CHAPTER 9 Imaging Evaluation of Trauma

ROLE OF IMAGING IN TRAUMA

Imaging plays an important role in the early evaluation of trauma patients. Current trauma management protocols use imaging more liberally than in the past in an effort to detect injuries that might otherwise be overlooked because of distracting pain or inaccurate historical information. Knowledge regarding patterns of injury in the abdomen and pelvis helps the radiologist detect subtle injuries or associated injuries that might be missed.

It is important to keep the role of imaging in the trauma setting in perspective. The diagnostic acumen of the trauma physician remains the most important factor in directing therapy. Patients with hypotension following severe blunt or penetrating injury often go directly to surgery. In such cases, the time required for imaging may cause unacceptable delay of life-saving therapy. When imaging is used, it must be interpreted within its clinical context to avoid errant interpretation leading to unnecessary intervention. When a trauma-related finding is present on imaging, management options include observation, surgery, and transcatheter intervention. Injuries that require emergent therapy must be immediately identified and accurately communicated to the trauma team.

Ultrasound and Computed Tomography

Despite earlier literature that implied competition between computed tomography (CT) and ultrasound in the trauma setting, the two have evolved as complementary imaging tools. CT has emerged as the cornerstone of trauma imaging of the abdomen and pelvis because of high sensitivity for injuries of the solid and hollow organs, fractures of the spine and pelvis, hemorrhage, and extraluminal gas.

Ultrasound, in contrast, can be used in the trauma room while members of the trauma team continue to assess and support the patient. Focused abdominal sonogram for trauma (FAST) has largely replaced diagnostic peritoneal lavage by providing a rapid noninvasive method for identifying hemoperitoneum. Box 9-1 shows the search pattern for FAST examination. Up to 40% of patients with injuries to the abdominal viscera are taken to the operating room immediately after sonographic determination of hemoperitoneum, and injuries are diagnosed during laparotomy rather than with CT.

A key limitation of the FAST examination is that nearly one in three injuries to the liver or spleen do not result in hemoperitoneum and are more likely to be identified on CT. Understanding the relative strengths and weakness of imaging modalities is an important step in creating effective trauma protocols. Table 9-1 summarizes the advantages and disadvantages of ultrasound and CT.

Table 9-1 Comparison of Ultrasound and Computed Tomography in Trauma Imaging

Imaging Type	Advantages	Disadvantages
Ultrasound	• Does not require patient transport	• Operator dependent
	• Other procedures can be performed simultaneously	• Requires sonographic window
	• No radiation or contrast media	• Limited in obese patients
	• Can be repeated	• May miss injuries without associated hemoperitoneum
Computed tomography	• Accurate characterization of solid organ injuries	• Requires contrast media
	• Sensitive for extraperitoneal injuries	• Ionizing radiation
	• Less operator dependent	• Requires patient transport and transfer to scanner

Blunt versus Penetrating Trauma

Until recently, CT played a limited role in the initial management of patients with penetrating injury to the abdomen and pelvis because most such patients received emergent surgery. Early in its history, CT had only limited sensitivity for detecting bowel injury. Therefore, CT was reserved for those patients in whom the depth of injury remained in question.

As advancements in multidetector computed tomography (MDCT) technology have improved sensitivity for bowel injury, the number of CT examinations performed for penetrating injuries has increased. Pneumoperitoneum, focal bowel wall thickening, isolated bowel dilatation, and mesenteric hematoma are CT findings that suggest bowel injury. Rarely, the site of discontinuity within the bowel wall can be seen. Some centers administer water-soluble enteric contrast media via a nasogastric tube in the trauma room, improving sensitivity for injuries to the stomach and proximal small bowel. Patients with hemodynamic compromise are still likely to be taken directly to surgery without any imaging beyond FAST ultrasound.

Imaging is routinely performed for blunt force trauma. Injury in blunt force trauma is the result of direct crush injury, sheer forces, or burst injury from an abrupt increase in intraluminal pressure. The most common source of blunt force trauma that requires imaging is motor vehicle collision. Victims of severe blunt force trauma with hemodynamic compromise may still require immediate surgical exploration, but occasionally even unstable patients require imaging if the site of injury is not apparent by physical examination. Imaging surveys often include CT examinations of the head, cervical spine, chest, abdomen, and pelvis.

