Whenever a fetal abnormality is identiied, indings from the afected organ and other organ systems should be thoroughly characterized. Accordingly, a fetal anatomical survey is generally completed during each MR examination. In a recent prospective study, nearly 95 percent of the anatomical components recommended by the Inter- national Society of Ultrasound in Obstetrics and Gynecology were visible at 30 weeks' gestation (Millischer, 2013). he aorta and pulmonary artery were the most diicult to evaluate. Zaretsky and coworkers (2003a) similarly found that with the exclusion of cardiac structures, fetal anatomical evaluation was possible in 99 percent of cases.
For intracranial anomalies, very fast T2-weighted images produce excellent tissue contrast, and eSF-containing structures are hyperintense or bright. This allows exquisite detail of the posterior fossa, midline structures, and cerebral cortex. T1-weighted images are used to identiy hemorrhage.
eNS biometry obtained with MR imaging is comparable with that obtained using sonography (Twickler, 2002). Nomograms have been published for multiple intracranial structures, including corpus callosum and cerebellar vermis lengths (Garel, 2004; Tilea, 2009).
MR imaging provides valuable added information for cerebral abnormalities suspected sonographically (Benacerraf, 2007; Li, 2012). In early studies, MR imaging changed the diagnosis in 40 to 50 percent of cases and afected management in 15 to 50 percent (Levine, 1999b; Simon, 2000; Twickler, 2003). Additional information is more likely to be gained when the examination is performed beyond 24 weeks' gestation. More recently, Griiths and associates (2017) reported that MR evaluation of suspected fetal brain anomalies identified additional indings in nearly 50 percent and changed prognosis in 20 percent.
Fetuses with a cerebral abnormality may have a signiicant lag in cortical development. Levine and colleagues (l999a) demonstrated that MR imaging accurately portrays cerebral gyration and sulcation patterns (Fig. 10-54). Sonography permits limited evaluation of subtle migrational abnormalities, and MR imaging provides greater accuracy, particularly later in gestation.
For ventriculomegaly, fetal MR imaging is selected to help identiy associated underlying eNS dysmorphology (p. 193). In cases of septo-optic dysplasia, MR imaging may confirm absence of the septum pellucidum and display hypoplastic
FIGURE 10-54 Axial images of the fetal brain at 23 weeks' gestation (A) and at 33 weeks (8) demonstrate the normal gyration and sulcation progression during fetal development. These images were obtained using a Half Fourier Acquisition Single Shot Turbo Spin Echo (HASTE) sequence because it is relatively motion insensitive.
optic tracts (Fig. 10-55). In other fetuses, MR imaging can also assist with identiying agenesis or dysgenesis of the corpus callosum and characterizing migrational abnormalities (Benacerraf, 2007; Li, 2012; Twickler, 2003).
Another fetal MR imaging indication is evaluation of suspected intraventricular hemorrhage (IVH). Fetal IVH risk factors include atypical-appearing ventriculomegaly, neonatal alloimmune thrombocytopenia, and a monochorionic multifetal gestation complicated by demise of one fetus or by severe twin-twin transu