A method of investigating a specimen using a tomographic imaging apparatus , by performing , in multiple iterations , the following steps : ( i ) Using a Back Projection technique to produce an initial tomogram from a set of initial images ; ( ii ) Subjecting said initial tomogram to a mathematical filtering operation , thereby producing an adjusted tomogram ; ( iii ) Using a Forward Projection technique on said adjusted tomogram to dissociate it into a set of calcu lated images ; ( iv ) Repeating steps ( i ) - ( iii ) until said calculated images satisfy an acceptance criterion .