The American Association for the Surgery of Trauma (AAST) provides a scoring system that categorizes the severity of solid organ injury based on imaging findings. Familiarity with that system allows the radiologist to report specific imaging findings that may affect the surgeon’s classification of a particular injury.

KEY COMPUTED TOMOGRAPHIC FINDINGS IN THE TRAUMA PATIENT

Lacerations

As with most disease processes, the key radiographic findings in trauma include both direct and indirect findings. Most direct CT findings are relatively straightforward but can be subtle at times. Low-attenuation parenchymal defects indicating laceration of a solid organ are perhaps the most common direct sign on enhanced CT images. The location of the laceration should be noted, with particular attention to involvement of the capsule of the organ in addition to any other vital structures such as blood vessels, bile ducts, or urinary collecting system. In addition, the approximate length of the laceration should be noted, particularly if it results in complete or near-complete transection of the organ involved.

Contrast Extravasation

Active contrast extravasation (ACE) describes the presence of vascular contrast media outside the vascular space and is a direct sign of hemorrhage. Attenuation of extravasated contrast medium is as high as in major arterial structures nearby and is often located within or adjacent to a parenchymal laceration (Fig. 9-1). When ACE is present, transcatheter or surgical intervention is often required to stop the hemorrhage. Before the significance of ACE was known, some patients were managed with observation that included prolonged replacement of blood products and fluids. In this scenario, the patient can become coagulopathic by the time the intervention is performed, increasing risk of the procedure and potentially decreasing its efficacy. ACE is now considered to be a risk factor independent of other AAST criteria.

Figure 9-1 Axial image from a contrast-enhanced computed tomographic scan demonstrates active contrast extravasation in the jejunal mesentery after blunt force trauma.

Pearl: ACE is a sign that urgent transcatheter or surgical intervention may be necessary to prevent prolonged hemorrhage.

Leakage of excreted contrast material from the urinary tract is a highly specific sign of urinary tract injury. It is seen most often with injuries to the intrarenal collecting system and bladder, and only rarely with ureteral injuries. Because urine will be opacified only in the excretory phase, delayed (>2-3 minutes) CT images are helpful in confirming the diagnosis (Fig. 9-2).

Figure 9-2 Importance of delayed images for urinary trauma. A, Axial image from initial contrast-enhanced computed tomographic examination performed after a motor vehicle collision demonstrates minimal fluid adjacent the right ureter. B, The scan was repeated 20 minutes later at the radiologist’s request, demonstrating excreted contrast leaking into the periureteral tissues. IVC, Inferior vena cava.

Some centers administer enteric contrast as part of the CT trauma evaluation in an attempt to increase sensitivity for gastrointestinal injury (Fig. 9-3). Because this technique can delay imaging (up to an hour for complete small-bowel opacification) and carries some increased risk for aspiration during surgery, many institutions no longer routinely use enteric contrast media in trauma patients.

Figure 9-3 Axial computed tomographic image demonstrates extravasated enteric contrast caused by blunt force injury to the bowel during motor vehicle collision. A jejunal perforation was confirmed at surgery.

Extraluminal Fluid and Air

Extraluminal fluid and air are important indirect signs of injury. It is critical to characterize the precise location of fluid and air in the abdomen because pathways of intraperitoneal and extraperitoneal spread are usually predictable and help direct the search for the site of injury (see Chapter 6).

Large amounts of hemoperitoneum are often associated with injuries to the spleen or liver. Fluid in the lesser sac is usually associated with injury to the structures that form its borders (i.e., spleen, stomach, or pancreas) (Fig. 9-4). When extraperitoneal fluid or hemorrhage is present, look for injuries to the urinary tract, pancreas, and vessels such as the aorta and its major branches. Fluid within the mesentery and between bowel loops is better seen with CT than with ultrasound. When mesenteric injury is present, also look for evidence of bowel injury such as wall thickening, absent mural enhancement, or extraluminal gas. Remember to adjust window/level settings to make fat appear gray to facilitate detection of pneumoperitoneum (Fig. 9-5).

Figure 9-4 Axial image from contrast-enhanced computed tomography performed after a motor vehicle collision demonstrates hematoma in the lesser sac as the result of active hemorrhage from the hepatic artery. ACE, Active contrast extravasation.

