter along said line, to data derived from said tomographic imaging data set; wherein said fitting comprises holding said first tissue modelling parameter substantially at said constraining value and allowing variation of said blurring parameter and said at least one parameter defining said thickness of said tissue structure to determine said at least one parameter defining said thickness of said tissue structure; and wherein said at least one parameter defining said thickness of said tissue structure comprises said data defining said estimate of said thickness of said tissue structure.