CT scans are used to image a wide variety of body structures and internal organs. Since the 1990s, CT equipment has become more affordable and available. In some diagnoses, CT scans have become the first imaging exam of choice. Because the computerized image is so sharp, focused, and three-dimensional, many tissues can be better differentiated than on standard x rays. Common CT indications include:
- Sinus studies. The CT scan can show details of sinusitis, and bone fractures. Physicians may order CT of the sinuses to provide an accurate map for surgery.
- Brain studies. Brain scans can detect hematomas, tumors, and strokes. 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 body will often be used to observe abdominal organs, such as the liver, kidneys, adrenal glands, spleen, and lymph nodes, and extremities.
- Aorta scans. CT scans can focus on the thoracic or abdominal aorta to locate aneurysms and other possible aortic diseases.
- 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 unless the diagnostic benefits outweigh the risks. Pregnant patients should particularly avoid full body or abdominal scans. If the exam is necessary for obstetric purposes, technologists are instructed not to repeat films if there are errors. Pregnant patients receiving CT 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 and is not blocked by the apron.
Contrast agents are often used in CT exams and the use of these agents should be discussed with the medical professional prior to the procedure. Patients should be asked to sign a consent form concerning the administration of contrast media. One common ingredient in contrast agents, iodine, can cause allergic reactions. Patients who are known to be allergic to iodine (or shellfish) should inform the physician prior to the CT scan.
Computed tomography, also called CT scan, CAT scan, or computerized axial tomography, is a combination of focused x-ray beams, a detector array, and 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 narrow table that slides into the center of the scanner, called the gantry. The scanner looks like a square doughnut with a round opening in the middle, which allows the x-ray beam to rotate around the patient. The scanner's gantry section
The patient will feel the table move 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 monitor.
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 a special viewing console. 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 gantry portion of the scanner is narrow enough to preclude problems with claustrophobia, in most instances.
The CT image
While traditional x rays image organs in two dimensions, with the possibility that organs in the front of the body are superimposed over those in the back, CT scans allow for a more three-dimensional effect. Some have compared CT images to slices in a loaf of bread. Precise sections of the body can be located and imaged as cross- sectional views. The technologist's console displays a computerized image of each section captured by the xray beam and detector array. Thus, various densities of tissue can be easily distinguished.
Contrast agents are often used in CT exams and in other radiology procedures to demonstrate certain anatomic details which, otherwise, may not be easily seen. Some contrast agents are natural, such as air or water. Other times, a water-based contrast agent is administered for specific diagnostic purposes. Barium sulfate is commonly used in gastrointestinal procedures. The patient may drink this contrast medium, or receive it in an enema. Oral and rectal contrast are usually given when examining the abdomen or gastrointestinal tract, and not used when scanning the brain or chest. Iodine based contrast media are the most widely used intra- venous contrast agents and are usually administered through an antecubital (in front of the elbow) vein.
If contrast agents are used in the CT exam, these will be administered several minutes before the study begins. Abdominal CT patients 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 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.
Spiral CT, also called helical CT, is a newer version of CT scanning which is continuous in motion and allows for three-dimensional recreation 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. The procedure also speeds up the imaging process, meaning less time for the patient to lie still. 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.
Some facilities will have both spiral 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 must be administered, the patient may be asked to fast from 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.
No aftercare is generally 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. Hives may develop later and usually do not require treatment.
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 exposure to other adults is minimal and should produce no effects. Although severe contrast reactions are rare, they are a risk of many CT procedures. There is also a small risk of renal failure in high-risk patients.
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 and darker than fat tissue. Intravenous, oral, and rectal contrast appear as white areas. The radiologist can determine if tissues and organs appear normal by the different gradations of the gray shadows. In CT, the images which can cut through a section of tissue or organ provide three-dimensional viewing for the radiologist and referring physician.
Abnormal results may show different characteristics of tissues within organs. Accumulations of blood or other fluids where they do not belong may be detected. Radiologists can differentiate among types of tumors throughout the body by viewing details of their makeup.
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 location of areas of sinus infection, particularly chronic infection, and is useful for planning prior to functional endoscopic sinus surgery. CT scans can show the extent and location of tiny fractures of the sinus and nasal bones. Foreign bodies in the sinus and nasal area are also easily detected by CT. CT imaging of the sinuses is important in evaluating trauma or disease of the sphenoid bone (the wedge-shaped bone at the base of the skull). 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 scanning can clearly show tumors, strokes, or other lesions in the brain area as altered densities. These lighter or darker areas on the image may indicate a tumor or hemorrhage 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. Congenital abnormalities in children, such as hydrocephalus, may also be confirmed with CT. Hydrocephalus is suggested by enlargement of the fluid structures called ventricles of the brain.
The body scan can identify abnormal body structures and organs. Throughout the body, a CT scan may indicate tumors or cysts, enlarged lymph nodes, abnormal collections of fluid, blood, or fat, and metastasis of cancer. Fractures or damage to soft tissues can be more easily seen on the sensitive images produced by CT scanning, though CT is not usually done for these types of examinations because of cost. Liver conditions, such as cirrhosis, abscess, and fatty liver, may be observed with a CT body scan.
CT of the aorta
CT provides the ability to see and measure the thickness of the aortal wall, which is very helpful in diagnosing aortic aneurysms. The use of contrast will help define details within the aorta. In addition, increased areas of density can identify calcification, which helps differentiate between acute and chronic problems. An abnormal CT scan may indicate signs of aortic clots. Aortic rupture is suggested by signs such as a hematoma around the aorta or the escape of blood from its cavity.
In addition to those findings which may indicate aortic aneurysms, chest CT studies can show other problems in the heart and lungs. The computer will not only show differences between air, water, tissues, and bone, but will also assign numerical values to the various densities. Mass lesions in the lungs may be indicative of tuberculosis or tumors. CT will help distinguish between the two. Enlarged lymph nodes in the chest area may indicate lymphoma. Spiral CT is particularly effective at identifying pulmonary emboli (clots in the lung's blood vessels).
Aneurysm—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 or impossible to see on the radiograph (film).
Gantry—A name for the portion of a CT scanner which houses the x-ray tube and detector array used to capture image information and send it to the computer.
Hematoma—A collection of blood that has escaped from the vessels. It may clot and harden, causing pain to the patient.
Hydrocephalus—A collection of fluid on or around the brain. The pressure from the spinal fluid causes the ventricles to widen.
Metastasis—Secondary cancer, or cancer that has spread from one body organ or tissue to another.
Radiologist—A medical doctor specially trained in radiology (x-ray) interpretation and its use in the diagnosis of disease and injury.
Spiral CT—Also referred to as helical CT, this method allows for continuous 360-degree x-ray image capture.
Thoracic—Refers to the chest area. The thorax runs between the abdomen and neck and is encased in the ribs.
Springhouse Corporation. Illustrated Guide to Diagnostic Tests. Springhouse, PA: Springhouse Corporation, 1998.
Papatheofanis, Frank J. "Helical CT and Pulmonary Disease." Decisions in Imaging Economics (January/February1997): 61-63.
American College of Radiology. 1891 Preston White Drive, Reston, VA 20191-4397. (800)ACR-LINE. <http://www.acr.org>.
Stephen John Hage, AAAS, RT(R), FAHRA