Figure 8. Fetal MRI: Brain Development. (A) and (B) Sagittal and axial T2W HASTE images of the brain in a fetus obtained during the third trimester demonstrate normal intracranial anatomy. (C) and (D) MR images obtained in a different third-trimester fetus with questionable abnormal ultrasound demonstrate absence of the corpus callosum and associated ventriculomegaly (arrows).
Figure 9. Congenital Diaphragmatic Hernia: Prenatal Diagnosis with US and MRI. (A) Ultrasound image obtained at the level of the fetal thorax demonstrates displacement of the fetal heart to the right (arrow) and the presence of bowel loops in the fetal chest (arrowhead). This was the patient’s initial prenatal sonogram performed at 38 weeks gestational age as the patient had received no prenatal care. A congenital diaphragmatic hernia was diagnosed. (B) In order to further characterize the hernia and assist in preoperative planning, MRI was performed. Axial T2W TrueFISP image through the fetal thorax again demonstrates that the heart (arrow) is displaced to the right. The left hemithorax is filled with loops of bowel (arrowheads). (C) Coronal T2W HASTE image demonstrates all of the fetal bowel (arrow) located in the thorax. The hernia was repaired immediately following delivery and the patient did well postoperatively.
THE ADNEXA
MRI serves as a useful adjunct to ultrasound for diagnosis of the problematic adnexal mass, largely due to its ability to provide tissue characterization. Fat-containing lesions, such as a mature cystic teratoma, can be diagnosed using fat-suppression techniques (FIGURE 6). Lesions containing blood or blood products, such as hemorrhagic ovarian cysts or endometriomas, may also be identified based on signal intensity characteristics. Gadolinium is used to identify solid, enhancing elements, indicative of high malignancy risk (FIGURE 7).
Figure 10. Appendicitis in Pregnancy. A pregnant patient
presented at 20 weeks gestational age with right lower quadrant pain.
Ultrasound findings were equivocal for acute appendicitis as the appendix
could not be definitively visualized. Coronal T2W HASTE images demonstrate
a dilated, fluid-filled appendix (arrow) located medial to the cecum (C). The
diagnosis of acute appendicitis was made and confirmed at surgery.
THE PREGNANT PATIENT
Ultrafast MR imaging sequences provide subsecond image acquisition times which enable evaluation for fetal abnormalities, since degradation of image quality related to fetal motion is minimized. The advantages of MRI in the setting of complex abnormalities of the fetal central nervous system and thorax, and MRI’s impact on pregnancy management and postnatal planning, have been demonstrated in numerous studies (FIGURE 8). Furthermore, MRI can be a very helpful problem-solving tool when ultrasound assessment is limited, such as in the setting of oligohydramnios, unfavorable fetal position or maternal body habitus, and advanced gestational age (FIGURE 9). MRI can also be used to confirm placental location, or to evaluate for placental implantation abnormalities, such as placenta accreta, when ultrasound is not definitive. In many circumstances, MRI of the fetus is a collaborative effort involving various subspecialties, including body imaging, neuroradiology, and Women’s Imaging. The use of MRI in the evaluation of non-pregnancy-related complications in the pregnant patient is also expanding. MRI is an important adjunct to ultrasound and an alternative to CT in the assessment of acute appendicitis in pregnancy (FIGURE 10). Other applications include characterization of adnexal masses and evaluation of urolithiasis and choledocholithiasis (FIGURE 11). At this time, there are no known adverse effects on the fetus resulting from in utero exposure to MR. MRI is performed on the pregnant patient when the potential benefits are felt to outweigh any theoretical risk, and informed consent is obtained. Gadolinium is never administered, as its safety during pregnancy is not established.
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