Ionising Radiation (IR) In Diagnostic Imaging
The risks of IR incurred at diagnostic imaging levels are presumptive and based on the 'linear/no lower threshold' (LNLT) model and extrapolated from data collected after the atomic bomb explosions in Japan. 1,2 However, it is important to note that all major responsible authorities believe it prudent to work to that model, although there is no shortage of opinion that disputes it. 3
The LNLT model indicates that no dose of IR, however small, is entirely without risk. This model estimates the average lifetime risk of induction of a fatal cancer from exposure to 5milliSieverts (mSv) to be approximately 1 in 4000 and that to 20mSv to be 1 in 1000. The risk is considerably greater than average in children and young adults and becomes diminishingly smaller with age.
If we accept this model of risk of ionizing radiation, that is a no lower threshold - and it is important to stress that all international regulatory authorities do - then all imaging procedures need to be justified before being performed.
The process of justification requires that the potential benefit of the procedure outweighs the risk. In the case of ionizing radiation, this risk is related to the induction of cancer in the exposed individual. The size of that risk depends on patient factors (in particular the age - children and young adults are especially susceptible), the extent and part of the body exposed (since some organs are more sensitive to IR than others) and to the nature of the examination and the imaging protocol used to perform it.
The risk of cancer induction by IR is a deferred risk that may occur from 5 to 15 years after exposure. The underlying clinical context in the individual patient is important, since, for example, in a patient who is undergoing imaging for an incurable cancer and in, say, an 80 year old patient, the risk may be irrelevant.
In recent decades there has been a marked increase in population exposure to IR. Most of this is related to medical procedures and especially to CT scans.The radiation dose received during a CT scan depends on the protocol used - that is the radiographic factors and the number of series obtained. For example scans may be obtained before intravenous iodinated contrast injection and in one or more phases post-contrast.
A CT scan of the abdomen and pelvis, depending on the protocol used may expose the patient to about 20mSv of IR which, on average, increases the risk of fatal cancer by about 1 in 1000. However, this risk may be doubled in young patients, but halved in elderly patients. Remember, though, that the risk is cumulative if the patient undergoes repeated scans. This risk must be put into the clinical context and compared against other common risks. For example the risk of being killed on Western Australian roads in a ten year period is approximately 1 in 1000.
In summary, if the potential benefit of the scan outweighs the risk, then the scan is justified. If the patient needs a scan for treatment or management then they should not be put off having one. Appropriate CT scans are good; inappropriate scans are bad.
Assessing the Risk / Benefit Ratio
Essentially the rules are:
- The potential benefit of the test should always outweigh the risk.
- A diagnostic imaging examination is indicated only if it is likely to be useful in the management of the patient and if the risk of the procedure is less than the risk of missing a treatable disorder.
- It is the responsibility of the imaging specialist to ensure radiation dosage during imaging is kept to a minimum according to the ALARA principle (As Low As Reasonably Achievable), while maintaining the diagnostic quality of the examination.
Before requesting an imaging investigation, the referring doctor must ask him/herself the following questions:
- Have I taken a history, performed a physical examination and come to a provisional clinical diagnosis? (the significance of the result of a test cannot be assessed without a pre-test probability of the disease being tested for)
- Is imaging indicated?
- Am I duplicating recent tests?
- Will it change my diagnosis?
- Will it effect my management?
- Will it do more harm than good?
- If imaging is indicated, is a test that does not employ IR a feasible option (Ultrasound or MRI)?
Thus it is the responsibility of both the referring clinician and the radiologist to minimise exposure of the individual patient and the community as a whole to ionising radiation. The principles that need to be adhered to achieve this at the individual patient level are also outlined in the page titled Requesting Imaging Investigations: General Principles.
Ionising Radiation Tutorial
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Radiation Training Module - A new online module on the use of radiation in medicine. It includes an interactive module and a self-test module. Note: The link will open in a new window. |
Measurement of Radiation Dose
- Absorbed dose (Gy - Gray): Represents the energy deposited in tissue per unit mass. This unit of measurement can be used for any form of radiation, but does not account for the different biological effects for various types of radiation.
- Equivalent dose: The equivalent dose for a particular tissue or organ equals the absorbed dose multiplied by the appropriate tissue weighting factor.
