IAEA

Radiation protection of patients during DXA

» What are the typical dose levels in DXA?

Dose data for the types of equipment currently employed for DXA is care, in comparison to data available for other radiographic modalities. For a spine plus femur DEXA scan, the effective dose varies from less than 1 μSv to about 15 μSv. In pencil beam systems the dose is generally less than 1 μSv, while values between 1 to 10 μSv have been reported for fan beam systems. One study estimates the effective dose from a cone beam system as about 18 μSv. For modern pencil beam scanners, recent data indicate entrance surface doses in the range 9 – 50 μGy and 9 – 200 μGy for fan beam systems. By comparison the effective dose from a chest radiograph is in the range 20 – 50 μSv. 

In addition, historical figures are available from UNSCEAR. These show, for example, that single-photon absorptiometry, which is now used little if at all, the entrance surface doses was 50 μGy and effective doses <1 μSv. UNSCEAR also estimates the range of entrance doses for pencil beam systems is in the range 2-1,400 μGy with effective doses of 0.1-8 μSv per examination. In fan beam DXA scanners the dose is somewhat increased (entrance surface dose of about 900 μGy and effective dose of 7-75 μSv). However, the higher ends of these ranges are probably due to equipment that no longer enjoys a dominant position in the market place and is now little used. 

» Does the design of DXA equipment affect patient dose?

Yes.

The patient dose from a DXA examination is determined by system dependent parameters such as source spectra, source-detector geometry, beam collimation, beam filters, beam filtration, tube current and scan speed. 
These factors vary significantly between systems made by different manufacturers. Broadly similar levels of dose are achieved when the radionuclide source is replaced by an X-ray tube. The dose depends on the precision of the BMD measurement as well as the site of investigation. This is commonly the spine, femur, hip, or whole body. 

The latest DXA scanners with fan beams provide improved images that approach diagnostic radiographic quality. However, this increases the patient dose. Pencil beam scanners give lower doses than fan beam systems. So far, there have been few investigations of cone-beam and C-arm systems, but initial reports indicate the dose is higher than that from typical fan-beam systems.

» How does the patient dose in DXA compare to the dose from Bone Mineral Density (BMD) estimates using Quantitative Computerised Tomography (QCT)?

The typical patient effective dose from a dual energy QCT examination is 50 - 100 μSv. Dose from dual energy QCT is higher than the effective dose from DXA scan. Dose from QCT depends upon scan parameters like kVp, mAs, slice thickness, number and spacing of slices, and on patient size, X-ray source/detector geometry and detector collimation and calibration. This means that the dose is scanner and technique specific.

The dose from QCT is debated in the literature. Sometimes skin doses rather than effective doses are quoted, which is problematic. Analysis shows that if only one image in DXA is produced, the dose may be lower, but additional localizer images are often taken. This increases the DXA dose and changes the comparison. The bottom line is that with reasonably performed DXA, with current technology, and with reasonably performed QCT, the DXA dose will be lower. However, this very much depends on technique. In addition the technology for both is evolving. These factors can obviously conspire to alter the ratio. Finally, at present, DXA is recommended as the “gold standard” method despite some well recognized deficiencies in this approach.

» How does the patient dose from DXA compare with natural background and the dose from common radiological procedures?

The limited data available confirms that patient doses from DXA are low. Even with the increase in dose associated with fan beam technology this remains so. It is comparable to a few days natural background and is generally similar to or less than dose from plain chest radiography. The effective dose we receive from natural background radiation during one day is about 10 μSv. For a typical DXA examination, using technology that has become available over the last decade, the patient dose will be comparable to or (especially in the case of pencil beam scanners) less than this daily background dose. The dose for a chest radiograph usually lies in the range 20 – 50 μSv. Doses from most DXA systems are less than that from a chest X-ray, although some fan or cone beam systems approach this.

» Should patients wear any protective devices during a DXA scan?

No.

There is no indication that protective devices should be used in DXA. These do not protect against internal scatter, which is the main source of radiation to organs outside the area of clinical interest. However, as with dental radiography, where a patient wishes to avail of a protective device, even after advice to the contrary, it is wise to try and accommodate them. This is particularly so in the case of screening studies or in cases without direct medical indications.

» Women who are informed about the risk of osteoporosis often wish to have their BMD checked as a precaution. Is it acceptable to perform a DXA scan in such patients without a referral letter from a medical practitioner?

The question here is whether or not the scan is medically justified. The system of radiation protection for medical exposures relies on patient exposure being justified by a net benefit to the individual involved. This is a required for the legal exemption of medical exposures from dose limits. Thus, in the question posed, the answer is yes. The scan may be performed, provided the exposure is justified and there is a net benefit to the individual. 

It is difficult to demonstrate that a scan is justified without a referral from a medical practitioner in which the reasons for the scan are stated. In some clinics relatively simple protocols may be used to determine whether or not an examination is justified. Some countries have defined the legal responsibilities with regard to justification of medical X-rays. Thus, for example in the EU, the person who takes responsibility for the protocol and ensuring that it is applied correctly has, in conjunction with the person in charge of the DXA service, legal responsibility for ensuring that the benefits outweigh the risks for each patient.

