Pictorial essay: MRI of the fetal brain

MRI is a useful supplement to USG for the assessment of fetal brain malformations. Superior soft tissue contrast and the ability to depict sulcation and myelination are the strengths of MRI. Subtle or inconclusive USG abnormalities can be confirmed or ruled out by MRI. In some cases, additional findings detected with MRI often help in arriving at a definitive diagnosis, which is necessary for parental counseling and for guiding management. Fast T2W sequences form the basis of fetal MRI. There have been no reports of deleterious effects of MRI on the fetus. A few case examples are presented to illustrate the advantages of MRI.

needs to consider the natural history of the fetal disorder in question.For example, in the case of fetal cytomegalovirus infection and tuberous sclerosis, the cranial fi ndings may become apparent only in the third trimester and MRI done too early may do more harm than good.Since most USG abnormalities are generally detected in an anomaly scan done at around 20 weeks of gestation, most cases are referred for MRI around the same time.

Safety issues
Many studies have shown that fetal MRI examination is not associated with any major deleterious eff ects. [7]No health risks have been reported at fi eld strength of 1.5-Tesla (1.5-T).No adverse outcomes have been observed in pregnant MRI workers. [8]The Safety Committ ee of the Society for MRI -has an advantage, -Is at a disadvantage has concluded that prenatal MRI is indicated when other nonionizing diagnostic imaging methods are inadequate or when MRI examination can provide important information that would otherwise require the use of ionizing radiation. [9]There is currently no data regarding the level of acoustic noise experienced by the fetus during the MRI procedure. [10,11][14] The National Radiation Protection Board and the Food and Drug Administration have approved MRI only aft er the fi rst trimester. [15,16]The safety of the newer techniques of diff usion-weighted imaging (DWI), diff usion tensor imaging (DTI) , MRI spectroscopy (MRS), and functional MRI (F-MRI) has not yet been proven. [17]

MRI technique
Informed consent from the mother has to be obtained before the procedure.Fetal MRI is performed on a high-fi eld strength MRI scanner (1.5-T); a phased-array surface coil is placed over the abdomen and pelvis, with the patient in the supine or lateral decubitus position.Polyhydramnios and   multiple gestations may increase the distance between the region of interest and the surface coil, resulting in a reduction in the signal strength. [18,19]Maternal premedication has been used to achieve fetal sedation, although the fast sequences available on the newer scanners obviate the need for sedation.Maternal breathholding during sequences is desirable. [5,20]tal motion signifi cantly degrades image quality.[23][24] Such fast acquisition sequences provide good contrast and spatial resolution and are suitable for surface delineation, sulcational analysis, and biometry [Figure 1].Studies mention a lag of one week of the brain development compared to the neuroanatomic fi ndings. [25]Gradient-echo T2W images (T2*) are accurate in the detection of chronic hemorrhagic lesions and calcifications [5] [Figure 2].T1W images are well suited for demonstrating small hyperintense lesions like tubers, calcifi cation, lipomas, and laminar necrosis [21,[26][27][28][29] [Figure 3].[32] An initial fast localizing sequence is performed; sequences are then acquired in orthogonal planes relative to the immediately preceding plane.Imaging planes are chosen to represent sections relative to the fetus. [5,19] normally use the following sequences [Table 2] on our 1.5-TMRI (Avanto, Siemens, Erlangen, Germany) unit:

