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.
Operation
After the technologist properly preps and positions the patient on the scanning table, the technologist goes to the adjacent control room and begins the scan using the control computer workstation. Usually, the computer has preprogrammed scanning protocols for common types of scans (e.g., abdomen and pelvis, chest, head) and some computers allow customized scan protocols to be entered. During scanning, the technologist instructs the patient via an intercom system regarding breathholds and positioning. The controlling computer automatically moves the patient table according to the scanning parameters selected. The scan itself may only take five to 15 minutes, but total examination time may be up to 30 minutes, since the patient must be prepped and positioned.
When the examination is completed, the technologist processes the image data using the computer workstation. Depending on the facility, images may be sent to an x-ray film processor or laser imager to be printed as hard copy and taken to a reading room, or they may be put on a
computer diskette or transferred via a digital image management system (picture archiving and communication system) for interpretation on a viewing monitor.
Before the patient is moved off the table, the radiologic technologist should review the acquired images to be sure they are of sufficient diagnostic quality. Motion artifacts, which are streaks, blurs, or other inconsistencies in the image, may occur if the patient moves during the scan or when imaging moving structures (e.g., heart, lungs). Decreasing the acquired image slice thickness, changing the timing of the contrast material injection, and shortening the patient's breathhold time can help reduce the occurrence of motion artifacts.
The radiologic technologist should choose the scanning protocol that will provide optimal image quality with minimal radiation dose. Typical radiation doses for a CT scan are approximately equal to the amount of natural background radiation the average person is exposed to over a year. The patient radiation dose from a CT scan is slightly higher than that of a typical x-ray procedure. Newer multislice scanners may deliver a significantly higher radiation dose than single-slice spiral scanners; this higher dose is of special concern for pediatric patients. The American Society of Radiologic Technologists (ASRT) has issued a statement regarding scanning protocols for pediatric scanning and recommends that specific scanning protocols be developed for pediatric patients and that CT equipment manufacturers develop a range of suggested parameters for pediatric patients based on weight. In addition, ASRT encourages technologists to be aware of radiation doses for their pediatric cases by using radiation shielding when necessary, adjusting patient positioning, using special dose filters, and increasing the pitch ratio (the table speed per gantry rotation) on spiral scans.
Maintenance
Due to its high cost ($500,000 to over $1,000,000) and technical sophistication, CT imaging equipment is usually purchased with a service contract from the manufacturer or third party service provider that covers x-ray tube and other parts replacement and emergency service repair. The facility's biomedical engineering department and medical physicist may also conduct annual preventive maintenance checks, as well as monthly calibration, image quality testing, and radiation dose monitoring.
A comprehensive quality control program that includes evaluation of image resolution, patient radiation dose, accuracy, image processing, patient table movement, and other overall system performance and image quality features should be followed. The radiologic tech- nologist may be required to assist engineering staff with maintenance checks and service repairs.
Most CT manufacturers offer remote diagnostic features on their equipment that facilitate repair of system problems. Communication via modem and telephone with service personnel and diagnostic software allows, for example, ordering of replacement parts, downloading of software to fix a problem, or immediate notification of an operational problem to repair personnel.
Health care team roles
A radiologic technologist trained in computed tomography positions the patient on the table, administers any contrast material (intravenously, oral, or by enema), and operates the CT scanner and computer workstation. Before administering any contrast material, the technologist will screen the patient for any allergies to medications or iodine and take a medical history to determine whether the patient has any medical condition (e.g., diabetes, asthma, heart disease, kidney or thyroid problems) that may interfere with CT imaging or indicate a higher risk of reaction to the contrast material. In addition, the technologist will ask female patients whether there is a possibility of pregnancy. The technologist will position lead aprons on appropriate areas of the patient to minimize unnecessary radiation exposure and provide lead aprons and other shielding for any individuals who must remain in the scan room (e.g., parents with children, staff monitoring a critical patient).
During the CT scan, the technologist controls the imaging scan parameters using the computer workstation, and communicates instructions to the patient via an intercom system. The technologist is responsible for acquiring the requested images and ensuring that they are of diagnostic quality. A radiologist will interpret the CT images and compile a report that is sent to the requesting physician.
Depending on the condition of the patient, other clinical staff may be present during the CT scan. Because CT is frequently used for trauma imaging, emergency medicine staff (nurses, emergency medical technicians) may be required to transport and monitor the patient.
Training
Radiologic technologist education programs include specialized training on CT principles of operation, radiation dose, patient positioning and anatomy, and CT imaging techniques. Specific training for a particular CT scan- ner is provided by the manufacturer upon installation and/or a workshop at the manufacturer facility. Usually, training for technologists and physicians is included in the cost of the CT system and consists of three to four days of technical and clinical instruction. The manufacturer often provides follow-up on-site visits after installation.
KEY TERMS
Angiography—Imaging of the blood vessels of the body conventionally performed using an x-ray system and invasive catheterization, but that is performed on some CT scanners as a noninvasive alternative, since only an intravenous injection of contrast material is required.
Contrast material—A chemical mixture injected intravenously, swallowed, or administered by enema before and/or during a CT scan to enhance imaging of the area of interest.
Digital image management system; picture archiving and communication system—Systems of computer networking that allow exchange of images over the network or Internet, archiving of images for on-line access, and viewing of patient images and other data on a display monitor. These may encompass just the radiology department or an entire facility. CT scanners are frequently networked as part of these larger systems.
Gantry—The large square unit that houses the xray system and related components and has an opening in the center through which the patient table is moved.
Laser imager—A device that uses laser technology to produce hard copies of CT images; used instead of an x-ray film processor.
BOOKS
Robb, Richard A. Biomedical Imaging, Visualization, and Analysis. New York: J. Wiley & Sons, 1999.
Harvey, Dan. "Preventive CT Screening: Health Boon or Bane?" Radiology Today 2, no. 6 (March 12, 2001): 8-11.
Napel, Sandy. "Design: Primer on Multislice CT Scanner Technology." Diagnostic Imaging, November 1999 Supplement. <http://www.dimag.com/db_area/archives/1999/9911mnapel.8-9.di-.html>.
ORGANIZATIONS
American College of Radiology. 1891 Preston White Drive, Reston, VA 20191-4397. (800)227-5463. <http://www.acr.org>.
American Registry of Radiologic Technologists. 1255 Northland Drive. St. Paul, MN 55120-1155 (651) 687-0048. <http://www.arrt.org>.
American Society of Radiologic Technologists (ASRT). 15000 Central Avenue SE, Albuquerque, NM 87123-2778. (800) 444-2778. <http://www.asrt.org>.
