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Quality control of test bodies used for image quality assurance in X-ray diagnosis

23.05.2011

When X-ray examinations are made on humans, it must be assured that the required image quality is achieved with as low a radiation exposure as possible (section §16 paragraph 2 sentence 1 RöV [1]). For this purpose, X-ray equipment must be subjected to an acceptance test before it is put into operation and, after that, to constancy tests at regular intervals. Here, test bodies play a central role. The general design of the test bodies as well as the geometry and the material of the single components have been specified in standards. Examples of test bodies for use in conventional X-ray diagnosis, mammography and computer tomography are shown in Figures 1 to 3.

 

Legend:

  1. Direction of the tube axis, 
  2. dynamic levels 
  3. Detail contrast object 4 mm diameter 
  4. kV test facility 
  5. Line cross as the center mark 
  6. Detail contrast object 10 mm in diameter 
  7. Infield, free of structures 
  8. dissolution test 
  9. Plaintext digits to denote the dynamic levels
Source: DIN 6868-4

Figure 1 : Test body for fluoroscopy and exposure modes in accordance with DIN 6868 part 4 [2]. The test body contains a perspex plate in which copper stairs 0.18 mm – 3.48 mm in thickness for the determination of the contrast dynamics range of the imaging system, lead grids (100 µm in thickness) for the determination of the spatial resolution with up to 5 line pairs per mm and low contrast objects of perspex (0.4 mm to 4 mm in thickness) for estimating thecontrast resolution are incorporated.

Source: Company Artinis

Figure 2 : The so-called "CDMAM" phantom is used to assure the image quality in digital mammography. The artefact is an aluminium carrier plate 0.5 mm in thickness to which a matrix with circular gold layers of different diameters and thicknesses is applied. The nominal diameters of the small circular gold plates vary from 60 µm and 2 mm, the nominal thicknesses from 30 nm to 2 µm. The test body is covered with a perspex plate 5 mm in thickness.

Source: DIN 6809-3:2010-08

Figure 3 : In computer tomography, the so-called "CTDI phantom" is used for quality control. The larger cylinder serves as a body phantom, the smaller as a head phantom. The cylindrical body consists of pure perspex and contains cylindrical holes on the central axis and on the periphery for the accommodation of CT dose measuring chambers. The phantom with the measuring chambers serves to measure the so-called computer tomography dose indices (CTDIs).

Recently, doubts have arisen as to whether all the test bodies available on the market comply with the specifications of the associated standards. Test bodies with relevant deviations from the specifications of the standards may lead to potentially unsuitable X-ray equipment remaining in use or suitable X-ray equipment being classified as defective. In addition, the question arises as to what extent the requirements of the standards to be met by the test bodies comply with those which seem to be reasonable with regard to the requirements to be met by the imaging system or by dosimetry. Both questions are to be answered within the scope of a project funded by the Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) dealing with test bodies for the acceptance and constancy test of conventional projection and fluoroscopy facilities, mammography devices and computer tomographs. The project is aimed at developing procedures for the non-destructive test of such test bodies which are suited for later use in routine operation.

Within the scope of the project, sophisticated coordinate measurements, thickness determinations, material tests, digital X-ray radiography, image data analyses and dosimetry of X-rays are carried out and test procedures are implemented in practical applications. Therefore, only an interdisciplinary approach is possible. The project is directed by the Institute for X-ray Diagnosis and Nuclear Medicine of the clinical centre Braunschweig. PTB is significantly involved with its competences in multisensor coordinate metrology (WG 5.34), scanning probe metrology (WG 5.25), X-ray radio-metry (WG 7.11) and dosimetry for diagnostic radiology (WG 6.25). The Federal Institute for Materials Research and Testing in Berlin and the TÜV NORD Ensys GmbH are also involved. The project is limited to a duration of two years and will be concluded by the end of September 2011.

Initial results of the project were presented in 2010 on the occasion of the 41st annual meeting of the German Society for Medical Physics (DGMP) [3]. At that meeting, methods for the examination of the CTDI test body in accordance with DIN EN 60601-2-44 were presented and reasonable tolerances to be demanded discussed. The investigation criteria applied in the project allow the nondestructive investigation of the CTDI test body to be carried out with low effort. The geometry as well as the density distribution of the test body can be determined with sufficient accuracy. Radiation transport calculations show that the measurement of the CTDI value is less sensitive to changes of the geometry and the electron density of the material. Only a change of the test body diameter of 1 mm or of the density by 3 % causes a change of the CTDI value of 1 %. Such an analysis of reasonable tolerances has not yet been taken into account in the present standards.

Literature

  1. Decree on the protection against injury or damage due to X-rays (RöV)
  2. DIN 6868-4, Image quality assurance in diagnostic X-ray departments – part 4: Constancy testing of medical X-ray equipment for fluoroscopy
  3. Markus Borowski, Sarah Wrede, Ulrich Neuschaefer-Rube, Hans-Ulrich Danzebrink, Michael Krumrey, Ludwig Büermann, Jürgen Goebbels and Helmut Kreienfeld:
    Development of procedures for the non-destructive quality control of the CTDI test body in accordance with DIN EN 60601-2-44.
  4. Conference volume on CD of the 41st annual meeting of the DGMP in Freiburg/Breisgau, 29.09.-02.10.2010