Ionising radiation such as that used in diagnostic imaging has biological effects. These have been characterised as deterministic (where the severity of the effect increases with dosage above some threshold) and stochastic (the probability of the occurrence of an effect is related to the dosage; the smaller the dose, the less the probability. In theory, there is no minimum threshold).
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. At the individual patient level this is achieved by adherence to the principles outlined in the section titled "Requesting Imaging Investigations: General Principles". As a general rule, 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.
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 |
|
| 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 2x10 -4 (1 in 5000), or about one-sixth the risk of dying on the roads in Western Australia in the next ten years.
Consumer Information
Click on icon below to download consumer information in PDF format.
Last reviewed March 2008.
File Formats
Some documents for download on this website are in a Portable Document Format (PDF). To read these files you might need to download Adobe® Acrobat Reader.
