CT Imaging Equipment Health Article

Definition

CT imaging equipment includes conventional, spiral, multi-slice, and electron-beam computed tomography full-body scanners, which use x rays to acquire cross-sectional images and computer workstations to reconstruct acquired image data for display on a viewing monitor or printed on film. Also referred to as computerized axial tomography (CAT) scanning equipment.

Purpose

Computed tomography is an x-ray imaging modality used for a variety of clinical applications. CT imaging equipment is used for spine and head imaging, gastrointestinal imaging, vascular imaging (e.g., signs of stroke, detection of blood clots), cancer staging and radiotherapy treatment planning, screening for cancers and heart disease, rapid imaging of trauma and pediatric patients, measuring bone mineral density for diagnosing osteoporosis, imaging of musculoskeletal disorders, detection of signs of infectious disease, and guidance of certain interventional procedures (e.g., biopsies). CT is the preferred imaging exam for diagnosing several types of cancers. CT scanners are also used to perform noninvasive angiographic imaging to assess the large blood vessels. Three-dimensional (3-D) image reconstruction, a feature available on many CT scanners, allows surgical procedure simulation and planning, postoperative evaluation, 3-D angiography, and virtual colonoscopy. Because computed tomography can clearly image soft tissue, bones, the lungs, and blood vessels, and can be used to diagnose so many diseases and conditions, CT scanners are often considered the backbone of a radiology department, and large hospitals may have multiple scanners to meet imaging demand. Because CT scanners are valuable in aiding in the evaluation of trauma and other emergency medical conditions, hospitals with large emergency volumes and major trauma centers may have a CT scanner located in and dedicated to the emergency department.

Some types of CT scanners (electron-beam and multislice, see below) have begun to be used for whole- body scanning for preventive screening purposes; that is, asymptomatic individuals can have a full-body scan to see if heart disease, cancer, or other conditions are present. This application is primarily offered by independent imaging centers and is not reimbursed by insurance companies.

Description

In general, a computed tomography scanner consists of a gantry, an x-ray system, a patient table, and a computer workstation. The gantry is a large square unit with an opening in the center through which the patient is moved during the scan. The gantry contains the x-ray system, which includes an x-ray tube, detectors, x-ray beam collimators, circuitry, and an x-ray generator. In some older CT scanners, the x-ray generator may be separate from the gantry. The patient table is designed for both vertical and horizontal motion to accommodate various types of patient positions during the scan.

During a CT scan, the x-ray generator supplies power to the x-ray tube. X rays are produced by the x-ray tube and emitted as it is rotated around the patient. The x rays pass through the patient's body to the detectors, which, depending on the CT scanner type and model, may consist of xenon gas ionization chambers or solid- state crystals (such as cesium-iodide or cadmium- tungstate). During each rotation, the detector produces electrical signals, which are generated after exposure to the x rays. These electrical signals are transferred to the computer, processed, and reconstructed into images using preprogrammed algorithms. Each rotation of the xray tube and detectors is reconstructed into an image that is referred to as a slice. The slice represents a cross-section of anatomical detail, and allows the inside of anatomical structures to be visualized, which is not possible with general radiography. Collimators are located near the x-ray tube and at each detector to minimize scatter radiation and to properly define the x-ray beam for the scan. The height of the collimators determines the slice thickness.

There are several types of CT scanners currently in use that differ in configuration and scanning features. Conventional CT scanners, which were introduced in the 1970s, have cables attached to a detector array, and therefore, at the end of one x-ray tube rotation, the assembly must reverse to avoid tangling the cables. Conventional scanners, then, have the slowest scanning speed. Spiral CT scanners, also called helical or volumetric scanners, have a slip-ring configuration that allows continuous one-way rotation. In spiral scanning, the patient table is moved through the gantry while the x-ray tube and detector rotate in a spiral around the patient. Scanning speed is faster, thinner slices are acquired, and shorter patient breathholds are required than for conventional CT. Spiral CT scanners were introduced in 1989, and have since been considered a revolutionary advance in CT imaging due to the improvements in scanning speed and image quality that were possible compared to conventional CT scanners.

Multislice scanners, which were introduced in 1998 and are considered the next revolution in CT imaging, have multiple rows of detectors that allow acquisition of multiple image slices during one x-ray tube rotation. Depending on the model and manufacturer, a multislice scanner may be up to eight times faster than a single-slice spiral scanner, and slices half as thin as those acquired on a spiral scanner are attainable. Multislice technology was still under development as of 2001.

Electron-beam CT scanners, also called ultrafast CT scanners, use a different scanning technology than other CT scanners, where x-ray tube rotation is mechanical. Electron-beam CT scanners have no moving parts, which makes such a fast scanning speed possible. An electron gun produces a focused electron beam that generates a rotating x-ray fan beam after being steered along tungsten target rings. Scan times are approximately ten times faster than multislice scanners because only the electron beam moves during scanning. Electron beam CT scan- ners were introduced in the mid-1980s and were designed for cardiac imaging and imaging of other moving structures (e.g., lungs, colon) due to their fast scanning speed.

CT imaging equipment is often supplied with image archiving devices (e.g., compact disk jukebox, tape drive), image hard copy devices (e.g., x-ray film processor, laser imager), and networking capabilities, depending on the needs of the facility. Because CT is a digital modality, CT scanners are frequently networked with other digital equipment, such as ultrasound and magnetic resonance imaging (MRI) systems, to facilitate comparison of images on viewing monitors.

In small hospitals or hospitals in rural areas, a CT scanner may not be installed; rather, a mobile CT scanning service may be contracted. A spiral CT scanner is installed in a specially designed trailer, which is driven to the hospital contracting the service. Prescheduled CT scans are then performed for the day or days the scanner is available at the hospital. Obviously, mobile CT only accommodates imaging in cases where the exam is not urgent.