Diagnostic Imaging Pathways - About Imaging: Ionising Radiation In Diagnostic Imaging

Ionising Radiation In Diagnostic Imaging

Ionising Radiation In Diagnostic Imaging

Ionising radiation (IR) is employed in x-rays, mammography,CT scans, fluoroscopic procedures and nuclear medicine examinations. Ultrasound and Magnetic Resonance Imaging (MRI) do not use ionising radiation.


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 some opinions dispute 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 5 milliSieverts (mSv) to be approximately 1 in 4000 and that to 20 mSv to be 1 in 1000. The risk is considerably greater than average in children and young adults and becomes smaller with age over the age of 40 years.


If we accept this model of risk of ionising 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.


In discussions of radiation exposure the terms stochastic and deterministic effects are often used. Stochastic effects are considered to be unpredictable and random in nature. Malignancy is the most significant stochastic effect where there is considered to be no threshold point at which this occurs. the risks of stochastic effects are considered to increase with dose but severity of effect is independent of this, with the development of a particular effect an all or nothing concept. 11,12 Deterministic effects are defined by a cause and effect relationship between radiation exposure and measure outcome. Above a certain threshold of exposure the measure outcome can be predictably appreciated; as the level of dose increases, the severity of the effect increases as well. 13


The process of justification 7 requires that the potential benefit of the procedure outweighs the risk. In the case of ionising 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 since 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 20 mSv 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 and technologist 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: 5

  1. 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 accurately assessed without a pre-test probability of the disease being tested for.
  2. Is imaging indicated?
    • Am I duplicating recent tests?
    • Will it change my diagnosis?
    • Will it affect my management?
    • Will it do more harm than good?
  3. 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 article titled Requesting Imaging Investigations: General Principles.


Ionising Radiation Tutorial

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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

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
Brain 0.01
Salivary Glands 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 4, number of films required, etc.) the following figures for dosage in milliSieverts (mSv) are given for some more common procedures.

Typical effective doses for common procedures. 2,3,6,8


IMAGING INVESTIGATION EFFECTIVE DOSE (mSv) EQUIVALENT NUMBER OF CHEST X-RAYS 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

*The average world-wide natural radiation dose is 2.4 mSv per year. 9,10


Within this website, the relative radiation level of each imaging investigation is displayed as below.


SYMBOL RRL EFFECTIVE DOSE RANGE
No radiation None 0
Minimal radiation Minimal < 1 millisieverts
Low radiation Low 1-5 mSv
Medium radiation Medium 5-10 mSv
High radiation High >10 mSv

The excess relative risk of cancer per Sv is 5.5%-6.0% in the population; with this being 4.1%-4.8% in the adult population. 1


Information for Consumers

For information for consumers at this website about ionising radiation, Click here.


Alternatively, for information published by the Royal Australian and New Zealand College of Radiologists, Click here.

Date reviewed: November 2014

Date of next review: November 2016