Figure 9-5 Impact of window/level settings on detecting pneumoperitoneum. A, Axial image from computed tomographic scan viewed in soft-tissue windows demonstrates a small pneumoperitoneum in the upper abdomen after a motor vehicle collision. B, Viewing the same image using bone windows increases conspicuity of the air.

Pearl: Increase your sensitivity for detecting extraluminal fluid with CT by including a directed search for fluid using the FAST pattern. Increase your sensitivity for detection of extraluminal air by reviewing the entire scan with bone or lung windows.

Pitfall: Window/level settings that make fat appear nearly black decrease the conspicuity of pneumoperitoneum. Adjusting settings to make the fat appear gray will facilitate detection of small amounts of extraluminal gas.

Signs of Hemodynamic Compromise

Aside from ACE, other CT findings indicate life-threatening hemorrhage. The most common are flattening of the inferior vena cava (IVC) and increased enhancement of the bowel, kidneys, and adrenal glands. Box 9-2 lists key findings of hemodynamic compromise. Figure 9-6 shows some of these findings.

Figure 9-6 Axial images from contrast-enhanced computed tomographic examinations in four different patients demonstrate findings of hemodynamic compromise. A, Hemoperitoneum with flattening of the inferior vena cava (IVC). B, Halo of low attenuation around the IVC. C, Bright enhancement of the kidneys and adrenal glands. D, Shock bowel appearance. ACE, Active contrast extravasation.

HEPATIC TRAUMA

The liver is the largest solid organ in the abdominal cavity, and its relatively brittle parenchyma makes it susceptible to laceration and fracture from the compressive and shear forces of blunt abdominal trauma. The size of the liver also makes it the most commonly injured solid organ from penetrating injury. Although the posterior segment of the right hepatic lobe and the interlobar region are most commonly injured, injuries can occur anywhere in the liver. Therapeutic options most commonly used to treat liver injury include transcatheter embolization, hepatotomy with vessel ligation, segmentectomy, and lobectomy. Despite these options, the mortality rate from liver injuries remains near 10%.

Types of Injury to the Liver

Injuries to the liver are classified into several types, listed in Box 9-3. The most common injuries are lacerations, which are parenchyma tears that extend to the liver surface. On contrast-enhanced CT, most lacerations appear as linear or curvilinear hypoattenuating defects. Characterization of a laceration should include its location and any important structures (particularly vascular structures) involved. A superficial laceration is sometimes called a capsular tear (Fig. 9-7). A laceration that continues from one liver surface to another (full thickness) is called a fracture.

BOX 9-3 Types of Hepatic Injury

Figure 9-7 Axial image from contrast-enhanced computed tomography performed after a motor vehicle collision demonstrates a small peripheral laceration extending to the capsule of the liver, commonly called a capsular tear.

Hematomas are nonlinear (often lentiform or round) collections of blood within the liver parenchyma (Fig. 9-8). Subcapsular hematomas occur between the liver capsule and the hepatic parenchyma, and are often curvilinear or lentiform.

Figure 9-8 Axial images from contrast-enhanced computed tomographic scans of two different patients with liver hematomas. A, Parenchymal hematoma with active contrast extravasation (ACE). B, Subcapsular hematoma.

Portosystemic shunting, arteriovenous fistula, ACE, and infarction near areas of laceration are all signs of vascular injury. ACE can be seen within any type of hepatic hematoma, and should be noted and reported immediately because prompt transcatheter or surgical intervention can be lifesaving. Infarction occurs with transection of arterial and portal venous structures but should not occur with hepatic venous injury alone. Vascular avulsions or sheer injuries can occur within the porta hepatis (hepatic artery, portal vein, or both) (Fig. 9-9), or at either the superior or inferior margin of the hepatic attachment of the inferior vena cava (Fig. 9-10). Subtle laceration to the central hepatic veins can result in dramatic blood loss if exacerbated during manipulation by an unsuspecting surgeon (Fig. 9-11).

Figure 9-9 Pedestrian struck by motor vehicle. Immediate exploratory laparotomy was unsuccessful at controlling liver hemorrhage. Postsurgery contrast-enhanced computed tomographic scan demonstrates active contrast extravasation (ACE) in the left hepatic lobe. Angiography confirmed active hemorrhage from a branch of left hepatic artery. Transcatheter embolization was successful at achieving hemostasis. IVC, Inferior vena cava.

