NATIONWIDE ANALYSIS OF PRACTICES IN COMPUTED TOMOGRAPHY WITH A VIEW TO IMPROVING QUALITY ASSURANCE MEASURES AND AVOIDING UNNECESSARY DOSES TO THE PATIENT.
An assessment of the collective effective dose equivalent from computerized tomography (CT) imaging techniques to the whole population was investigated by examining data collected over a period of time from all CT installations in Denmark. Evaluation of the relation between the usage of dose and the scanner performance was used as a base for formulating quality criteria for CT images and for optimizing the use of CT. The preliminary results from the dose measurements show that it is very important to know the details about the filter located in the X-ray beam. Shaped filters complicate the interpretation of the free in air axial dose measurements and if CT dose index values based on such measurements are used for organ dose calculations information about the filtration has to be integrated into the calculations. The deviation of the dose profiles from nominal values indicate for narrow slices, that in some CT systems the collimation is performed before the detectors, but after the patient. This was not clearly expressed to the user and therefore in some cases caused unnecessary high doses to be given to the patient.
The image quality depends on many parameters but primarly on dose which again depend on the scan parameters. In daily practice it is essential that simple and quick tests of the CT images can be performed by the radiologist or technician. The image noise which depend on dose in a wellknown fashion represents one of these parameters. A protocol for simple measurements should be established taking special requirements into consideration. These requirements typically depend on the contrast level within the actual anatomic region. It would be possible to perform such measurements at the start of each examination.
A. QUESTIONNAIRES ADAPTED FROM THE RUNNING PROGRAMME WILL BE SEND TO THE 24 INSTALLATIONS IN DENMARK, ALL LOCATED IN DIAGNOSTIC DEPARTMENTS IN HOSPITALS UNDER THE DANISH PUBLIC HEALTH CARE SYSTEM. DATA ON EACH EXAMINATION (SCANNING PARAMETERS, BODY AREA, ETC.) WILL BE COLLECTED FOR A GIVEN PERIOD IN ORDER TO GET A FULL DOCUMENTATION OF THE ACTUAL USE OF CT IN THE DANISH HEALTH SERVICE. IN THE SAME INQUIRY QUESTIONS ON THE ACTUAL PERFORMED QUALITY ASSURANCE ON EACH CT SYSTEM WILL BE ASKED.
B. A SET OF DOSE MEASUREMENTS WILL BE PERFORMED REGULARLY OF THE FREE-IN-AIR AXIAL DOSE PROFILE FOR A SINGLE SLICE AT THE CENTRE OF ROTATION OF EACH CT SCANNER FOR THE MOST COMMONLY USED SETS OF SCANNING PARAMETERS IN ACCORDANCE WITH SIMILAR INVESTIGATIONS IN THIS PROGRAMME.
C. A SITE VISIT AT ALL CT INSTALLATIONS WILL BE ARRANGED FOR PERFORMING IDENTICAL QUALITY CONTROL MEASUREMENTS OF PHYSICAL IMAGE PARAMETERS ON A STANDARD PHANTOM FOR WELL DEFINED SETS OF SCANNING PARAMETERS. AT THE SAME TIME DOSE MEASUREMENTS WITH TLD AND A PENCIL SHAPED IONIZATION CHAMBER WILL BE PERFORMED FOR COMPARISON.
D. ALL DATA COLLECTED IN A-C WILL BE PLACED IN A SIR DATABASE FOR FURTHER EVALUATION AND FOR CALCULATION OF ORGAN DOSES AND THE COLLECTIVE EFFECTIVE DOSE EQUIVALENT FROM CT IN DENMARK. THE PROCEDURE FOR CALCULATION OF ORGAN DOSES WILL BE IN ACCORDANCE WITH METHODS DEVELOPED IN THE RADIATION PROTECTION RESEARCH PROGRAMME UNDER THE SAME SUBJECT AREA.
E. ON THE BASIS OF THE EVALUATION IN D ELABORATION OF QUALITY CRITERIA OF CT EXAMINATION WILL BE PERFORMED. THE AIM, APPARATUS AND METHODS FOR SUCH MEASURES HAVE TO BE COORDINATED WITH THE RESEARCH WORK ON THE SAME SUBJECT AREA CARRIED OUT BY NRPB - CHILTON, GSF - MUNICH, CEPN - FONTENAY-AUX-ROSES AND USL NO7 - UDINE.
F. DOSE ASSESSMENT WILL BE DONE FOR CONVENTIONAL X-RAY EXAMINATIONS THAT HAS BEEN REPLACED BY CT IN ORDER TO CALCULATE THE COLLECTIVE EFFECTIVE DOSE EQUIVALENT FROM THESE EXAMINATIONS AND TO DETERMINE THE IMPACT FROM INTRODUCTION OF CT ON THE WHOLE POPULATION IN DENMARK.