Computed tomography (CT) scanning is a valuable diagnostic tool that provides physicians with views of internal body structures. During a CT scan, multiple x rays are passed through the body, producing cross-sectional images, or "slices, " on a cathode-ray tube (CRT), a device resembling a television screen. These images can then be preserved on film for examination.
CT scans are used to image bone, soft tissues, and air. Since the 1990s, CT equipment has become more affordable and available. CT scans have become the imaging exam of choice for the diagnoses of most solid tumors. Because the computerized image is sharp, focused, and three-dimensional, many structures can be better differentiated than on standard x rays.
Common indications for CT scans include:
- Sinus studies. The CT scan can show details of sinusitis, bone fractures, and the presence of bony tumor involvement. Physicians may order a CT scan of the sinuses to provide an accurate map for surgery.
- Brain studies. Brain CT scans can detect hematomas, tumors, strokes, aneurysms, and degenerative or infected brain tissue. The introduction of CT scanning, especially spiral CT, has helped reduce the need for more invasive procedures such as cerebral angiography.
- Body scans. CT scans of the chest, abdomen, spine, and extremities can detect the presence of tumors, enlarged lymph nodes, abnormal collection of fluid, and vertebral disc disease. These scans can also be helpful in evaluating the extent of bone breakdown in osteoporosis.
- Heart and aorta scans. CT scans can focus on the thoracic or abdominal aorta to locate aneurysms and other possible aortic diseases. A newer type of CT scan, called electron beam CT, can be used to detect calcium buildup in arteries. Because it is a new technology, it is not yet widely used and its indications are not yet well-defined.
- Chest scans. CT scans of the chest are useful in distinguishing tumors and in detailing accumulation of fluid in chest infections.
Pregnant women or those who could possibly be pregnant should not have a CT scan, particularly a full body or abdominal scan, unless the diagnostic benefits outweigh the risks. If the exam is necessary for obstetric purposes, technologists are instructed not to repeat films if there are errors. Pregnant patients receiving a CT scan or any x ray exam away from the abdominal area may be protected by a lead apron; most radiation, known as scatter, travels through the body, however, and is not blocked by the apron.
Contrast agents are often used in CT exams, though some types of tumors are better seen without it. Patients should discuss the use of contrast agents with their doctor, and should be asked to sign a consent form prior to the administration of contrast. One of the common contrast agents, iodine, can cause allergic reactions. Patients who are known to be allergic to iodine or shell-fish should inform the physician prior to the CT scan; a combination of medications can be given to such patients before the scan to prevent or minimize the reaction. Contrast agents may also put patients with diabetes at risk of kidney failure, particularly those taking the medication glucophage.
Computed tomography, also called CT scan, CAT scan, or computerized axial tomography, is a combination of focused x-ray beams and the computerized production of an image. Introduced in the early 1970s, this radiologic procedure has advanced rapidly and is now widely used, sometimes in the place of standard x rays.
A CT scan may be performed in a hospital or outpatient imaging center. Although the equipment looks large and intimidating, it is very sophisticated and fairly comfortable. The patient is asked to lie on a gantry, or narrow table, that slides into the center of the scanner. The scanner looks like a doughnut and is round in the middle, which allows the x-ray beam to rotate around the patient. The scanner section may also be tilted slightly to allow for certain cross-sectional angles.
The gantry moves very slightly as the precise adjustments for each sectional image are made. A technologist watches the procedure from a window and views the images on a computer screen. Generally, patients are alone during the procedure, though exceptions are sometimes made for pediatric patients. Communication is possible via an intercom system.
It is essential that the patient lie very still during the procedure to prevent motion blurring. In some studies, such as chest CTs, the patient will be asked to hold his or her breath during image capture.
Following the procedure, films of the images are usually printed for the radiologist and referring physician to review. A radiologist can also interpret CT exams on the computer screen. The procedure time will vary in length depending on the area being imaged. Average study times are from 30 to 60 minutes. Some patients may be concerned about claustrophobia but the width of the "doughnut" portion of the scanner is such that many patients can be reassured of openness. Doctors may consider giving sedatives to patients who have severe claustrophobia or difficulty lying still.
The CT image
While traditional x-ray machines image organs in two dimensions, often resulting in organs in the front of the body being superimposed over those in the back, CT scans allow for a more three-dimensional effect. CT images can be likened to slices in a loaf of bread. Precise sections of the body can be located and imaged as cross-sectional views. The screen before the technologist shows a computer's analysis of each section detected by the x-ray beam. Thus, various densities of tissue can be easily distinguished.
Contrast agents are often used in CT exams and in other radiology procedures to illuminate certain details of anatomy more clearly. Some contrasts are natural,
If contrast agents are used in the CT exam, these will be administered several minutes before the study begins. Patients undergoing abdominal CT may be asked to drink a contrast medium. Some patients may experience a salty taste, flushing of the face, warmth or slight nausea, or hives from an intravenous contrast injection. Technologists and radiologists have the equipment and training to help patients through these minor reactions and to handle more severe reactions. Severe reactions to contrast are rare, but do occur.
