title: How to measure CT image quality: variations in CT-numbers, uniformity and low contrast resolution for a CT quality assurance phantom.
abstract: OBJECTIVE
  Quality assurance (QA) phantoms for testing different image quality parameters in computed tomography (CT) are commercially available. Such phantoms are also used as reference for acceptance in the specifications of CT-scanners. The aim of this study was to analyze the characteristics of the most commonly used QA phantom in CT: Catphan 500/504/600.
  
  
  METHODS
  Nine different phantoms were scanned on the same day, on one CT-scanner with the same parameter settings. Interphantom variations in CT-number values, image uniformity and low contrast resolution were evaluated for the phantoms. Comparisons between manual image analysis and results obtained from the automatic evaluation software QAlite were performed.
  
  
  RESULTS
  Some interphantom variations were observed in the low contrast resolution and the CT-number modules of the phantoms. Depending on the chosen regulatory framework, the variations in CT-numbers can be interpreted as substantial. The homogenous modules were found more invariable. However, the automatic image analysis software QAlite measures image uniformity differently than recommended in international standards, and will not necessarily give results in agreement with these standards.
  
  
  CONCLUSIONS
  It is important to consider the interphantom variations in relation to ones framework, and to be aware of which phantom is used to study CT-numbers and low contrast resolution for a specific scanner. Comparisons with predicted values from manual and acceptance values should be performed with care and consideration. If automatic software-based evaluations are to be used, users should be aware that large differences can exist for the image uniformity testing.
authors: The Intervention Centre, Oslo University Hospital, Rikshospitalet, Oslo, Norway. Electronic address: krgull@ous-hf.no.
authors: Kristine
authors: K
authors: Gulliksrud
authors: The Intervention Centre, Oslo University Hospital, Rikshospitalet, Oslo, Norway; Faculty of Health Sciences, Oslo and Akershus University College of Applied Sciences, Oslo, Norway.
authors: Caroline
authors: C
authors: Stokke
authors: The Intervention Centre, Oslo University Hospital, Rikshospitalet, Oslo, Norway; Department of Physics, University of Oslo, Oslo, Norway. Electronic address: uxneti@ous-hf.no.
authors: Anne Catrine Trægde
authors: AC
authors: Martinsen
mesh_terms: D002648:Child; D002675:Child, Preschool; D006801:Humans; D007223:Infant; D019047:Phantoms, Imaging; D011786:Quality Control; D059629:Signal-To-Noise Ratio; D014057:Tomography, X-Ray Computed / Q000295:instrumentation*
keywords: CT; Catphan; Quality assurance