MRI in primary care — reducing radiation exposure

Published in Health News and Evidence

Date published: About this date

Clinical content may change after this date. This information is not intended as a substitute for medical advice from a qualified health professional. Health professionals should rely on their own expertise and enquiries when providing medical advice or treatment.

Summary

There is now a greater awareness of, and concern about, exposure to ionising radiation with the use of X-ray and computed tomography (CT).1 Magnetic resonance imaging (MRI) has the advantage of not using ionising radiation,1 but most new Medicare Benefits Schedule (MBS) items for MRI requested by general practitioners will require X-ray as a first investigation prior to reimbursement.2 Appropriate use of imaging technologies provides benefits for patients as well as our community.3

Key points

  • There are concerns about inducing cancer in individuals exposed to ionising radiation.
  • Imaging is justified when the potential benefits outweigh the possible risks.
  • The risk of radiation-induced cancer depends on patient age and imaging protocol.
  • CT is generally associated with higher effective radiation doses than X-ray.
  • The new MBS funding for GP-referred MRI and increased number of MRI facilities will give GPs the opportunity to choose MRI when appropriate.
  • Discuss options and findings in partnership with radiologists.

Concerns about exposure to ionising radiation inducing cancer1,2,4

People are exposed to ionising radiation through medical imaging with X-rays, CT and nuclear medicine scans, including positron-emission tomography (PET).5 While MRI has the advantage of not using ionising radiation4 most of the new MBS items for MRI requested by GPs will require X-ray as a first investigation.2

As there are no completed, large-scale epidemiological studies of cancer risk associated with CT, risk has been approximated using organ doses (or the distribution of dose in the organ) and application of organ-specific cancer incidence and mortality data derived from studies of atomic-bomb survivors on the peripheries of Hiroshima and Nagasaki.6 Risk estimates adjusted to take into account the greater use of CT since 2006 indicate that 1.5–2% of all cancers in the US may be due to radiation from CT.6 Estimating cancer risk from CT remains a contentious issue, and large-scale epidemiological studies are needed for a direct assessment of this risk.6

Imaging is justified if the potential benefits outweigh the risks7

Lower-dose imaging procedures such as X-ray require models to estimate associated cancer risk.8 There is consensus among major responsible authorities that the most appropriate model is the ‘linear/no lower threshold’,5which predicts that the risk of radiation-induced effects decreases linearly with decreasing dose.8,9 The absence of a threshold assumes any dose of ionising radiation carries risk.8,9 Balance the clinical benefit against the estimated risk — in many cases a delay in imaging may pose a greater risk to health than the risk from radiation.5

The risk of radiation-induced cancer depends on patient age and imaging protocol5

Children are particularly at risk of radiation-induced cancer, as they have higher organ sensitivity and more years of life remaining in which to develop radiation-induced cancer.10

Imaging protocols should be developed with the aim to help avoid exposing people to excessive radiation doses. This may require an estimate of a population total radiation risk associated with that procedure (effective radiation dose).10,11 The International Commission on Radiological Protection has made clear that effective radiation dose is not intended for use in epidemiological studies or predictions of risk to exposed individuals.11 The American College of Radiology Appropriateness Criteria specify a relative radiation level (RRL) indication to guide imaging protocol for each examination.10 RRL is based on effective radiation dose; dose estimate ranges for paediatric examinations are lower than those specified for adults.10 The Australian Radiation Protection and Nuclear Safety Agency, in collaboration with other stakeholders, is currently working to develop diagnostic reference levels that will serve as benchmarks that practices can adhere to in their imaging protocol.10,12

The Royal Australian and New Zealand College of Radiologists has recently published guidelines for imaging in children.

CT is associated with higher effective radiation doses than X-ray1,4,5

X-ray is the imaging modality of choice for first investigation of a range of conditions such as acute low back pain with possible infection13 and ankle injury;14 CT is the most appropriate choice for investigation of many clinical problems within the chest and abdomen.4

CT is generally associated with higher effective radiation doses than X-ray.1,4,5 For example, the effective radiation doses for imaging investigations of the chest are 8 milliSieverts (mSv) for CT and 0.02 mSv for X-ray.5 (See Figure 3 in Medicinewise News: MRI in Primary Care.)

Increased opportunities for GP-referred MRI

The new MBS funding items for MRI and an increase in the number of MRI facilities provide GPs with a greater opportunity to refer for MRI for certain conditions.1,2 MRI should be considered first in situations where it provides better or similar information to CT and where both modalities are available,1,2 such as when there is a high-suspicion of scaphoid fracture,15 hip fracture16 or a suspected thoraco-lumbar spine trauma with neurological signs.17 (See Figure 2 in Medicinewise News: MRI in Primary Care.) However, for all these examples guidelines recommend investigation with X-ray before MRI.15–17

In adults CT investigation generally remains the first imaging modality of choice for investigating secondary headache with ‘red flags’ (see Table 1 in Medicinewise News: MRI in Primary Care).18,19 In children the new MBS items allow GP referral for MRI for investigating unexplained headache when significant pathology is suspected.

