The ultrafast T2-weighted sequence is the recognized workhorse in fetal MRI [13], allowing excellent visualization of the fetal anatomy, especially cerebral anatomy. T2-weighted imaging allows optimization of tissue contrast in the fetus, due to the large volume of water in the fetus and adjacent amniotic cavity. Anatomic detail of the fetal brain, fluid-filled cavities, lungs, placenta, and fetal profile is consistently strong [15]. SSFSE allows image acquisition during a single TR interval with a total acquisition time of less than 2 s per image [16]. Images should be acquired with a 1–2 s time delay between each image to prevent saturation effects that will degrade the signal-to-noise ratio (SNR). Alternatively, the slices can be acquired in an interleaved fashion with a gap equal to the slice thickness to minimize the potential signal loss from bulk-fluid motion artifacts and cross talk. In neuroimaging, longer TEs allow better discrimination of the grey-white matter interface in the fetus since the T2 contrast increases with increasing TE times [17].

Fast imaging with balanced steady-state free precession (SSFP) provides images with T2/T1 contrast weighting with high temporal resolution [18–21]. These sequences are especially useful in demonstrating vasculature and fluid-filled cavities, specifically those surrounded by dense tissues, and these sequences can be used to visualize the fetal heart chambers [15]. Evaluation of the umbilical cord and its insertion may also be enhanced with this sequence.

SSFP sequences are obtained in a wide field of view, allowing visualization of the maternal abdominal anatomy and the uterus/placenta. Large field of view imaging limits usefulness of SSFP sequences in fetal brain imaging in early gestation [15].

SSFP provides similar image quality to SSFSE for brain imaging in the second trimester; however, axonal migration in the third trimester is best depicted with SSFSE [19]. SSFP is inferior to SSFSE in assessing the fetal lungs prior to 25 weeks gestational age since the lungs demonstrate hyperintense signal only in advanced gestation.

T1-weighted images are acquired using a gradient-echo sequence with breath-hold fast spoiled gradient-echo sequences used most commonly [15]. T1-weighted sequences provide little information over SSFSE sequences but can increase sensitivity for detection of fat, calcification, hemorrhage, or proteinaceous structures. Fat-suppressed T1-weighted images increase the dynamic range of fetal MRI and allow more specific detection of fat and hemorrhage [22].

T1-weighted imaging has proven beneficial when evaluating certain normal structures as well as a few common pathologies. The pituitary and thyroid glands can be seen at 20 weeks [15]. The liver is distinguished by its hyperintense T1 signal due to its high iron affinity, a fact exploited in evaluation for congenital diaphragmatic hernias or abdominal wall defects with displaced liver [23, 24].

Furthermore, the hyperintense T1-weighted appearance of meconium is essential to fetal abdominal imaging, both for position and caliber of the colon and for correlation with expected gestational age [25, 26]. T1-weighted imaging will highlight accumulating fetal adipose tissue as well.

Diffusion imaging is challenging in fetal MRI. Nevertheless, one important use is its sensitivity for detecting ischemic lesions, brain lesions, and regions of placental infarction [27, 28]. Certain diffusion sequences have been used to visualize the maturational processes of the cerebral cortex and premyelinating white matter, but this technique in fetal MRI is still in research [29–32]. Diffusion anisotropy of the white matter tracts increases constantly from premyelinating to myelinating stages, revealing maturing white matter tracts before there are visualized changes on T1- and T2-weighted images [13, 32].

Spectroscopy in fetal MRI has been applied to the older fetus whose head is relatively nonmobile within the maternal pelvis [33, 34]. Inversion recovery sequences may be used to obtain additional information involving the fetal brain, but these are sensitive to hemorrhage [15]. Dynamic fetal imaging allows assessment of fetal swallowing, palatal defects, diaphragm motion, peristalsis, and gross fetal motion [35].

Finally, echoplanar imaging may be considered on occasion. The speed at which echoplanar imaging (EPI) is acquired allows it to overcome typical motion artifacts seen in conventional MRI [36]. EPI has been used to produce ungated fetal cardiac movies, for volumetric measurements and for evaluating hepatic hematopoiesis [37–39]. EPI is unique in depicting the fetal skeleton and cartilaginous epiphysis in the fetus before 27 weeks gestation [15]. The magnetic susceptibility of EPI makes it valuable in detecting hemosiderin and certain placental pathologies [15].

The use of 3 T field strength to perform fetal MRI is currently under investigation. The most important advantage to 3 T imaging would be the gain in SNR, which is especially important in small part imaging [40]. On the other hand, 3 T field strength exaggerates artifacts, such as large field inhomogeneity, magnetic susceptibility, and chemical shift artifacts [40]. Lastly, fetal safety at 3 T, including static field exposure, gradient field switching, and radiofrequency power deposition, is a topic of research [40].

Routine use of gadolinium-based contrast agents is not a standard of care, since their biosafety in pregnancy has not been established [41]. Gadolinium-chelate agents cross the placenta, are filtered by the kidneys, are excreted into the amniotic fluid, and are resorbed by the fetus through the swallowing of amniotic fluid. With the cycle of excretion and reabsorption through swallowing, the biological half-life of gadolinium in the fetus is not known [41]. With the accumulation of gadolinium-chelate agents in the amniotic fluid, there is potential for dissociation of the toxic-free gadolinium ion, which poses a risk for the development of nephrogenic systemic fibrosis (NSF) in the mother or child. If gadolinium-based contrast agent is deemed necessary in a pregnant patient, then the agents with the lowest risk for development of NSF should be used at the lowest dose that allows for diagnosis.

