OA079] Development of a national guideline for yearly constancy testing of image quality on CT scanners in Denmark

Abstract

Purpose To develop a national guideline for yearly constancy testing of image quality on diagnostic CT scanners based on current IEC standards. Methods A national work group of experienced CT physicist was established to develop the guideline. The guideline should work for all diagnostic CT scanners, be based on the IEC standards and specify the details not given in the IEC standards, such as which scan parameters and phantoms to use, how to calculate and analyze a specific image quality parameter and how many different types of scan protocols to be tested taken the testing time into account. Beside the IEC standard, the following should be taken into consideration: 1) the methology should be easy to replicate minimizing inter operator errors knowing that typically more than one person takes part in testing a CT scanner over time 2) the methology should reflect the clinical use of the CT scanner along with meaningful tolerances 3) the need for an absolute test of CT number of water ensuring a well calibrated CT scanner. The final guideline is based on a comprehensive literature study, experience with performing and analyzing CT image quality control and thorough practical trials testing the outcome of different scan settings taking testing time and clinical relevance of parameters into account. Results A guideline with a complete set of information for testing the following image quality parameters for all kind of diagnostic CT scanners: Noise, CT number of water and its reproducibility, uniformity, CT number variation along z axis, CT number linearity, spatial resolution in scan plan and in scan direction and slice thickness. To lower the degree of methodic uncertainty several key points was discovered by the work group and incorporated in the guideline. 1) Image quality parameters evaluated by baseline comparison should be based on a method easily replicated. 2) Minimizing the number of and reducing the difference between scan protocols minimize errors. 3) Use of helical CT to reduce influences from slice to slice variation observed in axial CT. 4) Phantoms must be brought to room temperature to minimize variations in CT number from year to year.