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Computed tomography of bowel obstruction: tricks of the trade Expert Rev. Gastroenterol. Hepatol. Early online, 1–11 (2015)
Massimo Galia*, Francesco Agnello, Ludovico La Grutta, Giuseppe Lo Re, Giuseppe Cabibbo, Emanuele Grassedonio, Bruno Giuseppe Gioia, Gianvincenzo Sparacia, Antonio Lo Casto, Roberto Lagalla and Massimo Midiri Section of Radiological Sciences, DIBIMED, University of Palermo, 90127 Palermo, Italy *Author for correspondence: Tel.: +39 09 1655 2326 Fax: +39 09 1655 2324 [email protected]
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Bowel obstruction (BO) is a frequent cause of hospitalization and surgical consultation for acute abdominal pain. It is usually suspected at physical examination, but clinical and laboratory data are often nonspecific. Thus, computed tomography plays a crucial role in a correct diagnosis of BO. Indeed, computed tomography can confirm a diagnosis of BO, and identify the location and cause of the obstruction. In this review, the computed tomography appearances of common and uncommon causes of BO and pseudo-obstruction are reviewed. KEYWORDS: bowel obstruction . bowel pseudo-obstruction . Colon . CT . small bowel
Bowel obstruction (BO) is a frequent cause of hospitalization, and accounts for up to 20% of surgical consultations for acute abdominal pain [1]. BO is an interruption of the normal flow of bowel contents. The bowel proximal to the obstruction dilates, whereas the distal bowel is normal or collapses [2]. Severe bowel overdistension and strangulation may impair intestinal perfusion, and cause wall necrosis and perforation, complications that increase the mortality associated with BO [2]. The small bowel is involved in 60–80% of cases, whereas the colon is involved in 20–40% of cases [1]. Causes of BO are broad, and range from tumors and inflammatory conditions to ingested material. Most common symptoms and signs are nausea, vomiting, cramping abdominal pain, obstipation and abdominal distension. However, even if BO is usually suspected at physical examination, symptoms are often vague, and laboratory tests nonspecific. Thus, imaging plays a crucial role in a correct diagnosis of BO. Indeed, imaging can confirm or exclude a diagnosis of BO, and can identify the location and cause of the blockage. Abdominal radiography and computed tomography are the most commonly used imaging modalities for the evaluation of suspected BO. Abdominal radiography is traditionally the first modality because it is fast, inexpensive and widely available. Common radiographic findings include distended gas-filled bowel loops, 10.1586/17474124.2015.1051030
air–fluid levels greater than 2.5 cm in width, and differential air–fluid levels within the same small bowel loop [3,4]. However, abdominal radiography is not definitive in most patients with suspected BO, and rarely identifies the location and cause [5]. Computed tomography is more sensitive and specific than abdominal radiography, and is currently the modality of choice for a diagnosis of BO [5]. Computed tomography can depict the site and etiology of the obstruction, as well as potential complications, and evaluate adjacent soft tissues and abdominal organs. The use of thin computed tomography slices and multiplanar reformatted (MPR) images improves the identification of the location and cause of the obstruction [6,7]. Computed tomography results have important implication for patient care because they can influence the decision to proceed with rapid surgery or a conservative treatment. In this article, we describe our computed tomography protocol, the computed tomography findings of BO and bowel pseudoobstruction (BPO), and the causes. Computed tomography protocol
Computed tomography protocol is designed to answer specific clinical questions, and to minimize radiation exposure. Computed tomography images are acquired from the diaphragm to the symphysis pubis using the minimum collimation (0.625 mm) and isotropic data sets. Both the reconstruction slice thickness and reconstruction
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Galia, Agnello, La Grutta et al.
Table 1. Incidence of causes of bowel obstruction in the small bowel and colon. Small bowel
Colon
Peritoneal adhesions
Common
Less common
Abdominal hernias
Common
Uncommon
Neuroendocrine tumors
Uncommon
Extremely rare
Less common
Common
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Extrinsic causes
Intrinsic causes Neoplasia
intraluminal fluid and air provide intrinsic enhancement; and the administration of enteric contrast materials may interfere with the evaluation of bowel wall and increase the acquisition time [8]. One advantage of the use of enteric contrast is that it allows differentiation of partial BO from complete BO [1]. If the contrast passes through the site of obstruction, a complete BO is ruled out [1]. Bowel obstruction
The diagnosis of BO requires the presence of bowel dilatation (caliber > 2.5– Diverticulitis Extremely rare Common 3 cm in the small bowel; >9 cm in the Crohn’s disease Uncommon (terminal ileum) Uncommon (ascending colon) cecum; >6 cm in the remaining colon) and the presence of an abrupt transition Intraluminal causes zone [3]. The transition zone is the site of Fecaloma Not applicable Common caliber change from the proximal dilated Bezoar Uncommon Not applicable bowel to the distal normal or collapsed bowel, and indicates the site of obstrucGallstone ileus Uncommon (terminal ileum) Not applicable tion. The transition zone can be a point or a longer segment of narrowing. Identiinterval are 3 mm. Thinner collimation and isotropic acquisition fication of the transition zone is crucial because it facilitates the allow the creation of MPR images. We routinely administer 110– recognition of the underlying cause of the obstruction, and dif120 ml of an intravenous nonionic contrast material with an ferentiates BO from BPO. Causes of BO are multiple, and can iodine concentration of 350–370 mg/ml except in patients with be broadly classified into three categories: extrinsic, intrinsic allergy to iodinated contrast material and marked renal and intraluminal. Extrinsic causes originate adjacent to the impairment. Images are acquired about 70 s from the start of con- transition zone, and are associated with extraluminal findings. trast material injection. This delay is usually sufficient to evaluate Intrinsic causes result from thickening and/or dysfunction of bowel wall enhancement and major mesenteric vessels. Unen- the bowel wall. Intraluminal causes are located within the hanced images are useful to determine the basic bowel enhance- bowel lumen. The table summarizes the incidence of each cause ment, and to exclude mural hemorrhage. However, we perform of BO in the small bowel and colon (TABLE 1). the unenhanced phase only when bowel ischemia is suspected. Enteric contrast materials (dilute barium or water-soluble Extrinsic causes iodinated solutions) are not routinely used because retained Peritoneal adhesions Peritoneal adhesions are the most common cause of small BO [9,10]. Peritoneal adhesions are abnormal fibrous bands that connect adjacent tissues and organs in the abdomen and pelvis [9,10]. They are most commonly postoperative, whereas a minority is congenital or a consequence of peritoneal inflammation or carcinomatosis [9,10]. Adhesions are usually not clearly visualized at computed tomography unless they are complicated by inflammation or carcinomatosis [9]. Thus, the diagnosis of adhesive BO is usually one of exclusion based on the absence of another discernible cause [9,10]. Identification of indirect signs of extraluminal compression can allow for a correct diagnosis. These include a smooth, tapered appearance of the transition zone (beak sign), and an extrinsic impression on the bowel wall at the level of the transition zone (fat notch sign) (FIGURE 1) [9,10]. These findings are nonspecific, however, Figure 1. Small BO secondary to adhesions in a 43-year-old and should be evaluated in conjunction with history and physwoman with previous cesarean section. Axial contrastical data [10]. Several authors have reported a therapeutic role enhanced computed tomography image shows fluid-filled and dilated ileal loops (asterisk), and a smooth tapering of the bowel of water-soluble contrast materials (e.g., Gastrografin) in at the transition zone (arrow), corresponding to the beak sign. patients with adhesive small BO [11–13]. Due to high Intussusception
Common
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Less common
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Computed tomography of BO: tricks of the trade
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osmolarity, water-soluble contrast materials promote the passage of fluids into the bowel lumen, and increase the pressure gradient through the site of obstruction [11–13]. The passage of the contrast is usually monitored with serial abdominal radiographs. If the contrast reaches the colon within 24 h, a complete BO is ruled out, and conservative treatment is continued [11–13].
