Computed Medical Imaging

Abstract

In preparing this paper I realised that I would be speaking to a general audience and have therefore included a description of computed tomo- graphy (CT) and some of my early experiments that led up to the develop- ment of the new technique. I have concluded with an overall picture of the CT scene and of projected developments in both CT and other types of systems, such as Nuclear Magnetic Resonance (NMR). Although it is barely 8 years since the first brain scanner was construct- ed, computed tomography is now relatively widely used and has been extensively demonstrated. At the present time this new system is operating in some 1000 hospitals throughout the world. The technique has succesful- ly overcome many of the limitations which are inherent in conventional X- ray technology. When we consider the capabilities of conventional X-ray methods, three main limitations become obvious. Firstly, it is impossible to display within the framework of a two-dimensional X-ray picture all the information contained in the three-dimensional scene under view. Objects situated in depth, i. e. in the third dimension, superimpose, causing confusion to the viewer. Secondly, conventional X-rays cannot distinguish between soft tissues. In general, a radiogram differentiates only between bone and air, as in the lungs. Variations in soft tissues such as the liver and pancreas are not discernible at all and certain other organs may be rendered visible only through the use of radio-opaque dyes. Thirdly, when conventional X-ray methods are used, it is not possible to measure in a quantitative way the separate densities of the individual substances through which the X-ray has passed. The radiogram records the mean absorption by all the various tissues which the X-ray has penetrat- ed. This is of little use for quantitative measurement. Computed tomography, on the other hand, measures the attenuation of X-ray beams passing through sections of the body from hundreds of different angles, and then , from the evidence of these measurements, a computer is able to reconstruct pictures of the body's interior. Pictures are based on the separate examination of a series of contiguous cross sections, as though we looked at the body separated into a series of thin "slices". By doing so, we virtually obtain total three-dimensional infor- mation about the body.