CHAPTER 4: COMPUTED TOMOGRAPHY: BASICS

INTRODUCTION

Computed tomography (CT) has gained widespread acceptance for the evaluation of patients in the emergency setting. Its cross-sectional nature confers significant reliability and accuracy in detecting small anatomic changes, and it has several advantages over traditional two-dimensional radiographs.

CT completely eliminates the superimposition of images outside the area of interest that can occur with plain films. Additionally, its inherent high-contrast resolution allows for distinction between tissues that differ in physical density by less than 1%, permitting much finer detail of anatomic structures. Data from a single CT imaging procedure can be viewed as images in the axial, coronal, or sagittal planes, depending on the diagnostic task. Finally, CT scan was the first technology to marry a computer-based system to a diagnostic imaging technique, enabling a new era of digital imaging. It is a rapid and cost-effective modality that can provide invaluable diagnostic information and characterize the full extent of disease.

CT SCAN IN EMERGENT SCENARIOS

CT scans are completed with the use of a 360-degree x-ray beam and computer production of images. These features allow for cross-sectional views of organs and body structures yielding sharp, focused, and three-dimensional images. CT is most often indicated in patients who may require a form of immediate intervention and is most helpful when the patient has confusing clinical signs and symptoms. In fact, the accuracy of conventional CT has been shown to be as high as 95% in acute abdomen.¹ CT is considered the imaging modality of choice in many hospitals for patient triage in the emergency department, and now almost all hospitals have CT scanners on site in the emergency department.

CT technology growth has focused on dramatically increasing the speed of scanning and image reconstruction, both of which are critical in the emergency setting. Rapid speeds have been accomplished by simultaneous data acquisition through more extensive detector arrays, which facilitates the acquisition of multiple “slices” at the same time and, as a result, shortens acquisition time. Currently, CT systems have up to 64 rows of detectors, allowing up to 4 cm to be imaged per revolution (with each revolution being approximately 0.4 seconds).

INDICATIONS FOR CT SCAN

The applications of CT scan in emergencies are numerous, with contributions to diagnoses ranging from trauma to soft tissue infections to pulmonary embolism. The following list outlines a few major indications for CT scan in the acute surgical setting.

1. Imaging in patients with trauma:

   1. Significant mechanism of injury and normal clinical examination

   2. To localize a hematoma

   3. Traumatic aortic injury, as a normal chest film does not exclude aortic injury

   4. Nondisplaced subtle fractures

   5. To rule out hemorrhage in patients with head trauma

2. Excluding primary intracerebral hemorrhage as a cause of acute stroke (provided it is performed within about a week of onset)

3. Acute abdomen workup

BASIC ...