Making decisions about radiological imaging in pregnancy

What potential risk does imaging in pregnancy pose to the fetus?

Different imaging modalities have different effects on human tissues, and as such, pose different risks (and levels of risk) to the fetus (table 1). High dose ionising radiation such as CT is generally of more concern than ultrasound, MRI, and low dose ionising radiation such as plain radiography.

Is there a role for abdominal shielding?

Abdominal lead shielding was historically used for ionising radiation that did not directly expose the fetus (eg, chest radiography and CT pulmonary angiography (CTPA)). However, as most of the fetal dose in these investigations is from internal (rather than external) scatter, abdominal shielding is unlikely to be of benefit.18 Additionally, if the shield is inadvertently in the field of view during the scan, automatic exposure control can cause increased dose.18

Guidelines from the American Association of Physicists in Medicine19 and British Institute of Radiology20 suggest avoiding routine shielding, but recommend considering it case by case if it offers patient reassurance after adequate counselling about the above.

Ultrasound

A meta-analysis performed on behalf of the World Health Organization found no evidence of adverse maternal outcome following ultrasound in pregnancy and it can be considered safe.2

Ionising radiation

Breast tissue is particularly susceptible to the effects of ionising radiation21 and pregnant (and breastfeeding patients) are theoretically at increased risk because breast tissue is actively undergoing glandular proliferation.18 However, a retrospective population based cohort study with a short term follow-up period (<12 years) did not show an increased risk of breast cancer in patients exposed to CT chest or VQ scanning in pregnancy.22

MRI

Usual MRI safety considerations for all patients apply. In pregnancy, the relatively long scan times (20 minutes to one hour) and claustrophobia may be difficult, particularly in late pregnancy. Consider reduced scan times and alternative (eg oblique) positioning.

Difficulties in interpretation

False negative and false positive findings can occur in pregnancy because of altered anatomy and physiology. For example, haemodynamic circulation causing difficulties in pulmonary artery opacification on CTPA,23 and displacement or compression of structures by the gravid uterus can make visualising suspected acute abdominal pathology on ultrasound difficult.24

Incidentalomas

As with all patients, imaging presents a risk of detecting incidental findings, which may lead to further investigations and may increase patient anxiety unnecessarily.25

Is written consent needed?

Consent for imaging processes vary. The ACR suggests that consent can be obtained in both verbal and written forms but in either case should be documented. ACR offers a sample patient consent form in its guidelines.29

Trauma

A 27 year old woman is brought to the emergency department by ambulance following a high speed road traffic collision. She is 30 weeks pregnant.

She has a Glasgow coma scale score of 15, pulse rate 125 beats/min, blood pressure 120/70 mm Hg, and respiratory rate 25 breaths/min.

She has pain in the left lower chest wall and left flank and left flank guarding and bruising. She says her baby is moving normally and the obstetric team has no immediate concern about the fetus. The surgical team is called and imaging is considered. The patient wants to know about risks to her baby from CT scanning.

Trauma is the leading non-obstetric cause of maternal mortality and can also cause fetal loss, so early identification of injury is important.30 Consider radiology when the risk from trauma is likely to be higher than the risk from imaging.

Chest radiography and focused assessment with sonography in trauma may help to rule in some injuries (eg, pneumothorax, large volume haemoperitoneum) and expose the fetus to minimal or no radiation. However, these modalities may not rule out potentially life threatening injury, which could put the patient and fetus at risk (eg, active bleeding, organ injury, fractures).

When potentially life threatening thoracic or abdominal injury is suspected, CT scan is the imaging modality of choice and intravenous (IV) contrast is usually required (fig 1).

Fig 1

CT scan of the abdomen and pelvis (with IV contrast) of a pregnant patient involved in a high speed road traffic accident. (a) Axial image (soft tissue windowing) showing the gravid uterus and anterior placenta (arrows), (b) axial image (bone windowing, arrows), (c) coronal image (bone windowing) showing a right sacral fracture (arrows)

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Discuss with the patient the risk of missing life threatening pathology versus the relatively small risk of childhood cancer induction from CT. Table 2 gives approximate risks (note, doses, and risk in table 2 are for single scans, but trauma protocols can involve dual phase scanning31). Offer reassurance that the fetus is not at definitive risk from any IV contrast administration, but because of the theoretical risk of hypothyroidism, neonatal heel prick is advised (table 1).

Once life threatening injury to the patient is excluded, consider obstetric ultrasound to look for fetal, placental, and uterine injury.30

Headache

A 35 year old woman attends an out-of-hours GP service with headache and vomiting that started suddenly, 24 hours earlier. She is 17 weeks pregnant and has no headache history. She has diplopia but no fever or meningism. Vital observations and urine dipstick are unremarkable. The GP recommends urgent assessment in the emergency department and advises that this will likely include imaging. The patient wishes to discuss imaging options.

