Small Bowel Anatomy
The wall of the small intestines is made of what layers?
What creates the characteristic small bowel fold pattern?
What artery supplies the jejunum and ileum?
Which small bowel features larger, more feature-full folds and larger villi?
- The wall of the small intestine is made of four layers, from outside in:
- Serosa.
- Muscularis (thin longitudinal and thick circumferential smooth muscle).
- Submucosa.
- Mucosa (consists of intestinal villi, circular folds, glands, and lymphoid tissue).
- Valvulae conniventes create the characteristic small bowel fold pattern.
- The superior mesenteric artery SMA supplies both the jejunum and ileum. A common small bowel mesentery anchors the jejunum and ileum to the posterior abdominal wall.
- The jejunum features larger, more feature-full folds and larger villi compared to the ileum.
SBO
MCC? Other causes?
Radiographic findings of small bowel obstruction
What view(s) are generally necessary to confidently diagnose obstruction?
Potential false positives for diagnosing SBO on plain radiographs include?
- Small bowel obstruction SBO is common and most often due to adhesions from prior surgery or hernia. Neoplasm, stricture, and intussusception are less common causes.
- An abdominal radiograph is often the initial imaging evaluation for suspected obstruction.
- Radiographic findings of SBO include small bowel distention and multiple air-fluid levels at different heights seen on the upright view. In addition, the lack of gas in the colon is especially suggestive of obstruction.
- An upright or decubitus view is generally necessary to confidently diagnose obstruction.
- Potential false positives for diagnosing SBO on plain radiographs include:
- Ileus with prior colectomy: Would not see gas in the colon.
- Ileus with ascites: Ascites often compresses the ascending and descending colon and rectum as these structures are not on a mesentery. However, gas in the transverse colon and sigmoid colon is
still apparent.
CT Imaging of SBO
What is the highly specific finding to dx SBO?
In addition to dx, CT can show what else?
Provide a systematic approach for diagnosing SBO on CT!
What is the “never miss lesion”?
What are the signs of ischemia or impending ischemia in rough order of severity?
- CT is highly sensitive and specific for diagnosis of SBO. Small bowel distention >3 cm with a transition point to collapsed bowel is highly specific for a small bowel obstruction.
- In addition to diagnosing obstruction, CT can show the transition point, the cause of obstruction, and potential complications of obstruction such as ischemia or strangulation.
- First, look for the transition point to decompressed bowel to determine the cause.
- Second, always determine if the obstruction is simple or closed-loop. A closed-loop obstruction is a never miss lesion as there is very high risk for bowel ischemia and severe morbidity and mortality.
- Third, evaluate for signs of ischemia or impending ischemia, which include (in rough order of severity):
- Engorged mesenteric vessels.
- Ascites surrounding the bowel, due to increased capillary permeability.
- Wall thickening , due to submucosal edema.
- Lack of bowel wall enhancement , due to vasoconstriction or under-perfusion. Note that the presence or absence of bowel wall enhancement can only be assessed if positive oral contrast was not given.
- Pneumatosis intestinalis, which is gas in the bowel wall due to necrosis. Pneumatosis produces multiple small locules of gas seen circumferentially in the bowel wall.
Other than small bowel distention >3cm and a transition point to decompressed bowel, what is an additional helpful CT finding of SBO?
Where is this “sign” often seen? Why is this helpful?
In what case is this sign especially helpful?
What is this sign thought to be due to?
- In addition to small bowel distention >3 cm and a transition point to decompressed bowel, an additional helpful CT finding of SBO is the small bowel feces sign, which describes particulate feculent material mixed with gas bubbles in the small bowel that resembles the CT appearance of stool.
- The small bowel feces sign is often seen just proximal to the transition point and is helpful to localize the site of transition.
- The small bowel feces sign may be especially helpful in subacute or partial obstruction, which can otherwise be difficult to diagnose.
- The small bowel feces sign is thought to be due to bacterial overgrowth and undigested food.
Closed Loop Obstruction
Why is this so important?
What does it represent?
What may it be secondary to? Formation of what can lead to volvulus? What’s the big deal?
CT imaging features?
- Closed loop obstruction is a surgical emergency that may lead to bowel ischemia.
- Closed loop obstruction represents obstruction of both the efferent and afferent segments of a single loop of bowel.
- Closed loop obstruction may be secondary to adhesions or hernia. The formation of a narrow pedicle can lead to volvulus, which predisposes to ischemia.
