Diagnostic Imaging Pathways - Diagnostic Imaging During Pregnancy

The following measures can be taken to avoid inadvertant exposure or minimise exposure to ionising radiation in a patient of child-bearing age

• Raising the awareness of patients for the need to inform the Imaging Specialist or technologist of the possibility of pregnancy.
• Adherence to the "28 day rule". If a patient of child-bearing age has missed a period, the test may need to be delayed until pregnancy is excluded.
• When appropriate in patients of child-bearing age, perform tests that do not employ ionizing radiation in preference to those that do.
• Ensure the radiation dosage during imaging is kept to a minimum according to the ALARA principle (As Low As Reasonably Achievable).
  • Avoid duplication of tests and ensure optimum views by overseeing quality control of radiography, etc)
  • Adhere to strict principles of radiation protection such as shielding, appropriate technical factors, appropriate film/screen combinations and obtaining the minimum number of exposures required for adequate diagnosis.
  • When appropriate, choose tests that do not employ ionizing radiation in preference to those that do.

Effects on the Foetus Resulting From Irradiation

The biological effects of radiation are dependent on the foetal absorbed dose and the stage of foetal development. The foetus is much more sensitive to ionising radiation during the main period of organogenesis (weeks 3 to 8) and the first trimester compared to the 2nd and 3rd trimesters.

Table 3: Estimated foetal doses from common diagnostic procedures. 1

1. Miscarriage or foetal death - in the first few weeks after conception, there is an increased risk of early foetal death and failure of implantation with radiation doses of 100-500 mGy.
2. Malformations - within the first 8 weeks after implantation, radiation doses of 100-200 mGy are associated with malformation in developing organs.
3. Neurological effects - Central nervous system abnormalities are associated with ionising radiation doses in excess of 100 mGy between weeks 8 to 25, with the period during weeks 8 to 15 carrying the highest risk. The relationship between radiation dose and malformations is non-linear. The severity of abnormalities range from a slight reduction in IQ with doses of 100-200 mGy, to severe mental retardation and microcephaly at doses of 1000 mGy.
4. Carcinogenesis - there is an increased risk of developing childhood cancer and leukaemia with doses greater than 10 mGy. The risk of carcinogenesis after weeks 3 to 4 of gestation is considered equal throughout pregnancy for a given foetal radiation dose. The relative risk from exposure to 10 mGy is estimated at 1.4. As the background rate of childhood cancer is 0.2% to 0.3%, this translates to an absolute increase in risk of approximately 0.1%.
5. Genetic effects - the risk due to irradiation is extremely low, compared to both radiation-induced carcinogenesis and the natural risk of heritable effects.
6. Pre-conception gonadal irradiation - gonadal irradiation of parents prior to conception has not been shown to cause an increased risk of carcinogenesis or malformations in children.

Informed Consent

It is the responsibility of the Imaging Specialist performing the imaging investigation and the referring clinician to take all reasonable steps to inform the patient about the estimated foetal absorbed dose and the potential risks to the foetus. Whilst the information should be provided prior to any diagnostic imaging investigation involving ionising radiation, this may not be possible in certain emergency situations. For imaging investigations where the foetal absorbed dose is less than 1 mGy, it is sufficient to advise the pregnant patient that the risks are negligible. Imaging investigations associated with doses greater than 1 mGy require a more thorough discussion.

Justification for Diagnostic Imaging

Each case should be assessed individually, involving a risk-benefit analysis weighing up the potential maternal and foetal benefits against potential harm to the foetus. Imaging investigations involving ionising radiation should be avoided or delayed until after the pregnancy unless there are strong clinical indications. The urgency of the investigation should be assessed against the gestational age, with special care taken to avoid the use of ionising radiation especially during the first trimester. The responsibility for performing the test should be shared after consultation with the referring clinician.

For properly performed diagnostic imaging investigations, the foetal absorbed dose is unlikely to exceed 100 mGy even when the uterus is in the direct beam. Therefore for a clinically indicated diagnostic imaging investigation, the potential benefits will usually outweigh any potential risks from irradiation. The imaging modality with the lowest level of ionising radiation should be chosen. Consideration should be given to modalities such as ultrasound and MRI, which do not involve exposure to ionising radiation. If computed tomography is deemed necessary, the absorbed dose should be minimised through the following methods:

• Shielding of organs or the foetus.
• Reduction in mAs values.
• Limiting the volume scanned.
• Eliminating unnecessary scout images.
• Choosing appropriate image reconstruction parameters.
• Use of filtering in the z-axis with multidetector CT.
• Thicker detector collimation
• Increase pitch factor with software calculated overlapping images.

Inadvertent Radiation of Pregnant Patients

If the foetus has been inadvertently exposed to high-dose irradiation, the foetal absorbed dose must be measured. The risk to the foetus should be calculated by accounting for all fetal absorbed doses of previous imaging investigations during the pregnancy. The patient should then be informed about the potential risks to the foetus by the radiologist and the referring clinician or the patient's obstetrician.

Last reviewed in May 2009.