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Abdominal hernias
Abdominal hernias are the second most common extrinsic cause of BO, and are most frequent in elderly patients [14]. An abdominal hernia is the protrusion of any abdominal organ or tissue through a congenital or acquired (iatrogenic or post-traumatic) orifice [15]. Computed tomography is useful in determining the location, the size of the neck, the measurement of the sac, the content and the shape of abdominal hernias. Abdominal hernias are classified into two main groups: external and internal. External hernias occur through an orifice in the abdominal or pelvic wall. The most common external hernias are inguinal, umbilical, paraumbilical, epigastric and incisional [15]. Other, less frequent, external hernias include femoral, Spigelian and hypogastric hernias [15]. Inguinal hernias account for the vast majority of BO, and, as the name implies, extend into the inguinal canal (FIGURE 2) [16]. The principal differential diagnosis for inguinal hernias is femoral hernias, because of their close origin. Sukuzi et al. reported that femoral hernias are usually located lateral to the pubic tubercle, and compress the femoral vein, whereas most inguinal hernias extend medial to the pubic tubercle, and, if large, can compress femoral vein [16]. Umbilical and paraumbilical hernias protrude through the umbilical ring and the linea alba (in proximity of the umbilicus), respectively [17]. Epigastric hernias occur in the linea alba, above the umbilicus, whereas incisional hernias are located at the site of surgical incision or, more rarely, in the vicinity of the stoma [17]. Internal hernias are less common than external ones, occur through a mesenteric orifice, and, as the name implies, are located within the abdominal cavity [17]. The most common types are paraduodenal and Petersen hernias [17]. Paraduodenal hernias develop through congenital peritoneal anomalies, the paraduodenal fossae, and are located under the right or left mesocolon [17]. Petersen hernias are a complication of abdominal surgery, especially after the creation of a Roux-en-Y anastomosis, and protrude through the transverse mesocolon [17,18]. The hallmarks in computed tomography diagnosis of internal hernias include a cluster of distended bowel segments with an abnormal location, and stretching and displacement of mesenteric vessels [18]. Abdominal hernias can contain fatty tissue, solid organs and/or bowel loops. When bowel loops migrate to the hernial sac, and become entrapped, BO and strangulation can occur. The likelihood of obstruction and strangulation correlates inversely with the size of the neck [17]. Internal, inguinal and femoral hernias have a greater tendency toward incarceration and/or strangulation. informahealthcare.com
Figure 2. Small bowel obstruction secondary to right inguinal hernia in a 64-year-old man. Axial contrast-enhanced computed tomography image show a right inguinal hernia containing ileal loops and omentum. The hernia sac extends medial to the pubic tubercle (arrowhead), and does not compress the common femoral vein (asterisk). Also note a coexisting left inguinal hernia.
Neuroendocrine tumors
Neuroendocrine tumors are rare gastrointestinal tumors of neuroendocrine origin, and most commonly occur in the small bowel, but rarely in the colon [19]. Both primary neuroendocrine tumors and mesenteric neuroendocrine metastases can be an extrinsic cause of BO [20]. Primary carcinoid tumors are intrinsic lesions of the bowel, but are classified as an extrinsic cause of BO because the obstruction is caused largely by a mesenteric desmoplastic reaction [20]. Typical computed tomography findings include strong enhancement, ill-defined, spiculated margins, encasement and narrowing of mesenteric vessels, and calcifications [19].
Figure 3. Colon adenocarcinoma in a 68-year-old man. Axial contrast-enhanced computed tomography images show a circumferential mass in the sigmoid colon (arrows) that causes distension of proximal colon.
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Figure 4. Colonic intussusceptions due to adenocarcinoma in a 62-year-old woman. Axial contrast-enhanced computed tomography image shows the typical appearance of intussusception, with the intussuscipiens (arrow), the intussuscepted mesentery (dotted arrow), and the intussusception (asterisk). Note colonic dilatation.
Intrinsic causes Neoplasia
Neoplasia is a frequent cause of BO, and is most common in elderly patients [14]. Colorectal cancer should be suspected in any patient with colon obstruction. At computed tomography, colorectal cancer typically appears as a focal and heterogeneously enhancing mass with eccentric or annular extension (FIGURE 3) [21]. Computed tomography appearance may mimic that of several benign processes. Macari et al. [21] reported that stratified attenuation, symmetric thickening and segmental (10–30 cm) or diffuse longitudinal extension suggest a benign process, whereas heterogeneous attenuation, asymmetric thickening, and a focal longitudinal extension suggest a malignant process. However, there are numerous overlaps. For
Figure 5. Small bowel obstruction secondary to active Crohn’s disease in a 44-year-old woman. Axial contrastenhanced computed tomography image shows the thickened terminal ileum with a target appearance (arrows), and the prestenotic dilated ileum (asterisk), containing feculent material. Also note peritoneal-free fluid.