Newer types of CT scans
The spiral CT scan, also called a helical CT, is a newer version of CT. This type of scan is continuous in motion and allows for the continuous re-creation of images. For example, traditional CT allows the technologist to take slices at very small and precise intervals one after the other. Spiral CT allows for a continuous flow of images, without stopping the scanner to move to the next image slice. A major advantage of spiral CT is the ability to reconstruct images anywhere along the length of the study area. Because the procedure is faster, patients are required to lie still for shorter periods of time. The ability to image contrast more rapidly after it is injected, when it is at its highest level, is another advantage of spiral CT's high speed.
Electron beam CT scans are another newer type of CT technology that can be used to detect calcium buildup in arteries. These calcium deposits are potential risk factors for coronary artery disease. Electron beam CT scans take pictures much more quickly than conventional CTs, and are therefore better able to produce clear images of the heart as it pumps blood. Because it is a newer and expensive test, electron beam CT scanning is not widely used.
Some facilities will have spiral, electron, and conventional CT available. Although spiral is more advantageous for many applications, conventional CT is still a superior and precise method for imaging many tissues and structures. The physician will evaluate which type of CT works best for the specific exam purpose.
If a contrast medium is administered, the patient may be asked to fast for about four to six hours prior to the procedure. Patients will usually be given a gown (like a typical hospital gown) to be worn during the procedure. All metal and jewelry should be removed to avoid artifacts on the film. Depending on the type of study, patients may also be required to remove dentures.
Generally, no aftercare is required following a CT scan. Immediately following the exam, the technologist will continue to watch the patient for possible adverse contrast reactions. Patients are instructed to advise the technologist of any symptoms, particularly respiratory difficulty. The site of contrast injection will be bandaged and may feel tender following the exam.
Radiation exposure from a CT scan is similar to, though higher than, that of a conventional x ray. Although this is a risk to pregnant women, the risk for other adults is minimal and should produce no effects. Severe contrast reactions are rare, but they are a risk of many CT procedures.
Normal findings on a CT exam show bone, the most dense tissue, as white areas. Tissues and fat will show as various shades of gray, and fluids will be gray or black. Air will also look black. Intravenous, oral, and rectal contrast appear as white areas. The radiologist can determine if tissues and organs appear normal by the sensitivity of the gray shadows.
Abnormal results may show different characteristics of tissues within organs. Accumulations of blood or other
The increasing availability and lowered cost of CT scanning has lead to its increased use in sinus studies, either as a replacement for a sinus x ray or as a follow-up to an abnormal sinus radiograph. The sensitivity of CT allows for the location of areas of sinus infection, particularly chronic infection. Sinus tumors will show as shades of gray indicating the difference in their density from that of normal tissues in the area.
The precise differences in density allowed by CT scan can clearly show tumors, strokes, or lesions in the brain area as altered densities. These lighter or darker areas on the image may indicate a tumor or hematoma within the brain and skull area. Different types of tumors can be identified by the presence of edema, by the tissue's density, or by studying blood vessel location and activity. The speed and convenience of CT often allows for detection of hemorrhage before symptoms even occur.
The body CT scan can identify abnormal body structures and organs. A CT scan may indicate tumors or cysts, enlarged lymph nodes, abnormal collections of fluids, blood or fat, or cancer metastasis. Tumors resulting from metastasis are different in makeup than primary (original) tumors.
In addition to those findings which may indicate aortic aneurysms, chest CT studies can show other problems in the heart and lungs, and distinguish between an aortic aneurysm and a tumor adjacent to the aorta. CT will not only show differences between air, water, tissues and bone, but will also assign numerical values to the various densities. Coin-sized lesions in the lungs may be indicative of tuberculosis or tumors. CT will help distinguish among the two. Enlarged lymph nodes in the chest area may indicate Hodgkin's disease.
Teresa G. Norris
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American College of Radiology. 1891 Preston White Drive, Reston, VA 22091. (800) ACR-LINE. <http://www.acr.org>.
—The bulging of the blood vessel wall. Aortic aneurysms are the most dangerous. Aneurysms can break and cause bleeding.
Contrast (agent, medium)
—A substance injected into the body that illuminates certain structures that would otherwise be hard to see on the radiograph (film).
—A collection of blood that has escaped from the vessels. It may clot and harden, causing pain to the patient.
—Secondary cancer, or cancer that has spread from one body organ or tissue to another.
—A medical doctor specially trained in radiology (x ray) interpretation and its use in the diagnosis of disease and injury.
—Also referred to as helical CT, this method allows for continuous 360-degree x-ray image capture.
—Refers to the chest area. The thorax runs between the abdomen and neck and is encased in the ribs.
QUESTIONS TO ASK THE DOCTOR
- Why is a CT scan recommended in my case?
- What are the benefits associated with this procedure?
- What are the risks associated with this procedure?
- How do I prepare for the CT scan?
- When will I know the results?