The objective of this study is to separately analyse the effects of detection and image reconstruction on computed tomography (CT) performance to characterise standard and new CT systems. The focus here was on the determination of quantifiable parameters, such as the modulation transfer function, noise power spectrum and quantum efficiency of the detector and the entire system, considering the CT image and the raw data set. Because of the conversion of raw data and image data to the absolute scale of the photon number, a quantitative comparison between the quality parameters of both data sets is possible in this approach. The effort of the proposed method using simple, standardised test phantoms is comparable with the effort in the quality control in classical projection radiography. For the first time, the quantum efficiency of a CT detector and the entire system that is used in the daily clinical practice could be determined. This system reached a quantum efficiency up to 12 %.