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example, a scirrhous carcinoma can sometimes be characterized by symmetric wall thickening and stratified attenuation [21]. Administration of contrast material via a rectal tube can help identify the site of obstruction and differentiate collapsed bowel loops from mural thickening due to the neoplasia. Metastases are the first malignant cause of small BO, but are rare in the colon [3]. Metastases result from intraperitoneal seeding, hematogenous spread from distant tumors (e.g., melanoma), and direct extension of advanced abdominal tumors [3]. Computed tomography appearance is not pathognomonic. Metastases can appear as a focal wall thickening; a circumferential, stenosing mass; or a polyp that causes intussusception [3]. Benign intestinal tumors (e.g., lipoma, non-neoplastic polyps) are frequently asymptomatic, and most commonly occur in the small bowel [22]. They can often cause intussusception and consequent BO [22]. Intussusception
Intussusception is a rare cause of BO in adults [23]. It most commonly occurs in the small bowel, but any part of the gastrointestinal tract can be involved [24]. Intussusception is the invagination of a bowel segment (intussusceptum) and its mesentery into the lumen of the contiguous bowel (intussuscipiens) [23]. Intussusception can occur with or without a lead point [23]. A lead point is an irritant that alters bowel peristalsis, and causes bowel intussusception and obstruction [23]. Several factors, including benign and malignant tumors, and suture materials can serve as a lead point [23]. The exact pathogenesis of non-leadpoint intussusception is not well understood; dysrhythmic contractions are thought to be the most likely mechanism [24]. Nonlead-point intussusceptions usually resolve spontaneously, do not cause BO, and are frequently found incidentally [23,24]. Computed tomography imaging appearance of bowel intussusception varies with the stage of the disease [24]. Early intussusception shows a characteristic ‘target appearance’, with an intraluminal soft tissue mass and an eccentric fat density area, the latter representing the intussuscepted mesentery and the former representing the intussusception (FIGURE 4) [24]. As the disease progresses, a ‘sausage-shaped’ pattern develops [24]. This appearance results from alternating areas of low and high attenuation representing bowel wall, mesenteric fat, fluid and air [24]. When intussuscepted wall thickening occurs because of ischemia and edema, a reniform pattern can be found [24]. Computed tomography identification of the lead mass is often difficult [24]. Evaluation of intussusception length and width can be useful in predicting the need for surgery. In particular, an intussusception length of 1 cm), and often has a loops (asterisk). (B) Axial contrast-enhanced computed tomography image obtained at a more cranial level shows an ovoid mass similar to that of (A, arrow) floating in the stratified appearance, with two or three stomach, consistent with gastric bezoar. symmetric layers (FIGURE 5) [31,32]. The term informahealthcare.com
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Appreciation of what is called the ‘comb sign’ (engorgement of the vasa recta) and enlarged mesenteric nodes can suggest that the disease is active [32]. Assessing the degree of inflammation is crucial in directing patients to the most appropriate treatment. Patients with predominantly inflammatory lesions are usually treated medically, whereas patients with fibrostenotic lesions can be candidates for surgery [30]. The imaging appearance of Crohn’s disease can overlap with that of ulcerative colitis [33]. However, a limited extension of the inflammation (mucosa and submucosa only), a left side or diffuse colonic involvement, and the absence of skip areas are more suggestive of ulcerative colitis than Crohn’s disease [33]. Another important differential diagnosis is bowel neoplasia [33]. Intraluminal causes Fecaloma
Figure 8. Small bowel feces sign in a 78-year-old woman with a large adenocarcinoma of the cecum. Oblique axial computed tomography image shows a large heterogeneously enhancing mass (arrowhead) in the cecum, and feculent material (arrow) within the pre-stenotic dilated terminal ileum. The small bowel feces sign helps recognize the transition zone and cause of obstruction.
Fecaloma is the most common intraluminal cause of colonic obstruction [34]. It occurs predominantly in elderly patients, but can also occur in neurologically impaired young patients [34]. Causes include chronic constipation, diets poor in fiber and fluids, colonic motility disorders and medications that slow intestinal transit (e.g., narcotics, NSAIDs, antidepressants) [35]. A fecaloma is an intraluminal collection of dehydrated feces with a diameter equal to or greater than the colon. Fecalomas usually occur in the rectosigmoid colon, where the bowel lumen is narrowest and the feces are most dehydrated. Computed tomography shows a gas containing, heterogeneously hypoattenuating mass within the colon lumen (FIGURE 6). Fecalomas can sometimes cause colonic obstruction, stercoral colitis, stercoral ulcers, colon perforation, and fecal peritonitis [34]. Fecaloma must be distinguished from a fecal impaction that can accumulate proximally to a stenosing tumor. Bezoar
Bezoar is a rare cause of BO [36]. A bezoar is composed of ingested material and is classified according to its composition. Trichobezoar and phytobezoar are the most common types. The former is composed of hairs, whereas the latter is composed of fruit and/or vegetables. Predisposing factors include previous gastric surgery, inadequate mastication and high fiber diets [36,37]. A bezoar usually forms in the stomach, and can transit in the small bowel, where it can cause intraluminal obstruction. The site of obstruction is often the terminal ileum, where the bowel lumen is narrowest [36,37]. On computed tomography, the bezoar appears as a round or ovoid, intraluminal, gas containing, hypoattenuating mass with mottled appearance (FIGURE 7) [36,37]. The most important differential diagnosis for bezoar is small bowel feces that accumulate proximal to the transition zone (FIGURE 8) [36]. This location helps to differentiate small bowel feces from bezoar, because the latter accumulates in the obstruction zone [36]. Figure 9. Gallstone ileus in a 64-year-old woman. Sagittal MDCT image shows a large gallstone (arrow) impacted in the distal ileus and proximal fluid filled, dilated bowel. Also note aerobilia (arrowheads).
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Other intraluminal causes
Gallstone ileus is an extremely uncommon complication of cholelithiasis [38]. It occurs when a large gallstone (>2.5 cm in Expert Rev. Gastroenterol. Hepatol.
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diameter) migrates to the bowel through a cholecystoenteric fistula, located between the gallbladder and the stomach or duodenum, and becomes impacted in the ileus (FIGURE 9) [38]. Characteristic computed tomography findings include a large, calcified stone impacted in the terminal ileum and pneumobilia [38]. Other less common intraluminal causes include ingested foreign bodies and endoscopic capsule retention.
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Complicated BO
The final step is to differentiate a simple BO from a complicated one, such as a closed-loop BO or a strangulated BO. Simple BO is characterized by single or multiple transition points located along the bowel course [2]. When transition points are adjacent (e.g., hernia), a closed loop (or incarcerated) BO occurs [2]. The most common causes are peritoneal adhesions and abdominal hernias. Closed-loop BO is characterized by progressive distension of the isolated bowel due to progressive accumulation of secreted fluid, and variable distension of the proximal bowel (FIGURE 10) [2]. If untreated, the involved bowel loops can rotate around their mesentery, resulting in a volvulus (FIGURE 11) [2]. Volvulus can occur in any part of gastrointestinal tract, but the most common sites are the small bowel, the cecum and the sigmoid colon. Imaging appearance partially varies depending on the site. Small bowel volvulus appears as dilated and dislocated small bowel loops. Recognition of the whirl sign can confirm a diagnosis of volvulus [39]. The whirl represents the rotation of the mesentery and the afferent and efferent bowel loops [39]. However, the whirl sign is nonspecific and a similar appearance can be found in patients with colonic volvulus and internal hernia. In sigmoid volvulus, the sigmoid colon is typically displaced to the right upper quadrant, and resembles a coffee bean on conventional radiograph and topogram [40]. Appreciation of two crossing transition zones projecting at a single location (X-marks-the-spot sign) suggests a complete twist, whereas the separation of sigmoid walls by surrounding mesenteric fat suggests an incomplete twist [40]. Cecal volvulus is less common than sigmoid volvulus. Three types of cecal volvulus have been described: the axial torsion type, loop type and bascule type [37]. The axial torsion type results from cecum rotation along its axis, and the dilated cecum appears in the right lower quadrant [41]. The loop type, in which the cecum both rotates and inverts, leads to displacement of the dilated cecum to the left upper abdominal quadrant [41]. The bascule type is the less frequent variant of cecal volvulus. It results from anterior folding of the cecum and is not associated with torsion of the cecum [41]. The dilated cecum is usually displaced in the mid abdomen [41]. Associated involvement of the ileum (ileocecal twist sign) is found in approximately 50% of cases of cecal volvulus [42]. Other useful, but nonspecific, signs of cecal volvulus are the coffee bean sign, the whirl sign and the X-marks-the-spot sign [42]. Strangulation indicates a BO with impairment of blood flow [2]. The severity is variable. Strangulation occurs most commonly in closed loop BO, and that has been attributed to direct venous compression and twisting of the mesentery. informahealthcare.com
Figure 10. Closed loop obstruction secondary to a left inguinal hernia in a 63-year-old man. Oblique axial contrastenhanced computed tomography image shows a dilated C-shaped segment of the ileum (curved line) and two adjacent transition zones (arrows). Luminal dilatation and free fluid within the hernia sac suggest impending strangulation. Note the proximal dilated bowel (P) and the distal collapsed bowel (D).
