Imaging During Pregnancy and Lactation

Introduction

Exposure to ionizing radiation during pregnancy is associated with high levels of anxiety in pregnant patients.

The effects of ionizing radiation on the developing fetus include teratogenic and carcinogenic risks and are dose-dependent and related to the stage of pregnancy. The most vulnerable period is 8-15 weeks of gestation.

Exposure to 1 The actual quantitative carcinogenic risk to the fetus is controversial but has been estimated by the ICRP at one cancer per 500 fetuses exposed to 30 mGy. 2

The following measures can be taken to avoid inadvertant exposure or minimise exposure to ionizing 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
• 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

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 ionizing radiation during the main period of organogenesis (weeks 3 to 8) and the first trimester compared to the 2nd and 3rd trimesters. Between weeks 8 and 15 this represents the highest risk of radiation induced mental retardation. 3

ESTIMATED FOETAL DOSES FROM COMMON DIAGNOSTIC PROCEDURES 4

The foetal effects of ionizing radiation include:

• 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
• Malformations - within the first 8 weeks after implantation, radiation doses of 100-200 mGy are associated with malformation in developing organs
• Neurological effects - central nervous system abnormalities are associated with ionizing 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.
• The above effects occur at radiation doses in excess of those used in diagnostic imaging
• 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%
• A recent case-control study by Rajarman et al. examined childhood cancer risks associated with exposure to diagnostic radiation and ultrasound in over 7000 patients. They found that there was a slight increase in risk for all cancers and leukaemia after in utero radiation exposure, however this finding was not statistically significant. There was no evidence of ultrasound being associated with an increased risk of childhood cancer 5
• Genetic effects - the risk due to irradiation is extremely low, compared to both radiation-induced carcinogenesis and the natural risk of heritable effects
• 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 ionizing 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 ionizing 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 ionizing 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 ionizing radiation should be chosen. Consideration should be given to modalities such as ultrasound and MRI, which do not involve exposure to ionizing 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. This usually requires referral to a medical physicist. 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.

Use of Iodinated Contrast in Pregnancy and Lactation

The literature regarding the safety of iodinated contrast and effects during pregnancy and lactation is limited, though no deleterious effects have been reported. 1 Therefore the administration of iodinated contrast during pregnancy and lactation should be limited to where clinically indicated. The theoretical risk of contrast induced hypothyroidism within the foetus has not been validated and foetal exposure to iodinated contrast media and any associated free iodide is likely to be small and relatively short-lived. Although no harmful effects to the foetus from iodinated contrast or IV Gadolinium administered to the mother have been documented, theoretical considerations indicate that all neonates should receive thyroid function testing in the first week of life where the mother has received iodinated contrast material in accordance with current standard paediatric care. 6

European guidelines have stated that cessation of breast feeding following iodinated contrast material is not required. 6 Less than 1% of contrast agent administered to a lactating mother is excreted into the breast milk and less than 1% of this absorbed by the infant. 1 Neither direct toxicity or allergic reaction have been reported. However, the mother may choose to discard the breast milk for 24 hours after receiving intravenous contrast.

Alternative Imaging Modalities in Pregnancy

The use of ultrasound in pregnancy is regarded as safe with no deleterious effects from in utero exposure. 3

There is no evidence to suggest that fetal MRI is responsible for harmful effects but this must be considered against the fact that long term safety has not been conclusively demonstrated. There are theoretical risks of biological effects from the effects of local electric fields and currents from magnetic fields associated with MRI though there has been no conclusive evidence of this reported in the literature. 3 The use of MRI imaging during the first trimester should be limited to where clinically imperative with differing recommendations in the literature in regards to conducting imaging in this time frame. 7 Sequences with relatively low specific absortion rates (with less potential harmful heating effects on the foetus), such as gradient-recalled echo sequences are preferable. 1 During the second and third trimesters it is generally accepted that MRI up to 3.0 tesla is considered safe. 7 Whilst there are no studies conclusively linking the use of gadolinium to deleterious effects in pregnancy the use of it during pregnancy is generally not advised unless a specific clinical indication exists. 8, 9

Mammography if clinically indicated exposes the fetus to miniscule amounts of radiation so can be performed any time during pregnancy. 7 The use of nuclear imaging in pregnancy is generally a rare occurrence and generally when indicated the radiation risk to the foetus is small when appropriate methods are employed to reduce the absorbed dose. 10 The use of radioiodine based isotopes during pregnancy is generally contraindicated due to the risk of inducing thyroid cancer in the foetus, 11 where scintigraphic imaging of the thyroid is thought to be necessary Tc-99m should be used in the place of I-131. 11