Nowadays, fetal anomaly scanning relies more on MRI technology. It has gone from being a supplemental modality to being an essential part of fetal imaging procedures in the past several years [5].
Fetal MRI is usually used if the fetal US revealed a suspicious finding or if the sonographically detected fetal abnormalities necessitate further evaluation.
We compared the diagnostic accuracy of prenatal MRI to that of the US in identifying non-CNS anomalies between 18 and 36 weeks of gestation.
According to our study, the sensitivity of prenatal US alone in the detection of congenital anomalies was inferior to the sensitivity of MRI alone, and this is in accordance with Gonçalves et al. [6] study that reported higher sensitivity of MRI more than that of 2DUS and 3DUS.
As regards diagnostic accuracy, several studies revealed superiority of MRI in identifying congenital fetal anomalies compared to the US [7]. This was in agreement with our study that showed higher accuracy for MRI than for the US.
In our study, in 5/30 cases (16.6%), the US made the diagnoses and proved them by MRI. There were 24/30 cases (80%) in which MRI imaging provided more information than the US, which modified the diagnosis and, hence, the postnatal management plan. Still, MRI missed the correct diagnosis in 1/30 cases (3.5%) revealed by ultrasound as a case of unilateral renal cystic dysplastic kidney, even though MRI detected a renal pathology, still misdiagnosed it as PUJO, which was established postnatal as unilateral cystic dysplastic kidney.
Matching with Hosny IA et al. in 2010 [8] investigated 25 pregnant females with US followed by MRI after discovering congenital fetal anomalies. In 8% of cases (2 out of 25), the diagnosis was modified after using MRI because of the recognition of additional findings. MRI confirmed the diagnosis of US in the remaining 18/25(72%) cases.
Gastrointestinal anomalies
It is important for timing of fetal GI MR imaging studies, which should be scheduled at a gestational age when one would expect a normal distribution of meconium. Meconium should be seen in the left colon by 24 weeks gestation and is usually identified in the entire colon by 25 weeks gestation, so better after 24 weeks of gestation [9].
Regarding cases with anterior abdominal wall defects (Gastroschisis and omphalocele) are two of the most prevalent types of anterior abdominal wall abnormalities. The umbilical cord insertion lies on the apex of the omphalocele, which is a midline supra-umbilical abdominal wall defect with herniation of abdominal content and peritoneum. Gastroschisis usually affects small intestinal loops, which are herniated and floating with no covering membrane. Gastroschisis may be accompanied by oligohydramnios and intrauterine growth restriction [10].
Our study showed four cases of omphalocele. MRI was better in the assessment of the extent of herniation of the solid organs and the intestines out of the abdominal cavity and useful to assess the abdominal wall defect more precisely. MRI easily identified the amniotic sac covering and the herniated contents, which are typical of omphalocele. It easily identified other associated anomalies such as CNS anomalies and assessed the lung in case of giant defect.
In cases of duodenal atresia, duodenal atresia is considered one of the most frequent causes of fetal bowel obstruction. The incidence of duodenal atresia reached up to one in 10,000 births in normal karyotype fetuses.
Because an association was found between duodenal atresia and Trisomy 21 in up to 33% of cases and also with the VACTERL, fetal MRI may be valuable to detect and characterize these associated anomalies (anorectal malformation, cardiac disease, renal anomalies, vertebral anomaly, trachea–esophageal fistula, limb anomalies) [11].
Our study showed three cases presenting with duodenal atresia, two cases diagnosed by ultrasound with no additional findings on MRI, and one case showed double bubble sign on ultrasound still MRI proved to be of added value as suggested duodenal stenosis rather than atresia as it was able to assess the distal bowel loops excluding distal bowel atresia or malrotation which was confirmed postnatal.
In the setting of esophageal atresia in our study, MRI confirmed the diagnosis of esophageal atresia with better evaluation of the esophageal pouch than ultrasound.
In situations with a small or non-visualized stomach, especially if associated with polyhydramnios, fetal US imaging can diagnose esophageal atresia. However, MRI provides a more accurate diagnosis of esophageal atresia, showing the contracted stomach easily. It is important to mention that other anomalies in the VACTERL association should be suspected in the case of esophageal atresia [9].
Regarding the hepatic and splenic pathology in our study, MRI was superior to ultrasound in the diagnosis of a splenic cyst, assessment of extension of the hepatic tumor, and intrahepatic dilation (Caroli disease).
This matched with Chen CP, Cheng SJ, Sheu JC et al. reported the added value of MRI in diagnosing the choledochal cyst prenatally [12]. MRI is more accurate than the US in diagnosing exophytic behavior of large hepatic and splenic cysts and delineating their relation with the surrounding parenchyma [12].
Genitourinary anomalies
The genitourinary structures can already be correctly identified and examined from about 20 weeks onwards with MRI [13]
Regarding cases of Renal agenesis, Our study matched with several previous which demonstrated the MRI's usefulness in assessing a wide range of fetal urinary anomalies.
In our study, in three fetuses, MRI confirmed the findings of US concerning non-visualization of kidneys and non-filling of the urinary bladder. Fetuses who are diagnosed with bilateral renal agenesis are incompatible with life. The parents' decision to terminate the pregnancy is supported by prenatal proof of bilateral renal agenesis. In such circumstances, severe oligohydramnios or anhydramnios are almost always present, and the US assessment is inaccurate. In comparison, MRI can be very useful in evaluating bilateral renal agenesis [14].
The absence of the bright signals in renal fossa can be detected easily by DWI sequence [14], which plays an important role in confirming the diagnosis. Bilateral renal agenesis is frequently associated with pulmonary hypoplasia, which also can be detected more accurately on MRI.
