Pitfall. Secondary thickening of the ileum caused by appendicitis. If the prominent ileum is appreciated, but the inflamed appendix ( arrow ) is overlooked, an erroneous diagnosis of Crohn’s disease or infectious ileocolitis can be made, leading to surgical delay. 

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... immediate appendectomy is advisable, even if the patient is again completely free of symptoms at that time. Histology in such cases confirms acute inflammation. If conservative management is opted for, keep in mind that there is a recurrence rate of approximately 40% [8]. Patients who are admitted with considerable delay may present with a palpable mass and relatively mild peritonitis. In these patients, who usually have a high erythrocyte sedimentation rate, US shows a large mass of noncompressible fat around the appendix, inter- spersed with echolucent streaks. These patients are diagnosed as ‘‘appendiceal phlegmon’’ and are usually managed conservatively because the surgeon knows that appendectomy in such cases is technically difficult or even impossible [9]. In some patients, the surgeon might be uncertain about the right policy: immediate operation or conservative management. In these cases the clinical symptoms prevail over the US image. If treated conservatively, follow-up US shows a decrease in size of the appendiceal phlegmon within the course of weeks (Fig. 16). If next to the inflamed appendix a fluid collection is found, this is suggestive for an appendiceal abscess. The collection often contains air and is surrounded by inflamed noncompressible hyperechoic tissue representing omentum and mesentery and secondarily thickened neighboring bowel loops, attempting to seal-off the abscess from the peritoneal cavity. If an appendiceal abscess is demonstrated and there is no frank peritonitis, percutaneous drainage is the treatment of choice (Fig. 17). In stable patients who have no fever and only mild pain, it is wise to await spontaneous drainage of the abscess to neighboring bowel. Finally, there are some patients with an appendiceal abscess who are better off with immediate surgery: this goes in general for children and for those patients with severe peritonitis, which indicates that the walling-off process is failing. Immediate surgery also is indicated for patients who have a small abscess with a history of only a few days of symptoms, in whom appendectomy with evacuation of the abscess is usually technically easy (Fig. 18). Before percutaneous drainage, CT is necessary to delineate the extent of the abscess and to determine the safest access route. If expertise is available in US- guided puncture, the combination US plus fluoroscopy has several advantages over CT-guided drainage: it is rapid, allows continuous control, any angulation, and can be performed as a bedside procedure. A false-positive diagnosis can be made if the normal appendix is mistaken for an inflamed one. Not infrequently the normal appendix is larger than 7 mm, especially in children when caused by lymphoid hyperplasia and in adults when caused by fecal im- paction. Appendiceal compressibility, the absence of a Doppler signal, and the absence of inflamed fat are the most important features in deciding if it is normal or inflamed. Mistaking a normal appendix for an inflamed one may also occur if there is secondary thickening of the appendix associated with cecal carcinoma. In the latter case, the appendiceal lumen is obstructed giving rise to sterile accumulation of mucus in the lumen. The patient often has remarkably mild symptoms and is managed conservatively under the erroneous diagnosis of an appendiceal phlegmon. If the underlying tumor is small and is not recognized, this may lead to considerable delay in surgical treatment. The combination of a relatively large appendix with paradoxically mild and atypical symptoms should raise suspicion of underlying malignancy. Other conditions with secondary thickening of the appendix are perforated peptic ulcer, Crohn’s disease, and sigmoid diverticulitis. The most important reason for a false-negative ultrasound examination is overlooking the inflamed appendix. In experienced hands the inflamed appendix can be visualized in 90% of patients with acute appendicitis. Generalized peritonitis hampers graded compression, which may account for a lower score in patients with free appendiceal perforation. Air-filled dilated bowel loops from adynamic ileus may hide the appendix from view. Air in the lumen can make it difficult to identify the inflamed appendix (Fig. 19). Another pitfall is demonstration of the normal proxi- mal part of the appendix while the distal inflamed tip is overlooked, because it is obscured by bowel gas. Rarely, the inflamed appendix has a maximal diameter of less than 7 mm. In those cases rigidity, hyper- vascularity, and the presence of inflamed fat must give the clue (Fig. 20). Another pitfall is advanced appendicitis where there is secondary wall thickening of the ileum. Often the ileal thickening is more prominent and conspicuous on US than the underlying inflamed appendix. If only the ileum is