HISTORY

Computerized axial tomography was developed in the late 1960s and early 1970s, independently, by Hounsfield and Cormack. CT was first clinically applied in trauma situations in the mid-1970s. Initially, it was only possible to image the head and brain. After a period in which CAT scanners were not common, by the mid-1980s these devices were available in hospitals throughout the United States. This availability, as well as advances in CT technology, spurred the development of new applications and techniques.

Computed tomography is performed by passing rotating fan beams of x-rays through the patient in an axial plane. In the early scanners, the x-ray tube rotated around the patient to obtain a single image or “slice.” The table then moved for the next slice. This process was quite time consuming. In modern scanners, the tube rotates continuously while the table is in motion, yielding a spiral or helical scan. The more recent scanners also have multiple rows of detectors that can obtain as many as 64 slices simultaneously. This makes it possible to scan the entire body from head to pelvis in only a few minutes.

The continuous data from the spiral scanning are stored in computer memory, which allows the data to be manipulated in various ways. If, for example, the chest and abdomen have been scanned, detailed reconstructions of images of the thoracic and lumbar spine can be obtained without exposing the patient to additional radiation. The computer can also generate three-dimensional (3D) rotating images, which can be useful in the interpretation of CT angiography and complex fractures.

Similar to plain radiographs, there are four basic densities on a CT image. Air is black; fat is dark gray; soft tissue is light gray; and bone/calcium and contrast agents are white. The x-ray absorption of a specific tissue can be measured in Hounsfield units (HU). The density of water is zero. Blood measures 40–70 HU. Urine, ascites, and bowel content measure close to water, 0–30 HU. Various “windows” for bone, brain, lung, abdomen, and so on, are used to best display tissues of various densities.

COMPUTED TOMOGRAPHY OF HEAD/BRAIN (CRANIUM)

Computed tomography of the head is indicated in patients with clinical evidence of traumatic brain injury, including loss of consciousness, amnesia, depressed level of consciousness, a mental state that is difficult to evaluate due to recreational or therapeutic drug administration, hemotympanum or cerebrospinal fluid leak, suspected skull fracture, severe headache, persistent nausea and vomiting, or post-traumatic seizures.

Scanning protocols may vary slightly from institution to institution. In adults the scan is usually performed with 5-mm cuts from the skull base to the vertex. In infants, the scan is commonly performed with finer cuts starting at the second cervical vertebra.

The study is performed without contrast. Acute hemorrhage appears white on the scan (Figure 1) except in very anemic patients, where it may appear isodense with brain. Active intracranial bleeding may also appear as gray swirls within the white clot, indicating hyperacute hemorrhage.

Figure 1 Computed tomography scan demonstrating a classic lenticular-shaped, acute epidural hematoma.

Computed tomography is very sensitive for intracranial hemorrhage, edema, and mass effect. As CT technology has advanced, detection of smaller lesions has become possible, leading to controversy regarding the significance of these lesions, and the indications for CT in mild head injury.¹ CT of the brain may be normal and not correlate well with the clinical picture in patients with diffuse axonal injury.

Computed tomography of the head may also detect fractures of the upper facial bones and orbits; however, dedicated CT of the face is required to provide adequate resolution to plan operative intervention. High-resolution CT of the base of the skull may be required to evaluate basilar skull fractures.

COMPUTED TOMOGRAPHY OF FACE AND ORBITS

Computed tomography is the best method to study facial fractures. Patients with clinical suspicion of fractures due to facial swelling, deformities, visual changes, or bony tenderness or instability on examination should undergo CT of the facial bones, concomitant with head scan (if necessary). If facial fractures or fluid in the sinuses are seen on CT of head, a formal CT of the face should be performed, as the resolution of head CT is insufficient.

Axial images are obtained using 3-mm slices from below the mandible to just above the frontal sinuses. The mandible may be excluded if no suspicion of injury is present. In a spiral CT scanner, coronal views can be reconstructed from the initial data. With multidetector CT scanners, 3D reconstructions (Figure 2) can be generated that are of value to the maxillofacial surgeon in planning reconstruction, and particularly in complex fractures.

Figure 2 Three-dimensional reconstruction of the facial bones demonstrating multiple fractures, including a right-sided depressed skull fracture and bilateral orbital fractures.

COMPUTED TOMOGRAPHY OF SPINE

The cervical spine needs to be evaluated radiographically in victims of blunt trauma who present with cervical spine tenderness, acute neurologic deficit, or significant brain injury; or who cannot be adequately evaluated due to distracting injury, intoxication, or therapy (particularly those who are intubated and sedated). Debate continues regarding some of the “softer” indications for such imaging.² The mainstay of cervical spine imaging has been plain radiography for decades, with CT if the plain films are incomplete or inadequate, or the anatomy of injury requires delineation. Improvements in technology and technique have led some to advocate CT as the primary imaging modality for the cervical spine.³ The scan is done with 2.5–3-mm cuts from the skull base through the first thoracic vertebrae. In addition to the axial views, sagittal and coronal reconstructions are performed.

Computed tomography of the cervical spine has a sensitivity of 95%–100% and a specificity of 98%–100% for detecting cervical spine injury.⁴ Indications for imaging the thoracic and lumbar spine are similar to those for the cervical spine. Patients with spine tenderness, deformity, distracting injuries, neurologic deficit, or spine fractures at other levels, or who cannot be properly evaluated due to depressed mental status (with appropriate mechanism of injury) should have anteroposterior and lateral x-ray views of their thoracic and lumbar spine. If the plain films are inadequate or if the patient has an impressive clinical exam with normal plain radiographs, the area of concern should be further evaluated with CT. If the chest and abdomen have been scanned, detailed images of the spine can be reconstructed from that data. Images with a 2.5-mm slice thickness, including two normal vertebral bodies above and below the injury, should be obtained with sagittal and coronal reconstruction.

