Evaluation of bowel and mesenteric blunt trauma with multidetector CT: spectrum of signs beyond pneumoperitoneum
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Multidetector computed tomography (CT) is the primary imaging modality for diagnosing bowel and mesenteric blunt trauma. Pneumoperitoneum is the commonly observed for imaging sign among radiologists. There are, however, other CT findings with different clinical significances. The aim of this article is to heighten radiologist awareness regarding the diagnostic capability of CT in bowel and mesenteric blunt trauma, and to illuminate potential pitfalls to be avoided in this patient population. This guide was developed to enhance the knowledge and awareness of non-expert radiologists that may be less aware of the nuances of abdominal CT imaging in a blunt trauma setting. This enhanced knowledge and awareness will improve patient diagnosis, treatment, and outcomes.
KeywordsEvaluation Bowel and mesenteric blunt trauma Multidetector CT Spectrum of signs Pneumoperitoneum
Bowel and mesentery are injured in an estimated 1–5% of cases after blunt trauma , and this is the third most common type of injury from blunt trauma to the abdomen . Delayed or missed diagnosis is multifactorial. Symptoms may be absent on initial presentation and, when present, may be nonspecific. Moreover, clinical assessment alone can be unreliable due to the presence of concomitant injuries .
Investigation by cross-sectional imaging in blunt trauma depends on the hemodynamic status of the patient. If the patient is hemodynamically unstable with overt signs of abdominal injury, the patient should undergo surgery immediately with no need to perform any imaging method. When three abdominal solid organs are injured, the risk of bowel injury is 34% . Bowel and mesenteric injury can be difficult to detect for several reasons, including the presence of concurrent injuries, injury to multiple bowel segments, and the presence of subtle imaging features.
Diagnostic tools in patients with abdominal injury include peritoneal lavage, sonography, and computed tomography. Of these three modalities, CT is the most sensitive and specific for diagnosis of bowel and coexisting mesenteric injury. Among the spectrum of imaging findings in bowel injury, pneumoperitoneum is a commonly observed imaging sign among radiologists. However, there are other CT findings with differing levels of clinical significance. The aim of this article was to heighten radiologist awareness regarding the diagnostic capability of CT in traumatic bowel injury with and without coexisting mesenteric injury, and to illuminate potential pitfalls to be avoided in this patient population.
The role of the radiologist
In this clinical setting, the role of the radiologist is to provide all essential related information, including detection of bowel injury and/or mesenteric injury, identification of patients requiring immediate surgical management, and assessment of severity and related complications, including active bleeding, ischemic complications, and peritonitis.
Common sites of bowel injury
The most common site of bowel injury is the small intestine, with the proximal jejunum near the duodenojejunal junction or ligament of Treitz, and the terminal ileum being most commonly affected. Both are relatively mobile bowel sections that share a common border with fixed segment, which can result in shearing injury. Large intestinal injury is comparatively uncommon, with this type of injury found in only about 20% of cases , and most of these injuries are partial-thickness tears. The duodenum is the segment least frequently involved in blunt abdominal trauma; however, cases that involve the duodenum are frequently associated with pancreatic injury .
Importance of early detection of bowel and mesenteric injury
Identification of bowel and mesenteric injuries that require surgery
Full-thickness tear or perforation, devascularized bowel, and serosomuscular tear are findings that require early recognition and surgical treatment [6, 7]. A potential pitfall is that perforations and tears are sometimes too small to identify on CT. Diagnosis, therefore, relies on indirect imaging signs. Active mesenteric bleeding and mesenteric vascular injuries that cause bowel devascularization are emergent surgical conditions. The imaging signs of mesenteric injuries are more subtle than those of bowel injury, but early recognition is important to ensure appropriate surgical management.
Diagnostic imaging modalities in bowel trauma
Plain radiograph is not sensitive enough to exclude surgically significant bowel and/or mesenteric injury, but it remains a useful tool for detecting pneumoperitoneum in certain cases. Plain radiographs require larger amounts of free intraperitoneal air for detection, but small amounts of extraluminal air or pneumoperitoneum may not be detectable. Nonspecific findings of bowel injury may be detected, including dilated bowel loops, soft tissue density, and mass effect, which suggest fluid and loss of psoas margin. More than 800 ml of free intraperitoneal fluid must be present to be detected radiographically . Plain radiographs are of little utility in the setting of blunt trauma, but they are often obtained following penetrating trauma to detect metallic bullets, shrapnel, or foreign bodies.
