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- About Imaging: Ionising
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Diagnostic Imaging Pathways - About 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 postcontrast.
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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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Radiation Training Module - An online module on the use of radiation in medicine. It includes 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
Tissue weighting factors for specific organs. 1
TISSUE
ORGAN
Gonads
Red Bone
Marrow
Colon
Lung
Stomach
Bladder
Breast
Liver
Oesophagus
Thyroid
Skin
Bone Surface
Brain
Salivary Glands
Remainder
TISSUE
WEIGHTING
FACTOR
0.20
0.12
0.12
0.12
0.12
0.05
0.05
0.05
0.05
0.05
0.01
0.01
0.01
0.01
0.05
Typical Effective Doses of Imaging Investigations
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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)
PLAIN RADIOGRAPHY
Extremities
Chest
Skull
Cervical Spine
Thoracic Spine
Lumbar Spine
Hip
Pelvis
Abdomen
IVP
Barium Swallow
Barium Meal
Barium Follow through
Barium Enema
COMPUTED TOMOGRAPHY
Head
Cervical Spine
Thoracic Spine
Chest
Lumbar Spine
Abdomen
Pelvis
NUCLEAR MEDICINE
Bone Imaging (Tc-99m)
Cerebral Perfusion
(Tc-99m)
Lung Ventilation (Xe-133)
Lung Perfusion (Tc-99m)
Myocardial Perfusion
(Tc-99m)
Myocardial Imaging (FDGPET)
Thyroid Imaging (Tc-99m)
DTPA Renogram
DMSA Renogram
HIDA Hepatobilliary
Imaging
EQUIVALENT
NUMBER OF
CHEST X-RAYS
EQUIVALENT
PERIOD OF
NATURAL
RADIATION
0.01
0.02
0.07
0.10
0.70
1.30
0.30
0.70
1.00
2.50
1.50
3.00
3.00
7.00
0.50
1.00
3.50
5.00
35.0
65.0
15.0
35.0
50.0
125
75.0
150
150
350
1.5 days
3 days
11 days
15 days
4 months
7 months
7 weeks
4 months
6 months
14 months
8 months
16 months
16 months
3.2 years
2.30
1.50
6.00
8.00
3.30
10.0
10.0
115
75.0
300
400
165
500
500
1 year
8 months
2.5 years
3.6 years
1.4 years
4.5 years
4.5 years
4.00
5.00
200
250
1.6 years
2.0 years
0.30
1.00
6.00
15.0
50.0
300
7 weeks
6 months
2.5 years
10.0
500
4.0 years
1.00
2.00
0.70
2.30
50.0
100
35.0
115
6 months
10 months
3.5 months
1.0 years
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*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
None
EFFECTIVE DOSE RANGE
0
Minimal
< 1 millisieverts
Low
1-5 mSv
Medium
5-10 mSv
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
References - Ionising Radiation in Diagnostic Imaging
1. The 2007 Recommendations of the International Commission on Radiological Protection.
ICRP publication 103. Ann ICRP. 2007;37(2-4):1-332. (Guideline document).
2. Little MP, Wakeford R, Tawn EJ, Bouffler SD, Berrington de Gonzalez A. Risks associated with
low doses and low dose rates of ionising radiation: why linearity may be (almost) the best
we can do. Radiology. 2009;251:6-12.
3. Tubiana M, Feinendegen LE, Yang C, Kaminski JM. The linear No-threshold ralationship is
inconsistent with Radiation Biologic and Experimental Data. Radiology. 2009;251:13-22.
(Commentary article)
4. Huda W, Ravenel JG, Scalzetti EM. How do radiographic techniques affect image quality and
patient doses in CT? Semin Ultrasound CT MR. 2002;23:411-22. (Review article)
5. European Commission. Referral guidelines for imaging. Luxembourg. Office for Official
Publications of the European Communities. 2001.
6. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). UNSCEAR
2000 Report to the General Assembly. Annex D: Medical Radiation Exposures. 2000. [cited
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7.
8.
9.
10.
11.
12.
13.
2005 August 24]. (Guideline document)
ICRP.The 2007 recommendations of the International Commission on Radiological
Protection. ICRP Publication 103. Ann ICRP. 2007;37(2-4):1-332. (Guideline document)
Mettler FA Jr, Huda W, Yoshizumi TT, Mahesh M. Effective doses in radiology and diagnostic
nuclear medicine: a catalog. Radiology. 2008;248(1):254-63. (Review article)
Australian Radiation Protection and Nuclear Safety Agency. ARPANSA Fact Sheet 27 - Cosmic
radiation exposure when flying [Document on the Internet]. Updated: May 2011. Accessed:
August 2011. Available from: http://www.arpansa.gov.au/pubs/factsheets/027.pdf (Information
pamphlet)
United Nations Scientific Committee on the Effects of Atomic Radiation.Sources and effects of
ionising radiation.UNSCEAR 2008 Report to the General Assembly with Scientific Annexes.
Accessed on 6 Dec'2012 at: http://www.unscear.org/docs/reports/2008/0986753_Report_2008_GA_Report_corr2.pdf (General assembly report)
Hall EJ, Brenner DJ. Cancer risks from diagnostic radiology. Br J Radiol. 2008;81(965):362-78.
(Review article)
Sarma A, Heilbrun ME, Conner KE, Stevens SM, Woller SC, Elliott CG. Radiation and chest ct
scan examinations: what do we know? Chest. 2012;142(3):750-60. (Review article)
Edwards AA, Lloyd DC. Risks from ionising radiation: deterministic effects. J Radiol Prot.
1998;18(3):175-83. (Review article)
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