Understanding Dental X-Ray Compliance Testing: Part 1 – Beam Collimation

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Have you ever wondered what is involved in dental x-ray compliance testing? Do you know which elements of the x-ray machine are tested for compliance with the regulations and how these relate to the performance and safety of the equipment? This series of articles will explain the assessments that are performed in routine compliance testing of dental x-ray systems.

Part 1 of this series focuses on evaluation of the x-ray beam collimation system.

Relevance

The collimation system of an x-ray machine is responsible for controlling the size and shape of the x-ray beam for successful alignment with the relevant patient anatomy and the image detector/receptor.

The integrity of the collimation system has important implications for patient radiation exposure, image quality and image retake rates. Consider the fact that an under-collimated beam unnecessarily irradiates tissue lying outside of the imaging field-of-view. Radiation-sensitive tissues that can be unnecessarily exposed include the thyroid, brain, and red bone marrow in the mandible. Furthermore, large beams generate additional scatter-radiation that can noticeably degrade image-contrast. On the other hand, beams that are over-collimated or poorly aligned are more likely to produce cut-off artefacts and non-diagnostic images.

Collimation system

A collimator is simply a lead-shielded plate with a central aperture that defines the size and shape of the beam. Radiation is absorbed in the lead plate and passes freely through the aperture. 

Figure 1 below shows how x-rays are produced at a small focal spot within the x-ray tube. A small segment of this radiation is allowed to pass freely through a tube ‘window’ to form an initial but poorly defined beam, while the remaining x-rays are absorbed by shielding within the tube head. A primary collimator works to define the size and shape of this initial beam while also helping to eliminate unwanted extra-focal radiation that is produced in a broader area around the focal spot.

In intraoral units, the primary collimator is included at the proximal end of the position indicating device (PID) / cone to produce either a circular or rectangular shaped beam. Panoramic and scanning (CCD) cephalometric systems use slit-shaped collimator apertures that form narrow vertical beams to scan the patient while minimising scatter (Figure 2). In contrast, flat-panel detector (FPD) based systems have rectangular collimators with motorised shutters that generate changeable field sizes.

Intraoral Collimation

Figure 1: Intraoral tube head collimation system.

A secondary collimation system is also included on some types of equipment to further restrict the beam and also reduce the amount of penumbral blurring at the edges of the x-ray field. On intraoral units, a lead-lined PID or attachable rectangular collimator device works to this effect. Scanning cephalometric systems have a secondary collimator slit mounted prominently on the cephalostat prior to the patient-entry point (Figure 2). Finally, the image receptors of extraoral equipment may be self-collimated to reject scatter radiation generated outside of the imaging volume, while the receptor housing is shielded to intercept and stop the beam.

Figure 2: OPG unit collimation system for scanning cephalometry.

Compliance testing standards

In Western Australia, compliance testing standards are set under the Radiation Safety (General) Regulations 1983 and the relevant Radiological Council of WA compliance testing workbooks for digital dental radiography equipment and dental cone beam computed tomography (CBCT) equipment. These compliance criteria are derived from international recommendations and Australian standards. Many states/territories have adopted similar requirements.

Intraoral systems

Criteria

The following compliance assessment criteria apply to the collimation of intraoral units registered in Western Australia.

  1. The maximum diameter of the beam shall not exceed 60 mm. In the case of rectangular collimation this applies to the diagonal width of the beam.
  2. The area of the beam shall not be greater than the cross-sectional area of the PID / cone.
  3. The beam shall not pass through the walls of the PID.
  4. There must not be evidence of any primary radiation outside of the collimated area. 

Note that these requirements apply to the x-ray field as measured at the open end of the PID. Figure 3 shows examples of compliant and non-compliant x-ray beams in intraoral radiography.

Figure 3: Examples of compliant and non-compliant intraoral beams.

Assessment method

To evaluate the collimation system, the compliance tester uses a device that allows them to visualise a cross-section of the x-ray field. For this there are several options which include fluorescent screens, self-developing film or PSP receptors. Our preference is to use a fluorescent screen device which converts x-ray energy to visible green light that is viewed directly to ‘see’ the x-ray field (Figure 4). With this method the room lighting should be dimmed if possible to help with the visualisation.

