QUALITY-CONTROL AND IMAGE QUALITY CRITERIA IN COMPUTED-TOMOGRAPHY

Technical and diagnostic quality assurance are particularly important in computed tomography due to the complexity of the technology and the variability of scan parameters. The effect of the scanning parameters on image quality and patient dose has been assessed quantitatively by phantom measurements on seven CT systems in Denmark brought into clin ical operation during recent years. Image quality criteria have been f ormulated for the mediastinum and retroperitoneum. These criteria are a modified version of the criteria defined by the German Federal Chamb er of Physicians which have been tested in a prefatory pilot study. A total of 190 examinations for the two types have been evaluated by an expert radiologist trained in CT and a mean score given for fulfilment of the selected pure anatomical criteria equally weighted. The doses for the examinations are defined by the measured multi-slice average d ose (MSAD) multiplied with the number of slices and the slice thicknes s, giving a value related to the volume dose. For the same type of exa mination a variation up to a factor of five in dose is registered with out any significant improvement of the image quality or diagnostic inf ormation. There seems to be a need for optimisation of the dose value and the number of slices for each examination protocol. The new fast s canners with high capacity of the X ray tube invites an unnecessarily high patient exposure. More detailed calculations of effective doses w ill be performed.