The evolution of CT technology and its application

Andy Scally and Mandy Webster, from the Division of Radiography at University of Bradford, give a brief outline of the evolution of Computed Tomography (CT) technology and its main clinical applications.

The history of the development of the science and technology underpinning CT is almost as complex as the science itself The story of its development is told in Webb1. The first clinical scan was performed on I October 1971 at the Atkinson Morley Hospital in London, using a scanner designed by Sir Godfrey Hounsfield who was, at the time, working for the company EMI. Sir Godfrey later (1979) shared the Nobel Prize for Physiology and Medicine with Allan MacLeod Cormack for their work in this field.

There were many variations in the basic design scheme of the earlier scanners but for the sake of simplicity it is useful to break down the evolution of the CT scanner up to the late 1980s/early 1990s into generations determined by the geometrical arrangement and type of movement of the x-ray tube and detectors.

First generation (translate-rotate, pencil beam)

The first generation of scanners utilised a narrow `pencil' beam of x-rays that was carefully aligned with a single detector. The xray tube and detector assembly moved (translated) slowly across the patient until the whole width of the patient had been scanned. The tube and detector assembly was then rotated through a small angle and the linear translation movement of the tube/detector was repeated. This process was performed many times through a 180' rotation. Initial total scan times (including time for image reconstruction) were approximately 20 minutes but, with improvements in the speed of image reconstruction calculations, this was reduced to a little under five minutes.

Second generation (translate-rotate, fan beam)

The second generation of scanners worked on similar principles to the first generation but utilised a fan beam and a bank of detectors. This effectively meant that each detector was collecting data from a different projection angle which meant that fewer rotation stops were required and therefore scan acquisition time was decreased. Examination times for second generation scanners were still quite long (10-90 seconds) and the translate-rotate movements required complex mechanical design.

Third generation (rotate-rotate)

In third generation scanners, a large bank of detectors is used, arranged...