The imaging appearance depends on the stage of strangulation. At earlier stages, computed tomography shows bowel wall thickening with a target or double halo appearance because of submucosal edema and/or delayed enhancement (FIGURE 12) [43].
Figure 11. Small bowel volvulus secondary to postoperative adhesions in a 45-year-old woman with previous left hemicolectomy. Axial contrast-enhanced computed tomography image shows the whirling of the mesentery (arrow) and the dilated small bowel loops (asterisks). Also note the rotation of the body and the tail of the pancreas (P).
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venous outflow. Imaging appearance is similar to that caused by closed loop BP. Bowel ischemia can lead to intestinal infarction and perforation. Diagnosis of perforation is based on the identification of extraluminal air. Locating the site of perforation is usually difficult because the discontinuity of the bowel wall is extremely small [44]. In most instances, however, the presence of free air bubbles in proximity of the bowel wall can suggest the site of perforation because free air bubbles tend to accumulate close to the site of perforation [44]. The use of wide or lung window settings can improve the detection of small amounts of free peritoneal gas because it accentuates the contrast between extraluminal air and mesenteric fat. Bowel pseudo-obstruction
Several conditions can manifest as bowel dilatation without an obstructive lesion and mimic BO [4]. These conditions can occur in any part of the bowel, but are most commonly found in the colon. BPO includes toxic megacolon, Ogilvie’s syndrome, and adynamic ileus. Clinical symptoms and laboratory values can overlap with those of BO. Toxic megacolon is a rare but life-threatening complication Engorgement of small mesenteric veins, mesenteric fluid and mesenteric fat stranding are commonly found. Areas of intramu- of severe acute colitis and is characterized by segmental or total ral hyperattenuation can also be observed and suggest submuco- colonic distension of at least 6 cm [45]. The presence of signs of sal hemorrhage. With progression of the disease, absent or systemic toxicity differentiates toxic megacolon from other reduced bowel wall enhancement is found. This finding is the forms of colonic dilatation [45]. Although toxic megacolon is most specific indicator of bowel ischemia. In more severe cases, considered classically a complication of inflammatory bowel pneumatosis intestinalis and portomesenteric gas can occur, and diseases (predominantly ulcerative colitis), it can complicate represent the propagation of luminal gas in the bowel wall and infectious colitis of various causes (e.g., Clostridium difficile) as the portomesenteric venous system, respectively. Bowel ischemia well as ischemic and metabolic colitis [41]. Computed tomogracan also occur without the rotation of the mesentery. In these phy findings include segmental wall thinning, nodular pseudocases, impairment of blood flow is induced by intestinal overdis- polyps, abnormal austral pattern and signs of severe acute tention, which can compromise mucosal microcirculation and colitis [45]. Ogilvie’s syndrome is characterized by proximal colonic dilatation, with an A B intermediate transition zone typically located at or adjacent to the splenic flexure (FIGURE 13) [46]. Ogilvie’s syndrome is often associated with severe illness, recent surgery, electrolyte imbalance and use of narcotics or anticholinergics. The exact pathogenesis of Ogilvie’s syndrome is still unknown, but an autonomic neuronal imbalance is considered the most likely cause. Ogilvie’s syndrome occurs in an acute or chronic form [46]. The acute form is usually transient and reversible [46]. The chronic form can be recurrent or persistent [42]. Adynamic (paralytic) ileus is a functional dilatation of the bowel and is associated with recent surgery, peritonitis or medications [46]. Computed tomography Figure 13. Ogilvie’s syndrome in a 22-year-old man. (A) Coronal contrast-enhanced shows coexisting dilatation of the small computed tomography image shows a markedly dilated transverse colon, containing fecal material. (B) Sagital contrast-enhanced computed tomography image shows an and large bowel, and, in more severe intermediate transition zone (arrow) located at the splenic flexure. cases, gastric distension [46]. Figure 12. Bowel ischemia in a 72-year-old woman with small bowel obstruction secondary to postoperative adhesions. Axial contrast-enhanced computed tomography image shows thickened bowel wall with target (arrows) and double halo appearance (arrowhead).
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Computed tomography of BO: tricks of the trade
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Expert commentary & five-year view
The role of computed tomography in patients with suspected BO is well established. Of course, computed tomography can identify the site and etiology of the obstruction, as well as potential complications; and computed tomography results can influence the decision to proceed with rapid surgery or conservative treatment. There is, however, evidence that ionizing radiations may increase the risk of cancer [47,48]. One study reported that about 0.6–3% of the cumulative risk of cancer in developed countries can be attributable to diagnostic ionizing radiations [48]. This study and previously published studies got attention of media and scientific community and led to several efforts to minimize exposure to medical ionizing radiations [48–51]. Two ways to reduce exposure to medical ionizing radiations have been proposed. The first is to replace computed tomography with other imaging techniques, such as magnetic resonance and ultrasound, which do not use ionizing radiations. Although these techniques can be of value in certain situations, their limitations preclude the usage of ultrasound in patients with suspected BO. Specifically, MRI is limited by high cost and increased acquisition time; while operator dependency and limitations in the evaluation of gas-containing structures restrict the usage of ultrasound. The second and most effective method is to minimize computed tomography-related dose. Dose
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reduction is a challenging process. In general, higher radiation dose results in a better image quality, whereas low radiation dose can cause image noise, and consequent poor image quality. Moreover, the benefit of performing an indicated computed tomography usually overbalances any potential risk of cancer. The ALARA (as low as reasonably achievable) principles recommend to reduce the computed tomography radiation as low as possible (i.e., without compromising image quality). Computed tomography protocols can be modified by modulating the peak voltage and tube current time product; by decreasing the number of scanning phases or using techniques such as automated exposure control and iterative reconstruction [52]. By reducing computed tomography radiation dose, we hope to see a decrease in computed tomography radiation-induced cancer in the next 20–30 years. Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties. No writing assistance was utilized in the production of this manuscript.
Key issues .
Computed tomography is the modality of choice in patients with suspected bowel obstruction (BO).
.
Computed tomography can identify the location and cause of the obstruction.
.
Computed tomography differentiates BO from bowel pseudo-obstruction.
.
Computed tomography identifies complications of BO.
.
There is evidence that medical ionizing radiations may increase the risk of cancer.
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Exposure to medical ionizing should be reduced by minimizing computed tomography-related dose without compromising image quality.
