Computed Tomography (CT) is a method of diagnostic imaging in medicine where from multiple X-ray images of an object taken from different angles using computer processing is obtained a three-dimensional image. The examination is carried out in successive slices. When performing the standard X-ray image the patient's body is irradiated from an X-ray source (X-ray tube) and the image is obtained as a projection onto a plane (the X-ray film). In this type of projection there is a loss of information about the thickness of the objects and additionally it is not possible to determine whether a particular bright area in the image is due to the passage of the rays through a denser material (with greater light absorption) or through thicker object. In Computed Tomography this problem is solved by making multiple X-ray images from different positions around the patient's body and then from these images using computer algorithms is recovered the information about the density of each element within the volume of the scanned object (in this case the patient). One of the disadvantages in the use of computed tomography is the exposure of the patients to ionizing radiation (X-rays). The resulting dose for a single examination can be 1000 times higher than that in taking a conventional X-ray image of the lungs. According to a study published in the medical journal "New England Journal of Medicine" is assumed that the examinations of this type conducted nowadays will be the cause for 1,5-2% of all cancers in a few decades.
What is the risk to the patient?
Risk assessment gives the magnitude "effective dose of radiation".
Here are some typical values of the effective dose:
|
Examination
|
Effective dose in mSv
|
Equivalent exposure to the natural background
|
|
An X-ray of the bones of extremities
|
< 0.01
|
less than 1.5 days
|
|
An X-ray of a tooth
|
0.01 – 0.04
|
1.5 – 7 days
|
|
An X-ray of lungs
|
0.02 – 0.06
|
3 – 10 days
|
|
An X-ray of the skull and cervical vertebrae
|
0.07 – 0.08
|
11 – 14 days
|
|
An X-ray of the hip
|
0.3
|
7 weeks
|
|
An X-ray of thoracic vertebrae
|
0.7
|
4 months
|
|
An X-ray of the pelvis or abdomen
|
0.7
|
4 months
|
|
An X-ray of the lumbar vertebrae
|
1.3
|
7 months
|
|
X-rays of the lungs (duration 1 minute)
|
2.3
|
12 months
|
|
Intravenous urography (kidneys’ examination)
|
2.5
|
14 months
|
|
Contrast examination of the stomach
|
3.0
|
16 months
|
|
Mammography
|
3.0
|
16 months
|
|
Irigography (examination of the colon)
|
7.0
|
3 years
|
|
Natural radioactive background for one year
|
2.0
|
1 year
|
|
Computed tomography of the head
|
2.3
|
1 year
|
|
Computed tomography of the chest
|
8.0
|
3.6 years
|
|
Computed tomography of the abdomen or pelvis
|
10.0
|
4.5 years
|
|
Cardiac catheterization, coronary angiography, coronary angioplasty
|
> 14.0
|
over 6 years
|
Caution! The numerical values in the table are only indicative. The effective dose for the same examinations made with different X-ray installations and in other diagnostic centers may differ dozens or even hundreds of times. The reason is the divergent technical condition of the equipment and the differences in the methods of examination. The likelihood of radiation carcinogenesis increases with the increase of the dose and the number of X-ray examinations.
What is the risk of irradiation during pregnancy?
• The risk depends on the radiation dose delivered to the fetus and the stage of pregnancy.
• The risk is relatively highest during organogenesis and the early fetal period (2 - 12 week) and then decreases.
The equivalent dose of irradiation of the fetus depends on the type of examination:
- very low risk <0.1 mSv
- low risk 0.1 - 4 mSv
- increased risk >4 mSv
