Intramural air suggests an infection with anaerobic bacteria. Pericolic fluid is a sign of a (possible) perforation. Sonography may demonstrate free abdominal air, which is usually detected on the right side, e.g., perihepatic. In advanced disease with catastrophic prognosis, air bubbles in the portal vein may be demonstrated, resembling late phases after application of contrast enhancing agents. Another feature may be pneumatosis cystoides intestinalis, as seen in premature infants with necrotizing enterocolitis. It is noteworthy that the hydrogen content of the expired air in these patients is massively increased (Dietrich et al. 2006).

A few prospective studies used ultrasound to evaluate patients after chemotherapy. Bowel wall thickness (>4 mm) was seen only in those patients who fulfilled symptom criteria for NPC, and not in those patients with mucositis alone or in those who remained asymptomatic (Gorschluter et al. 2002). Besides the wall thickness, its overall echotexture and appearance of surrounding structures should be considered (Nuernberg et al. 2008; Dietrich et al. 1998b).

Bowel wall thickness has also been shown to be an important prognostic factor. Cartoni et al. (2001) evaluated neutropenic patients with fever and abdominal pain. A bowel wall thickness of >5 mm was considered as diagnostic of NPC. The mean duration of symptoms was significantly longer among patients with (7.9 days) than among patients without (3.8 days) bowel wall thickening and the related mortality was also higher (29.5 versus. 0 %). The degree of bowel wall thickness correlated with the outcome of patients: 60 % of patients with bowel wall thickness >10 mm died from this complication, compared with only 4.2 % of those with mural thickness ≤10 mm (Cartoni et al. 2001). These results are similar to those of our (not yet published) prospective study. Recently, 14 neutropenic patients (absolute neutrophil count <500/mm³) with sonographic features of NPC were evaluated regarding clinical findings, sonographic features, and outcome (8 male, mean age 48 years). Bowel wall thickness was measured at terminal ileum, cecum, sigmoid colon, and small intestine by high resolution sonography with moderate pressure. A thickness <2 mm was considered normal, a thickness >2 and <5 mm was considered as non-specific bowel wall thickening, and a thickness >5 mm was considered abnormal confirming the diagnosis of NPC (Dietrich et al. 2006). The morphology and the five layers of ileum and colon were evaluated assessing a mucosal or transmural pattern of inflammation. Bowel wall vascularity was examined by color Doppler imaging (CDI) and graded as ischemic (no flow pattern), normal, or hypervascular as per standard GvHD criteria (Klein et al. 2001).

In all 14 patients treatment consisted of bowel rest and total parenteral nutrition and multiple antibiotics. G-CSF was given in eight patients to shorten the time of neutropenia. Three patients died due to sepsis with multiorgan failure. High C-reactive protein (P < 0.001) and the sonographically demonstrated bowel wall thickness (P < 0.03) were associated with lethal outcome. The surviving 11 patients recovered from NPC and the reduction of abdominal symptoms was accompanied by progressive reduction of intestinal mural thickening in follow-up ultrasound examinations. Nearly, all patients with NPC (13 of 14) presented a transmural inflammatory pattern of thickened bowel wall. In all patients, the ileocecal region was involved. The thickness of the inflamed bowel wall was at least 10 mm. Hypervascularity on CDI and the detection of small amounts of free fluid in the abdomen were documented in the majority of patients with NPC. Typical sonographic characteristics in NPC are depicted in Fig. 1a.

Other imaging methods include computed tomography (CT) and abdominal X-ray whereas barium enema and enteroclysis are relatively contraindicated as they increase the luminal pressure and the risk of perforation. Although most authors favor CT as the most sensitive diagnostic tool for NPC, sonography has the advantage of being easily performed with repeated examinations (e.g., at the bedside) even in severely ill patients in intensive care or transplantation units. The findings on abdominal X-ray are often nonspecific and may show small bowel ileus, an ill-defined soft tissue density in the region of the cecum, thickened air-filled loops of bowel or signs of pneumatosis intestinalis. Other methods such as scintigraphy are not routinely used in clinical practice.

Several conditions should be taken into account in the differential diagnosis. Appendicitis is very often the main diagnosis. Because of the high perioperative mortality in these patients [most often from 50 to 100 % (Wade et al. 1992)] the operative approach should be avoided. Other acute or chronic inflammatory diseases of the ileocecal area are viral (e.g., CMV colitis) and bacterial ileocecitis, Crohn′s disease, pseudomembranous and ischemic colitis, and colonic pseudo-obstruction (Nuernberg et al. 2008). In post allogeneic stem-cell transplantation patients, one has to think of graft versus host disease (GvHD), although this usually occurs after engraftment (Klein et al. 2001). A neoplastic (leukemic of lymphocytic) infiltration of the ileocecal region must be excluded especially in case of a palpable mass in this area. In pancytopenic patients one also has to think of an acute hemorrhage into the mucosal wall (Dietrich et al. 2006).

Penicillin-induced segmental hemorrhagic colitis shows characteristic sonographic features with asymmetric wall thickening of the colon (typically in the right colon) with loss of intestinal wall layers due to edema and bleeding. Relative hypovascularization can be detected with CDI. The surrounding colon may appear normal. The past medical history is decisive. Intestinal tuberculosis has become increasingly observed since the occurrence of the AIDS pandemic. As with NPC, many regions of the intestine may be involved, although the ileocecal region is affected in up to 90 % of patients. The ultrasound image displays a hyperechoic (sometimes asymmetrically thickened) mucosa with loss of folds and unclear demarcation from (sometimes asymmetrically) thickened submucosa, so that a pseudo-polypoid image prevails. Short stenoses, mesenteric hyperechoic inflammatory signs, development of fistula, and abscesses can also be seen. Other characteristic signs include severe mesenteric lymphadenopathy (sometimes more than 10 hyperechoic round- to oval-shaped lymph nodes are detected) and a distended ileocecal valve, with asymmetrical (sometimes tumorous) thickening, especially affecting the medial cecal wall. Patients with peritoneal tuberculosis may also have ascites and thickening of the peritoneum (Barreiros et al. 2008).