appreciated and the appendix is overlooked, an erroneous diagnosis of infectious ileocolitis or Crohn’s disease can be made, leading to ill- advised surgical delay (Fig. 21). Similarly, if in an adult patient enlarged mesenteric lymph nodes are the sole US finding, one should be cautious to diagnose mesenteric lymphadenitis because these nodes could be secondarily enlarged because of acute appendicitis, while the inflamed appendix is overlooked. If in a patient with appendicitis only the fecalith in the appendiceal base is visualized and the rest of the appendix is overlooked, this may lead to an erroneous diagnosis of cecal diverticulitis. If in a woman a relatively large right-sided ovarian cyst is found, this is not necessarily the cause of her symptoms and one should still search for appendicitis. Finally, if in advanced appendicitis only the hyperechoic noncompressible inflamed fat of omentum and mesentery is visualized, and the inflamed appendix is overlooked, this may lead to an erroneous diagnosis of omental infarction or epiploic appendagitis [10,11]. In patients with equivocal US findings, CT scan is indicated. A fortunate circumstance is that these are often obese patients. Patients with ileocecal Crohn’s disease often have protracted and atypical symptoms causing marked diagnostic delay. Crohn’s disease may also present with acute, appendicitis-like symptoms and lead to an ill-advised operation. In both scenarios US may play an important role in establishing the initial diagnosis [12,13]. The sensitivity of US for detecting ileocecal Crohn’s disease is over 95%. Sonographically, there is marked mural thickening of the ileum, which shows decreased or no peristalsis and is not compressible. Classically, all layers are involved and layer structure is often locally disturbed, the earliest sign being echolucent changes in the submucosa. There is inflammation of the fatty mesentery and omentum, recognizable as hyperechoic, noncompressible tissue adjacent to the ileum. In the echolucent wall bright eccentric foci may indicate deep ulceration. Echolucent streaks within the hyperechoic tissue indicate liponecrotic tracts, which may herald fistula formation. Cecum and appendix may also show mural thickening. Mesenteric lymph nodes are often markedly enlarged, but hypovascular. In long-standing Crohn’s disease, ‘‘creeping fat’’ is found, which is recognized as a large, moderately well-compressible fatty mass encompassing most of the circumference of the ileum and isoechoic to normal fat. Eventually, there are often US signs of prestenotic dilatation, abscess formation, or fistula formation (Figs. 22, 23). Infectious ileocolitis is a bacterial infection of terminal ileum and colon, which is characterized by diarrhea and abdominal pain. The most frequently cultured bacteria are Campylobacter , Salmonella , and Yersinia . The infection is generally limited to the mucosa, is self-limiting, and rarely poses diagnostic problems. There is an interesting variant of infectious ileocolitis in which the infection is mainly limited to the ileocecal area and has been termed ‘‘infectious ileoce- citis’’ [14]. It is usually caused by the same bacteria and the importance of this variant is that its clinical symptoms are dominated by acute right lower abdominal pain, whereas diarrhea is absent or only mild. These symptoms masquerade as the clinical signs of appendicitis and explain why infectious ileocecitis often leads to an unnecessary laparotomy. The symptoms of Yersinia are often more protracted and both the clinical symptoms and the US features may mimic those of Crohn’s disease. The absence of a transmural component, the self-limiting course, and positive stool cultures or serology yield the correct diagnosis. The frequency of infectious ileocecitis is fairly high and has a ratio of 1 to 8 compared with appendicitis [14]. An US shows fairly characteristic features: there is diffuse thickening of mucosa and submucosa of the terminal ileum and the cecum and enlargement of mesenteric lymph nodes (Fig. 24). The appendix is so- nographically normal. In contrast to ileocecal Crohn’s disease, in infectious ileocecitis the wall layers are always intact and the muscularis and serosa are never affected. Omentum and mesentery are never involved and there are never signs of bowel obstruction or abscess or fistula formation. The various microorga- nisms have a slightly different pattern of affecting the ileocecal area (Fig. 25). This is an ill-defined entity, probably of viral origin, in which the mesenteric lymph nodes become inflamed and enlarged. It is a typical disease of childhood and is only rarely seen in young adults. It mimics the clinical signs of appendicitis and may lead to an unnecessary appendectomy. The US findings are solely enlarged, hypervascular mesenteric lymph nodes. If these are the only US findings in a symptomatic young adult, however, it is well possible that these nodes are in fact secondarily enlarged because of acute appendicitis and the inflamed appendix is overlooked. Patients with cecal carcinoma can present with acute or subacute abdominal symptoms in ...