- Effective dose (Sv - Sievert): A summation of the equivalent doses to all organs and tissues, adjusting for varying radiosensitivity in different tissues. It gives an indication of the overall risk to the patient due to radiation. The effective dose provides a measure of the absorbed dose in human tissue in terms of the effective biological damage of the radiation.
Table 1: Tissue weighting factors for specific organs. 1
| TISSUE/ORGAN | TISSUE WEIGHTING FACTOR |
|---|---|
| Gonads | 0.20 |
| Red Bone Marrow | 0.12 |
| Colon | 0.12 |
| Lung | 0.12 |
| Stomach | 0.12 |
| Bladder | 0.05 |
| Breast | 0.05 |
| Liver | 0.05 |
| Oesophagus | 0.05 |
| Thyroid | 0.05 |
| Skin | 0.01 |
| Bone Surface | 0.01 |
| Remainder | 0.05 |
Typical Effective Doses of Imaging Investigations
As a general guide (and it should be noted that the figures are subject to a great deal of variability dependent on equipment, technique, number of films required, etc.) the following figures for dosage in milliSieverts (mSv); are given for some more common procedures.
Table 2: Typical effective doses for common procedures. 2,3
| IMAGING INVESTIGATION | EFFECTIVE DOSE (mSv) | EQUIVALENT NUMBER OF CHEST XRAYS | EQUIVALENT PERIOD OF NATURAL RADIATION |
|---|---|---|---|
| PLAIN RADIOGRAPHY | |||
| Extremities | 0.01 | 0.50 | 1.5 days |
| Chest | 0.02 | 1.00 | 3 days |
| Skull | 0.07 | 3.50 | 11 days |
| Cervical Spine | 0.10 | 5.00 | 15 days |
| Thoracic Spine | 0.70 | 35.0 | 4 months |
| Lumbar Spine | 1.30 | 65.0 | 7 months |
| Hip | 0.30 | 15.0 | 7 weeks |
| Pelvis | 0.70 | 35.0 | 4 months |
| Abdomen | 1.00 | 50.0 | 6 months |
| IVP | 2.50 | 125 | 14 months |
| Barium Swallow | 1.50 | 75.0 | 8 months |
| Barium Meal | 3.00 | 150 | 16 months |
| Barium Follow through | 3.00 | 150 | 16 months |
| Barium Enema | 7.00 | 350 | 3.2 years |
| COMPUTED TOMOGRAPHY | |||
| Head | 2.30 | 115 | 1 year |
| Cervical Spine | 1.50 | 75.0 | 8 months |
| Thoracic Spine | 6.00 | 300 | 2.5 years |
| Chest | 8.00 | 400 | 3.6 years |
| Lumbar Spine | 3.30 | 165 | 1.4 years |
| Abdomen | 10.0 | 500 | 4.5 years |
| Pelvis | 10.0 | 500 | 4.5 years |
| NUCLEAR MEDICINE | |||
| Bone Imaging (Tc-99m) | 4.00 | 200 | 1.6 years |
| Cerebral Perfusion (Tc-99m) | 5.00 | 250 | 2.0 years |
| Lung Ventilation (Xe-133) | 0.30 | 15.0 | 7 weeks |
| Lung Perfusion (Tc-99m) | 1.00 | 50.0 | 6 months |
| Myocardial Perfusion (Tc-99m) | 6.00 | 300 | 2.5 years |
| Myocardial Imaging (FDG-PET) | 10.0 | 500 | 4.0 years |
| Thyroid Imaging (Tc-99m) | 1.00 | 50.0 | 6 months |
| DTPA Renogram | 2.00 | 100 | 10 months |
| DMSA Renogram | 0.70 | 35.0 | 3.5 months |
| HIDA Hepatobilliary Imaging | 2.30 | 115 | 1.0 years |
Within this website, the relative radiation level of each imaging investigation is displayed as below.
| SYMBOL | RRL | EFFECTIVE DOSE RANGE |
 | None | 0 |
 | Minimal | < 1 millisieverts |
 | Low | 1-5mSv |
 | Medium | 5-10 mSv |
 | High | >10 mSv |
The lifelong risk of death from medical ionising radiation is calculated at approximately 4% per Sievert. Therefore, for example, an examination giving a dose of 5mSV has an attendant risk of 2x104 (1 in 5000), or about one-sixth the risk of dying on the roads in Western Australia in the next ten years.
Consumer Information
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Radiation Risks of X-Rays and Scans (May 2009) |
Last reviewed December 2011
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