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Computed tomography of BO: tricks of the trade
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Computed tomography of bowel obstruction: tricks of the trade Expert Rev. Gastroenterol. Hepatol. Early online, 1–11 (2015)
Massimo Galia*, Francesco Agnello, Ludovico La Grutta, Giuseppe Lo Re, Giuseppe Cabibbo, Emanuele Grassedonio, Bruno Giuseppe Gioia, Gianvincenzo Sparacia, Antonio Lo Casto, Roberto Lagalla and Massimo Midiri Section of Radiological Sciences, DIBIMED, University of Palermo, 90127 Palermo, Italy *Author for correspondence: Tel.: +39 09 1655 2326 Fax: +39 09 1655 2324 [email protected]
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Bowel obstruction (BO) is a frequent cause of hospitalization and surgical consultation for acute abdominal pain. It is usually suspected at physical examination, but clinical and laboratory data are often nonspecific. Thus, computed tomography plays a crucial role in a correct diagnosis of BO. Indeed, computed tomography can confirm a diagnosis of BO, and identify the location and cause of the obstruction. In this review, the computed tomography appearances of common and uncommon causes of BO and pseudo-obstruction are reviewed. KEYWORDS: bowel obstruction . bowel pseudo-obstruction . Colon . CT . small bowel
Bowel obstruction (BO) is a frequent cause of hospitalization, and accounts for up to 20% of surgical consultations for acute abdominal pain [1]. BO is an interruption of the normal flow of bowel contents. The bowel proximal to the obstruction dilates, whereas the distal bowel is normal or collapses [2]. Severe bowel overdistension and strangulation may impair intestinal perfusion, and cause wall necrosis and perforation, complications that increase the mortality associated with BO [2]. The small bowel is involved in 60–80% of cases, whereas the colon is involved in 20–40% of cases [1]. Causes of BO are broad, and range from tumors and inflammatory conditions to ingested material. Most common symptoms and signs are nausea, vomiting, cramping abdominal pain, obstipation and abdominal distension. However, even if BO is usually suspected at physical examination, symptoms are often vague, and laboratory tests nonspecific. Thus, imaging plays a crucial role in a correct diagnosis of BO. Indeed, imaging can confirm or exclude a diagnosis of BO, and can identify the location and cause of the blockage. Abdominal radiography and computed tomography are the most commonly used imaging modalities for the evaluation of suspected BO. Abdominal radiography is traditionally the first modality because it is fast, inexpensive and widely available. Common radiographic findings include distended gas-filled bowel loops, 10.1586/17474124.2015.1051030
air–fluid levels greater than 2.5 cm in width, and differential air–fluid levels within the same small bowel loop [3,4]. However, abdominal radiography is not definitive in most patients with suspected BO, and rarely identifies the location and cause [5]. Computed tomography is more sensitive and specific than abdominal radiography, and is currently the modality of choice for a diagnosis of BO [5]. Computed tomography can depict the site and etiology of the obstruction, as well as potential complications, and evaluate adjacent soft tissues and abdominal organs. The use of thin computed tomography slices and multiplanar reformatted (MPR) images improves the identification of the location and cause of the obstruction [6,7]. Computed tomography results have important implication for patient care because they can influence the decision to proceed with rapid surgery or a conservative treatment. In this article, we describe our computed tomography protocol, the computed tomography findings of BO and bowel pseudoobstruction (BPO), and the causes. Computed tomography protocol
Computed tomography protocol is designed to answer specific clinical questions, and to minimize radiation exposure. Computed tomography images are acquired from the diaphragm to the symphysis pubis using the minimum collimation (0.625 mm) and isotropic data sets. Both the reconstruction slice thickness and reconstruction
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Table 1. Incidence of causes of bowel obstruction in the small bowel and colon. Small bowel
Colon
Peritoneal adhesions
Common
Less common
Abdominal hernias
Common
Uncommon
Neuroendocrine tumors
Uncommon
Extremely rare
Less common
Common
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Extrinsic causes
Intrinsic causes Neoplasia
intraluminal fluid and air provide intrinsic enhancement; and the administration of enteric contrast materials may interfere with the evaluation of bowel wall and increase the acquisition time [8]. One advantage of the use of enteric contrast is that it allows differentiation of partial BO from complete BO [1]. If the contrast passes through the site of obstruction, a complete BO is ruled out [1]. Bowel obstruction
The diagnosis of BO requires the presence of bowel dilatation (caliber > 2.5– Diverticulitis Extremely rare Common 3 cm in the small bowel; >9 cm in the Crohn’s disease Uncommon (terminal ileum) Uncommon (ascending colon) cecum; >6 cm in the remaining colon) and the presence of an abrupt transition Intraluminal causes zone [3]. The transition zone is the site of Fecaloma Not applicable Common caliber change from the proximal dilated Bezoar Uncommon Not applicable bowel to the distal normal or collapsed bowel, and indicates the site of obstrucGallstone ileus Uncommon (terminal ileum) Not applicable tion. The transition zone can be a point or a longer segment of narrowing. Identiinterval are 3 mm. Thinner collimation and isotropic acquisition fication of the transition zone is crucial because it facilitates the allow the creation of MPR images. We routinely administer 110– recognition of the underlying cause of the obstruction, and dif120 ml of an intravenous nonionic contrast material with an ferentiates BO from BPO. Causes of BO are multiple, and can iodine concentration of 350–370 mg/ml except in patients with be broadly classified into three categories: extrinsic, intrinsic allergy to iodinated contrast material and marked renal and intraluminal. Extrinsic causes originate adjacent to the impairment. Images are acquired about 70 s from the start of con- transition zone, and are associated with extraluminal findings. trast material injection. This delay is usually sufficient to evaluate Intrinsic causes result from thickening and/or dysfunction of bowel wall enhancement and major mesenteric vessels. Unen- the bowel wall. Intraluminal causes are located within the hanced images are useful to determine the basic bowel enhance- bowel lumen. The table summarizes the incidence of each cause ment, and to exclude mural hemorrhage. However, we perform of BO in the small bowel and colon (TABLE 1). the unenhanced phase only when bowel ischemia is suspected. Enteric contrast materials (dilute barium or water-soluble Extrinsic causes iodinated solutions) are not routinely used because retained Peritoneal adhesions Peritoneal adhesions are the most common cause of small BO [9,10]. Peritoneal adhesions are abnormal fibrous bands that connect adjacent tissues and organs in the abdomen and pelvis [9,10]. They are most commonly postoperative, whereas a minority is congenital or a consequence of peritoneal inflammation or carcinomatosis [9,10]. Adhesions are usually not clearly visualized at computed tomography unless they are complicated by inflammation or carcinomatosis [9]. Thus, the diagnosis of adhesive BO is usually one of exclusion based on the absence of another discernible cause [9,10]. Identification of indirect signs of extraluminal compression can allow for a correct diagnosis. These include a smooth, tapered appearance of the transition zone (beak sign), and an extrinsic impression on the bowel wall at the level of the transition zone (fat notch sign) (FIGURE 1) [9,10]. These findings are nonspecific, however, Figure 1. Small BO secondary to adhesions in a 43-year-old and should be evaluated in conjunction with history and physwoman with previous cesarean section. Axial contrastical data [10]. Several authors have reported a therapeutic role enhanced computed tomography image shows fluid-filled and dilated ileal loops (asterisk), and a smooth tapering of the bowel of water-soluble contrast materials (e.g., Gastrografin) in at the transition zone (arrow), corresponding to the beak sign. patients with adhesive small BO [11–13]. Due to high Intussusception
Common
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Less common
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osmolarity, water-soluble contrast materials promote the passage of fluids into the bowel lumen, and increase the pressure gradient through the site of obstruction [11–13]. The passage of the contrast is usually monitored with serial abdominal radiographs. If the contrast reaches the colon within 24 h, a complete BO is ruled out, and conservative treatment is continued [11–13].
