Role of Ultrasound and MRI in Assessment of Fetal GIT and Anterior Abdominal Wall Anomalies

Abstract

Abnormalities involving the gastrointestinal tract (GIT) and abdominal wall anomalies account for about 15% of all the congenital abnormalities that are identifiable by ultrasound in antenatal period. Ultrasonography (US) is the primary imaging modality for the evaluation of the fetus. It is safe for both fetus and mother, relatively inexpensive, allows real-time imaging and is readily available. Anomalies detected by ultrasound include: Esophageal atresia And Tracheo-esophageal fistula, Duodenal Atresia, Small Bowel Atresia, Meconium Ileus, Anorectal Anomalies, Hepatomegaly/Splenomegaly, Omphalocele and Gastroschisis . Although ultrasonography is the method of choice for evaluating the fetus, magnetic resonance imaging complements ultrasonography in the accurate diagnosis of fetal abnormalities. The advantages of MRI include excellent tissue contrast, a large field of view, relative operator independence and overcoming the poor image quality in oligohydramnios , maternal obesity and inadequate fetal lie. With the advent of fast magnetic resonance applications such as echo planar imaging and single-shot fast spin echo prenatal MR imaging became possible and popular in clinical settings. There is no consistent or convincing evidence to suggest that short-term exposure to electromagnetic fields, such as that which occurs during MR imaging, harms the developing fetus. Compliance with the united states food and drug administration and international commission on Non-Ionizing radiation Protection Guidelines requires control of specific absorption rates (SAR) values. The most frequently used sequences specifically echo planer imaging and single-shot fast spin echo operate at the (SAR) limits imposed by these specific guide lines The importance and diagnostic value of both Ultrasound and MRI in scanning and detection of fetal GIT and Abdominal wall anomalies was highlighted. Comparative results of each modality in different anomalies scanning and which modality is better in diagnostic results could be classified such as : Esophageal Atresia: Magnetic resonance imaging appeared to be accurate for establishing or ruling out a prenatal diagnosis of esophageal atresia and should be considered in fetuses that are at high risk based on ultrasound findings. Prenatal MR visualization of a distended esophagus in fetuses with an absent stomach has been reported to be 100% sensitive and specific for esophageal atresia and by that MRI took the upper hand over ultrasound. Duodenal Atresia: Ultrasound and MR appeared to have equivalent values in detection showing the characteristic double bubble sign. Small bowel Atresia: MR imaging could provide additional information beyond that provided by ultrasound regarding the level of obstruction. In proximal small bowel obstruction, the dilated proximal intestine appeareshyperintense on T2-weighted images and hypointense on T1-weighted images, whereas in distal bowel obstruction the dilated meconium-filled distal intestine appeareshypointense on T2-weighted images and hyperintense on T1-weighted images.