Despite the stigma surrounding diagnostic imaging in pregnancy, such procedures are sometimes unavoidable and actually stand to bring more benefit than harm. Medical Imaging Technology explores the safety issues related to CT and X-rays during pregnancy, and determining the most appropriate course of action for these patients, with Dr Manjiri Dighe, associate professor of radiology at the University of Washington.

It's a trope of TV medical dramas – a woman has an accident and is rushed into ER, where the radiologist prepares to administer an X-ray. Waiting until her anxious partner leaves the room, she looks at the doctor in panic. "I'm pregnant," she says, protecting her unborn child from radiation and her husband from the truth, while revealing the plot twist to viewers.

As narrative devices go, this one is certainly hammy, but it does make one thing clear: the risks from radiation exposure during pregnancy are common knowledge. In fact, for many pregnant women, X-rays are up there with alcohol and cigarettes in their mental checklist of 'things that may harm the baby'. However, the reality of the situation is somewhat more nuanced. Although diagnostic imaging during pregnancy is a sensitive and complex issue, it is not a no-go zone, and the risks are considerably more minor than many patients may anticipate.

The lowdown
"Currently, all imaging modalities can be used on pregnant women; however, some have the issue of radiation that can affect the foetus – for example, CT scans and fluoroscopy. Ultrasound and MRI do not use radiation and are considered the safest modalities to use in pregnant patients," explains Dr Manjiri Dighe, an associate professor of radiology at the University of Washington and an expert in obstetric imaging.

While ultrasounds are routinely used to check the development of the baby and are not associated with any adverse effects, it may come as more of a surprise that radiation-based modalities are mostly safe. When CT scans and X-rays are kept away from the abdomen and pelvis – such as during a chest or head CT, or a dental X-ray – the developing foetus is exposed to very little radiation.
For instance, in the case of CT scans to the head, with a lead apron over the abdomen, it would take approximately 50 tests to reach a potentially harmful dose – 20 times higher than environmental radiation, at which level the baby has a one in 500 chance of developing a childhood cancer.

However, when the mother's lower torso is X-rayed, the risks do start to climb. For instance, it would only take a few abdominal CT scans to reach a similar threshold, meaning the decision of whether to administer a scan may become especially loaded. For fluoroscopically guided interventional procedures in the pelvis, the risk is higher still.

"Maternal safety is always paramount and, hence, in trauma, the concern for foetal radiation is secondary. If the exam is not needed – if suspicion for disease in the mother is very low – the harm to the foetus will be unnecessary," says Dighe. "Radiologists need to first make sure that the exam is needed. If yes, the safest and best technique should be used. The lowest radiation should be used and non-radiation exams should be used before exams with radiation are used."

Mantra rays
Radiation concerns are not unique to pregnant patients. Even though we live in a radioactive world, with natural exposure to background radiation in day-to-day life and particularly in scenarios such as air travel, any slight increase in cancer risk is liable to give pause for thought. While the average X-ray is virtually harmless, equivalent in dose to several days' worth of background radiation, for CT scans, safety is more of an issue.

According to FDA, out of every 2,000 patients receiving a CT examination with an effective dose of 10mSv – equivalent to more than three years' typical background radiation – one of them will succumb to a fatal cancer they would not otherwise have developed.
A foetus is more vulnerable still. Because its cells are fast dividing and proliferating, any damage sustained could lead to birth defects or problems further down the line. Here, the precise balance of risk will depend on the stage of pregnancy and the strength of the dose. "The risk of harm is very low – for example, the risk of developing cancer is 0.25% from low-dose radiation," says Dighe. "The risk changes based on the amount of radiation received."

Last year, the American College of Radiology (ACR), amended its guidelines for imaging pregnant or potentially pregnant patients. It states the risk to the baby is highest between three and 15 weeks after conception, with a chance of possible malformations at very high doses. At medium doses (typical of CT scans), the "potential effects are scientifically uncertain and probably too subtle to be clinically detectable" according to the guidelines, but there may be sufficient grounds to postpone the procedure.

Once the foetus is more than 15 weeks post-conception, imaging procedures are unlikely to pose a risk to its central nervous system. For this to be an issue, the radiologist would need to administer an extremely high radiation dose, well beyond the scope of most diagnostic scans.

Possible cancer induction, however, does remain an issue, and while the precise figure is difficult to pin down – the ACR estimates there is at least a 98% likelihood that the child will be unaffected – it is still important to take precautions.

The guidelines therefore recommend that all girls and women of reproductive age, approximately 12-50, be screened for the possibility of pregnancy, generally through a standardised form or direct questioning from the technologist.

With higher-risk procedures, more intensive screening may be necessary: according to one study, 1% of women of childbearing age who underwent abdominal images were unknowingly in the first trimester of pregnancy.

If the patient is indeed pregnant and consent to proceed is obtained, the physician needs to look into how to minimise radiation exposure. One way to do this is simply to decrease the dosage, although this may render the scan less diagnostically accurate as it also increases image noise. Other options include reducing the number of images or limiting CT phases through the pelvis, as well as better targeting the scan where possible to avoid unnecessary exposure to the uterus.

Finding the balance
The current generation of CT scanners is designed to make dose reduction simpler. Today's multirow-detector scanners come with automatic exposure-control software that may modify X-ray tube output to produce diagnostic images at a preset level of noise. Other scanners, meanwhile, have emerged with iterative reconstruction techniques that improve image quality at low doses of radiation.

"There have been new safety advances such as ultra-low-dose CT, faster scanning sequences in MR and better imaging in ultrasound," says Dighe.

It is unlikely that the risks associated with medical imaging will ever be eliminated altogether and, certainly when you're dealing with radiation of any dosage, 'safety' is a nebulous term. This applies doubly so in pregnancy, where the foetus must be considered a patient too. Here, we are dealing with grey areas, a delicate balancing of harms and benefits that are nearly impossible to quantify.

However, during a well-managed procedure that follows proper protocol, there is no reason for these risks to climb especially high. Potential health conditions, left unchecked, could be far more deleterious to mother and baby, and accurate diagnosis is critically important. This means that, whatever the dubious wisdom of TV dramas, medical imaging during pregnancy is here to stay.