Imaging In Pregnancy

A common diagnostic challenge faced by both radiologists and clinicians involved radiation exposure in imaging during pregnancy as it involves both the mother and the fetus. Often, recognised or unrecognised pregnancies are likely to undergo X-ray, ultrasonography (USG), CT, nuclear medicine imaging, and MRI, as they are part of routine clinical evaluation for various conditions. There remains a lack of understanding around safety, leading to unnecessary avoidance of tests that may be valuable in clinical management. The American College of Gynecology and Obstetrics (ACOG) recommends a case-to-case basis and a risk-benefit assessment to be made by all physicians involved. The decision should be driven by the medical necessity of the examination and the likelihood of the information impacting the patient’s care during the gestation or if it can be delayed until the completion of pregnancy without compromising the maternal or fetal outcome or if the diagnosis can be established with a non-ionising radiation investigation. USG and MRI are not associated with risk and therefore are the frontline choice for imaging in pregnancy.  X-ray, CT, or nuclear imaging techniques may be associated with a radiation exposure lower than that associated with harm to the fetus and hence may be readily used when indicated.  It is always advised to have informed consent in place before any imaging procedure. It educates the patient on both risks and benefits to maternal and fetal health, alleviates the mother’s anxiety during the procedure, and can be beneficial if any future medicolegal issues arise.

Non-Ionising Radiation Imaging

Ultrasound

Ultrasound lacks any adverse effects, and hence remains a frontline option for fetal structural and chromosomal screening, biometry, and diagnostic surveillance for obstetrical conditions during pregnancy. Although ACOG, the American Institute of Ultrasound in Medicine (AIUM), and the Society of Radiologists in Ultrasound (SRU) back the use of ultrasound images for fetuses irrespective of gestational age, the acoustic output should be as low as reasonably achievable. A nuchal translucency scan is a routine first-trimester screening to evaluate for chromosomal anomalies. Non-Chromosomal conditions such as neural tube defects, abdominal wall defects, limb abnormalities, and some congenital heart diseases can also be identified early in pregnancy. Down’s syndrome can be predicted with an accuracy of 75% even without a blood test. Uterine artery Pulsatility index (PI) obtained at the time of the nuchal translucency scan in the first trimester is strongly associated with the risk of pre-eclampsia and pre-term delivery, and knowledge of the same helps in instituting prophylactic treatment protocols. Doppler, in conjunction with biometry scans, has been proven to predict adverse perinatal outcomes and can be used to assess clinical management. 

Magnetic Resonance Imaging

MRI can be a reliable alternative over modalities using ionising radiation in pregnant women especially when an ultrasound shows equivocal findings. It has the ability to image deep tissue structures and is not operator-dependent. As per the American College of Radiology (ACR) guidelines, MRI can be performed at a 3.0-T scale or less irrespective of the gestational age. There are no documented harmful effects on a developing fetus when exposed to MRI fields used in clinical practice. Hence the risk remains theoretical. MRI plays a role in obstetric and non-obstetric imaging. It can be used as a problem-solving tool, in cases with concern regarding invasive placenta or indecisive cases of congenital anomalies. In cases with non-obstetric causes of acute abdominal pain, MRI is advised to be utilised as an alternative to CT.

Ionising Radiation Imaging

Computed Tomography

The use of CT in pregnancy is limited due to ionising radiation at a significantly higher dose than those of conventional X-rays. Despite no clear evidence of known harm, the use of contrast is recommended only when there is potential for clinical impact. For a clinical condition demanding a CT scan, with the gravid uterus out of the field of view, the concern for the harmful effects of radiation on the fetus is negligible. It may be performed without any risk to the fetus. The concern for the teratogenic effects on the fetus is highest up to 15 weeks of gestation. Intravenous and oral contrast may be used as per the clinical indication. 

Conventional X-ray

Diagnostic X-ray does not have significant radiation to cause any adverse effect on the developing fetus even when the fetus is included in the view. The risk of ionising radiation impacting the fetus depends on the time of exposure (gestational age) and the radiation dose. Only high-dose exposure over 1 GY during early embryogenesis can be lethal to the embryo. Diagnostic radiographic imaging never uses these dose levels. Termination of pregnancy should not be contemplated unless consulted with a radiation physicist in the event of multiple imaging studies using ionising radiation. 

Imaging using ionising radiations such as conventional x-rays, or CT is always undertaken to bear in mind the possibility of an unpredicted pregnancy, by taking into account the last menstrual cycle date and incorporating the 28-day rule in non-emergent cases. It is the responsibility of the female to inform the medical personnel of the pregnancy status and the responsibility of the medical personnel to proactively enquire regarding the same. 

Nuclear Medicine Imaging

Nuclear medicine imaging uses a radioisotope which is used in varied scenarios. During pregnancy, it is used to study pulmonary ventilation-perfusion, thyroid, bone, and renal scans. Technetium 99m is commonly used for these procedures, which result in embryonic or fetal exposure of less than 5 mGy (safe dose). However, not all radioisotopes are safe for use during pregnancy.

Conclusion

Pregnancy is that phase where clinicians constantly have to consider the clinical benefit versus the impact of radiation when suggesting an imaging procedure. Imaging procedures have become a great adjuvant to clinical diagnosis and are increasingly deployed in steering early and appropriate management. Recommending the right procedure during pregnancy can minimise radiation exposure and enhance clinical outcomes. USG and MRI being non-ionising, are the preferred choices, however, X-rays, CT, and nuclear medicine imaging may be deployed where indicated with adequate caution.