X-ray computed tomography, derived from the Greek meaning 'to write a slice or section', has been commercially available from 1972, after its invention and development by G.N. Hounsfield at the central research laboratories of EMI in 1967. Since then, its impact on the medical world has been as great as the discovery of X-rays itself, and despite the inherent high cost of the equipment there are already over 1000 systems in hospitals around the world.