- CT imaging features include a U-shaped distribution of the bowel loop with radially oriented vessels. If volvulus is present, the whirl sign may be seen, due to twisting of mesenteric vessels.
Obstruction due to Adhesions
Why do adhesions occur? What is the most common cause of SBO?
How do you diagnose adhesions on CT?
The vast majority of patients with SBO due to adhesions have had what in the past?
- Adhesions from prior surgery or intra-peritoneal inflammatory process are the most common cause of small bowel obstruction.
- Adhesions are an imaging diagnosis of exclusion. on CT, a transition point is seen, but no obvious cause for the transition (e.g., no mass or hernia, etc.) is identified.
- The vast majority of patients with SBO due to adhesions have had prior abdominal surgery.
Obstruction due to External Hernia
Where do most hernias occur? The majority of which are what kinds?
An inguinal hernia can be either what two things? Describe them both.
- Protrusion of bowel through the abdominal wall is the second most common cause of small bowel obstruction. Approximately 75% of external hernias occur in the groin, with the majority being inguinal hernias.
- An inguinal hernia may be either indirect or direct, depending on the relation of the hernia to the inferior epigastric vessels.
- Indirect : Indirect inguinal hernia is the most common type and is more common in males. The neck of the hernia is lateral to the inferior epigastric vessels. Hernia contents travel with the spermatic cord, often into the scrotum. Indirect inguinal hernias are considered a congenital lesion due to a patent processus vaginalis.
- Direct : The neck of an indirect inguinal hernia is medial to the inferior epigastric vessels, protruding through a weak area in the anterior abdominal wall. The hernia contents do not go into the scrotum.
- In an obturator hernia, bowel herniates through the obturator canal. obturator hernias are almost always seen in elderly women due to pelvic floor laxity.
- The key imaging finding is bowel located between the pectineus and obturator muscles.
- It is important to correctly diagnose an obturator hernia preoperatively. An obturator hernia requires a very different surgery from inguinal hernia, and has an especially high morbidity and mortality if incarcerated.
- Ventral hernia is often due to prior laparotomy.
Transmesenteric Hernia
What is it?
What are the three true mesenteries?
What is the most common type of transmesenteric hernia? Where is this most commonly seen?
What makes it difficult to diagnose? What are some clues to help dx?
Internal hernias have a high rate of what? What sign can be present?
What is the most common type of hernia in children? What is the cause?
- Transmesenteric hernia is a broad category of bowel herniation through defects in any of the three true mesenteries (small bowel mesentery, transverse mesocolon, and sigmoid mesentery).
- The most common type of transmesenteric hernia is the transmesocolic hernia, due to a defect in the transverse mesocolon (mesentery of the transverse colon). Transmesocolic hernia is seen most commonly post RYGB or biliary-enteric anastomosis from a liver transplant.
- The lack of confining hernia sac and variable imaging appearance make diagnosis difficult. A clue on imaging may be posterior displacement of the colon, with small bowel located anterior to the colon. The SMA & SMV may be displaced and engorged.
- Internal hernias carry a high rate of volvulus. If volvulus is present, the whirl sign may be visible.
- Transmesenteric hernias are also the most common type of hernia in children, not due to surgery but secondary to a congenital mesenteric defect thought to be from prenatal intestinal ischemia. In children, the mesenteric defect has a variable position.
Paraduodenal Hernia
These were the most common internal hernias until what came along?
What are paraduodenal hernias and what are they due to?
Which side are they more common on?
What are they associated with?
What is a common clinical complaint?
What is Landzert’s fossa? What is a key imaging finding for paraduodenal hernia?
- Paraduodenal hernia was previously the most common internal hernia (older literature states 53% of internal hernias were paraduodenal), prior to the rise in gastric bypass surgery. Paraduodenal hernias are congenital anomalies, due to embryologic failure of mesenteric fusion and resultant mesenteric defect. They more commonly occur on the left.
- Paraduodenal hernia is associated with abnormal rotation of the intestine.
- A common clinical complaint described by patients with paraduodenal hernia is chronic postprandial pain often relieved by massaging, which reduces the hernia.
- In the more common left paraduodenal hernia, the bowel can herniate through a mesenteric defect named Landzert’s fossa, located behind the ascending (fourth) duodenum. The key imaging finding is a cluster of small bowel loops between the pancreas and stomach.
Foramen of Winslow Hernia
What communicates between this foramen?
Key imaging features of a Formen of Winslow hernia?