Headache in pregnancy is common and usually the result of primary headache disorders (eg, tension headache or migraine).

After 20 weeks’ gestation, pre-eclampsia is also a common cause and associated with hypertension with or without proteinuria.32

Consider urgent radiological investigations if there is clinical suspicion of a life threatening diagnosis (eg, subarachnoid haemorrhage, venous sinus thrombosis).

CT is quick, widely available, can readily detect acute haemorrhage, and post contrast imaging is possible. MRI is more sensitive than CT for detecting most intracranial pathology; however, it is slower, less readily available, more prone to artefact, and post contrast imaging (using gadolinium) is generally not recommended in pregnancy1017 (table 1).

If CT is clinically indicated and would be the usual first choice in a non-pregnant patient, the authors recommend not delaying CT (ie, avoid considering MRI as first line purely on the basis of fetal risk).

Advise the patient that the radiation dose to the fetus from CT scanning of the head is negligible (table 2).11

If CT is negative, further investigation with MRI, post contrast CT, or lumbar puncture may be required, depending on the suspected diagnosis.

Suspected pulmonary embolism

A 32 year old woman attends the emergency department following sudden onset chest pain and dyspnoea six hours earlier. She is 25 weeks pregnant. She has no relevant medical history, fever, cough, or alteration in her sense of taste or smell.

Her pulse rate is 105 beats/min and her respiratory rate 18 breaths/min. Other observations, examination, electrocardiogram, routine blood tests, and chest radiograph are normal. The clinical team suspects pulmonary embolism, and imaging is discussed. The patient is concerned about implications for her baby. She asks, “What difference will a scan make if I have a blood clot in my lungs?”

The risk of missing pulmonary embolism or of anticoagulating a patient without pulmonary embolism puts the patient (and therefore fetus) at risk of illness or death, so imaging is recommended.35

If the patient has clinical signs of deep vein thrombosis (DVT), perform lower limb vein ultrasound.3637 In the absence of signs of DVT, guidelines for imaging of suspected pulmonary embolism in pregnancy are conflicting.38 Some suggest lower limb vein ultrasound to avoid ionising radiation. Others advise doing this only if the patient also has clinical signs of DVT.

Direct imaging is most commonly performed with CTPA (fig 2) or VQ scanning. Choosing between these studies is controversial and varies between institutions because each modality has relative risks and benefits (table 3).

Fig 2

CTPA in a patient in the first trimester of pregnancy showing acute pulmonary embolism in the left pulmonary artery (arrow). Pregnancy makes the breast tissue dense and glandular (ie, at increased risk from ionising radiation)

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Reassure the patient that whichever modality is used, the risks of harm to the pregnant woman and fetus are likely to be less than those of missing a pulmonary embolism.

Acute abdominal pain

A 30 year old woman attends the emergency department with right iliac fossa pain which started in the mid abdomen. She is 30 weeks pregnant. On examination, she has tenderness and guarding in the right iliac fossa. Blood tests show elevated white cell count and C reactive protein levels. Urine dipstick is normal. Acute appendicitis and adnexal torsion are considered the most likely differential diagnoses. The patient asks, “Do I need an MRI? Isn’t ultrasound good enough?”

Early diagnosis of acute abdominal or pelvic pathology can prevent maternal and fetal harm. The risk of not treating some conditions (eg, acute appendicitis or adnexal torsion) while difficult to quantify, is likely to be higher than the risk of imaging.

Ultrasound is safe in pregnancy. It is often the first modality used for abdominal and pelvic pain in a non-emergency situation. However, its usefulness is dependent on the skill or experience of the operator and views of deep structures can be poor owing to anatomical displacement. Ultrasound for acute appendicitis has 50-100% sensitivity and 33-92% specificity in pregnant patients.42

MRI (fig 3) is potentially more accurate42—eg, for acute appendicitis, sensitivity is 92% and specificity 98%.43 It is, however, more time consuming and not as readily available as ultrasound, particularly out of hours.

Fig 3

MRI in a pregnant patient showing right adnexal torsion. (a) and (b) Coronal balanced fast field echo images, (c) axial T2 weighted images. The patient has a right sided abdominal mass with a thick irregular wall and central fluid signal (large arrow in (a)) and abdominal free fluid (arrowheads in (c)), both of which had been detected on ultrasound. The known intrauterine pregnancy is also shown (small arrow (a)). An additional finding on MRI, not detected on ultrasound, was the twisted adnexal pedicle (arrows in (b) and (c)) connecting the uterus to the mass, confirming the diagnosis of adnexal torsion for which surgery in pregnancy was required

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If MRI is being considered, advise patients that there is no known harm to the fetus (table 1) but that scan times are at least 20 minutes (potentially longer) and scanners are small, which may be uncomfortable. Avoid IV contrast (table 1).

If MRI is not available, CT can be considered but involves pelvic irradiation, so only if there is no alternative. The risks are discussed above, under “Trauma.”