The most common indication of the fetal MR evaluation is for suspected central nervous system abnormalities on sonographic evaluation and accounts for approximately 70% of our practice. MR images of the fetal CNS are hypothetically superior to that of ultrasound because of the improved resolution. Ultrasonography remains the modality of choice for diagnosing CNS abnormalities. However, MR is a very useful adjunct in the antenatal diagnosis of some suspected CNS anomalies. Fast T2-weighted images produce excellent tissue contrast, and cerebrospinal fluid–containing structures are bright. This allows exquisite detail of the posterior fossa, midline structures, and cortex. Near-field attenuation secondary to the fetal skull on ultrasonography is not encountered with MR and allows the easy determination of bilateralism of CNS lesions.
Another major advantage of MR is the ability to acquire images in the axial, coronal, and sagittal planes in reference to the fetus or the maternal pelvis. Sagittal fetal images are very helpful, for example, in evaluating the corpus callosum in cases referred for mild ventriculomegaly (Figure 46-3). T1-weighted images are also used to assess whether a CNS lesion contains fat or represents hemorrhage (Figure 46-4).
Callosal dysgenesis at 21 weeks. A: Axial image with the "tear-drop" appearance of the laterally displaced frontal horns (white arrows) and colpocepahly (yellow arrows). B: No corpus callosum is identified (arrows) bridging the frontal horns in the coronal plane. C: Sagittal image shows absence of the corpus callosum.
Intraventricular hemorrhage at 31 weeks. A: T2-weighted image reveals areas of low and inhomogeneous signal of the choroid plexus (arrows), which correspond to (B) bright signal on T1-weighted images (arrows) and represent early subacute blood.
Similar to sonographic scans, CNS biometry can be measured and is routinely done in our cases. Routine measurements include biparietal diameter, occipital frontal distance, cerebellar width, cisterna magna depth, and bilateral atrial measurements.12,16,17 Ventricle and cisterna magna measurements on 60 fetuses beyond 14 weeks to term gestation found atrial measurements to be slightly smaller on MR compared with ultrasonography, but determined the abnormal sonographic cutoff of greater than 10 mm was greater than 2 SD above the mean on MR (Figure 46-5).16 Cisterna magna measurements were gestational age dependent with cutoff values less than 4 and greater than 11, also similar to ultrasonography (Figure 46-6). Nomograms for multiple components of brain biometry, including the corpus callosum length and cerebellar vermis, have also been published.13,18 The measurements of corpus callosum and vermis lengths are routinely done on all MR studies of the central nervous system (see Figure 46-1C).
Ventriculomegaly at 22 weeks. Coronal (A) and axial (B) images show measurements of the enlarged lateral ventricles (yellow lines). Measurements are similar in both planes.
Large cisterna magna at 24 weeks (arrow) with intact vermis.
Cortical maturation can be evaluated by MR images, and cerebral gyration and sulcation patterns reflect the embryologic development.19,20 Fetuses with a CNS abnormality may have significant lag time in cortical development.19 Ultrasonography is very limited in evaluating very subtle migrational abnormalities at early gestational age. These migrational CNS disorders such as Walker-Warburg are problematic, but MR is becoming more reliable in assessing for cortical maturation, especially later in gestation (Figure 46-7).20,21, and 22
Fetus at 29 weeks with sonographic suspicion of Dandy-Walker malformation. A: Axial MR and (B) US images of enlarged ventricle (yellow markers), with MR demonstration of extensive lissencephaly. C: Axial sonographic scan of the absent inferior vermis (arrow) and large cisterna magna. D: Sagittal MR demonstrates the communication of the cisterna magna with the fourth ventricle as well as the abnormalities of the brain stem classic for Walker-Warburg.
Four large studies assessed the second-opinion MR examination in the setting of a suspected CNS abnormality demonstrated on ultrasonography in regard to its ability to confirm, change the diagnosis, and possibly alter clinical management. In the first study with 66 cases of abnormalities found on confirmatory sonogram, MR added additional information in 57.6% of cases, and changed the diagnosis in 40% of cases.23 Clinical management was clearly changed in 9 cases. In 73 MR examinations performed for CNS anomalies, another group found that 46% of pregnancies were managed differently than the way they would have been managed if the diagnosis had been based only on a sonographic basis.24 Additional information was provided in 46 of 72 pregnancies (64%), and the diagnosis was changed in 20 of those 46 cases. Clinical management was altered in 8 cases. MR was more likely to confirm sonographic diagnosis prior to 24 weeks, but beyond 24 weeks a change of diagnosis or additional information was gleaned more frequently.25 A British study confirmed the findings of the other studies, where MR imaging either changed the diagnosis or gave additional information that altered management in 35 of 100 cases.26
Presently, the most common reason for fetal MR referral is isolated ventriculomegaly, which may be associated as a highly variable outcome.27,28,29, and 30 Additional abnormalities commonly associated with ventriculomegaly may not be defined on sonographic examination. The most common change in diagnosis was from marked ventriculomegaly to a more precise diagnostic category, such as aqueductal stenosis or hydrancephaly (Figure 46-8), and from mild ventriculomegaly to more specific diagnoses, including agenesis of the corpus callosum and migrational abnormalities (Figure 46-9). A more exact diagnosis impacts patient counseling and, to a lesser degree, clinical management.25,26,27,28,29,30, and 31 This finding was confirmed in a recent paper, which found that MR imaging provided important information in the setting of ventriculomegaly, especially in fetuses with other findings of the central nervous system by sonographic evaluation.29 At the present time there is considerable variability in the central nervous system diagnosis in the setting of ventriculomegaly secondary to modality differences (ultrasonography or MR) and observational errors. Agreement was 60% with sonography and 53% with MR in this series.31
Axial (A), sagittal (B), and coronal (C) images of hydranencephaly at 33 weeks' gestation with minimal cortical development it the region of the thalami (arrows). Absence of calvarial cortex in all 3 planes confirmed the diagnosis.
coronal (B and C) images of a fetus with ventriculomegaly on US reveals multiple abnormal sulcations and gyrations best characterized as polymicrogyria with associated ex-vacuo ventricular and cavum vergae enlargement.
MR evaluation of the fetal spine has been performed. In one series additional spinal cord abnormalities were seen on 10% of cases in the setting of sonographically detected bony anomalies of the spine.32 Two of the 3 defects were diastematomyelia, and one was in the setting of lumbosacral myelomeningocele. The third was a segmental spinal dysgenesis.