Figure 9-10 Axial image of the liver dome from contrast-enhanced computed tomographic scan demonstrates a venous pseudoaneurysm at the superior margin of the intrahepatic inferior vena cava (IVC). At surgery, a partial hepatic avulsion from the IVC was repaired successfully.

Figure 9-11 Axial image from contrast-enhanced computed tomographic scan demonstrates active contrast extravasation (ACE) from the middle hepatic vein. Note near-complete interlobar fracture of the liver.

Pearl: Warning the surgeon of injury to the large central hepatic veins can help prevent rapid, unexpected intraoperative hemorrhage caused by manipulation of the liver.

Grading Hepatic Trauma

Trauma surgeons use the classification system adopted by the AAST to stratify liver injuries. Table 9-2 summarizes the AAST classification. In most centers, the radiologist is not expected to provide the precise grading for the injury. However, knowledge of the classification system paired with precise observation of the relevant findings facilitates effective patient management. In most cases, the key imaging findings used for the AAST classification can be distilled to the short list included in Table 9-3.

Table 9-2 American Association for the Surgery of Trauma Classification of Hepatic Trauma

Table 9-3 Key Imaging Findings in Hepatic Trauma

Finding	Significance
Presence of active contrast extravasation	Likely to require embolization or surgery
Laceration to a central hepatic vein	Risk for severe hemorrhage when liver is moved during surgery
Central collection of low-attenuation fluid (rare)	Possible bile leak; consider endoscopic retrograde cholan-giopancreatogram or scintigraphy when stable
Regions of hepatic infarction (rare)	Vascular injury is present, increased risk for further hemorrhage

Pitfalls in Imaging Hepatic Trauma

Intravenous contrast media is essential for detecting lacerations of the solid organs. Timing of image acquisition is also important. Imaging the liver before the portal venous phase causes the hepatic veins to appear as low-attenuation linear structures that can be mistaken for or obscure a laceration.

Hepatic fissures can also mimic laceration. Similarly, a laceration in the expected location of a normal fissure can be overlooked. Multiplanar reformations are useful when differentiating between fissures and lacerations (Fig. 9-12). Aggressive volume resuscitation can affect the appearance of the liver on contrast-enhanced CT images, with periportal edema resulting in a halo of low attenuation around portal branches (Fig. 9-13).

Figure 9-12 Accessory fissure in right hepatic lobe mimics liver laceration. A, Axial image from contrast-enhanced computed tomographic (CT) scan demonstrates a linear low-attenuation defect in the right hepatic lobe, but fat attenuation and absence of perihepatic fluid suggest that this is not a laceration. B, Sagittal reformation of the CT scan demonstrates an anatomic (nontraumatic) cleft.

Figure 9-13 Axial image from contrast-enhanced computed tomographic scan after aggressive volume resuscitation demonstrates extensive periportal edema. Distention of the inferior vena cava (IVC) suggests volume overload.

SPLENIC TRAUMA

The spleen is the most commonly injured abdominal organ. A soft and relatively mobile but highly vascular organ, the spleen is quite susceptible to injury from blunt force trauma. In addition to motor vehicle collisions, the spleen is commonly injured in fights, falls, and athletic activities.

With increasing understanding of postsplenectomy sepsis, nonoperative management of splenic injury has become increasingly emphasized in recent years. Because the risk for death from sepsis in patients who have undergone splenectomy is approximately 10 times that in the general population, efforts are made to preserve functioning splenic tissue whenever possible. Currently, at least 70% of patients with splenic injury who are hemodynamically stable are treated without surgery. Because delayed hemorrhage can be life threatening, patients with splenic injuries are often observed in an inpatient setting for at least 72 hours.

Types of Injury to the Spleen

As with the liver, most splenic injuries can be described as lacerations, hematomas, or vascular injuries. Small, superficial lacerations are commonly called capsular tears (Fig. 9-14). Hematomas contained by the splenic capsule are called subcapsular hematomas (Fig. 9-15). Although sometimes difficult to differentiate from areas of parenchymal hematoma, regions of devitalized spleen signify vascular injury (Fig. 9-16). ACE indicates severe vascular injury with active hemorrhage and usually requires immediate surgical or transcatheter intervention.