Although focused ultrasound assessment in trauma has been widely accepted as a valid tool for evaluating patients with abdominal trauma, US has a sensitivity of 86% for detection of free intra-abdominal fluid, but is nonspecific for organ injury and bowel injury . Potential pitfalls include physiologic fluid, such as intra-abdominal fluid found in young woman in puberty, aggressive hydration, and other pathologic conditions, such as hemoperitoneum that can be caused by various diseases. Other potential pitfalls include bowel wall thickening or disruption and intramural bowel wall hematoma, because they are difficult to identify for reasons that include anatomical factors relating to the location of the injury (e.g., retroperitoneal location), sectoriality of the examination, and lack of patient cooperation in an acute trauma setting .
Magnetic resonance imaging (MRI)
Although MRI has the advantage of no patient radiation exposure, MRI is not routinely used for the initial evaluation of patients with blunt abdominal trauma or traumatic bowel injury due to prolonged scanning time, presence of artifacts from bowel movement, limited sensitivity for detecting pneumoperitoneum, and the requirement for an experienced interpreter.
Computed tomography (CT)
In hemodynamically stable patients, multidetector CT is the imaging modality of choice in this clinical setting. It provides more information than diagnostic peritoneal lavage (DPL) and US, to include grading, sites of injury, and presence of complication. The retroperitoneum, which is difficult to assess by US or DPL is also well visualized.
CT imaging protocol
- The density of luminal fluid and bowel wall will be assessed at baseline before contrast enhancement to facilitate differentiation between intramural hematoma and bowel wall enhancement (Fig. 2).
Comparison of fluid density between non-enhanced and enhanced phase helps to distinguish hemoperitoneum from other intra-abdominal fluid collection.
Mesenteric stranding can be easily identified in non-enhanced phase of mesenteric vessels .
A biphasic technique, including arterial and venous assessment after intravenous infusion of 120–150 ml of iodinated contrast agent at a flow rate ≥ 3 ml/s, is recommended to detect active bleeding and abnormal enhancement of the bowel loops . The delayed phase can be useful for excluding low-flow active bleeding. Reformatted images on coronal and sagittal view are also obtained.
The advantages of using oral contrast include improved delineation of bowel wall thickness and improved ability to identify contrast leak; however, administration of oral contrast in blunt abdominal trauma remains controversial. Oral contrast administration is currently discouraged or is not routinely used in patients with blunt abdominal trauma, because it is time-consuming process that may significantly adversely affect the patient’s prognosis by delaying the identification of active bleeding that requires urgent intervention. Time-related factors include the time required to prepare and administer the contrast, and the relatively long transit time that is required to completely opacify bowel loops. Potential pitfalls include spread of the extravasation of contrast from intraperitoneal bladder rupture that may mimic the spillage of oral contrast material from bowel loops, and extraluminal oral contrast material from traumatic bowel loop may mimic extravasated contrast material from ruptured vessel [11, 14, 15, 16, 17].
Common features in bowel and mesenteric injuries
Intra-abdominal fluid identified in a trauma patient can be related or not related to bowel and/or mesenteric injury. A careful search for other more specific findings is needed to identify the source of the fluid. Observed fluid could be one or more of the followings: blood from bowel or solid organ injury, urine from urinary tract injury, bile from biliary tract injury, fluid from prior DPL, pancreatic fluid from pancreatic duct disruption, and/or bowel content from bowel injury .
About 3% of male patients may have a small amount of hypoattenuating simple fluid in the pelvis without an associated intra-abdominal injury . Female patients, in particular premenopausal female patients, frequently have a small amount of simple physiologic fluid . Some trauma centers will admit patients for clinical observation when isolated free fluid is observed in a trauma setting . Surgery may be indicated in patients with an increase in free fluid or in those who remain hemodynamically unstable over time.
