This part of IEC 61223 provides assistance in performing constancy tests on a CT scanner. It applies to those components of CT scanners which influence the image quality, patient dose and positioning. This standard - defines the essential parameters which describe the performance of CT scanners with regard to image quality, patient dose and positioning; the list of parameters to be tested can be found in section 4.5; - defines the methods of testing the essential parameters; - provides criteria to be applied in the evaluation of data for compliance with the tolerances of the parameters specified by the accompanying documents and with respect to baseline values. These methods rely on non-invasive measurements, using appropriate test equipment, performed to ensure that the functional performance of equipment meets established criteria or to enable the early recognition of changes in the properties of components of the equipment. The aim is to verify compliance with specifications affecting the image quality, patient dose and patient positioning.