Imaging modalities used with neonates during the active phase of NEC include plain abdominal radiography and, recently, abdominal sonography. Radiographic imaging is essential in the diagnosis of NEC. Bell’s clinical staging of NEC was published in 1978 and was a major force that facilitated grouping of NEC into patient cohorts rather than as case reports and series [
35]. Bell’s Criteria has been the mainstay for the diagnosis and staging of NEC for the last three decades. However, Bell’s staging has been modified by Walsh and Kliegman [
36], who divided each stage into two subcategories, and included signs that differentiate between milder and more severe courses of disease, such as absent bowel sounds, abdominal tenderness and ascites, as well as laboratory parameters indicative of acidosis, thrombocytopenia, neutropenia and disseminated intravascular coagulation [
36]. Successively, a new ANID taxonomy, sometimes called Gordon’s classification was ideated [
37]; although, Bell Staging continues to be used as the standard of practice to diagnose, stage, and treat NEC [
35]. The most important radiographic findings of plain abdominal radiography to confirm diagnosis of NEC are pneumatosis intestinalis, portal venous gas and pneumoperitoneum [
38]. Seminal breakthroughs have occurred in this decade, such as ventilation support and total parental nutrition, and with them a “new NEC” has emerged in increasingly premature infants who have survived because of these technologies. Once patients are diagnosed with definitive NEC (Bell’s stage 2), significant intestinal injury is likely to occur. Therefore, it is possible that earlier detection of intestinal injury and appropriate treatment might prevent the progression of this disease [
39]. Plain abdominal radiography remains the main diagnostic tool in the diagnosis and follow-up of NEC. However, it is sometimes impossible to expose patients to consecutive episodes of radiation. Ultrasound examination (US) seems to be an alternative to current standard usage of radiography [
40]. US is an ideal modality for evaluating bowel necrosis as it is non-invasive, does not involve the use of ionizing radiation and can be performed readily at the bedside. Many studies emphasized the numerous advantages of US over plain abdominal radiography, including no exposure to ionizing radiation, no limitation on frequent use, availability, possibility of use at patient’s bedside, possibility of evaluating indices of bowel dynamics, bowel wall thickness, echogenicity, pneumatosis intestinalis, rate of bowel wall perfusion, ability of determining the nature, and estimating the amount, of intra-abdominal liquid [
41,
42], but its role is still underestimated and plain radiology remains the gold standard modality for diagnostic purposes. Pneumatosis intestinalis and portal venous gas are both seen during abdominal US, detected earlier than by plain radiography [
43]. Moreover, US permits visualization of the bowel wall in a much more detailed way than is possible with plain radiographs, with an assessment of the thickness and perfusion of the intestinal wall and peristalsis; moreover, other information, such as the presence of free intraabdominal gas and the presence of any free intraabdominal fluid, indicative of intestinal perforation, can be obtained from abdominal US [
44]. Ultrasonic findings can also contribute to the prediction of the severity of NEC. Yang et al. investigated the value of abdominal US in diagnosing NEC and its significance in evaluating disease severity [
45]. According to the modified Bell-NEC staging criteria, authors enrolled 84 neonates, 44 with suspected NEC and 40 with confirmed NEC; according to clinical prognosis, they were divided into a medical treatment (
n = 58) and a surgery/death group (
n = 26), and changes in the results of abdominal ultrasound and abdominal X-ray plain film between groups were compared. In the confirmed NEC group, abdominal US showed significantly higher detection rates of portal venous gas and dilatation of the intestine than abdominal X-ray plain film; compared with the medical treatment group, the surgery/death group had significantly higher detection rates of dilatation of intestine, bowel wall thickening, peritoneal effusion and free intraperitoneal air (
P < 0.05). Furthermore, dilatation of the bowel and free intraperitoneal air shown by abdominal X-ray plain film were more common in the surgery/death group. Therefore, authors concluded that ultrasonic findings could permit predicting the severity of NEC [
45]. In another study, it has been stated that abdominal US and radiography in patients with NEC can help predict the outcome [
46].
Pneumoperitoneum, detected by abdominal radiography, is the only sign that has been universally agreed on as an indication for surgery [
48]; however, this is not present in all babies with bowel necrosis and perforation. Furthermore, colour Doppler US is more accurate than abdominal radiography in depicting bowel necrosis in NEC [
49]. The absence of bowel wall perfusion at colour Doppler US is more sensitive and specific than the presence of free air at abdominal radiography in the detection of necrotic bowel in NEC [
49]. Faingold et al. showed that colour Doppler US is more sensitive and specific than abdominal radiography in the detection of necrotic bowel [
38]. Abnormal US and radiography findings were the most powerful predictors of the need for surgical intervention, including persistent dilation of the bowel loops and evidence of portal venous gas, which were detected by radiography, and bowel wall thickening, absent peristalsis and echogenic-free fluid or focal fluid collection, which were detected by ultrasonography. Therefore, the presence of both echogenic fluid and focal fluid have been reported to be indicators for surgical intervention [
50]. Other studies have also evaluated the use of computed tomography and magnetic resonance imaging, but these modalities have not been found to be useful in clinical practice [
51,
52].