Figure 4: (a) Image of a fluorescent collimation test object, (b) Illustration of fluorescent screen usage in intraoral collimation testing. 

Panoramic or scanning cephalometric systems

Criteria

Scanning-type extraoral systems have separate requirements to be evaluated by the compliance tester.

Panoramic OPG systems: 

  1. The size of the x-ray beam must not be significantly greater than the image receptor. 
  2. The image must be symmetrical and must not show evidence of cut-off related to misalignment of the x-ray beam with the receptor. 

Cephalometric systems: 

  1. The beam shall be confined to the dimensions of the image detector or to within the tolerances specified by the manufacturer. 
  2. The total area scanned at the level of the image receptor shall not be greater than stated in the manufacturer’s specifications. 

Assessment method

Again there are multiple ways to conduct these assessments. An accurate and objective method is with self-developing film called radiochromic or Gafchromic film that spontaneously undergoes polymerisation and darkens when exposed to x-rays. When placed at the corners of the marked area of the image detector, these films appear in the image and reveal the size and placement of the beam with respect to the image receptor (Figure 5). 

Figure 5: (a) Strip of self-developing radiochromic film, (b) Radiochromic film placed at the marked edges of the image detector can reveal the size and alignment of the beam. 

Flat panel detector (FPD) cephalometric and CBCT systems

Criteria

The requirements for FPD cephalometric systems are: 

  1. The size must not exceed the size of the dimensions of the image receptor. For circular beams the diameter must not be greater than the smallest dimension of the image receptor. 
  2. Interchangeable collimators must be labelled with the relevant beam dimensions.  
  3. The beam shall be aligned with the image receptor. 
  4. The beam shall be fully intercepted by the image receptor support structure. 

The requirements for CBCT systems are: 

  1. The beam shall conform to the nominal size of the image receptor specified by the manufacturer to within ±10 mm or ±10%, whichever value is smaller. 
  2. The beam shall be fully contained within the detector housing. 

Assessment method

In evaluating flat panel detector systems it is convenient to use fluorescent screens to visualise the x-ray beam at the image receptor. Figure 6 shows how fluorescence screens placed at the corners of the flat panel detector can be used to confirm whether the beam is correctly collimated to the detector.

Figure 6: Examples of acceptable and unacceptable beam alignment to the FPD, as revealed by fluorescent screens placed at the corners of the image receptor markings.

What can go wrong with collimation?

Accurate beam collimation relies on maintaining precise geometrical relationships between the x-ray focal point, the collimator apertures, and the image receptor. Non-compliances relating to collimating are relatively rare in practice, but can have a wide range of possible causes. Potential causes for non-compliance include installation of non-conforming equipment, manufacturing or assembly errors, improper re-assembly following servicing, mechanical slop or sag of components, component damage, or movement of parts caused by physical vibration or shock.

Summary

In summary, accurate beam collimation is important for optimising patient radiation exposure and and image quality while reducing the likelihood of image cut-off artefacts. A number of assessment criteria apply to each class of dental x-ray equipment. Compliance testing of collimation involves the use of devices such as fluorescent screens or self-developing films that are used to determine the size, shape and position of the x-ray beam.

The next article in this series will cover performance testing of the x-ray generator and control circuitry.

Bibliography

  • Dance, D. R., Christofides, S., Maidment, A. D., McLean, I. D., & Ng, K. H. (2014). Diagnostic Radiology Physics. Vienna: International Atomic Energy Agency.
  • Iannucci, J., & Howerton, L. J. (2016). Dental Radiography, 5th Edition. Saunders.
  • McClelland, I. R. (2004). X-Ray Equipment Maintenance and Repairs Workbook for Radiographers and Radiological Technologists. Geneva: World Health Organisation.
  • Radiological Council of Western Australia. (2007). Diagnostic X-Ray Equipment Compliance Testing, Workbook 11, Dental Digital Radiographic Equipment. Perth: Radiological Council WA.
  • Radiological Council of Western Australia. (2015). Diagnostic X-Ray Equipment Compliance Testing, Dental Cone Beam Computed Tomography Equipment. Perth: Radiological Council WA.
  • Western Australia Radiation Safety Act 1975 – Radiation Safety (General) Regulations 1983. www.legislation.wa.gov.au.

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