COMPUTED TOMOGRAPHY OF NECK AND GREAT VESSELS

Patients with clinical signs of blunt neck injury, including laryngeal tenderness, subcutaneous emphysema, hemoptysis, hoarseness, hematomas, or significant contusions of the neck related to seat restraints should undergo CT of the soft tissues to evaluate the aerodigestive tract. CT is the study of choice to detect and define injuries or the larynx. Patients with cervical spine fractures (especially with transverse foramen involvement), severe facial fractures, basal skull fracture involving the carotid foramen, expanding hematoma in the neck, or with neurologic status not explained by traumatic brain injury, are at risk for cervical vascular injury. Although four-vessel cerebral angiography is considered the gold standard for diagnosis of blunt cerebrovascular injury, CT angiography has shown promising results in some series, and is a rapid approach to identification of carotid or vertebral artery injuries in this setting.⁵ For stable patients who have suffered penetrating neck trauma, CT-angiogram may be used to identify vascular, aerodigestive, and bony injuries, and may be useful to guide invasive studies and surgery.⁶

The study is carried out on multidetector scanner, with administration of 75–150 ml (1.5–2 ml/kg) of contrast material delivered at a rate of 3–5 ml/sec. When necessary, 3D images are reconstructed for interpretation and surgical planning.

COMPUTED TOMOGRAPHY OF CHEST

Computed tomography of the chest is performed routinely following blunt trauma in many institutions, whenever an abdominal scan is obtained. Chest CT is specifically indicated when the initial chest radiography or mechanism of injury suggests likelihood of blunt thoracovascular injury. CT angiography has replaced traditional angiography as the initial study of choice for blunt aortic injury (Figure 3), and may completely eliminate the need for arteriogram for both operative and “nonoperative” management.⁷ Chest CT is also more sensitive for pneumothorax, hemothorax, pneumomediastinum, parenchymal lung injury, and bony thoracic injury, than plain chest radiography.

Figure 3 Axial image in the arterial phase of a patient suffering from blunt chest trauma demonstrating large mediastinal hematoma thoracic aortic intimal disruption near the ligamentum arteriosum (white arrow).

In hemodynamically stable transmediastinal gunshot wounds, a CT scan of the chest may be useful for initial screening to determine if further work-up or interventions are required.⁸

The chest is typically scanned in 2.5–3-mm slices with a contrast load of 150 ml (1.5–2 ml) given at a rate of 3–5 ml/sec. A delay of 20 seconds is used in order to optimize great artery opacification during the scan.

COMPUTED TOMOGRAPHY OF ABDOMEN AND PELVIS

Computed tomography has become the study of choice for evaluating the abdomen in hemodynamically stable blunt trauma victims. It is sensitive to the presence and extent of injuries of solid organs (e.g., liver, spleen, and kidneys), but is less sensitive in detecting injuries of the diaphragm, pancreas, and the gastrointestinal tract. Indirect findings, such as free fluid without solid viscus injury, or mesenteric streaking, are important when suspicion of these injuries exists.

The scan is performed in 5-mm slices from the lower chest through the perineum. In the trauma setting, there is no place for CT of the abdomen without concomitant pelvic CT. Intravenous contrast is used to enhance the solid organs and to detect active bleeding. A “blush” suggests active arterial bleed when seen in the area of a pelvic fracture, or in a solid organ injury, even in a stable patient (Figure 4). This finding should prompt an invasive approach to care, including interventional angiography and embolization and/or surgery.

Figure 4 Abdominal computed tomography demonstrating a splenic injury with extravasation of contrast (“blush”) (black arrow).

Oral contrast is also frequently used, although many studies have demonstrated that it is not necessary in evaluating the gastrointestinal tract for injury. Radiologist preference and patient compliance should determine its use.⁹

In penetrating trauma, CT of the abdomen and pelvis may be useful in evaluating asymptomatic patients with stab wounds of the back or flank. In these situations, the triple-contrast technique yields sensitivity of 97%–100% and specificity of 96%–98% for retroperitoneal or abdominal visceral injury and peritoneal penetration. It may also be of value in identifying tangential gunshot wounds, at low risk for intra-abdominal or retroperitoneal injury. In institutions where nonoperative management of gunshot wounds of the liver is an option, CT of the abdomen of stable patients with suspicion of such injuries is a requirement.^10,11

A CT cystogram can be performed in patients at risk for bladder injury. This has a diagnostic accuracy approaching 100%. Bladder distention is achieved by instilling at least 350 ml of diluted contrast solution via a Foley catheter under gravity. Postdrainage images are unnecessary.¹²

COMPUTED TOMOGRAPHY OF ORTHOPEDIC INJURIES

Other than for initial evaluation of pelvic fractures, CT study of orthopedic injuries is not part of initial evaluation. In otherwise stable patients, it may be convenient to perform them concomitantly with other studies.

Computed tomography of the acetabulum is indicated after hip dislocation to detect intra-articular bone fragments or acetabular fractures and for presurgical evaluation of unstable fractures. It is also useful in the evaluation of shoulder/scapular injury, tibial plateau fractures, and complex fractures of the ankle involving the articular surfaces and talus/calcaneal fractures.