Sir Godfrey Newbold Hounsfield KT CBE. 28 August 1919 - 12 August 2004: Elected F.R.S. 1975

Within a few months of the discovery of X-rays by Wilhelm Conrad Rntgen in Wrzburg, Germany, on 8 November 1895, the practice of medicine had been revolutionized. The ability to see inside the intact human body advanced the diagnosis of disease from the art of guesswork into a new era based on knowledge and logic. However, there are limitations to what can be seen in X-ray shadow images. The different soft tissues of the body attenuate X-rays at very similar rates. Consequently, although the anatomical positions of many organs and structures can be seen on plain radiographs, it is often difficult to identify pathological tissue changes. On plain radiographs, the shadows of all the structures within the body are superimposed, so there is no means of depth discrimination in the images. Over the years, several technical innovations emerged that helped to mitigate these problems. First, contrast agents-compounds of iodine and barium, and air itself-can be introduced into the vessels and cavities of the body. This may make it possible to identify the corresponding structures in their images: examples of this approach include angiography and pneumoencephalography. Second, by moving the Xray source and the film by a system of mechanical linkages, the shadows of the tissues in a particular plane within the patient can be recorded in fixed positions on the film, whereas the shadows of tissues not lying in that plane are blurred by the motion. These two-dimensional images-so-called tomograms, from the Greek, tomos (meaning 'a slice') and graphein (meaning 'to draw') -can be made in contiguous planes, to build up a three-dimensional picture of the internal structures of the body. Third, a pair of X-ray images obtained at slightly different angles can be viewed stereoscopically. This is another way of obtaining three-dimensional information.