Bowel Trauma Imaging

For centuries, bowel trauma was associated with a high mortality rate, and survivors escaped death by withstanding hemorrhage and sepsis. With the introduction of radiography, diagnostic accuracy improved. Multirow helical detector CT scans are capable of scanning the abdomen in less than 30 seconds and can detect free air, free fluid, abnormal bowel wall enhancement, bowel wall thickening, and mesenteric infiltration. These advances in CT have brought the debate of conservative management of abdominal trauma full circle. Currently, many patients with blunt abdominal trauma or retroperitoneal penetrating trauma can be managed without surgery and can avoid unnecessary laparotomy. ^{[1, 2, 3, 4, 5, 6, 7, 8]}

(See the related images below.)

Multidetector CT (MDCT) scanning using 16- and 64-slice CT has increased the speed in which trauma patients are scanned. In addition, 16- and 64-slice MDCT imaging allows the creation of isotropic voxels that allow reformats to be performed in sagittal and coronal planes, which can allow better localization of bowel injuries.

Imaging modalities

Bedside ultrasonography is a rapid, portable, noninvasive, and accurate examination that can be performed by emergency clinicians and trauma surgeons to detect hemoperitoneum. In many medical centers, focused assessment with sonography for trauma (FAST) examination is the procedure of choice in the evaluation of hemodynamically unstable trauma patients. FAST has gained acceptance in the evaluation of blunt and penetrating abdominal trauma because of its speed, noninvasiveness, and reproducibility in diagnosing intraperitoneal injury that requires laparotomy.  The American College of Radiology (ACR) suggests that it can be useful for triage in hemodynamically stable patients with suspected bowel injury due to blunt trauma. ^[9]   

CT is the first-line diagnostic modality for abdominal trauma and can offer visceral organ information and help determine therapeutic options. The shift to nonoperative management in many cases has made accurate imaging necessary for conservative management. ^[1] CT of the abdomen is the preferred diagnostic examination for the evaluation of blunt abdominal trauma in the hemodynamically stable patient with blunt abdominal trauma and in selected instances of penetrating trauma to the posterior abdomen. Unstable patients or patients with penetrating injuries to the abdomen undergo exploratory laparotomy. ^{[2, 3, 10, 11, 12, 1, 13, 14, 15, 16]}

Abdominal CT examination should be systematic. Traumatic injury to the bowel is rarely isolated. First, evaluate the more commonly injured organs, such as the liver and spleen, as well as the pancreas, adrenals, kidneys, blood vessels, spine, and skeletal structures.

Evaluate traumatic injury to the bowel in the context of the mechanism and location of injury, as follows:

• Right upper quadrant: examine the right lung base, right hemidiaphragm, liver, gallbladder, right colon, right adrenal, right kidney, and right abdominal small bowel loops

• Midline: evaluate the base of the heart, diaphragm, pancreas, duodenum, aorta, inferior vena cava, and small bowel mesentery

• Left upper quadrant: evaluate the base of the heart, base of the left lung, left hemidiaphragm, stomach, spleen, tail of the pancreas, left adrenal, left kidney, left colon, and small bowel loops

• Lower abdomen and/or pelvis: evaluate the bladder, rectosigmoid, and small bowel loops.

Each imaging modality (eg, plain abdominal radiograph, US, CT) demonstrates typical findings that suggest a diagnosis of bowel trauma. ^[17] In the hemodynamically stable patient with abdominal trauma, CT is the study of choice.

The only role of angiography in acute bowel trauma is to identify the site of visceral bleeding. ^[18]

Limitations of techniques

The accuracy of CT for the evaluation of bowel injury is as high as 97.6%.

CT can be limited if diagnostic peritoneal lavage (DPL) is performed prior to the CT. Free intraperitoneal fluid and air from the DPL observed on CT makes the evaluation for bowel injury very difficult.

Although CT can suggest bowel injury by demonstrating free intraperitoneal air, free fluid, or thickened bowel wall, in many instances it cannot reliably localize the exact location of bowel injury.

Delayed presentation of bowel injury occasionally occurs. Patients returning with continued symptoms several hours or days after a negative trauma should undergo repeat CT. ^[19]

WSES guidelines

The World Society of Emergency Surgery (WSES) has published guidelines on bowel trauma diagnosis and management, including the following ^[20] :

• Management of the awake and oriented patient with blunt abdominal trauma starts with the primary survey, E-FAST, physical examination, secondary survey, blood chemistry, vital signs, and contrast-enhanced abdominal CT.
• The presence of a seatbelt sign should prompt a CT scan and a high index of suspicion for bowel injury.
• In selected cases. repeat CT might be considered. Patients with equivocal signs on initial CT scan should be reimaged after 6 hours. In patients who demonstrate evolving clinical signs suspicious for bowel injury, reimaging should be considered. 
• The presence of highly specific CT findings such as extraluminal air, extraluminal oral contrast, or bowel-wall defects warrants prompt surgical exploration.
• The presence of highly sensitive CT findings such as free fluid in the absence of solid-organ injury, abnormal enhancement of bowel wall, and mesenteric stranding can be used as an adjunct to the clinical picture but should not solely determine management. 
• Following penetrating trauma, highly sensitive CT findings for bowel injury include free fluid in the absence of solid-organ injury, abnormal enhancement of bowel wall, and mesenteric stranding. These can be used as an adjunct in the clinical picture but should not solely determine management.
• IV contrast-enhancing CT scanning has equal sensitivity to triple contrast in detecting bowel injury and is favorable in time-sensitive trauma situations.

