Technical notes| Volume 30, ISSUE 4, P521-526, June 2014

Download started.


How to measure CT image quality: Variations in CT-numbers, uniformity and low contrast resolution for a CT quality assurance phantom

Published:February 14, 2014DOI:



      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.


      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.


      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.


      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.


      To read this article in full you will need to make a payment

      Purchase one-time access:

      Academic & Personal: 24 hour online accessCorporate R&D Professionals: 24 hour online access
      One-time access price info
      • For academic or personal research use, select 'Academic and Personal'
      • For corporate R&D use, select 'Corporate R&D Professionals'


      Subscribe to Physica Medica: European Journal of Medical Physics
      Already a print subscriber? Claim online access
      Already an online subscriber? Sign in
      Institutional Access: Sign in to ScienceDirect


      1. The Norwegian Radiation Protection Authority. Regulation for radiation protection and use of radiation, FOR 2010-10-29 nr 1380: Forskrift om strålevern og bruk av stråling. 29-10-2010.

        • Birnbaum B.A.
        • Hindman N.
        • Lee J.
        • Babb J.S.
        Multi-detector row CT attenuation measurements: assessment of intra- and interscanner variability with an anthropomorphic body CT phantom.
        Radiology. 2007; 242: 109-119
        • IEC 61223-3-5
        International standard IEC 61223-3-5. Evaluation and routine testing in medical imaging departments – part 3– 5: acceptance tests – imaging performance of computed tomography X-ray equipment.
        • IEC 61223-2-6
        International standard IEC 61223-2-6. Evaluation and routine testing in medical imaging departments – part 2–6: acceptance tests – imaging performance of computed tomography X-ray equipment.
        • IPEM
        Report 91.
        • Colli V.
        • Mangini M.
        • Strocchi S.
        • Lumia D.
        • Cani A.
        • Boffano C.
        • et al.
        Performance assessment of four 64-slice computed tomographic devices for a typical clinical protocol.
        J Comput Assist Tomogr. 2011; 35: 57-64
        • IPEM
        Report 32.
        • Mieville F.A.
        • Gudinchet F.
        • Brunelle F.
        • Bochud F.O.
        • Verdun F.R.
        Iterative reconstruction methods in two different MDCT scanners: physical metrics and 4-alternative forced-choice detectability experiments – a phantom approach.
        Phys Med. 2013; 29: 99-110
        • Fleiss J.
        • Cohen J.
        The equivalence of weighted kappa and the intraclass correlation coefficient as measures of reliability.
        Educ Psychol Meas. 1973; : 613-619
        • Thitaikumar A.
        • Krouskop T.A.
        • Ophir J.
        Signal-to-noise ratio, contrast-to-noise ratio and their trade-offs with resolution in axial-shear strain elastography.
        Phys Med Biol. 2007; 52: 13-28
        • Garcia-Ramirez J.L.
        • Mutic S.
        • Dempsey J.F.
        • Low D.A.
        • Purdy J.A.
        Performance evaluation of an 85-cm-bore X-ray computed tomography scanner designed for radiation oncology and comparison with current diagnostic CT scanners.
        Int J Radiat Oncol Biol Phys. 2002; 52: 1123-1131
        • Levi C.
        • Gray J.E.
        • Mccullough E.C.
        • Hattery R.R.
        The unreliability of Ct-numbers as absolute values.
        Am J Roentgenol. 1982; 139: 443-447
        • Zerhouni E.A.
        • Boukadoum M.
        • Siddiky M.A.
        • Newbold J.M.
        • Stone D.C.
        • Shirey M.P.
        • et al.
        A standard phantom for quantitative Ct analysis of pulmonary nodules.
        Radiology. 1983; 149: 767-773
        • Bosniak M.A.
        The current radiological approach to renal cysts.
        Radiology. 1986; 158: 1-10
        • Hamrahian A.H.
        • Ioachimescu A.G.
        • Remer E.M.
        • Motta-Ramirez G.
        • Bogabathina H.
        • Levin H.S.
        • et al.
        Clinical utility of noncontrast computed tomography attenuation value (Hounsfield units) to differentiate adrenal adenomas/hyperplasias from nonadenomas: Cleveland clinic experience.
        J Clin Endocrinol Metab. 2005; 90: 871-877
        • Ilias I.
        • Sahdev A.
        • Reznek R.H.
        • Grossman A.B.
        • Pacak K.
        The optimal imaging of adrenal tumours: a comparison of different methods.
        Endocr Relat Cancer. 2007; 14: 587-599
        • Wilson D.O.
        • Leader J.K.
        • Fuhrman C.R.
        • Reilly J.J.
        • Sciurba F.C.
        • Weissfeld J.L.
        Quantitative computed tomography analysis, airflow obstruction, and lung cancer in the pittsburgh lung screening study.
        J Thorac Oncol. 2011; 6: 1200-1205
        • Sande E.P.S.
        • Martinsen A.C.T.
        • Hole E.O.
        • Olerud H.M.
        Interphantom and interscanner variations for Hounsfield units-establishment of reference values for HU in a commercial QA phantom.
        Phys Med Biol. 2010; 55: 5123-5135