The typical sonographic appearance of the normal bowel wall consists of five concentric, alternately echogenic and hypoechoic layers that we describe from the lumen outward.
The classic sonographic feature of Crohn's disease is the "target" sign.
which means a strong echogenic center surrounded by a relatively sonolucent rim of more than 5 mm. This transmural inflammation or fibrosis can lead to complete circumferential loss of the typical gut wall layers, which results in a thick hypoechoic rim on axial images. Strictures are shown as marked thickening of the gut wall with a fixed hyperechoic narrowed lumen, dilatation, and hyperperistalsis of the proximal gut. Peri-intestinal inflammation leads to the "creeping fat" sign, which appears as a uniform hyperechoic mass typically seen around the ileum and cecum. Mesenteric lymphadenopathy is seen as multiple oval hypoechoic masses, usually in the right lower quadrant. In contrast to other forms of colitis, Crohn's disease is suggested by skip areas and involvement of the distal ileum. Possible complications of Crohn's disease comprise fistulas, abscess formation, mechanical bowel obstruction, and perforation. Abscesses are seen as poorly defined, mostly hypoechoic focal masses that can contain hyperechoic gas. Fistulas are a hallmark of Crohn's disease and are seen in as many as one third of patients with advanced disease as hypoechoic tracts with gas inclusions connecting bowel loops or adjacent structures (bladder, abdominal wall, vagina, psoas muscle). Detection of gas bubbles in abnormal locations raises the possibility of fistulous communication.
Circular hypoechoic wall thickening and loss of stratification
"thumbprinting" and narrowing of jejunal lumen in left lower abdomen. Crohn's disease
Transverse sonogram of ileum (arrows) shows severe narrowing of small hyperechoic central lumen caused by excessively echolucent wall thickening and loss of stratification, indicating scarring of entire bowel wall.
Large-bowel enema with fine granularity of mucosa reflecting hyperemia and edema confirms suspected sonographic diagnosis of early changes in ulcerative colitis.
NON-HODGKIN LYMPHOMA IN THE GIT:
The gut is the most commonly involved extranodal site of lymphoma. The most common sites, in order of descending frequency, are stomach, small intestine, and colon, especially cecum. Eighty percent of gastrointestinal lymphomas are of B-cell origin.
Sonography classically shows transmural circumferential, profoundly hypoechoic wall thickening up to 4 cm in diameter, with loss of normal stratification. This pattern, also known as the "pseudokidney" sign in longitudinal views. The pseudokidney sign is often seen in lymphoma because of extensive hypoechoic bowel wall thickening, but it can be seen in any bowel disorder leading to marked bowel wall thickening. Other findings include nodular or bulky tumor spread caused by extraluminal involvement. Sonographic patterns favoring the diagnosis of a non-Hodgkin's lymphoma over adenocarcinoma are transmural circumferential, profoundly hypoechoic wall thickening with preserved peristalsis; lack of intestinal obstruction, because narrowing of the lumen is uncommon; involvement of a long stretch of the gut; and the presence of multiple prominent regional lymph nodes.
Other findings include nodular or bulky tumor spread caused by extraluminal involvement. Mesenteric tumor spread and bulky tumor growth need biopsy for definite diagnosis because they cannot be reliably differentiated from other diseases such as primary bowel tumors or metastases. Isolated mucosal involvement is rare and leads to hyperechoic thickening of the mucosa. Sonographic patterns favoring the diagnosis of a non-Hodgkin's lymphoma over adenocarcinoma are transmural circumferential, profoundly hypoechoic wall thickening with preserved peristalsis; lack of intestinal obstruction, because narrowing of the lumen is uncommon; involvement of a long stretch of the gut; and the presence of multiple prominent regional lymph nodes.
