Defines variables which describe the imaging performance of radiological installations with diagnostic X-ray equipment for computed tomography and methods of checking whether variations in measured variables meet established criteria in order to ensure that the conditions for adequate imaging performance of the equipment are maintained whilst unnecessary irradiation of the patient is avoided. Guidance is given to indicate the degree of variation in any variable which might require appropriate remedial action. The test methods are concerned with noise, mean CT numbers and uniformity, spatial resolution, slice thickness, dose, positioning of the patient support.