Plain radiography findings in bowel injury include the following ^[21] :

• Nonspecific findings may include small and/or large bowel dilation suggestive of ileus or obstruction secondary to peritonitis

• Soft tissue density and/or mass effect on bowel gas loops suggest fluid collections, hematoma, or scoliosis resulting from splinting toward the side of injury

• Loss of right psoas shadow suggests a retroperitoneal fluid collection from a duodenal injury

• Free intraperitoneal air suggests bowel perforation

• Upright abdominal films and left lateral decubitus films can detect as little as 1 mL or 2 mL of free intraperitoneal air under the diaphragm or over the liver edge, respectively

• On supine abdominal radiographs, free air can be observed outlining the serosa of the bowel loops ("Rigler" or serosa sign) or the falciform ligament

• Retroperitoneal air from duodenal or sigmoid injury can outline the diaphragmatic crura or the kidneys

• GI studies using contrast can identify areas of extravasation from perforated bowel. GI studies also can help identify a duodenal hematoma

Plain radiographs are not highly specific or sensitive for evaluating bowel injury. Findings of free air, abdominal fluid, scoliosis, and psoas shadow obliteration are observed in fewer than 43% of patients with intestinal trauma. Fluid collections must be large (>800 mL) to be visible on plain radiographs.

Pneumoperitoneum does not always indicate bowel rupture and can be observed in patients with pneumomediastinum or pneumothorax and in patients on mechanical ventilation.

CT is the modality of choice for evaluating abdominal trauma in the hemodynamically stable patient (see the images below). ^{[22, 23, 24, 4, 25, 5, 26, 27, 28, 29, 30, 31]} CT evaluation for blunt abdominal trauma can be difficult and requires strict attention to meticulous techniques. ^{[1, 32, 33, 34, 35]} Multidetector CT scanning using a 16- or 64-slice CT allows for coronal and sagittal reformats. Examination of these off-axis images allows for improved detection and localization of bowel injuries. ^{[6, 7]} 7 The coronal plane is especially helpful because it provides an anatomic view and helps referring physicians/surgeons better understand the location and extent of injury.

The sagittal plane is helpful in the evaluation of the thoracic and lumbar spine and can detect associated compression fractures of the spine, which may be missed when viewed in the axial plane alone.

Administer oral and intravenous contrast. Extend sections from the base of the lungs to below the symphysis pubis. View these sections in the "abdominal, lung, liver" and "bone" windows. Carefully search for associated injuries, including Chance or vertebral burst fractures; abdominal bruising; and pancreatic, liver, spleen, adrenal, and kidney injury. Consider administering rectal contrast in patients with suggested penetrating injury to the rectum or retroperitoneal colon. ^[14]

 

CT findings in bowel injury include the following (also see Table 1, below):

• Bowel injury is suggested by free intraperitoneal air, free intraperitoneal or retroperitoneal fluid, focal areas of bowel wall thickening, abnormal bowel wall enhancement, bowel wall hematoma (ie, duodenal hematoma), and intramural air.

• The most specific finding is the visualization of oral contrast extravasation and bowel wall disruption.

• A pattern of more diffuse bowel wall thickening, abnormal enhancement, and mesenteric infiltration can suggest mesenteric vascular injury resulting in ischemic bowel.

• In intestinal vascular injury, evaluate the celiac axis, superior mesenteric artery, and superior mesenteric vein. A mesenteric hematoma or a focal area of higher density clotted blood (ie, "sentinel clot") can suggest vascular injury.

• Focal contrast extravasation can indicate active hemorrhage.

Table 1. CT Findings in Bowel Injury (Open Table in a new window)

 

The accuracy of CT for evaluating bowel injury is 82%, with a sensitivity of 64% and a specificity of 97%.

Some findings on CT suggesting bowel injury can represent false-positive findings, such as the following:

• Free intraperitoneal air in a trauma patient can also be observed in a patient with pneumomediastinum, pneumothorax, recent DPL, laparotomy, or barotrauma with no associated bowel injury

• Free fluid in a trauma patient can originate from injury to other organs, including the liver, spleen, gallbladder, and urinary bladder, without any injury to the bowel

• Bowel wall thickening and abnormal wall enhancement can also be observed in patients with hypotension or hypoperfusion without direct bowel injury

In a study of blunt small bowel perforation in patients from 39 centers, initial CT showed free air in 43% of cases and none in control patients. Mean time to surgery was 8.7 hours (median, 3.7 hr). Following analysis, the study authors noted that intraperitoneal abnormalities on CT scan should always evoke high suspicion and that there should be strong consideration for additional diagnostic/therapeutic intervention by 8 hours after arrival in patients who continue to pose a clinical challenge. ^[34]

Bowel and/or mesentery injuries are the third most common type of injury in patients who sustain blunt abdominal trauma. In a study by Firetto et al, 34 (4.1%) of 831 patients who sustained blunt abdominal trauma had blunt bowel and/or mesenteric injury on CT. Of the 34 patients, 21 (61.8%) underwent surgical repair and 13 received conservative treatment. Free fluid was shown to have a significant statistical association with surgery. The highest positive predictive values were found to be (1) abdominal guarding with bowel wall discontinuity and (2) extraluminal air (100% and 83.3%, respectively). ^[13]

In study by Liao et al of 188 patients with  blunt bowel and mesenteric injury, the most common characteristics on CT were free fluid (71.3%), free air (43.6%), and mesenteric infiltration (23.4%). ^[32]

Typically, the role of US in evaluating bowel trauma is limited to detecting free intraperitoneal fluid in trauma patients who are not sufficiently hemodynamically stable to undergo CT. However, the identified fluid cannot be further defined. Considerations include benign ascites, blood, urine, or bile and must be confirmed with CT. ^{[36, 17]}

Other findings of bowel injury include dilated bowel loops secondary to an ileus or obstruction. US is insensitive in detecting intraperitoneal free air.