The potential pitfall is that abdominal fluid can accumulate from a combination of injury sources, such as combined bowel/mesenteric and solid organ injury. It is important to point out that diagnosis of bowel or mesenteric injury often goes unrecognized in combined injury settings. Another important potential interpretation pitfall is failure to differentiate interloop fluid from fluid within bowel loops. Shape or configuration of fluid is a helpful feature for distinguishing between these two types of fluid accumulation. Mesenteric or interloop fluid frequently manifests as triangular or V-shaped between mesenteric leaves [20, 21], which is easily distinguished from the more rounded shape of fluid within bowel loops. In equivocal cases, follow-up CT with oral contrast agent is recommended to opacify the bowel loops. Preexisting ascites may also cause confusion, since ascites accumulation may mimic fluid observed in bowel or mesenteric injury. Another pitfall is that free peritoneal fluid will accumulate in the peritoneal cavity following massive fluid resuscitation without any evidence of bowel injury .
Pneumoperitoneum: significance and pitfalls
In addition, no visualized pneumoperitoneum on cross-sectional imaging or CT does not exclude the presence of bowel perforation, because small extraintestinal air can be spontaneously reabsorbed by the peritoneum, the development of ileus can prevent gas leakage, and the perforation can be partially sealed off.
Radiologists must, therefore, remember to observe for associated mesenteric injury and other specific and less specific signs—not just for pneumoperitoneum.
Direct CT findings in bowel injury
Bowel discontinuity or interruption of the bowel wall
Extraluminal contrast extravasation
Indirect CT findings in bowel injury
Bowel wall thickening and abnormal enhancement
Bowel wall thickening may take concentric or eccentric form, and can be observed in 45–75% of cases . A potential pitfall is that actual bowel wall thickening needs to be differentiated from artifactual thickening or inadequate distention, which makes this interpretation largely subjective.
Bowel walls were considered thick if they were greater than 3 mm for the small bowel and 5 mm for the colon [23, 25, 31]. Bowel wall thickening was considered as focal if it was less than 10 cm in length, and non-focal if it was longer than 10 cm . However, isolated mesenteric vascular injury can cause localized or segmental bowel wall thickening, which may indicate bowel ischemia. It is, therefore, important to evaluate both bowel wall enhancement and bowel wall thickening to rule out bowel ischemia.
Abnormal bowel wall enhancement is not a specific sign of injury. Abnormal bowel wall enhancement can be classified as homogeneous, patchy/inhomogeneous, or absent. Homogeneous increased enhancement of long segments of bowel that is caused by increased vascular permeability can be observed in trauma patients with prolonged hypoperfusion. Patchy/inhomogeneous increased enhancement, although uncommon, can also be observed in bowel injury. Absent bowel wall enhancement can occasionally be observed in bowel injury, but more often in bowel ischemia associated with mesenteric vascular injury.
Direct CT findings in mesenteric injury
Active contrast extravasation
Injury to the mesenteric vasculature
Indirect CT findings in mesenteric injury
Mesenteric infiltration or stranding is characterized by an ill-defined area of increased attenuation in fatty mesenteric folds, which is usually caused by perivascular microhemorrhage in a trauma setting. This sign has high sensitivity, but low specificity, because it can be found in various conditions, such as mesenteric panniculitis due to mesenteric inflammatory infiltrate. Mesenteric infiltration can be associated with mesenteric injury alone or combined bowel and mesenteric injury. Mesenteric infiltration or stranding is more common when bowel injury occurs along the mesenteric border .
Mesenteric hematoma in the absence of other mesenteric or bowel injury is considered a nonsurgical condition. However, the presence of mesenteric hematoma should alert the radiologist to carefully search for any evidence of mesenteric vascular injury or associated significant bowel injury. Interloop hematoma was reported to be significantly associated with bowel or mesenteric injury, because it is not a common feature in solid organ injury [21, 36]. Mesenteric hematoma that borders a thickened bowel wall is a finding suggestive of bowel injury, and is an indication for surgery.
Radiologist familiarity with all CT findings in bowel and mesenteric blunt trauma, as well as the pitfalls described herein, is essential for making a timely diagnosis and providing essential information to the emergency team to help guide proper and timely treatment. This guide was developed to enhance the knowledge and awareness of non-expert radiologists that may be less aware of the nuances of abdominal CT imaging in a blunt trauma setting. This enhanced knowledge and awareness will improve patient diagnosis, treatment, and outcomes.
Compliance with ethical standards
Conflict of interest
All authors declare no personal or professional conflicts of interest, and no financial support from the companies that produce and/or distribute the drugs, devices, or materials described in this report.
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