An early study using the Cavalieri method to estimate fetal and whole brain volumes in a small cohort of third trimester fetuses [30] described a linear relationship between gestational age and whole brain volume, with a growth rate averaging 2.3 mL/day. This impression was dispelled as soon as newer sequences and image analysis methods, allowing volumetric measurements of smaller regions, were applied to larger cohorts of second and third trimester normal fetuses ([17, 26, 32, 34, 42, 90]). The findings from these studies consistently demonstrate region-specific, nonlinear growth trajectories which can be obtained with manual drawing of regions of interest on 2D images ([32]; Fig. 2.10) as well as automatic segmentation of motion-corrected, 3D-reconstructed MRI scans [90]. In the clinical setting, volumetry was successfully used to describe changes in brain and body volumes in fetuses exposed to cytomegalovirus (CMV) infection in early pregnancy ([38], Fig. 2.10), as well as fetuses with ventriculomegaly [77, 91] and intrauterine growth restriction [19].

Recent studies of fetal cortical development using advanced techniques reiterate the theme of region-specific, nonlinear maturation ([16, 35, 42, 43, 82]). Thus, Hu et al. (2011) provide a regional quantification of cortical shape development reporting faster shape changes in the occipital lobe than in other regions, while findings by Clouchoux et al. [16] demonstrate an exuberant third trimester gyrification process and suggest a nonlinear evolution of sulcal development. The same group also reported on delayed cortical development in fetuses with congenital heart disease [15]. Adaptation of tensor-based morphometry revealed that fetal brain development exhibits a distinct spatial pattern of anisotropic growth, with the most significant changes in the directionality of growth occurring in the cortical plate at major sulci. The authors also report significant directional growth asymmetry in the perisylvian region and the medial frontal lobe of the fetal brain [82].

Regional differences and developmental changes in apparent diffusion coefficients (ADC) of normal fetuses have been the subject of several studies conducted and published during the last decade [7, 8, 10, 11, 39, 64, 83, 88]. All of the published studies report absolute values for ADC in a similar range and detect a trend toward a reduction in ADC with increased GA (Figs. 2.11 and 2.12), which could be explained by progressive myelination. However, the relationship between ADC and GA appears to be region dependent and nonlinear. Thus, in a study of 50 normal fetuses between 19 and 37 weeks’ gestation, ADC values remained constant in the basal ganglia, frontal, parietal, temporal, and occipital white matter and in the centrum semiovale while significant decreases were observed in the cerebellum, pons, and thalamus with advancing gestational age [7]. In the clinical setting, ADC values were reported to be decreased in specific brain regions of hydrocephalic [21] and CMV-infected fetuses [108].

Several groups have demonstrated the ability to obtain useful information on fetal metabolism using H1 spectroscopy (e.g., [37, 55, 56]). Short TEs (~35 ms) are used to detect metabolites such as glutamine, glutamate, glucose, taurine, and lipids, while longer TEs (~140 ms) are useful in the detection of N-acetylaspartate (NAA, considered to be a neuronal marker), choline (Cho, a marker of membrane formation), and creatine (Cr, a marker of mitochondrial activity). Lactate, a marker of anaerobic metabolic, can be detected as well. The relatively long imaging time and large voxel size required for MRS limit the widespread use of this technique in clinical fetal imaging (Fig. 2.13).

Development of the normal fetal brain in utero using MRS has been studied by a number of groups. The size of the voxel necessary to acquire reliable information limits the possibility of regional measurements, so these studies mostly reflect whole brain maturation. With this caveat, levels of choline (Cho), creatine (Cr), myo-inositol (myo-ins), and N-acetylaspartate (NAA) have been more recently measured in utero in fetuses in the age range of 22–41 weeks [28, 29, 99]. Brain maturation was most clearly reflected by increasing levels of the neuronal marker NAA, with a parallel decrease in choline. In clinical populations, Limperopoulos and colleagues found significantly slower increase in the NAA:choline ratio in fetuses with congenital heart disorder. Predictors of lower NAA:choline included diagnosis, absence of antegrade aortic arch flow, and evidence of cerebral lactate [60]. A study of fetuses with intrauterine growth restriction (IUGR, [2, 12, 84]) detected a lactate peak in the brain of the most severely affected IUGR fetus, which was consistent with low oxygen content and high lactic acid concentration in umbilical blood obtained at delivery.

DTI is a relatively new technique, only recently applied to fetal imaging ([44, 49, 52, 54, 69, 71, 81, 86, 97]; Fig. 2.14). It may be used to map and characterize the 3D diffusion of water as a function of spatial location. The diffusion tensor describes the magnitude, the degree of anisotropy, and the orientation of diffusion anisotropy. Estimates of white matter connectivity patterns in the brain from white matter tractography may be obtained using the diffusion anisotropy and the principal diffusion directions. The technique is also highly sensitive to changes at the cellular and microstructural level [81, 86]. Unfortunately, DTI presents an even bigger challenge for in utero fetal imaging relative to other techniques since acquisition times are longer and therefore studies are more susceptible to motion artifacts. Consequently, only a few recent studies provide quantitative data from in utero studies of neuronal pathways. As an example, Kasprian et al. [52] examined a group of fetuses ranging in age from 18 to 37 weeks and reported that only in 40 % of examined fetuses, DTI measurements were robust enough to successfully calculate and visualize bilateral, craniocaudally oriented (mainly sensorimotor), and callosal trajectories in utero. However, the successful studies resulted in a wealth of quantitative information on fiber lengths, ADC, fractional anisotropy, and eigenvalues at different anatomically defined areas with high potential utility in understanding the etiology of neuropsychiatric disorders (e.g., [104]).

MRI allows for the noninvasive imaging of regional activation and functional connectivity. The feasibility of studying fetal brain activity with functional magnetic resonance imaging [96] (fMRI, or BOLD (blood oxygen level-dependent) MRI) was demonstrated by Hykin et al. [45] just before the turn of the century, reporting on fetal brain responses to maternal speech. This was followed by additional studies reporting the detection of responses to various acoustic [23, 47, 48, 70] and visual [22] stimuli, which were detected between 33 and 34 weeks of gestation. Functional connectivity (FC) at rest was subsequently demonstrated in the fetal brain [89, 103] in fetuses from 20 to 36 gestational weeks of age, documenting the presence of bilateral fetal brain FC as well as regional and age-related variation in the strength of FC between homologous cortical brain regions, which increased with advancing gestational age. Sorensen et al. [95] examined the BOLD response in the fetus and placenta following maternal hyperoxia, demonstrating an increased oxygenation in a number of fetal organs and in the placenta while oxygenation of the fetal brain remained constant. These studies together with findings from other modalities like fetal EEG and MEG [1] are truly revolutionary since unlike information on maturation of brain morphology and microstructure/chemistry which can be obtained postmortem, the development of function can only be studied in vivo.