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Abdominal hernias
Abdominal hernias are the second most common extrinsic cause of BO, and are most frequent in elderly patients [14]. An abdominal hernia is the protrusion of any abdominal organ or tissue through a congenital or acquired (iatrogenic or post-traumatic) orifice [15]. Computed tomography is useful in determining the location, the size of the neck, the measurement of the sac, the content and the shape of abdominal hernias. Abdominal hernias are classified into two main groups: external and internal. External hernias occur through an orifice in the abdominal or pelvic wall. The most common external hernias are inguinal, umbilical, paraumbilical, epigastric and incisional [15]. Other, less frequent, external hernias include femoral, Spigelian and hypogastric hernias [15]. Inguinal hernias account for the vast majority of BO, and, as the name implies, extend into the inguinal canal (FIGURE 2) [16]. The principal differential diagnosis for inguinal hernias is femoral hernias, because of their close origin. Sukuzi et al. reported that femoral hernias are usually located lateral to the pubic tubercle, and compress the femoral vein, whereas most inguinal hernias extend medial to the pubic tubercle, and, if large, can compress femoral vein [16]. Umbilical and paraumbilical hernias protrude through the umbilical ring and the linea alba (in proximity of the umbilicus), respectively [17]. Epigastric hernias occur in the linea alba, above the umbilicus, whereas incisional hernias are located at the site of surgical incision or, more rarely, in the vicinity of the stoma [17]. Internal hernias are less common than external ones, occur through a mesenteric orifice, and, as the name implies, are located within the abdominal cavity [17]. The most common types are paraduodenal and Petersen hernias [17]. Paraduodenal hernias develop through congenital peritoneal anomalies, the paraduodenal fossae, and are located under the right or left mesocolon [17]. Petersen hernias are a complication of abdominal surgery, especially after the creation of a Roux-en-Y anastomosis, and protrude through the transverse mesocolon [17,18]. The hallmarks in computed tomography diagnosis of internal hernias include a cluster of distended bowel segments with an abnormal location, and stretching and displacement of mesenteric vessels [18]. Abdominal hernias can contain fatty tissue, solid organs and/or bowel loops. When bowel loops migrate to the hernial sac, and become entrapped, BO and strangulation can occur. The likelihood of obstruction and strangulation correlates inversely with the size of the neck [17]. Internal, inguinal and femoral hernias have a greater tendency toward incarceration and/or strangulation. informahealthcare.com
Figure 2. Small bowel obstruction secondary to right inguinal hernia in a 64-year-old man. Axial contrast-enhanced computed tomography image show a right inguinal hernia containing ileal loops and omentum. The hernia sac extends medial to the pubic tubercle (arrowhead), and does not compress the common femoral vein (asterisk). Also note a coexisting left inguinal hernia.
Neuroendocrine tumors
Neuroendocrine tumors are rare gastrointestinal tumors of neuroendocrine origin, and most commonly occur in the small bowel, but rarely in the colon [19]. Both primary neuroendocrine tumors and mesenteric neuroendocrine metastases can be an extrinsic cause of BO [20]. Primary carcinoid tumors are intrinsic lesions of the bowel, but are classified as an extrinsic cause of BO because the obstruction is caused largely by a mesenteric desmoplastic reaction [20]. Typical computed tomography findings include strong enhancement, ill-defined, spiculated margins, encasement and narrowing of mesenteric vessels, and calcifications [19].
Figure 3. Colon adenocarcinoma in a 68-year-old man. Axial contrast-enhanced computed tomography images show a circumferential mass in the sigmoid colon (arrows) that causes distension of proximal colon.
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Figure 4. Colonic intussusceptions due to adenocarcinoma in a 62-year-old woman. Axial contrast-enhanced computed tomography image shows the typical appearance of intussusception, with the intussuscipiens (arrow), the intussuscepted mesentery (dotted arrow), and the intussusception (asterisk). Note colonic dilatation.
Intrinsic causes Neoplasia
Neoplasia is a frequent cause of BO, and is most common in elderly patients [14]. Colorectal cancer should be suspected in any patient with colon obstruction. At computed tomography, colorectal cancer typically appears as a focal and heterogeneously enhancing mass with eccentric or annular extension (FIGURE 3) [21]. Computed tomography appearance may mimic that of several benign processes. Macari et al. [21] reported that stratified attenuation, symmetric thickening and segmental (10–30 cm) or diffuse longitudinal extension suggest a benign process, whereas heterogeneous attenuation, asymmetric thickening, and a focal longitudinal extension suggest a malignant process. However, there are numerous overlaps. For
Figure 5. Small bowel obstruction secondary to active Crohn’s disease in a 44-year-old woman. Axial contrastenhanced computed tomography image shows the thickened terminal ileum with a target appearance (arrows), and the prestenotic dilated ileum (asterisk), containing feculent material. Also note peritoneal-free fluid.
