Image Quality and Artifacts

Abstract

The physical attenuation values f (x,y)of the human tissue within an axial slice are distributed continuously in value and space. In the previous chapter it has become evident that in practice it is indispensable for computed tomography (CT) to digitize the acquired data in order to carry out the mathematical object reconstruction. Thismeans that the attenuation of the radiation must be measured, digitalized, and stored. Within this processing chain, the continuous physical signal is discretized in the spatial domain and the values have to be quantized. Figure 9.1 shows that this process can be modeled as a signal transmission chain. One can thus evaluate the changes – in general the deterioration – to which the signal f (x,y)is subjected during acquisition and discretization into a series of numbers g(n,m)and further on until the tomogram c(n,m)is displayed and presented to the clinician.