Suspicion of a CNS abnormality requires a multi-disciplinary approach that includes imaging by prenatal US and MRI, as well as laboratory and genomics assessments. With the diagnosis of a congenital malformation, decisions are guided by the parent’s personal background, education, and culture [7, 8].
Intrauterine MRI neuroimaging has multiple advantages, such as high soft-tissue contrast, a large field of view; additionally, it is unaffected by a suboptimal foetal position, oligohydramnios, ossified calvaria, and maternal habitus [9].
In this work, we evaluated the role and outcome of MRI and US as the primary imaging modalities in the evaluation of foetal midline cerebral anomalies in pregnant women. MRI provided additional information and changed the diagnosis to the previous US in 16 cases 43.2% of patients. However, US added information to MRI in 3 cases (8.1%), and MRI agreed with US in 21 cases (56.76%) and helped to terminate pregnancy in 4 cases (10.8%). The diagnosis was confirmed by postnatal MRI in live birth.
There are great controversies regarding the prenatal US versus MRI, as Wagenvoort et al. [10] stated that foetal brain MRI added more information and is more accurate than foetal US. Additionally, Jarvis et al. [11] concluded that when foetal brain abnormalities are suspected on US, MR imaging can significantly contribute to the diagnostic pathway by clarifying findings and significantly increasing the detection rate of abnormalities, particularly in the midline and posterior fossa anomalies. However, dedicated neuro-sonography studies concluded that MRI is for the confirmation of diagnosis and that each modality provided additional/different information only in a minority of cases [12].
In our study, pre- and postnatal MRI showed good agreement in 97.3% of cases. Similarly, in Dhouib and colleagues’ study [13], pre- and postnatal MRI data showed good agreement in 85% of cases, and there was disagreement with a prognostic impact in only 9% of cases.
The prenatal diagnosis of alobar holoprosencephaly is fairly easy; however, the recognition of the semilobar and lobar forms can present a diagnostic challenge. In some cases with US examination, the inability to detect the cavum septum pellucidum may be the only obvious finding at 18–20 weeks. MRI is useful for further evaluation when US findings are unclear or to better define associated brain anomalies when the cavum septum pellucidum is absent [9].
In holoprosencephaly, 3D and 4D US surpass MRI in detecting associated midline facial anomalies. 3D and 4D US have been used as an adjunctive imaging modality to 2D ultrasonography to generate accurate detailed images of facial surface anatomy [14].
Prenatal detection of corpus callosal malformations present a challenge as the US signs may be very subtle or appear late after the second-trimester scans [15]. Direct visualization of the corpus callosum and its abnormalities by US is difficult; thus, we mainly depend on the indirect signs of callosal development (such as the absence of the cavum septum pellucidum, colpocephaly, and the inferior orientation of the medial hemispheric sulci). Foetal MRI is helpful in the evaluation of suspected corpus callosal anomalies by US because the corpus callosum can be visualized directly. Additionally, the indirect signs can be seen in foetal MRI. Many studies, such as Wagenvoort et al. [10] and Malinger et al. [16] concluded that MRI is superior to US, particularly in the diagnosis of callosal anomalies. In this study, five cases of partial callosal agenesis were missed by US and diagnosed by MRI.
The diagnostic accuracy for detecting failed commissuration in our study was 86.49% for US and 100% for MRI. Griffiths et al. [17] calculated accuracy of 34.2% for US and 94.9% for MRI with significant improvement in diagnostic confidence for MRI. Therefore, these authors suggest that any woman whose foetus has failed commissuration as the only intracranial finding detected on US should have an MRI examination for further evaluation.
MRI helps to define the nature of cerebral cysts [16]. In this study, we reported nine cases with midline interhemispheric cysts detected by US; MRI changed the diagnosis to an arachnoid cyst in one case, and in another case, we could not determine whether the large midline cyst was a cystic expansion of third ventricle or an associated interhemispheric cyst with the ACC by US.
Craniopharyngioma is a benign CNS tumour that may present with enlarged head circumference and hydrocephalus secondary to cerebrospinal fluid drainage obstruction. This tumour is mostly found in the suprasellar region. In US, craniopharyngioma appears as an intracranial large echogenic mass (differential with teratomas). MRI helps in the exact localization of the tumour and the assessment of the remaining brain structures [18]. In this study, there was a case with craniopharyngioma, and US was superior in the detection of intra-tumoural vascularity by colour Doppler imaging; however, MRI helped in the assessment of the posterior fossa and the remaining brain structures (Fig. 3).
Posterior fossa anomalies comprise various malformations, such as Dandy–Walker malformation, various causes of vermian hypoplasia or aplasia, persistent Blake’s pouch, mega cisterna magna, and cerebellar hypoplasia [8]. The overall diagnostic accuracy for detecting posterior fossa abnormalities in our study was 97.3% for prenatal MRI and 76.92% for US. Griffiths et al. [19] showed comparable results with 65.4% diagnostic accuracy for US and 87.7% for MRI. Additionally, any woman whose foetus has a posterior fossa abnormality on US should have MRI for further evaluation to improve diagnostic accuracy and confidence.
US and foetal MRI are complementary techniques; when using one of these imaging modalities, we should be aware of their main advantages and limitations. We should determine the imaging strategy on a case-by-case basis depending on the maternal body habitus, the foetal presentation, and the type of abnormalities suspected. Notably, accurate prenatal diagnosis is of the utmost importance for prenatal counselling [20].
The results of the MERIDIAN cohort studies [21] strongly support the routine clinical use of intrauterine MRI as an adjunct to US when assessing foetuses with structural brain abnormalities. Therefore, intrauterine MRI was potentially beneficial when included in the diagnostic pathway for prenatal structural midline brain anomalies. This technique improves diagnostic accuracy and confidence for foetal brain anomalies and leads to better counselling and management changes in a high proportion of cases [3].
The main limitation of this study is the small number of the study group, as it would be more beneficial if it was performed in a large scale multicentre study.