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example, a scirrhous carcinoma can sometimes be characterized by symmetric wall thickening and stratified attenuation [21]. Administration of contrast material via a rectal tube can help identify the site of obstruction and differentiate collapsed bowel loops from mural thickening due to the neoplasia. Metastases are the first malignant cause of small BO, but are rare in the colon [3]. Metastases result from intraperitoneal seeding, hematogenous spread from distant tumors (e.g., melanoma), and direct extension of advanced abdominal tumors [3]. Computed tomography appearance is not pathognomonic. Metastases can appear as a focal wall thickening; a circumferential, stenosing mass; or a polyp that causes intussusception [3]. Benign intestinal tumors (e.g., lipoma, non-neoplastic polyps) are frequently asymptomatic, and most commonly occur in the small bowel [22]. They can often cause intussusception and consequent BO [22]. Intussusception
Intussusception is a rare cause of BO in adults [23]. It most commonly occurs in the small bowel, but any part of the gastrointestinal tract can be involved [24]. Intussusception is the invagination of a bowel segment (intussusceptum) and its mesentery into the lumen of the contiguous bowel (intussuscipiens) [23]. Intussusception can occur with or without a lead point [23]. A lead point is an irritant that alters bowel peristalsis, and causes bowel intussusception and obstruction [23]. Several factors, including benign and malignant tumors, and suture materials can serve as a lead point [23]. The exact pathogenesis of non-leadpoint intussusception is not well understood; dysrhythmic contractions are thought to be the most likely mechanism [24]. Nonlead-point intussusceptions usually resolve spontaneously, do not cause BO, and are frequently found incidentally [23,24]. Computed tomography imaging appearance of bowel intussusception varies with the stage of the disease [24]. Early intussusception shows a characteristic ‘target appearance’, with an intraluminal soft tissue mass and an eccentric fat density area, the latter representing the intussuscepted mesentery and the former representing the intussusception (FIGURE 4) [24]. As the disease progresses, a ‘sausage-shaped’ pattern develops [24]. This appearance results from alternating areas of low and high attenuation representing bowel wall, mesenteric fat, fluid and air [24]. When intussuscepted wall thickening occurs because of ischemia and edema, a reniform pattern can be found [24]. Computed tomography identification of the lead mass is often difficult [24]. Evaluation of intussusception length and width can be useful in predicting the need for surgery. In particular, an intussusception length of 1 cm), and often has a loops (asterisk). (B) Axial contrast-enhanced computed tomography image obtained at a more cranial level shows an ovoid mass similar to that of (A, arrow) floating in the stratified appearance, with two or three stomach, consistent with gastric bezoar. symmetric layers (FIGURE 5) [31,32]. The term informahealthcare.com
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Appreciation of what is called the ‘comb sign’ (engorgement of the vasa recta) and enlarged mesenteric nodes can suggest that the disease is active [32]. Assessing the degree of inflammation is crucial in directing patients to the most appropriate treatment. Patients with predominantly inflammatory lesions are usually treated medically, whereas patients with fibrostenotic lesions can be candidates for surgery [30]. The imaging appearance of Crohn’s disease can overlap with that of ulcerative colitis [33]. However, a limited extension of the inflammation (mucosa and submucosa only), a left side or diffuse colonic involvement, and the absence of skip areas are more suggestive of ulcerative colitis than Crohn’s disease [33]. Another important differential diagnosis is bowel neoplasia [33]. Intraluminal causes Fecaloma
Figure 8. Small bowel feces sign in a 78-year-old woman with a large adenocarcinoma of the cecum. Oblique axial computed tomography image shows a large heterogeneously enhancing mass (arrowhead) in the cecum, and feculent material (arrow) within the pre-stenotic dilated terminal ileum. The small bowel feces sign helps recognize the transition zone and cause of obstruction.
Fecaloma is the most common intraluminal cause of colonic obstruction [34]. It occurs predominantly in elderly patients, but can also occur in neurologically impaired young patients [34]. Causes include chronic constipation, diets poor in fiber and fluids, colonic motility disorders and medications that slow intestinal transit (e.g., narcotics, NSAIDs, antidepressants) [35]. A fecaloma is an intraluminal collection of dehydrated feces with a diameter equal to or greater than the colon. Fecalomas usually occur in the rectosigmoid colon, where the bowel lumen is narrowest and the feces are most dehydrated. Computed tomography shows a gas containing, heterogeneously hypoattenuating mass within the colon lumen (FIGURE 6). Fecalomas can sometimes cause colonic obstruction, stercoral colitis, stercoral ulcers, colon perforation, and fecal peritonitis [34]. Fecaloma must be distinguished from a fecal impaction that can accumulate proximally to a stenosing tumor. Bezoar
Bezoar is a rare cause of BO [36]. A bezoar is composed of ingested material and is classified according to its composition. Trichobezoar and phytobezoar are the most common types. The former is composed of hairs, whereas the latter is composed of fruit and/or vegetables. Predisposing factors include previous gastric surgery, inadequate mastication and high fiber diets [36,37]. A bezoar usually forms in the stomach, and can transit in the small bowel, where it can cause intraluminal obstruction. The site of obstruction is often the terminal ileum, where the bowel lumen is narrowest [36,37]. On computed tomography, the bezoar appears as a round or ovoid, intraluminal, gas containing, hypoattenuating mass with mottled appearance (FIGURE 7) [36,37]. The most important differential diagnosis for bezoar is small bowel feces that accumulate proximal to the transition zone (FIGURE 8) [36]. This location helps to differentiate small bowel feces from bezoar, because the latter accumulates in the obstruction zone [36]. Figure 9. Gallstone ileus in a 64-year-old woman. Sagittal MDCT image shows a large gallstone (arrow) impacted in the distal ileus and proximal fluid filled, dilated bowel. Also note aerobilia (arrowheads).
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Other intraluminal causes
Gallstone ileus is an extremely uncommon complication of cholelithiasis [38]. It occurs when a large gallstone (>2.5 cm in Expert Rev. Gastroenterol. Hepatol.
Computed tomography of BO: tricks of the trade
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diameter) migrates to the bowel through a cholecystoenteric fistula, located between the gallbladder and the stomach or duodenum, and becomes impacted in the ileus (FIGURE 9) [38]. Characteristic computed tomography findings include a large, calcified stone impacted in the terminal ileum and pneumobilia [38]. Other less common intraluminal causes include ingested foreign bodies and endoscopic capsule retention.
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Complicated BO
The final step is to differentiate a simple BO from a complicated one, such as a closed-loop BO or a strangulated BO. Simple BO is characterized by single or multiple transition points located along the bowel course [2]. When transition points are adjacent (e.g., hernia), a closed loop (or incarcerated) BO occurs [2]. The most common causes are peritoneal adhesions and abdominal hernias. Closed-loop BO is characterized by progressive distension of the isolated bowel due to progressive accumulation of secreted fluid, and variable distension of the proximal bowel (FIGURE 10) [2]. If untreated, the involved bowel loops can rotate around their mesentery, resulting in a volvulus (FIGURE 11) [2]. Volvulus can occur in any part of gastrointestinal tract, but the most common sites are the small bowel, the cecum and the sigmoid colon. Imaging appearance partially varies depending on the site. Small bowel volvulus appears as dilated and dislocated small bowel loops. Recognition of the whirl sign can confirm a diagnosis of volvulus [39]. The whirl represents the rotation of the mesentery and the afferent and efferent bowel loops [39]. However, the whirl sign is nonspecific and a similar appearance can be found in patients with colonic volvulus and internal hernia. In sigmoid volvulus, the sigmoid colon is typically displaced to the right upper quadrant, and resembles a coffee bean on conventional radiograph and topogram [40]. Appreciation of two crossing transition zones projecting at a single location (X-marks-the-spot sign) suggests a complete twist, whereas the separation of sigmoid walls by surrounding mesenteric fat suggests an incomplete twist [40]. Cecal volvulus is less common than sigmoid volvulus. Three types of cecal volvulus have been described: the axial torsion type, loop type and bascule type [37]. The axial torsion type results from cecum rotation along its axis, and the dilated cecum appears in the right lower quadrant [41]. The loop type, in which the cecum both rotates and inverts, leads to displacement of the dilated cecum to the left upper abdominal quadrant [41]. The bascule type is the less frequent variant of cecal volvulus. It results from anterior folding of the cecum and is not associated with torsion of the cecum [41]. The dilated cecum is usually displaced in the mid abdomen [41]. Associated involvement of the ileum (ileocecal twist sign) is found in approximately 50% of cases of cecal volvulus [42]. Other useful, but nonspecific, signs of cecal volvulus are the coffee bean sign, the whirl sign and the X-marks-the-spot sign [42]. Strangulation indicates a BO with impairment of blood flow [2]. The severity is variable. Strangulation occurs most commonly in closed loop BO, and that has been attributed to direct venous compression and twisting of the mesentery. informahealthcare.com
Figure 10. Closed loop obstruction secondary to a left inguinal hernia in a 63-year-old man. Oblique axial contrastenhanced computed tomography image shows a dilated C-shaped segment of the ileum (curved line) and two adjacent transition zones (arrows). Luminal dilatation and free fluid within the hernia sac suggest impending strangulation. Note the proximal dilated bowel (P) and the distal collapsed bowel (D).