Indications and case studies
MRI is indicated in situations when a brain anomaly has been detected on USG, but the diagnosis is not obvious or defi nite and needs confi rmation.
Marginal ventriculomegaly is one such instance.MRI may detect the presence of heterotopia or sulcational disorders, which have profound prognostic and counseling implications.In the case illustrated in Figure 4, the presence of fetal cardiac rhabdomyoma and unilateral marginal ventriculomegaly prompted the MRI examination.Multiple subependymal tubers were demonstrated on MRI, thus clinching the diagnosis of tuberous sclerosis.Bilateral  ventriculomegaly, especially of the occipital horns, was the indication for MRI in the case illustrated in Figure 5. Here, MRI clearly demonstrates bilateral, symmetric, neuroparenchymal loss in the parieto-occipital watershed regions as against the USG possibility of cleft ing; these fi ndings suggest an ischemic etiopathogenesis rather than a neuronal migrational disorder such as schizencephaly.MRI demonstrated the extent, location, symmetry, and morphology of the defects bett er than USG.In the case illustrated in Figure 6, USG detection of ventriculomegaly, periventricular calcifi cation, cortical thinning, and cerebellar hypoplasia led to a referral for MRI.MRI confi rmed the severity of cerebral and cerebellar atrophy but failed to demonstrate calcification.Congenital cytomegalovirus infection was then considered.
Uncertain fi ndings on USG can by supplemented by an MRI examination.In the case described in Figure 7, USG detected inferior vermian agenesis and postaxial polydactyly.Fetal MRI was done specifi cally to look for the 'molar tooth sign' which, however, was not seen.Postnatally, at 3 months of age, this infant presented with the clinical features of Joubert's syndrome.A repeat MRI demonstrated the presence of the 'molar tooth sign,' thus confi rming the diagnosis of Joubert's syndrome.This is an instance of a disease that evolved with the passage of time.
In some instances, overt intracranial fi ndings on USG may necessitate MRI examination as a second confi rmatory tool.The example of a case of USG-detected frontal horn fusion with agenesis of septum pellucidum is illustrated in Figure 9. Here, MRI confi rmed the USG fi ndings and also revealed an indistinct optic chiasma and possible hypophyseal hypoplasia.This clinched the diagnosis of septo-optic dysplasia.
MRI can confirm callosal agenesis or dysgenesis in cases suspected by USG.Additionally, MRI may also demonstrate heterotopia.USG-detected fetal intracranial tumors can be characterized by MRI 34 .When USG is unable to diff erentiate between fetal intracranial bleed and tumor, MRI can help resolve the issue. [35]tal MRI is increasingly being used as a means of surveillance in situations where fetal brain lesions are anticipated.An example of such a situation is the monitoring of the surviving twin after the co-twin's demise in monochorionic twinning. [36]MRI is also indicated when there is a history of a previous child with a genetic disorder, e.g., Miller-Dieker syndrome. [37]I examination of the fetal brain requires extensive training.As with USG examination, an MRI examination should also be performed in a systematic manner.The USG fi ndings and the history should be known to the radiologist.It is best if the radiologist performing the MRI is provided with the questions that have to be specifi cally answered.[32][33] The ability to accurately measure cerebral and bone biparietal diameters enables quantifi cation of any fetal cerebral neuroparenchymal loss.Callosal length and vermian height and area are measurements that MRI can provide with ease, though with the advent of three-dimensional studies these measurements may also be assessed by USG.A detailed discussion of sulcation and myelination analysis is beyond the scope of this article.
Newer techniques such as DWI, DTI, MRS, and F-MRI are being tried out.DWI may be useful to study hypoxia and ischemia aff ecting the fetal brain [36] and DTI for studying the development of neural tracts like the corpus callosum, the optic tracts, and the anterior commisure. [38]MRS has the potential to be useful in the evaluation of myelination, ischemia, hemorrhage, metabolic variations in brain damage and inborn errors of metabolism. [39]tal MRI is a powerful supplement to USG and enables us to demonstrate fi ndings that cannot be recognized on USG.Appropriate integration of fetal MRI into the prenatal evaluation algorithm can improve decisionmaking and patient care.Clinching of a diagnosis enables cross-specialty consultation amongst radiologists, obstetricians, neuropediatricians, and geneticists; this allows more targeted and meaningful counseling and management strategies.

Figure 3 :
Figure 3: Single live gestation at 30 weeks.T1W axial MRI image at the level of the lateral ventricles shows tubers as subependymal hyperintense nodules (arrows).

Figure 4 (
Figure 4 (A-D): Single live gestation at 37 weeks.USG scans of the heart (A) and brain (B) show a cardiac rhabdomyoma (arrow in A) and mild ventriculomegaly (arrow in B).Axial (C) and coronal (D) T2W MRI images show subependymal tubers (arrowheads), confi rming the diagnosis of tuberous sclerosis.The large left lateral ventricular subependymal nodule at the foramen of Monroe is probably responsible for the unilateral ventriculomegaly.The infant developed ash-leaf macules at 1 month and myoclonic jerks at 2 months of age.It died at 3 months (cradle death).

Figure 5 (Figure 6 (
Figure 5 (A-F): Single live gestation at 24 weeks.USG images (A-C) in different planes show ventriculomegaly with prominent occipital horns (arrows) and widened subarachnoid spaces.The possibilities of neuronal migrational and an atrophic disorder were considered.T2W axial (D), coronal (E), and sagittal (F) MRI images show bilateral symmetric total neuroparenchymal loss in the parieto-occipital watershed regions (arrowheads) but no clefting; this is suggestive of an ischemic etiopathogenesis rather than a neuronal migrational disorder.The couple opted for termination of pregnancy.They declined an autopsy

Figure 7 (Figure 8 (Figure 9 (
Figure 7 (A-D): Single live gestation at 28 weeks.USG images (A,B) show vermian agenesis (arrow in A) and postaxial polydactyly (arrow in B).T1W axial image (C) does not show the 'molar tooth sign' (arrowhead).Postnatal T1W axial MRI image at 4 months shows the 'molar tooth sign' (arrowhead), confi rming the diagnosis of Joubert's syndrome.In this case, the mother noticed abnormal eye movements and failure of fi xation of vision by the infant at 3 months of age