Profound hypoechoic wall thickening. Non-Hodgkin lymphoma
"Pseudokidney" sign in ileocecal region: marked hypoechoic thickening of bowel wall resembling form of kidney in longitudinal sonogram of cecum. Non-Hodgkin lymphoma
ACUTE TERMINAL ILEITIS
The clinical symptoms of acute ileitis are right-sided lower abdominal pain, diarrhea, and nausea, with an accelerated erythrocyte sedimentation rate, positive C-reactive protein, and leukocytosis. Caused by Yersinia species but Campylobacter and Salmonella species may also be cultured. Reported sonographic features include hypoechogenic mural thickening of the terminal ileum and cecum between 6 and 10 mm with hypoechoic swollen ileal folds in the edematous mucosa. Hypoechoic enlarged mesenteric lymph nodes are frequently seen. Color Doppler sonography in patients with infectious ileitis shows increased flow centrally rather than peripherally (as in appendicitis).
Tuberculous enteritis and Behçet's syndrome also predominantly affect the ileocecal region
The typical finding of acute appendicitis in transverse sonograms is the target sign with a hypoechoic center, an inner hyperechoic ring, and an external thicker hypoechoic ring. In sagittal images, the inflamed appendix is seen as a blind-ending noncompressible tubular structure. Focal or circumferential loss of the inner layer of echoes usually indicates gangrenous inflammation and ulceration of the submucosa.
The diagnosis can be established with confidence if the appendix is noncompressible, shows no peristalsis, and measures more than 6 mm in diameter on axial images, and if compression leads to a localized pain response. The surrounding mesentery is often inflamed, which can be seen as a hyperechoic diffuse halo sign around the appendix. If an appendicolith is identified in an appendix of any size, the findings of the examination are always considered positive. A simple additional color Doppler examination may be helpful in the diagnosis of early acute appendicitis. The presence of visible hyperemia or increased flow in the hypoechoic muscular layer of the bowel wall may be a marker of appendicitis, whereas increased flow in the mucosal layer most likely represents enteritis. Increased flow in the fat surrounding the appendix is indicative of transmural extension of the inflammation with mesenteric response. An inflamed appendix rarely measures more than 15 mm in transverse diameter, which usually allows differentiation from ileitis. A markedly enlarged or perforating appendix or dilated fallopian tubes may lead to interpretive pitfalls.
"Target" sign (curved arrows) in acute appendicitis. On transverse image, inflamed appendix is seen with hypoechoic center, inner hyperechoic ring, and outer hypoechoic ring. Note hyperechoic circular area (straight arrows) of inflamed mesentery ("halo" sign).
Longitudinal sonogram of inflamed appendix in same patient shows blind-ending tubular structure (arrow) of at least 6 mm in diameter.
Longitudinal section of inflamed appendix reveals five round hyperechoic appendicoliths with acoustic shadows.
A statistically significant association exists between perforation and two sonographic findings: loculated pericecal fluid and loss of the echogenic submucosa. Abscess formation is the major complication of perforating appendicitis. Abscesses may extend into the pelvis or into the peritoneal spaces of the upper abdomen. They may be sonolucent or appear as a complex mass.
SMALL BOWEL DISEASES:
Mesenteric infarction in its late stages leads to small-bowel wall thickening. In the early stages, however, no bowel wall thickening may be seen. Doppler sonography can aid in differentiating ischemic and inflammatory bowel wall thickening.
In approximately 90% of cases, small-bowel infarctions are due to arterial hypoperfusion; only 10% are caused by mesenteric vein occlusion. Acute intramural intestinal hematoma leads typically to a homogeneous hypoechoic symmetric thickening of a long stretch of the affected bowel segment, with reduced or absent peristalsis and marked luminal narrowing. In the subacute stage, strong internal echoes caused by thrombi may mimic an abscess.
Peritoneal carcinomatosis is the most frequent malignant lesion of the small bowel and may lead to irregular wall thickening with the typical contraction of several bowel loops to a conglomerate.
Lipomas are the second most common tumors of the small intestine and occur with greatest frequency in the distal ileum and at the ileocecal valve. The location of these tumors is submucosal, or, less frequently, is subserosal. Adenocarcinoma is the second most common small intestine malignancy and the peak incidence is in the seventh decade of life.
