Ultrasound is still the first-line modality for fetal scanning being readily available, of low cost, and provides the advantage of assessing blood flow of the fetus and providing a dependable data about fetal development. However, in cases like morbid maternal obesity, oligohydramnios, and ossified skull, in late pregnancy together with cases showing inconclusive results, US needs an adjuvant examination to collect more accurate information in case of indefinitive diagnosis .
Magnetic resonance imaging arised as a tool of fetal examination since 1983. Its inherent advantages, being operator independent, having high resolution, superior soft tissue contrast, wide field of view, multiplanar capability, as well as lacking ionizing radiation, have encouraged its use as a problem-solving tool in diagnosing fetal anomalies diagnosed by US .
The guidelines made by the American College of Radiology for MR practices in 2013 sanctions recommends doing fetal MRI at any time during gestation if the benefits exceeds the possible risks for the woman and the fetus . However, up to current moment, there is no proof that MRI is unsafe for the fetus, yet not enough studies were made to confirm its safety performance. Recommendations are to do MRI after 18 weeks of gestation due to unknown biological effects of high magnetic fields during organogenesis .
Our study is a prospective study dealing with fetal CNS anomalies diagnosed between 18 and 36 weeks gestation. We compared US and MRI results with fetal post natal outcome or autopsy of abortus and cases with IUFD, where the patients were consented for the autopsy.
We compared the diagnostic accuracy of prenatal MRI to that of ultrasound in the diagnosis of CNS anomalies between 18 and 36 weeks gestation, and this diagnosis was confirmed by the fetal post natal outcome; postnatal transcranial US, CT, or MRT and autopsy in cases of termination or IUFD if the patient consented for autopsy.
Obtained MRI confirmed US diagnosis, without additional information in 23/40 cases (57.5%), added an extra finding in 11/40cases (27.5 %), differentiated between 2 pathologies in 3/40cases (7.5%) and changed diagnosis in 3/40cases (7.5%).
These results are comparable to the results of Frates et al. , who performed fetal MRI for 28 cases; in which 14 (50%) of 28 cases, the diagnoses made by US and MR imaging were accurate when compared with post-natal diagnosis. MRI supplied more information than did US in 7 of the 14 (25% of total) fetuses but added no data in 7 others. There were no fetuses in which US added additional information to that provided with MRI when both were correct. In 7 (25%) of 28 cases, diagnoses with both US and MRI were incorrect when matched with postnatal diagnosis.
Amini et al.  made a study about fetal CNS abnormalities in the second trimester and found that fetal MRI in second trimester delivered additional diagnostic data compared to the ultrasound scan in 45% of the cases, which resulted in a change in the management in 10% of all cases.
In the present study, MRI had a sensitivity of 97.5% compared to 95% for US; this is in agreement with a study made by Gonçalves et al.  who found that MRI was more sensitive than 2DUS and 3DUS.
Several studies revealed a higher accuracy of MRI in comparison to US for the diagnosis of CNS anomalies [13,14,15,16]; this agrees with our study that revealed an overall accuracy for MRI 99.6%, with PPV=95.4% and NPV =99.9% compared to 98.8% for US with PPV=90.5% and NPV=99.9%. However, Malinger et al.  disagrees with these results they revealed a higher accuracy for fetal neurosonography in 7/39 (17.9%) cases in comparison to the accuracy for MRI in 3/39 (7.7%) cases.
Paladini et al.  examined 126 fetuses by 2D-US, 3D-US, and MRI where he concluded that MRI and ultrasound results were disagreeing in 12.7% (16/126), with additional diagnostic information provided by MRI in 7.9% (10/126) of cases.
In our study MRI added an extra-finding in 11/40 cases(27.5%), differentiated between 2 pathologies in 3 /40cases (7.5%), and changed diagnosis in 3 /40cases (7.5 %).
Strengths and limitations of the study
The strengths of this study are (1) the blinded and randomized image revision by experienced examiners, (2) specific comparison of diagnostic confidence rates between examiners, and (3) interpretation bias was reduced by preventing access to patient history or findings from prior diagnostic imaging studies.
Limitation of this study was the number of fetuses studied due to limited awareness of fetal MR imaging, as well as financial issues.