Fetal MRI of the central nervous system: clinical relevance

Contribution of fetal magnetic resonance imaging in the evaluation of neurosonographically detected cases of isolated mild and moderate cerebral ventriculomegaly

AIM

This study aimed to present the contribution of prenatal magnetic resonance imaging (MRI) in the diagnosis of fetuses that were previously identified as isolated mild and moderate cerebral ventriculomegaly (VM) by ultrasound (US).

METHODS

The data between February 2013 and August 2020 were collected for women who were diagnosed with isolated mild or moderate fetal VM by US and subsequently underwent a fetal MRI.

RESULTS

Among 321 women, 21 (6.5%) had a clinically important additional finding after MRI. Twelve of 276 (4.3%) fetuses with mild VM and 9 of 45 (20%) with moderate VM had turned out to have additional central nervous system abnormalities. Additional findings were detected more in fetuses with moderate VM, mothers with an anterior-located placenta, and mothers with higher body mass indexes (BMIs) with statistical significance (p = 0.001, p = 0.013, p = 0.036, respectively). The most common additional MRI finding was grade 3 or 4 germinal matrix hemorrhage, which was detected in 11 of 21 fetuses (52.3%).

CONCLUSIONS

Considering the countries' health policies, prenatal MRI would contribute mostly to the diagnosis of fetuses with moderate VM, pregnancies with anterior-located placenta, and mothers with high BMIs. According to our data, we believe that MRI will be valuable, especially in the diagnosis of grade 3 and 4 intracranial hemorrhage group.

Two Cases of Craniospinal Rachischisis Totalis: Role of Magnetic Resonance Imaging in Diagnosis and Review of Neural Tube Defects in the Indian Context with Implications for Folate Fortification

Craniospinal rachischisis is a rare and severe form of neural tube defects (NTDs), which is always fatal. It is characterized by anencephaly accompanied by a bony defect of the spine and exposure of neural tissue. We describe the two patients with ultrasonographic and magnetic resonance imaging appearance of craniospinal rachischisis totalis, detected antenatally at 22 and 25 weeks of gestation, and confirmed after termination of pregnancy. The multifactorial etiology of NTDs, with specific reference to folate deficiency, is discussed with possible role of folate fortification in the Indian context.

The Contribution of MRI after Fetal Anomalies Have Been Diagnosed by Ultrasound: Correlation with Postnatal Outcomes

Objective: The aim of this study was to investigate the additional value of fetal magnetic resonance imaging (MRI) in the assessment and management of fetuses with abnormal findings on ultrasound. Methods: A total of 257 patients who had fetal MRI following the ultrasound diagnosis of a fetal anomaly, or were at high risk, were included. The patients were grouped by referral category for fetal MRI. Fetal MRI was compared to ultrasound in the detection of anomalies, i.e. whether additional findings were identified and if this changed diagnosis, prognosis and management during pregnancy. Results: Ultrasound findings were confirmed on fetal MRI in 89% of the cases. Additional findings were seen with MRI in 28% of all patients. The diagnosis changed in 21% and the prognosis in 19% of the cases. Perinatal management changed in 8%. The antenatal findings were confirmed in all cases that had a postmortem examination following termination of pregnancy. In all the pregnancies that continued to delivery and for which the postnatal outcome is known, the findings correlated in 97% of the cases. Conclusion: Fetal MRI provided additional detection of fetal anomalies, leading to a change in diagnosis and prognosis in 19% of the cases. Neonatal and postmortem findings mostly confirmed the fetal MRI diagnosis, suggesting it to be a useful tool for clinical decision making in perinatal management.

Clinical Significance of Prenatal and Postnatal Heavily T2-Weighted Magnetic Resonance Images in Patients with Myelomeningocele

PURPOSE

To compare the utility and limitations of prenatal magnetic resonance (MR) imaging using half-Fourier acquisition single-shot turbo spin-echo (HASTE) with postnatal heavily T2-weighted imaging (hT2WI) for the evaluation of central nervous system abnormalities associated with myelomeningocele (MMC).

METHODS

Sixteen patients with MMC who had undergone pre- and postnatal MR imaging were included in this study. MR imaging, including HASTE, was undertaken in the 3rd trimester, and hT2WI was performed immediately after delivery. The precision with which each could distinguish MMC, hindbrain herniation and ventriculomegaly was compared retrospectively.

RESULTS

The skin defects and MMC sacs were clearly visible on prenatal HASTE images, although it was difficult to identify precisely the level of MMC compared with postnatal hT2WI, in which the detailed anatomical relationships of the spinal cord, neural placode and ventral nerve roots were evident in every case. Hindbrain herniation could be visualized on prenatal HASTE images, although its severity was difficult to evaluate because of the small size of the structures and neck flexion; again, the resolution was superior on postnatal hT2WI. For hydrocephalus, there were no significant differences in the diagnostic precision and ability to grade the severity between pre- and postnatal imaging.

CONCLUSION

Prenatal HASTE imaging permits the diagnosis and understanding of the gross anatomy of MMC and associated hindbrain herniation and ventriculomegaly, but postnatal hT2WI is superior for evaluating detailed anatomy.

Accuracy of neurosonography and MRI in clinical management of fetuses referred with central nervous system abnormalities

OBJECTIVE

To assess the accuracy of expert neurosonography (two- and three-dimensional NSG) in the characterization of major fetal central nervous system (CNS) anomalies seen at a tertiary referral center and to report the differential clinical usefulness of magnetic resonance imaging (MRI) used as a second-line diagnostic procedure in the same cohort.