Striking thickening of the colonic wall with a wide inner circle of heterogeneous medium echogenicity surrounded by a narrow hypoechoic muscularis propria is found in all patients, reflecting the gross submucosal edema. The lumen of the colon is almost completely effaced by the mural edema, and 64-77% of the patients have ascites. Pseudomembranous colitis shows typically a strong folding or gyral pattern of the swollen submucosa.
Sonographic features of diverticulitis include visualization of diverticula, thickening of the bowel wall, inflammatory changes in the pericolic fat (typically on the mesenteric side of the colonic wall) , intramural or periocolic abscess , and (usually) severe local tenderness induced by graded compression. Diverticula are round or oval echogenic foci seen in or right next to the gut wall, mostly with internal acoustic shadowing. Thickening of the bowel wall is usually considered present when the distance from the echogenic lumen interface to the hyperechogenic serosa and pericolic fat exceeds 4 mm. Inflammatory changes in the pericolic fat are seen as ill-defined echogenic areas surrounding the thickened colon segment.
Pericolic abscesses typically present as hypoechoic masses adjacent to the inflamed bowel. The major sonographic finding in patients with uncomplicated acute diverticulitis of the right colon has been found to be a hypoechoic round or oval focus protruding from the segmentally thickened colonic wall and representing small abscesses in the pericolic fat.
Massive hyperechoic inflammatory infiltration seen on mesenteric side of sigmoid colon.
Echolucent fistula is seen in mesentery with small, hyperechoic, gas-containing abscess (arrow).
Diverticulum of sigmoid colon seen as focal hyperechoic intramural structure with acoustic shadow
Colonic carcinomas have two typical sonographic appearances. The first type is seen as a localized hypoechoic mass up to 10 cm or more with an irregular shape and a lobulated contour. The intraluminal gas, seen as a cluster of high amplitude, is usually eccentrically located around the mass. The second type shows segmental eccentric or circumferential thickening of the colonic wall. The mural thickening may be irregular but not as severe as in the first type. The central echo clusters are small because the diseased lumen is usually narrow. This type leads frequently to colonic obstruction. Rectum carcinomas are seen only when the bladder is well-filled.
Other features: localized irregular thickening of the colonic wall with heterogeneous low echogenicity; irregular contour; lack of movement or change in configuration on real-time scanning; and absence of a layered appearance of the colonic wall. Other findings include lymphadenopathy in most patients and abscess formation in 10% of patients.
Only 5-10% of all intussusceptions occur in adults. The clinical symptoms may suggest partial obstruction of the intestine, but diagnosis may be difficult because symptoms are often nonspecific. The ileocecal region is the most commonly affected area in children, whereas there is no clearly preferred anatomic site in adults. Most intussusceptions in children are idiopathic and are presumed to be the result of enlarged lymphoid follicles in the terminal ileum. An organic cause can be shown in as many as 90% of cases in adults. The leading mass is nearly always a tumor of the intestinal wall, usually malignant in intussusceptions of the colon and benign in intussusceptions of the small intestine. The sonographic hallmark of intussusception has been described as the target, "doughnut," or "bull's-eye" sign. Typically, one finds two hypoechoic rings separated by a hyperechoic ring or crescent on axial images. On longitudinal images, a pseudokidney structure or layering of hypoechoic lines with hyperechoic areas is observed. The outer hypoechoic ring is formed by the intussuscipiens and the everted returning limb of the intussusceptum, with their mucosal surfaces face to face. The center of the intussusception varies with the scan level. At the apex, the center is hypoechoic because of the entering limb of the intussusceptum. At the base, the entering bowel wall forms a hypoechoic center that is surrounded by the hyperechoic mesentery. In a case of surgically proven triple jejunojejunocolonic intussusception, three hypoechoic rings separated by two hyperechoic rings were found on sonography.