The imaging appearance depends on the stage of strangulation. At earlier stages, computed tomography shows bowel wall thickening with a target or double halo appearance because of submucosal edema and/or delayed enhancement (FIGURE 12) [43].
Figure 11. Small bowel volvulus secondary to postoperative adhesions in a 45-year-old woman with previous left hemicolectomy. Axial contrast-enhanced computed tomography image shows the whirling of the mesentery (arrow) and the dilated small bowel loops (asterisks). Also note the rotation of the body and the tail of the pancreas (P).
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venous outflow. Imaging appearance is similar to that caused by closed loop BP. Bowel ischemia can lead to intestinal infarction and perforation. Diagnosis of perforation is based on the identification of extraluminal air. Locating the site of perforation is usually difficult because the discontinuity of the bowel wall is extremely small [44]. In most instances, however, the presence of free air bubbles in proximity of the bowel wall can suggest the site of perforation because free air bubbles tend to accumulate close to the site of perforation [44]. The use of wide or lung window settings can improve the detection of small amounts of free peritoneal gas because it accentuates the contrast between extraluminal air and mesenteric fat. Bowel pseudo-obstruction
Several conditions can manifest as bowel dilatation without an obstructive lesion and mimic BO [4]. These conditions can occur in any part of the bowel, but are most commonly found in the colon. BPO includes toxic megacolon, Ogilvie’s syndrome, and adynamic ileus. Clinical symptoms and laboratory values can overlap with those of BO. Toxic megacolon is a rare but life-threatening complication Engorgement of small mesenteric veins, mesenteric fluid and mesenteric fat stranding are commonly found. Areas of intramu- of severe acute colitis and is characterized by segmental or total ral hyperattenuation can also be observed and suggest submuco- colonic distension of at least 6 cm [45]. The presence of signs of sal hemorrhage. With progression of the disease, absent or systemic toxicity differentiates toxic megacolon from other reduced bowel wall enhancement is found. This finding is the forms of colonic dilatation [45]. Although toxic megacolon is most specific indicator of bowel ischemia. In more severe cases, considered classically a complication of inflammatory bowel pneumatosis intestinalis and portomesenteric gas can occur, and diseases (predominantly ulcerative colitis), it can complicate represent the propagation of luminal gas in the bowel wall and infectious colitis of various causes (e.g., Clostridium difficile) as the portomesenteric venous system, respectively. Bowel ischemia well as ischemic and metabolic colitis [41]. Computed tomogracan also occur without the rotation of the mesentery. In these phy findings include segmental wall thinning, nodular pseudocases, impairment of blood flow is induced by intestinal overdis- polyps, abnormal austral pattern and signs of severe acute tention, which can compromise mucosal microcirculation and colitis [45]. Ogilvie’s syndrome is characterized by proximal colonic dilatation, with an A B intermediate transition zone typically located at or adjacent to the splenic flexure (FIGURE 13) [46]. Ogilvie’s syndrome is often associated with severe illness, recent surgery, electrolyte imbalance and use of narcotics or anticholinergics. The exact pathogenesis of Ogilvie’s syndrome is still unknown, but an autonomic neuronal imbalance is considered the most likely cause. Ogilvie’s syndrome occurs in an acute or chronic form [46]. The acute form is usually transient and reversible [46]. The chronic form can be recurrent or persistent [42]. Adynamic (paralytic) ileus is a functional dilatation of the bowel and is associated with recent surgery, peritonitis or medications [46]. Computed tomography Figure 13. Ogilvie’s syndrome in a 22-year-old man. (A) Coronal contrast-enhanced shows coexisting dilatation of the small computed tomography image shows a markedly dilated transverse colon, containing fecal material. (B) Sagital contrast-enhanced computed tomography image shows an and large bowel, and, in more severe intermediate transition zone (arrow) located at the splenic flexure. cases, gastric distension [46]. Figure 12. Bowel ischemia in a 72-year-old woman with small bowel obstruction secondary to postoperative adhesions. Axial contrast-enhanced computed tomography image shows thickened bowel wall with target (arrows) and double halo appearance (arrowhead).
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Expert Rev. Gastroenterol. Hepatol.
Computed tomography of BO: tricks of the trade
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Expert commentary & five-year view
The role of computed tomography in patients with suspected BO is well established. Of course, computed tomography can identify the site and etiology of the obstruction, as well as potential complications; and computed tomography results can influence the decision to proceed with rapid surgery or conservative treatment. There is, however, evidence that ionizing radiations may increase the risk of cancer [47,48]. One study reported that about 0.6–3% of the cumulative risk of cancer in developed countries can be attributable to diagnostic ionizing radiations [48]. This study and previously published studies got attention of media and scientific community and led to several efforts to minimize exposure to medical ionizing radiations [48–51]. Two ways to reduce exposure to medical ionizing radiations have been proposed. The first is to replace computed tomography with other imaging techniques, such as magnetic resonance and ultrasound, which do not use ionizing radiations. Although these techniques can be of value in certain situations, their limitations preclude the usage of ultrasound in patients with suspected BO. Specifically, MRI is limited by high cost and increased acquisition time; while operator dependency and limitations in the evaluation of gas-containing structures restrict the usage of ultrasound. The second and most effective method is to minimize computed tomography-related dose. Dose
Review
reduction is a challenging process. In general, higher radiation dose results in a better image quality, whereas low radiation dose can cause image noise, and consequent poor image quality. Moreover, the benefit of performing an indicated computed tomography usually overbalances any potential risk of cancer. The ALARA (as low as reasonably achievable) principles recommend to reduce the computed tomography radiation as low as possible (i.e., without compromising image quality). Computed tomography protocols can be modified by modulating the peak voltage and tube current time product; by decreasing the number of scanning phases or using techniques such as automated exposure control and iterative reconstruction [52]. By reducing computed tomography radiation dose, we hope to see a decrease in computed tomography radiation-induced cancer in the next 20–30 years. Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties. No writing assistance was utilized in the production of this manuscript.
Key issues .
Computed tomography is the modality of choice in patients with suspected bowel obstruction (BO).
.
Computed tomography can identify the location and cause of the obstruction.
.
Computed tomography differentiates BO from bowel pseudo-obstruction.
.
Computed tomography identifies complications of BO.
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There is evidence that medical ionizing radiations may increase the risk of cancer.
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Exposure to medical ionizing should be reduced by minimizing computed tomography-related dose without compromising image quality.
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