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A medical CT scanner for dosimetry


Abstract: The number of CT examinations has more than doubled in Germany since 1996. In 2010, 61 % of the collective effective dose of the population could be attributed to X-ray examinations for CT imaging. This statistic shows how important CT dosimetry has become. The fundamentals of CT dosimetry were established at the beginning of the 1980s - shortly after the first CT scanner had been invented - and have hardly changed since then. In contrast, CT devices have experienced a fast-paced evolution. The old concepts of CT dosimetry have become obsolete and can no longer be applied to the new generations of scanners. New concepts are therefore being contemplated worldwide. PTB is responsible for realizing and disseminating the units for CT dosimetry and for the type approvals of CT dosimeters for radiodiagnostics according to the Verification Act. In order to maintain its competence and to deal with fundamental issues in this field of metrology for CT dosimetry in medicine, PTB has purchased a modern 64-slice CT scanner.

The invention of computed tomography (CT) can be considered as the main revolution in medical X-ray diagnostics in the last decades; this method has now established itself worldwide in hospitals and medical practices. Modern CT scanners provide 3D images of an unprecedented quality. They are physically possible because X-rays are attenuated differently depending on whether they hit a tissue or an organ. Hereby, the X-rays also deposit a certain dose in the patient’s body; the values of this dose have to be checked in order to minimize correlated risks for the patient.

After the BMU parliament report in 2011 [1], the number of CT examinations carried out per capita and per annum in Germany more than doubled between 1996 and 2010 - to be more precise, it increased by 130 %. The contribution of X-ray examinations to the collective effective dose of the population increased to 61 % in 2010 (see Fig. 1). Certain CT examinations cause considerable effective doses of up to 25 mSv. For comparison: for persons who are occupationally exposed to radiation, the limit value for the effective dose amounts to 20 mSv per legal year. This statistic shows how important dosimetry has become for CT.

In the "Medical Exposures" Directive (97/43/EURATOM), the European Commission introduced the concept of "diagnostic reference levels" (DRLs) of the International Commission on Radiological Protection (ICRP) in order to limit exposure within the scope of medical diagnostics.

Figure 1: Percentage of the different types of examinations contributing to the total incidence and to the collective effective dose in Germany in 2010 [1]

For computed tomography, the DRLs are stated on the basis of the measurand "computed tomography dose index" (CTDI). Acceptance and constancy tests are also based on this measurand. The CTDI was already proposed by Shope et al. [2] in the early 1980s to describe the absorbed dose in computed tomography slices. For the measurement, a pencil-shaped ionization chamber (CT chamber), 100 mm in length, is used, either in air or in a head or body phantom (see Fig. 2). Therefore, it is also referred to as the "CTDI100 concept" - and is currently used all over the world in CT dosimetry.

Figure 2: The phantoms for CTDI measurements are homogeneous cylinders made of plexiglass which have a diameter of 16 cm and 32 cm, respectively, and a length of 15 cm and are used as a head or body phantom. The 100 mm pencil-shaped ionization chamber must, alongside the active measuring volume, exhibit as homogeneous an efficiency as possible.

Since the CTDI was introduced, CT devices have experienced a fast-paced evolution. For the new generations of scanners with larger scanning widths, the "CTDI100 concept" can no longer be used consistently. The American Association of Physics in Medicine (AAPM) [3] has therefore developed an alternative concept for CT dosimetry which is based on smaller measuring chambers in large phantoms. Hereby, AAPM contradicts numerous manufacturers of CT devices and the IEC Working Group (e.g. IEC 60601-2-44 [4]) for acceptance tests on CT devices who would rather stick to the CTDI100 concept by making slight corrections. Also, the International Commission on Radiation Units and Measurements (ICRU) has looked thoroughly into the subject over the past few years. In its latest Report 87, which deals with "Radiation Dosimetry and Image Quality Assessment in Computed Tomography" [5], a new procedure is suggested; this procedure allows the dose and the image quality to be derived from a single CT image with the aid of a special, newly developed phantom. PTB is of the opinion that a uniform CT dose concept that is valid worldwide is indispensable for continuing to ensure the comparability of dose measurands.

Since PTB is responsible for not only realizing but also for disseminating the units of CT dosimetry as well as issuing type approvals for CT dosimeters for radiodiagnostics and, furthermore, cooperates in the respective national and international standardization committees, there is an urgent need for action in this field. For this reason, PTB has purchased its own medical CT scanner for CT dosimetry. The device is a scanner built by General Electric (GE). The GE Optima 660 CT scanner (see Fig. 3) is equipped with a collimation system with 6 different widths from 1.25 mm to 40 mm. The collimator contains two bow-tie filters which filter and shape the beam in order to optimize the dose and the image quality according to the size of the part of the patientxs body to be scanned (e.g. head or trunk). The GE Optima 660 CT scanner is equipped with a detector system consisting of 64 detector cells in z direction. The integrated X-ray tube can be operated at four different tube voltages (80/100/120/140 kV) with a maximum generator power of 72 kW [6]. The CT is to be characterized as a reference measuring facility for CT dosimetry. This means that the X-ray radiation fields of the CT are measured experimentally. In addition, the radiation transport of the X-rays is to be calculable from the source to the imaging system. For this purpose, GE makes the necessary data of the CT device available to PTB on the basis of a non-disclosure contract. The radiation transport calculations are to be performed by means of Monte Carlo methods. In this way, it is possible to measure dose values at certain points in special phantoms and to compare them with calculated values. If the agreement between the measured and the calculated dose values is sufficient, then the individual dose for the patient can, in the future, be determined with the aid of the scanned CT image as an input model for calculations, after which the individual dose can be displayed. The reference measuring facility will, however, first be used to test the proposed dose concepts and the novel detectors used in CT dosimetry, and also within the scope of type examinations. The CT device was successfully installed and approved in August 2013. First experiments have already started.

Figure 3: GE Optima 660 CT scanner. The CT scanner is operative and is located in the new annex of the Röntgen Building. It was delivered by General Electric in August 2013. This CT scanner is a model that is used in hospitals and medical practices; at PTB, it is used for research and for solving issues with regard to dose determination.



  1. Umweltradioaktivität und Strahlenbelastung: Jahresbericht 2011
    Internet: nbn-resolving.de/urn:nbn:de:0221-2013090511044
  2. T. B. Shope, R. M. Gagne, G. C. Johnson:
    A method for describing the doses delivered by transmission x-ray computed tomography

    Med. Phys. 1981; 8(4): 488-495
  3. Report of AAPM Task Group 111:
    The Future of CT Dosimetry, Comprehensive Methodology for the Evaluation of Radiation Dose in X-Ray Computed Tomography
  4. IEC 60601-2-44
    Medical electrical equipment - Part 2-44: Particular requirements for the basic safety and essential performance of X-ray equipment for computed tomography
  5. Radiation Dosimetry and Image Quality Assessment in Computed Tomography, ICRU Report 87, 2013
  6. GE Healthcare, OptimaTM CT660, Benutzerhandbuch, Version 12HW28.x für OptimaTM CT660, 2012