- Foramen of Winslow Hernia: The foramen of Winslow is the communication between the lesser sac and the greater peritoneal cavity.
- The key imaging features of a foramen of Winslow hernia are dilated loops of bowel in the upper abdomen and presence of mesentery between the IVC and main portal vein.
Obstruction due to Neoplasm
What are clues to detect the presence of a mass intrinsic to the small bowel?
What are the primary small bowel neoplasms that can cause intrinsic bowel obstruction?
What are metastatic causes that can cause intrinsic mass bowel obstruction?
Which met is known to cause intussusception?
How often does lymphoma cause obstruction?
- A mass intrinsic to the bowel or external compression from an extrinsic mass may cause small bowel obstruction. An extrinsic mass is usually straightforward to diagnose by CT.
- Although the presence of an intraluminal mass may be more difficult to detect on CT, clues to the presence of an intrinsic mass include irregular bowel wall thickening and/or regional lymphadenopathy.
- Primary small bowel neoplasm causing intrinsic bowel obstruction may be due to adenocarcinoma, GIST, and carcinoid. Metastatic causes of intrinsic bowel neoplasm include melanoma, ovarian, and lung cancer. Melanoma is known to cause intussusception.
- Lymphoma is generally a “soft” tumor and rarely causes obstruction. Aneurysmal expansion of the small bowel wall is a classic appearance, but presentation is highly variable.
Obstruction due to Intussusception
Imaging appearance?
What should this raise suspicion for?
- While transient intussusceptions are a common incidental finding, an intussusception causing obstruction should raise suspicion for an underlying lesion and prompt surgery.
Obstruction due to Crohn Disease
Which kind is of Crohn’s is most likely to cause an obstruction?
- Stricture or active enteritis is an important cause of bowel obstruction in Crohn disease, especially the fibrostenotic subtype.
Gallstone Ileus
What is the cause?
What is Rigler’s triad?
- Gallstone ileus is due to a gallstone that has eroded through into the small bowel, causing the classic Rigler’s triad of pneumobilia (from cholecystoduodenal fistula), small bowel obstruction, and ectopic gallstone within the small bowel.
Enteritis
What is enteritis? What is the most common CT manifestation of enteritis? What else may be present?
- Enteritis is inflammation of the small bowel. The most common CT manifestation of enteritis is bowel wall thickening. Mesenteric stranding or free fluid may also be present.
Crohn Disease
What is Crohn disease? Where is the most common site of involvement?
What are the earliest histologic changes?
Historically, what modalities were used to evaluate Crohn?
CT and MR enterography have what advantages over this. What disadvantages?
Most common imaging finding on all modalities?
Fluoroscopy findings?
- Crohn disease is a chronic granulomatous inflammatory condition that may affect any part of the gastrointestinal tract from the mouth to the anus. Involvement is discontinuous, with characteristic skip lesions of intervening normal GI tract. The most common site of involvement is the small bowel, especially the terminal ileum.
- The earliest histologic changes occur in the submucosa, seen on imaging as aphthous ulcers due to lymphoid hyperplasia and lymphedema.
- Endoscopy and barium fluoroscopy (small bowel follow-through, enteroclysis, and barium enema) have historically been the modalities to evaluate Crohn disease. More recently, however, CT and MR enterography are emerging as the exams of choice.
- The advantages of CT and MRI are the ability to visualize beyond the bowel lumen to evaluate the bowel wall, presence of extraintestinal complications, and the vasculature.
- The disadvantages of CT and MRI compared to fluoroscopy and endoscopy are reduced spatial resolution and limited sensitivity for detecting subtle early signs of disease.
- The most common imaging finding on all modalities is wall thickening of the terminal ileum.
- Fluoroscopic findings include thickened, nodular folds in the affected regions of small bowel, luminal narrowing, mucosal ulceration, and separation of bowel loops. The typical cobblestone appearance seen on endoscopy and fluoroscopy is a result of crisscrossing deep ulcerations.
Which subtype of Crohn can present with bowel obstruction?
What leads pseudosacculations, and what side of the bowel loop do they form?
What is the classic flouro sign?
What are some complications of Crohn disease?
- Complications of Crohn disease include:
- Bowel strictures
- Fistulae
- Abscesses.
Scleroderma
What is scleroderma?
What is the primary insult on the GI tract? What does it lead to, which subsequently leads to what sequela?
Radiographic findings?
Treatment?
- Scleroderma is a systemic disease characterized by the deposition of collagen into multiple internal organs and the skin.
