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Background
Internal hernia is defined as protrusion of abdominal viscera, most commonly small bowel loops, through a peritoneal or mesenteric aperture into a compartment in the abdominal and pelvic cavity. The most common manifestation of an internal hernia is strangulating small bowel obstruction (SBO), that occurs after a closed-loop obstruction. Multidetector CT (MDCT) represents the first-line imaging technique in these patients.

Educational Goals / Teaching Points
Internal hernias although are uncommon, they must be considered in differential diagnosis of acute abdominal pain, especially in patients with strangulated closed loop SBO. The most common types of internal hernias according to the classification of Welch in the descending order of frequency are as follows. 1) Paraduodenal hernias (left > right); 2) pericecal hernia; 3. through the foramen of Winslow; 4. transmesenteric; 5. intersigmoid; 6. supravesical and pelvic; and 7. transomental.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The most common manifestation of an internal hernia is strangulating SBO, that occurs after a closed-loop obstruction However, the clinical manifestations range from mild digestive symptoms to acute abdomen, as symptom severity relates to duration and reducibility of the hernia and the presence or absence of strangulation and incarceration. This nonspecific clinical presentation often leads to a delay in diagnosis, in most cases made at the time of laparotomy and consequently in proper treatment, carrying risk of serious complications; therefore, when the possibility of internal hernia is considered, a rapid imaging evaluation is necessary to aid an early diagnosis and a prompt intervention. MDCT, with its wide availability, has become the first-line imaging technique in these patients and plays an important role in the preoperative diagnosis and planning of surgical intervention. On CT, key imaging findings of internal hernia include a sac-like mass or cluster of dilated bowel loops in the setting of SBO; convergence, engorgement, crowding of vessels and mesenteric fat at the hernia orifice; and displacement of surrounding structures around the hernia sac.

Conclusion
In the acute setting, a prompt imaging diagnosis is mandatory to avoid intestinal ischemia and necrosis. Radiologists must familiarize themselves with specific CT signs and etiology to aid an accurate and quick preoperative diagnosis and improve patient’s outcome. Radiologists play a critical role in the acute setting in detection of these internal hernia as early surgical intervention reduce high mortality and high morbidity. An internal hernia is a potentially life-threatening condition and surgical emergency because the bowel entrapment in one of the defects can lead to acute intestinal obstruction with rapid evolution, if left untreated, into strangulation and ischemia. Urgent surgical intervention to prevent strangulation, which is responsible for high mortality, is imperative.