Letter From the Guest Editors: Multimodality Imaging of the Pregnant Patient

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It is well recognized that radiation exposure during pregnancy carries significant risk to the fetus. Many factors must be taken into consideration when faced with a pregnant patient who requires medical imaging. The risk/benefit ratio to the mother must always outweigh the risk of radiation exposure to the fetus. Once the benefit of imaging has been determined to outweigh fetal risk, careful thought must be given to the appropriate imaging modality to be used, as well as to the mandatory employment of radiation dose reduction techniques.

Where possible, ultrasound and magnetic resonance imaging (MRI) should be the primary imaging modalities used to evaluate the pregnant patient. There are several benefits of both of these imaging technologies, the most important of which is the lack of ionizing radiation. Ultrasound is readily available, cost-effective, and portable. It may be performed at the bedside for unstable or immobile patients and is a rapid screening and diagnostic tool in this patient population. Ultrasound plays a significant role in the evaluation of the newly confirmed pregnancy, particularly when there is concern for ectopic gestation or missed/threatened abortion. In second- and third-trimester pregnancy, ultrasound is the imaging modality of choice for evaluation of fetal anomalies and dating, as well as for assessment of placental abruption in symptomatic women. Beyond its obstetrical role, ultrasound is a valuable tool in the assessment of the pregnant patient presenting with abdominal or pelvic pain. Evaluation or surveillance of adnexal masses is typically performed with ultrasound imaging; MRI may be required if a confident diagnosis cannot be made with ultrasound. Imaging is often requested to evaluate the etiology of acute right lower quadrant pain, with the main diagnostic considerations including appendicitis and ovarian torsion. Ultrasound has a very high sensitivity and specificity for the identification of ovarian torsion; however, it is less reliable and reproducible in the identification of appendiceal pathology. This is typically because of the variable location of the appendix in pregnancy due to displacement of viscera by the gravid uterus.

The role of ultrasound in the evaluation of abdominal pain is well established, particularly for cholelithiasis, cholecystitis, and renal obstruction. Conventional ultrasound imaging techniques are used for the evaluation of abdominal pain in pregnancy. There is an additional role for ultrasound abdominal imaging in pregnancy, that being its use in the evaluation of the pregnant trauma patient. Although computed tomography remains the gold standard for trauma imaging, ultrasound and MRI have limited roles in detection of solid organ injury. Discussions must occur between the radiologist and the trauma team to determine if ultrasound or MRI is adequate for patient evaluation or if the benefit of computed tomography imaging to the mother outweighs the risk to the fetus.

MRI has played an emerging role in the evaluation of acute abdominal and pelvic abnormalities in pregnancy, as well as in the work up of nonacute findings identified on physical examination or on other imaging modalities (ie, ultrasound). MRI for the evaluation of appendicitis has come to the forefront in recent years and is the imaging study of choice for patients in whom appendicitis cannot be confirmed on clinical grounds or by ultrasound imaging. MRI for appendicitis is rapid, has no risk of radiation exposure, and does not require the administration of intravenous contrast agents. There are no proven adverse effects to the fetus, although attention must be paid to the specific absorption rate, which may cause fetal heating effects. MRI is also useful in the evaluation of adnexal masses, both for characterization and for surveillance of size during pregnancy and risk of obstructed delivery. MRI findings of ovarian torsion and ectopic pregnancy have been reportedly identified in small trials.

One groundbreaking application of MRI over the last decade is in fetal imaging. Fetal anatomic screening is performed with ultrasound at 18-20 weeks of gestation. Ultrasound is considered the gold standard for evaluation of fetal structural anomalies; however, it may not always be definitive in elucidation of the abnormalities in all cases. It is in these cases that MRI has the largest role, particularly in evaluation of fetal brain anomalies. There has been increasing use in the evaluation of lung, solid organ, and celomic cavity abnormalities.

When ionizing radiation must be used in the evaluation of maternal medical conditions, the individual performing/interpreting the examination should be very familiar with the fetal risks. A thorough discussion of fetal radiation exposure risks and the benefits of the procedure should occur with the mother and written informed consent should be obtained. Consideration for different modalities and their inherent risks should be given, and an educated decision should be made regarding which modality is best suited to make the diagnosis at the lowest radiation risk to the mother and fetus. This is particularly the case in the decision of nuclear medicine ventilation-perfusion imaging versus computed tomographic angiography for the evaluation of pulmonary embolism.

This volume consists of articles chosen to discuss many of the issues noted above, including fetal radiation risks, imaging of the acute abdomen and the trauma patient, and imaging of pregnancy-related neurologic emergencies. Topics also include imaging of breast diseases in pregnancy and the postpartum period and fetal MRI.