**10. Necrotizing enterocolitis**

Necrotizing enterocolitis (NEC) is one of the most common and lethal gastrointestinal emergencies of neonates, usually affecting the terminal ileum and ascending colon [22]. Although it affects primarily preterm babies, NEC can also be seen in term infants. The clinical presentation ranges from feeding intolerance, abdominal distention, emesis, diarrhea, rectal bleeding to more severe systemic findings including respiratory failure and fulminant shock [41]. Bowel necrosis occurs in NEC without any precise cause, which compromises the mucosal integrity [6]. Pathogenic organisms become dominant in the gut flora, leading to the pneumatosis intestinalis, which subsequently leads to portal venous gas and consequently leads to perforation and pneumoperitoneum. While the disease progresses, both early and late clinical signs and laboratory tests are often non-specific for diagnosis of NEC, therefore imaging plays crucial role for accurate diagnosis.

Radiographs are still primary modality of choice for evaluation of neonates suspected of having NEC [46]. Plain abdominal radiographs demonstrate pneumatosis, increased thickness of bowel wall, free intraperitoneal air and portal venous air [22, 46]. The role of US has been increasingly appreciated, owing to its higher sensitivity than plain films in the detection of early changes such as wall thickening, intestinal pneumatosis, portal venous air and disturbed bowel wall perfusion on color Doppler [5, 46]. Recent publications stated that diagnostic performance of US for detecting NEC is accurate with sensitivity of 100% and specificity of 90%. However, role of US in the follow-up of NEC is uncertain [6].

In the early phase of the disease, US can show the bowel wall thickening due to inflammation. Whereas, bowel wall thinning (<1 mm) may occur as it becomes necrotic and progresses toward perforation [47]. Similarly, Color Doppler may display hyperemia in the early stages due to inflammation, and avascular wall in the advanced disease with bowel wall necrosis [6]. Pneumatosis intestinalis is seen as punctate or granular echogenic foci with 'dirty' posterior acoustic shadowing or linear echogenic ring within the bowel wall. The gas bubbles create twinkling artifact on color Doppler which is useful in equivocal cases. To differentiate intramural gas from intraluminal air, nondependent bowel wall should be evaluated. Moreover, true pneumatosis would not change with the motion of the patient, whereas intraluminal air is freely mobile. Placing the patient in multiple positions may be useful to observe movement of the air. For the detection of pneumatosis, US is more sensitive than plain radiography [48].

Portal venous gas manifests on US as the presence of curvilinear or punctate mobile echogenic foci within the portal venous system. It is commonly seen in the neonates after umbilical catheterization, and may occur in different neonatal diseases. Therefore, in the absence of pneumatosis intestinalis, other etiologies should be considered rather than NEC. In the case of NEC, fluid-filled dilated bowel, complex hyperechoic intraperitoneal free fluid, focal fluid collections are suggestive of perforation and have been correlated with a poor clinical outcome [47, 49]. Evaluation of bowel peristalsis by real-time examination is an important component of US in infants with suspected NEC, because necrotic or inflamed bowel segments have decreased or absent motility [6]. US may also be considered in the follow-up to decide the appropriate time to restore oral feeding and to evaluate post-enterocolitis stenosis [5].