Fetal MRI struggles with artifacts as seen with MRI in other body regions. Motion artifacts, susceptibility, aliasing, partial volume averaging, and Gibbs artifacts are issues at times. Artifact from bulk fluid motion can complicate SSFSE sequence when excited spins in fluid change position with respect to slice and/or spatial encoding gradients before the signal is acquired (Fig. 3.3). If all of the excited fluid remains stationary from excitation to the time of signal acquisition, then the fluid signal will be uniformly hyperintense. Alternatively, if all or a partial volume of the excited fluid moves out of the imaging plane by the time of image acquisition, then the fluid will become hypointense. As long as the fetus is not moving continuously, only one or two adjacent slices should be degraded during a typical SSFSE acquisition.

Fetal MRI interpretation requires correlation with available US images and/or report. The US provides a road map for tailoring the subsequent MRI examination; valuable time in the scanner should focus on specific clinical questions and on anatomy not cleared by US. Also, the US evaluates structures that are often incompletely imaged during the fetal MRI, such as the extremities or digits.

Determining fetal situs may not be intuitive. Confirming situs solitus eliminates numerous pathologies and allows lateralization of asymmetric findings. To determine situs, one must orient the fetus relative to the maternal axis (Fig. 3.4). In relation to the maternal long axis, the fetus can thus be vertex (head directed toward the maternal pelvis) or breech (head directed toward maternal head). If the fetus is breech, then the fetal spine is down, and the fetal abdomen is up on the image, making the fetal lie similar to that of the mother. If the fetus is vertex, the fetal spine is down, and the fetal abdomen is up on the image, creating an opposite viewing orientation of fetal and maternal anatomy. Fetal situs can subsequently be determined for any rotation about the fetal axis with regard to breech or vertex positioning.

US can suggest situs ambiguity, but it may not completely describe all the anomalies that can accompany these syndromes. Furthermore, US may miss a situs abnormality, which may inadvertently be discovered on MRI by virtue of the large field of view imaging [42]. Recent work has shown that fetal MRI can demonstrate situs anomalies at least similarly to US as well as associated malformations that provide important information regarding perinatal management [42].

The entirety of fetal embryology and neuroanatomy is beyond the scope of this chapter. Dedicated textbooks explore the magnitude of the topic, and several references on brain development are available [43–51]. We will highlight key landmarks and developmental milestones in the fetal brain.

Fetal MRI is commonly used to investigate underlying etiology for ventriculomegaly and morphologic brain abnormalities that are incompletely assessed with US. The brain evolves throughout gestation, making evaluation challenging to those unaware of the normal appearance of the anatomy at a specific gestational age [6].

Beginning in the early second trimester, structures that arise from the telencephalon, mesencephalon, and rhombencephalon become visible with the gyral and sulcal pattern developing in a predictable sequence [52].

The early fetal brain is composed predominately of large ventricles with a thin layer of cerebral tissue having a smooth appearance [6]. The choroid plexus, seen as an echogenic structure filling the lateral ventricles on US, is difficult to visualize early in gestation by MRI [6]. At 14 weeks, the interhemispheric fissure separating the cerebral hemispheres is well developed [6]. At approximately 16 weeks, the Sylvian fissure begins to appear, but the cortex does not undergo infolding and opercular formation before 34 weeks gestation. Normal cortical maturation, while predictable and easily visualized on fetal MRI, often lags behind the maturation described in neuroanatomic specimens [6, 43, 48]. Table 3.3 provides a list of the commonly seen and evaluated sulcal developmental landmarks, their neuropathologic appearance, and the gestational week of detectability at MRI [43, 53]. Note that the neuroanatomic information provided in the tables describes the gestational ages at which 25–50 % of the brains demonstrate a specific cortical landmark [6, 54]. On a midsagittal plane, the cingular sulcus appears at 24 weeks gestation, the marginal at 27 weeks gestation, the parieto-occipital at 22 weeks gestation, and the calcarine at 24 weeks gestation. On a lateral sagittal plane, the central sulcus appears at 27 weeks gestation, the postcentral at 28 weeks gestation, and the precentral sulcus at 27 weeks gestation. On an anterior coronal plane, the superior frontal sulcus and the inferior frontal sulcus appear at the 29th week of gestation. On a coronal plane at the level of the third ventricle, the superior and inferior temporal sulci appear at 27 and 33 weeks, respectively. Sulcation continues throughout fetal life, with secondary sulci becoming evident at 32–35 weeks and tertiary sulci subsequently forming [6] (Fig. 3.5). If CNS imaging is necessary in early pregnancy, recent work is available to assess biometric data below 24 gestational weeks [55].

In addition to parenchymal folding, the size and morphology of certain parenchymal structures should be assessed. Tables 3.4, 3.5, 3.6, 3.7, and 3.8 and the associated figures can be used as a reference regarding size and location of measurement. Garel et al. provide reference data regarding additional parenchymal size landmarks [44]. Concomitant with sulcation and gyration, neural migration is occurring. At approximately 5 weeks post conception, the forebrain consists of two layers: a deep neuroepithelial layer (ventricular zone or germinal matrix) and a superficial layer (the preplate) [56]. Through complex radial migration, various layers of neural tissue are added to the developing parenchyma. The migration will give the appearance of distinct bands in the forebrain depending on the imaging time. For example, a multilayered appearance of the forebrain is typical between 23 and 28 gestational weeks [56, 57]. The following five distinct layers during these gestational weeks can be identified (from outside in): cortical plate, subplate zone, subventricular and intermediate zones, periventricular fiber-rich zone, and ventricular zone [57]. As neuronal migration begins, the germinal ventricular zone gradually regresses. By 29 weeks gestation, migration to the developing cortex is almost complete, and the ventricular zone disappears, resulting in the differentiation of only two layers: the cortex and the white matter. Persistent visualization of a multilayered appearance of the forebrain after 29 weeks signifies a migrational abnormality.