A significant concern with DXA is that scans may not always be medically justified. They may be undertaken for many reasons including: a misplaced understanding of how they will contribute to the management of osteoporosis; screening programmes that have not been formally justified; medico-legal reasons; reasons associated with insurance; reasons associated with athletic performance, sports injuries, sports medicine and sports contracts; unnecessary repeat examinations and economic reasons associated with the practice involved. In these circumstances careful consideration needs to be given to establishing good referral protocols. 

» Is there an upper limit on the number of DXA scans that can be performed on a person?

No, provided that each individual scan is medically justified.

There is no upper limit on the number of scans that can be performed, since there is no dose limits in the case of medical exposures. Instead, the radiation exposure for each patient and each scan must be justified in terms of the risk and benefit. The risk increases with the number of scans. If a number of repeat scans are required then the justification process must show that the benefit outweighs the risks for the total number of scans required. In addition, it must be borne in mind that DXA will not be sensitive to the small changes in bone density typically seen over short time periods. The real problems in this area arise when the justification is not strictly medical in the individual case involved. This may arise in unapproved screening programmes, sports clinics or with self-referral. In such circumstances special attention needs to be given to the justification protocol.

» Are DXA scans justified in children?

Even though the doses are low, good radiological practice should be followed. Thus, alternative non-ionizing methods for determining BMD (e.g. Quantitative Ultrasound, QUS) should be carefully considered in the case of children. Provided this is done, and the benefits outweigh the risks, a DXA scan may be justified for children.

» Should a patient be told to stop breastfeeding for a time after a DXA scan?

No.

The patient may breastfeed as normal after the DXA scan. Once the DXA scan is over the patient is no longer being exposed to radiation, and does not pose a radiation risk to others.

» Is it acceptable to perform a DXA scan on a patient as a part of a research study?

Policy for irradiation of individuals as part of research studies has been well developed by the WHO and more recently the EU has produced useful guidance  . In practice, this question should be addressed by a local research-ethics committee which examines the benefits of the research and determines whether these outweigh the risks to individuals. The ethics committee will require that all risks, including radiation risks be clearly explained to the patient or volunteer prior to the scan and that they give informed consent to the study. However, justification is essential. The criteria for justification for patients and volunteers will be different. 

Read more:

  • BOUDOUSQ, V., KOTZKI, P.O., DINTEN, J.M., BARRAU, C., ROBERT-COUTANT, C., THOMAS, E., GOULART, D.M., Total Dose incurred by patients and staff from BMD measurement using a new 2D digital bone densitometer, Osteoporos. Int. 14 3 (May 2003) 263-269. 
  • EUROPEAN COMMISSION, Council Directive 97/43/ EURATOM on Health Protection of Individuals against the Dangers of Ionising Radiation in relation to Medical Exposure. Official Journal of the European Communities, Luxembourg(1996). 
  • EU 2000; Publication 118: Referral Guidelines. 
  • EU 1998; Research Guidelines, Publication 99. 
  • HUDA, W., MORIN, R.L., Patient doses in bone mineral densitometry, Br. J. Radiol. 69 821 (May 1996) 422-425. 
  • LARKIN, A., SHEAHAN, N., GRAY, L., O’CONNOR, U., DOWLING, A., VANO, E., TORBICA, P., SALAT, D., SCHREINER, A., NEOFOTISTOU, V., MALONE, J.F., Commissioning and Quality Assurance of Dual Energy X-ray Absorptiometry (DEXA) systems, Rad. Prot. Dosim. 2008 (in press). 
  • LEWIS, M.K., BLAKE, G.M., FOGELMAN, I., Patient dose in dual X-ray absorptiometry, Osteoporos. Int. 4 1 (Jan.1994) 11-15. 
  • SHEAHAN, N.F., DOWLING, A., O'REILLY, G., MALONE, J.F., Commissioning and quality assurance protocol for dual energy X-ray absorptiometry systems, Rad. Prot. Dosim. 117 (2005) 288-290. 
  • UNITED NATIONS Scientific Committee on the Effects of Atomic Radiation, Sources and Effects of Ionizing Radiation, UNSCEAR Report Volume 1, United Nations, New York (2000). 
  • WILSON CR, 2003 AAPM. 
  • WORLD HEALTH ORGANISATION, in CHAN, K.M., ANDERSON, M., EDITH, LAU, M.C., Exercise interventions: Defusing the world’s osteoporosis time bomb. Bulletin of the World Health Organization 81 11 (2003) 827-830; and EL-HAJJ, FULEIHAN, G., BADDOURA, R., AWADA, H., OKAIS, J., RIZK, P., McCLUNG, M., Practice guidelines on the use of bone mineral density measurements: Who to test? What measures to use? When to treat? A Consensus report from the Middle East Densitometry Workshop, J. Med. Liban. 50 3 (2002) 89-104.