- The primary insult to the gastrointestinal tract in scleroderma is impaired motility due to the replacement of the muscular layers with collagen, which leads to slowed transit and subsequent bacterial overgrowth, progressive dilation, and pseudo-obstruction.
- Radiographic findings are sacculations on the antimesenteric border (sideopposite where the mesentery attaches) and a hidebound bowel due to thin, straight bowel folds stacked together.
- Treatment is with antibiotics for bacterial overgrowth and prokinetic drugs such as erythromycin or octreotide for bowel motility.
What can cause infectious small bowel enteritis?
Which organisms have a propensity to affect the terminal ileum, mimicking Crohn?
What is the most common food-borne gastroenteritis? What is the CT appearance? Fluoroscopic appearance?
- Several bacterial, viral, and fungal organisms may cause enteritis.
- Yersinia and tuberculosis have a propensity to affect the terminal ileum, mimicking Crohn disease.
- Salmonella is the most common cause of food-borne gastroenteritis and causes segmental distal small bowel thickening on CT and segmental nodular thickened folds on fluoroscopy.
Radiation Enteritis
What are the long-term effects of rations to the pelvis?
Clues to this diagnosis?
Image findings?
Complication of radiation enteritis?
- Long-term effects of radiation to the pelvis include adhesive and fibrotic changes to the mesentery and small bowel.
- Clues to the diagnosis of radiation enteritis include a history of radiation therapy and regional involvement of bowel loops not confined to a vascular territory.
- Imaging findings include mural thickening and mucosal hyperenhancement with narrowing of the lumen. Radiation enteritis may be a cause of small bowel obstruction.
Whipple Disease
What is this?
GI manifestation? What else may it cause?
Characteristic image finding? In what other disease may this be seen?
Radiographically, what does Whipple disease cause? Contrast to the main DDx.
- Whipple disease is due to infection by Tropheryma whippelii, which manifests in the GI tract as malabsorption and abdominal pain. Whipple disease may cause cardiac symptoms, arthralgias, neurologic symptoms, and increased skin pigmentation.
- MNEMONIC: PAS+ the Foamy WHIPPed cream in a CAN = PAS+ foamy macrophages - cardiac, arthralgias and neuro symptoms
- Whipple disease characteristically causes low attenuation adenopathy that may appear similar to the cavitating mesenteric lymph node syndrome seen in celiac disease.
- Radiographically, Whipple disease causes thickening and nodularity of duodenal and proximal small bowel folds. In contrast to celiac disease, there is typically no hypersecretion.
Graft Versus Host Disease
What is it? What organ systems are most commonly affected?
Image findings? What is the classic barium finding and how often is it seen?
- Graft versus host disease is a complication of bone marrow transplantation. The skin, liver, and gastrointestinal tract are most commonly affected.
- Imaging findings of GVHD include nonspecific wall thickening and effacement of the normal small bowel fold pattern. While the classic barium finding is the ribbon bowel, this is not often seen.
Complications of Celiac Disease
What is the malignant complication? How would this manifest on imaging?
What are the other GI manifestations?
What can happen to the spleen?
Veins?
Lab abnormalities? Skin abnormalities?
What is Cavitating mesenteric lymph node syndrome?!
The differential diagnosis of low attenuation mesenteric lymph nodes includes?
- An important complication of celiac disease is small bowel t-cell lymphoma, which may manifest as an exophytic mass, circumferential bowel wall thickening, or enlarged mesenteric lymph nodes.
- Intussusception, thought to be due to uncoordinated peristalsis, without a lead-point mass.
- Pneumatosis intestinalis, thought to be due to dissection of intraluminal gas through the inflamed bowel wall. Pneumatosis in the settng of celiac disease is not thought to reflect bowel ischemia.
- Splenic atrophy.
- Increased risk of venous thromboembolism.
- Lab abnormalities include anemia (secondary to malabsorption), leukopenia, and immunoglobulin deficiency.
- Skin abnormalities include the characteristic dermatitis herpetiformis rash.
- Cavitating mesenteric lymph node syndrome CMLNS is a very rare complication of celiac disease, with only 36 reported cases in the literature. The central portion of affected lymph nodes shows low attenuation due to liquid necrosis. CMLNS is thought to be highly specific for celiac disease when seen in combination with villous atrophy and splenic atrophy. The differential diagnosis of low attenuation mesenteric lymph nodes includes tuberculosis, Whipple disease, treated lymphoma, and CMLNS.
