Principles of CT

Abstract

This chapter summarises the basic technical principles which underpin computed tomography (CT). The key advantage of CT over conventional radiography is its ability to obtain 2D sections and 3D volume representations of the human body, with greatly improved contrast discrimination between tissues. This is enabled by a rotating X-ray tube and detector array which obtain multiple image projections during scanning. Much CT development occurred via a series of scanner generations, especially spiral (helical) scanning and multi-detector row designs. Imaging is based on the conversion of X-ray linear attenuation values to Hounsfield units which can be transformed to an extended greyscale of signal intensities. Windowing is a means of improving the visualisation of image contrast. Image resolution is determined by factors such as the slice width and pixel matrix. The effect of exposure factors such as kilovoltage peak (kVp) and milliampere seconds (mAs) is considered. Modern methods for CT image formation from raw data include back projection and iterative reconstruction.