METHODS

This was a retrospective analysis of all 773 fetuses with confirmed CNS abnormalities referred to our center between 2005 and 2012. The following variables were analyzed: gestational age at NSG and MRI, NSG and MRI diagnoses, indication for MRI (confirmation of NSG findings; diagnostic doubt; search for possible additional brain anomalies), association with other malformations, diagnostic accuracy of NSG vs MRI (no additional clinical value for either MRI or NSG; additional information with clinical/prognostic significance on MRI relative to NSG; additional information with clinical/prognostic significance on NSG relative to MRI, NSG and MRI concordant but incorrect) and final diagnosis, which was made at autopsy or postnatal MRI/surgery.

RESULTS

CNS malformations were associated with other anomalies in 372/773 (48.1%) cases and were isolated in the remaining 401 (51.9%) cases. NSG alone was able to establish the diagnosis in 647/773 (83.7%) cases. MRI was performed in 126 (16.3%) cases. The indication for MRI was: confirmation of NSG diagnosis in 59 (46.8%) cases; diagnostic query (in the case of inconclusive or uncertain finding on NSG) in 20 (15.9%) cases; search for possible additional brain anomalies in 47 (37.3%) cases. NSG and MRI were concordant and correct in 109/126 (86.5%) cases. Clinically relevant findings were evident on MRI alone in 10/126 (7.9%) cases (1.3% of the whole population) and on NSG alone in 6/126 (4.8%) cases; in all six of these cases, MRI had been performed at < 24 weeks of gestation. In one case, both NSG and MRI diagnoses were incorrect. The main type of malformation in w ich MRI played an important diagnostic role was space-occupying lesions, MRI identifying clinically relevant findings in 42.9% (3/7) of these cases.

CONCLUSIONS

(1) In a tertiary referral center with good NSG expertise in the assessment of fetal CNS malformations, MRI is likely to be of help in a limited proportion of cases; (2) MRI is more useful after 24 weeks of gestation; (3) the lesions whose diagnosis is most likely to benefit from MRI are gross space-occupying lesions.

Ultrasound vs MRI in Diagnosis of Fetal and Maternal Complications

Ultrasound is the screening modality of choice for the fetal imaging. However, there are circumstances in which an alternative imaging technique is needed for additional information regarding fetal anatomy and pathology as well as different maternal conditions.

Magnetic resonance imaging (MRI) is being increasingly used as correlative imaging modality in pregnancy because it uses no ionizing radiation, provides excellent soft-tissue contrast, and has multiple planes for reconstruction and large field of view, allowing better depiction of anatomy in fetuses with large or complex anomalies.

In this review, we attempted to identify strengths and weaknesses of each modality both from the literature and our own working experience, and to propose to some practical recommendations on when to use which imaging modality.

Both ultrasonography and MRI are operator-dependant and neither technique obviates the need for thorough knowledge of normal and abnormal anatomy. In early pregnancy, and where repeated assessment is needed, ultrasound has the obvious advantage. In circumstances where ultrasound examination is difficult, as in the obese patient or severe oligohydramnion, better images might be obtained by MRI examination. MRI might also identify early fetal ischemic lesions after an insult, such as maternal trauma or death of a monochorionic co-twin. From the published literature, it would appear that MRI may provide additional diagnostic information to that given by ultrasound in 25 to 55% of cases, which in turn may have influence on parental counseling and/or management of affected pregnancies. Individual circumstances and expertise influence the accuracy of both modalities. Ultrasound and MRI should be performed to the highest possible standard, and the final diagnosis should be made in a multidisciplinary setting.

3D and 4D sonography and magnetic resonance in the assessment of normal and abnormal CNS development: alternative or complementary

Advanced transvaginal neurosonography has revealed normal and abnormal intracranial morphology. Transvaginal three-dimensional (3D) sonography demonstrates bony structure, multiplanar analysis of inside detailed morphology, tomographic ultrasound imaging in any cutting sections, 3D sonoangiography and volume calculation of ventricles and/or intracranial lesions. Longitudinal assessment of normal and abnormal central nervous system (CNS) development is done by serial scanning. However, the transvaginal high-frequency approach has several limitations due to lack of penetration and cranial bone ossification with advanced gestational age. Magnetic resonance neuroimaging enabled observation of the whole intracranial cavity, brainstem and cortical gyral/sulcal development. On the other hand, neuro-sonography has advantages in detecting intracranial calcification, vascular abnormalities, intratumoral vascularity and bone dysplasia. Moreover, 3D ultrasound demonstrates extra CNS abnormalities, strongly associated with CNS abnormalities. Any less-invasive modalities can be used for a CNS anomaly screening scan and ultrasound is no doubt the first choice. Once CNS abnormality is suspected, it is suggested to use the different technologies according to what is looked for in each abnormal CNS case. Of course, MR and 3D ultrasound imaging should be complementary as well as alternative.

Fetal hydrocephalus caused by cryptic intraventricular hemorrhage

Cryptic intracerebral hemorrhage as an etiological factor in fetal hydrocephalus has been postulated but not described at autopsy. Four fetuses with overt hydrocephalus diagnosed by in utero ultrasound examination were examined at autopsy at 19-22 weeks gestation. Although a hemorrhagic etiology was not evident on ultrasound, hemosiderin-containing macrophages and associated reactive changes were found to obstruct the otherwise well-formed cerebral aqueduct in all four. Coagulopathy due to thrombocytopenia was implicated in one case. Anomalies involving other parts of the body were identified in two cases, although a direct link to the hydrocephalus was not obvious. The abnormality was isolated in one case. In three cases, possible sites of hemorrhage in the ventricles were identified. This abnormality represents a significant proportion of the fetuses examined for hydrocephalus in our referral center. We discuss the importance of careful autopsy examination in the diagnosis of cryptic intracerebral hemorrhage and the implications for counseling.