European Radiology

, Volume 29, Issue 1, pp 168–175 | Cite as

Statistical analysis for obtaining optimum number of CT scanners in patient dose surveys for determining national diagnostic reference levels

  • Mehdi Sohrabi
  • Masoumeh Parsi
  • Sanaz Hariri Tabrizi
Computed Tomography



To statistically determine an ‘optimum number of CT scanners’ for obtaining ‘diagnostic reference levels’ (DRLs) in CT examinations as close as possible to ‘ideal DRLs’ when all available CT scanners are considered.


First, six ‘ideal DRLs’ (CTDIVol and DLP) were determined for head, chest and abdomen/pelvis examinations by using patient-dose survey data of 100 CT scanners of different models in Tehran. Then, a ‘random sampling method’ was applied to different percent fractions of patient dose data of 100 CT scanners. The percent differences (PD) of the DRLs obtained from ‘ideal DRLs’ and their coefficients of variation (CVs) were calculated. The ‘optimum number of CT scanners’ determined met those of ‘ideal DRL’ criteria; i.e. precision (CV ≤ 10%) and accuracy (PD ≤ 10%).


‘Optimum number of CT scanners’ for determining DRLs as close as possible to ‘ideal DRLs’, fulfilling the stated criteria, is 43 instead of using 100.


‘Optimum number of CT scanners’ for obtaining DRLs as close as possible to ‘ideal DRLs’ was determined. This optimum number can be effectively applied in patient-dose survey situations with limited resources in a time- and cost-effective manner.

Key Points

• Ideal DRLs were determined by a CT patient-dose survey applied to available scanners.

• ‘Optimum number of CT scanners’ statistically determined for DRLs is 43%.

• Optimum number can be used for DRLs as if ‘ideal DRLs’ were determined by all scanners.


Surveys and questionnaires Radiation protection Statistics Radiation dosage Tomography scanners, X-ray computed 



Volume computed tomography dose index


Coefficient of variation


Dose length product


Diagnostic reference level


Percent difference


Percent fraction



The authors state that this work has been carried out under the current budget of the Amirkabir University of Technology with partial support from the Iranian National Science Foundation through Grant No. 96006106.

Compliance with ethical standards


The scientific guarantor of this publication is Prof. Mehdi Sohrabi from the Amirkabir University of Technology.

Conflict of interest

The authors of this manuscript declare no relationships with any companies whose products or services may be related to the subject matter of the article.

Statistics and biometry

One of the authors (Dr. Sanaz Hariri Tabrizi) has significant statistical expertise.

No complex statistical methods were necessary for this paper.

Informed consent

Written informed consent was not required for this study because the study was based on the ‘QC-based dose-survey method’, which required no information about the CT institutes or any particular patients.

Ethical approval

Institutional Review Board approval was not required because the study is based on CT scanner models, which do not involve ethical issues.


• retrospective

• observational

• multicentre study


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Copyright information

© European Society of Radiology 2018

Authors and Affiliations

  • Mehdi Sohrabi
    • 1
  • Masoumeh Parsi
    • 2
  • Sanaz Hariri Tabrizi
    • 3
  1. 1.Health Physics and Dosimetry Research Laboratory, Department of Energy Engineering and PhysicsAmirkabir University of TechnologyTehranIran
  2. 2.Department of Radiology, Faculty of ParamedicalRafsanjan University of Medical SciencesRafsanjanIran
  3. 3.Department of Medical Radiation EngineeringShahid Beheshti University, G.C.TehranIran

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