Regarding cases with Cystic kidneys in our study, cystic kidneys were shown in one fetus with unilateral multicystic dysplastic kidneys and two fetuses with bilateral autosomal recessive polycystic kidneys (ARPKD), representing one of the most frequent urinary abnormalities. As the recurrence rate of ARPKD represents 25% while that MCDK represents 3%, it is important to differentiate between both abnormalities [15]. When small cysts and marked enlargement of both kidneys are detected, the suspension of ARPKD is raised more than MCDK [15].
In two fetuses, MRI confirmed the US diagnosis of bilateral ARPKD. Fetal MRI showed obvious bilateral renal enlargement with a mottled hyperintense signal on T2-weighted images reflecting the innumerable small cysts in ARPKD [16]. When bilateral renal abnormalities and oligohydramnios are diagnosed, syndromal affection and other fetal anomalies are suspected such as Meckel–Gruber syndrome (MKS) [17, 18].
Concerning cases of pelvi-ureteric junction obstruction (PUJO), Urinary system obstruction at the pelvi-ureteric junction (PUJ), ureterovesical (U–V) junction, and/or urethra can be presented as a fetal abdominal cystic mass [19]. Correct diagnosis of urinary tract obstruction was made by fetal MRI in one fetus with accurate determination of the obstruction level. Due to extended view field, MRI delineated the massively dilated urinary bladder (megacystis) correctly in a fetus in our study; both dilated posterior urethra (Keyhole sign) and thickening of the wall of the urinary bladder contribute to the confirmation of the prenatal PUV diagnosis in this fetus [20].
Thoracic anomalies
Fetal MRI is a valuable tool in assessing chest malformation due to the high ability of T2WI MRI to visualize the anatomy and morphology of lung parenchyma. In addition, in the last few years, DWI can assess the physiological maturation of lung parenchyma throughout different gestational ages. It is proposed that the increase in the gestational age of pregnancy is accompanied by an increase in the ADC values of fetal lung parenchyma [21].
The normal fetal lung is homogenous and has a moderately high signal intensity of T2-weighted images. With maturation of the lungs, the signal intensity of the lungs increases, so better assessment of lung pathology after 24 weeks of gestation [21].
In the case of congenital diaphragmatic hernia, Out of four cases with a congenital diaphragmatic hernia (CDH), three of them were identified by ultrasound, and MRI confirmed the diagnosis with more additional information regarding the content and associated lung hypoplasia. One case was misdiagnosed on ultrasound as esophageal atresia, and MRI changed the diagnosis.
The commonest site of CDH is the posterior aspect of the left hemidiaphragm. CDH results from a failure in diaphragmatic leaflet development. In fetuses with left-sided CDH, the presence of the liver in the chest is evidence of a very poor prognosis. MRI can visualize liver position directly and distinguish meconium-filled herniated bowel loops from cystic lesions within the chest [22].
In the case of congenital lobar over inflation, prenatal fetal MRI has a great capability of diagnosing fetal thoracic malformations, such as congenital diaphragmatic hernia, extra lobar bronchopulmonary sequestration, congenital pulmonary airway malformation, congenital lobar emphysema, etc. Sometimes, the early exploration of the disease allows better patient counseling, pregnancy and delivery management, and neonatal care preparation [23]. As in our study, fetal MRI was superior to ultrasound in diagnosing congenital lobar over inflation.
Neck and facial anomalies
Although fetal MRI of the face, and neck can be performed during the second or third trimester, the preferred timing for fetal MRI is the 32nd week of gestation at that time one can make a comprehensive structural assessment [24]
Fetal MRI added additional information in cases of neck anomalies as in cases of cystic hygroma and cervical teratoma in which MRI was superior in assessing the nature of the lesion solid with cystic changes, the extension of the lesion, and the relation to oropharyngeal airway.
The nature of the lesion (cystic, vascular, neoplastic, etc.) and its extension can be detected accurately with fetal MRI. Accordingly, MRI can assess the severity of the malformation by assessing its actual extension, whether confined to the neck or had mediastinum invasion [25].
In the US, neck mass extension and upper airway scanning are challenging. On T2-W fetal MRI the airway appears typically as hyperintense fluid-filled passage so that T2WI can assess the degree of the obstruction, its invasion to the mediastinum, and also shows esophageal and/or vascular compression, which can negatively affect the prognosis [26]
Cleft lip/palate (CL/P) is the commonest congenital malformations affecting the face. Recently, there has been a wide acceptance to use the US in diagnosing CL/P. Jean-Marc et al. [27] described high rates of recognition (up to 85%) of fetal facial anomalies using the US. In comparison, MRI offers stronger tissue contrast, multiplanar capability, and higher resolution. Also, MRI is less affected by human influences. Moreover, it has been found that MRI can aid in the diagnosis of fetal cleft lip and/or cleft palate. Additionally, based on MRI, CL/P classification, and other integrated anomalies can be determined, helping in perinatal follow-up and management. [28]
The limitation of this study was the small number of fetuses studied due to limited knowledge about fetal MRI, as it would be more beneficial if it was performed on a large-scale multicenter study in addition to financial issues with MRI being an expensive technique in comparison to the US. Last but not least, the lack of postnatal autopsy in our country due to religious factors makes it impossible to compare MRI to open procedure confirming the proposed diagnosis.
As MRI is not conducted in negative US cases, there is deficient evidence in assessing the true negative and false negative cases on fetal MRI, leading to uncertainty of the sensitivity, specificity, and predictive values.