Ultrasound, another imaging modality that does not use ionising radiation, is the best modality for viewing dynamic images, separating solid from cystic masses and for viewing vascularity without the requirement for contrast media.4

Discuss options and findings in partnership with radiologists

Choose the right test

There are often several appropriate investigations in any clinical situation. Radiologists can provide guidance on selecting the right imaging pathway and advise when to refer onwards.7

Effective communication

Communicate background information about the patient to the radiologist to allow for a more definitive interpretation of test results.1

References
  1. Canadian Agency for Drugs and Technologies in Health. Appropriate Utilization of Advanced Diagnostic Imaging Procedures. 2012. http://cadth.ca/media/pdf/PF%20DI%20ES%20Lit%20Scan%200%208.pdf (accessed 9 August 2012).
  2. The Royal Australian and New Zealand College of Radiologists. Details on New GP Referred Paediatric MRI Items. September 2012. http://www.ranzcr.edu.au/news-a-events/latest-news/1183-details-on-new-gp-referred-paediatric-mri-items (accessed 12 September 2012).
  3. Australian Diagnostic Imaging Association. What Patients Need: Accesible, Accurate, Affordable and Early Diagnosis Through Quality Diagnostic Imaging. Federal Budget Submission 2012-13. 2012. http://www.adia.asn.au/sites/default/files/020211%20ADIA%20Federal%20Budget%20Submission%202011-12.pdf (accessed 24 September 2012).
  4. The Canadian Association of Radiologists. Diagnostic Imaging Referral Guidelines, a Guide for Physicians. Saint-Laurent, Quebec: Legal Deposit, Bibliothèque nationale du Québec, 2005. http://car.ca/uploads/membership/Guidelines_ENG.pdf (accessed 9 August 2012).
  5. Government of Western Australia, Department of Health. Diagnostic Imaging Pathways – About Imaging: Ionising Radiation. December 2011. http://www.imagingpathways.health.wa.gov.au/includes/aboutimaging/ionising-radiation.html (accessed 11 September 2012).
  6. Brenner DJ, Hall EJ. Computed tomography – an increasing source of radiation exposure. N Engl J Med 2007;357:2277–84. [PubMed]
  7. Government of Western Australia, Department of Health. Diagnostic Imaging Pathways. General Principles in Requesting Imaging Investigations. 2011. http://www.imagingpathways.health.wa.gov.au/includes/aboutimaging/general.html (accessed 26 October 2012).
  8. Hall EJ, Brenner DJ. Cancer risks from diagnostic radiology. Br J Radiol 2008;81:362–78. [PubMed]
  9. Little MP, Wakeford R, Tawn EJ, et al. Risks associated with low doses and low dose rates of ionizing radiation: why linearity may be (almost) the best we can do. Radiology 2009;251:6–12. [PubMed]
  10. American College of Radiology. ACR Appropriateness Criteria. 2009. http://gm.acr.org/SecondaryMainMenuCategories/quality_safety/app_criteria/pdf/ExpertPanelonPediatricImaging/SeizuresChildDoc7.aspx (accessed 19 November 2012).
  11. Hendee WR, O'Connor MK. Radiation risks of medical imaging: separating fact from fantasy. Radiology 2012;264:312–21. [PubMed]
  12. Australian Government. Australian Radiation Protection and Nuclear Safety Agency. National Diagnostic Reference Level Factsheet. June 2011. http://www.arpansa.gov.au/pubs/Services/NDRL/NDRLfactsheet.pdf (accessed 19 November 2012).
  13. Government of Western Australia, Department of Health. Diagnostic Imaging Pathways. Acute low back pain. 2010. http://www.imagingpathways.health.wa.gov.au/includes/pdf/backpain.pdf (accessed 9 August 2012).
  14. Government of Western Australia, Department of Health. Diagnostic Imaging Pathways. Acute ankle sprain. 2009. http://www.imagingpathways.health.wa.gov.au/includes/dipmenu/ankle/chart.html (accessed 30 October 2012).
  15. Department of Health Government of Western Australia Diagnostic Imaging Pathways. Suspected scaphoid fracture. 2009. http://www.imagingpathways.health.wa.gov.au/includes/pdf/sca_frac.pdf (accessed 24 September 2012).
  16. Government of Western Australia, Department of Health. Diagnostic Imaging Pathways. Suspected hip fracture. 2009. http://www.imagingpathways.health.wa.gov.au/includes/pdf/hipfract.pdf (accessed 24 September 2012).
  17. Government of Western Australia, Department of Health. Diagnostic Imaging Pathways. Thoraco-lumbar spine trauma. 2009. http://www.imagingpathways.health.wa.gov.au/includes/pdf/spine_inj.pdf (accessed 22 October 2012).
  18. National Prescribing Service. NPS News 79: Headache: Diagnosing, Managing and Preventing. Sydney: National Prescribing Service Limited, 2012. http://www.nps.org.au/publications/health-professional/nps-news/2012/headache (accessed 5 October 2012).
  19. Government of Western Australia, Department of Health. Diagnostic Imaging Pathways. Headache.  Department of Health Western Australia, 2010. http://www.imagingpathways.health.wa.gov.au/includes/pdf/headache.pdf (accessed 9 August 2012).