CAT scan

Definition

Computed tomography (also known as CT, CT scan, CAT, or computerized axial tomography) scans use x rays to produce precise cross-sectional images of anatomical structures.

Description

With the development of modern computers, the scans enhanced digital capabilities allowed the development of computed tomography imaging (derived from the Greek tomos, meaning "to slice"). The diagnostic potential of CT scans was first realized by English physician Godfrey Hounsfield.

CT scans differ from conventional x ray by collecting x rays that have passed through the body (those not absorbed by tissue) with an electronic detector mounted on a rotating frame rather than on film. The x-ray source and collector rotate around the patient as they emit and absorb x rays. CT technology then utilizes advanced computer-based mathematical algorithms to combine different readings or views of a patient into a coherent picture usable for diagnosis.

CT scans increase the scope and safety of imaging procedures that allow physicians to view the arrangement and functioning of the body's internal structures. With particular regard to neurology, CT scans are used to determine the presence or absence of brain tumors. CT scans usually take about an hour and a half, including preparation time, with the actual examination of neural tissue in a brain scan taking 15–45 minutes.

CT scanners are now often combined with positron emission tomography (PET) scanners into one unit. PETCT scanners have the ability to link the functional image created by a PET scan with the anatomical image produced by a CT scan. The combined scanning technique enhances a physician's ability to detect metabolic abnormalities (some no larger than 0.15 in [4 mm] in size) and to precisely map the location of the anomaly.

Increased accuracy reduces the number of unusable results and also results in less retesting.

The combined PET-CT scanners offer physicians the opportunity to differentiate, for example, between Alzheimer's disease and multi-infarct dementia. In addition, the enhanced images allow the differentiation of brain tumors from cerebral necrosis.

The physics

The physical basis of the CT scans lies in the fact that different tissues absorb x rays at different rates. The density and atomic number of the elements present are critical factors in determining whether a particular x ray is absorbed or passes through the body. The opacity of an image is related directly to the type of tissue or element. Dense bone appears white, while gaseous air in the lungs appears black.

CT scans are also used by some security agencies to examine packages and baggage.

CT scan procedures

CT scan allow the construction of detailed images and offer another, and in many cases, more affordable means of diagnosis without invasive surgical procedures. CT scans can also be used to guide the course of surgical procedures.

CT scans often utilize a medium or contrast enhancer, provided in the form of a drinkable liquid or via injection into the patient's bloodstream. Approximately 45 minutes before a patient is examined, the individual is given an intravenous injection of a radiopharmaceutical tracer. A brain scan and scan of the spinal cord can take less than 30 minutes.

Radiation exposure from a CT exam is roughly equal to a normal year's worth of exposure to natural background radiation—more than from a conventional x-ray examination, but less than that of other x-ray exams such as a skull x ray.

Because x rays are high energy rays that can damage critical cells in the developing embryo, women who suspect that they are pregnant should inform their doctor and the CT scan technologist prior to the exam. Nursing mothers are often advised to wait 24 hours after the injection of the contrast medium before resuming breast-feeding.

Because CT scans provide only axial cross-sections, an MRI test is often used to more carefully examine unusual or suspect findings.

Resources

WEBSITES

"Computed Tomography." EcureME. May 9, 2004 (May 27, 2004). http://www.ecureme.com/emyhealth/data/Computed_Tomography.asp.

The CT: Computed Tomography Test. University of Iowa Department of Neurology. May 9, 2004 (May 27, 2004). http://www.vh.org/adult/patient/neurology/computedtomographytest/.

Paul Arthur

Definition

Computed tomography (CT) scans are completed with the use of a 360-degree x-ray beam and computer production of images. These scans allow for cross-sectional views of body organs and tissues. Computed tomography is also known as computerized axial tomography or CAT scan.

Purpose

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 a CT scan of the sinuses to provide an accurate map for surgery.
• Brain studies. Brain scans can detect tumors, strokes, and hematomas (collections of blood that have escaped from the vessels). 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, pancreas, biliary tree and lymph nodes, and extremities.
• Aorta scans. CT scans can focus on the thoracic or abdominal sections of the 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.

Description

Computed 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.

Ct Equipment

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 may also be tilted slightly to allow for certain cross-sectional angles.

Ct Procedure

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 wide enough to preclude problems with claustrophobia, in most instances.

The Ct Image

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

Contrast agents are often used in CT exams and in other radiology procedures to demonstrate certain anatomic details that, otherwise, may not be seen easily. 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 contrasts 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 intravenous 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, 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

Spiral CT, also called helical CT, is a newer version of CT scanning that is continuous in motion and allows for three-dimensional 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 higher resolution and 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 the contrast medium more rapidly after it is injected and when it is at its highest level, is another advantage of spiral CT's high speed.

Some facilities 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.

Preparation

If a contrast medium must be administered, the patient may be asked to fast from about four to six hours prior to the procedure. This is so if a patient experiences nausea, vomiting will not occur. 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. Pregnant women or those who could possibly be pregnant should not have a CT scan unless the diagnostic benefits outweigh the risks. 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.

Aftercare

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.

Risks

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 Results

Normal findings on a CT exam show bone, the most dense tissue, as white areas. Tissues and fluid will show as various shades of gray, and fat will be dark 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 scale. In CT, the images that 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.

Sinus Studies

The increasing availability and lowered cost of CT scanning has led 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.

Brain Studies

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. Different types of tumors can be identified by the presence of edema, by the tissue's density, or by abnormal contrast enhancement. 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.

Body Scans

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. 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 visualize and measure the thickness of the aorta, 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 or contrast from its cavity.

Chest Scans

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).

Resources

Books

Springhouse Corporation. Illustrated Guide to DiagnosticTests. Springhouse, PA: Springhouse Corporation, 1998.

Periodicals

Beauchamp, N. "Imaging of Acute Cerebral Ischemia." Radiology 307 (August 1999).

Papatheofanis, Frank J. "Helical CT and Pulmonary Disease." Decisions in Imaging Economics (January/February 1997): 61–63.

Organizations

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 Lee Alan Shratter, MD

(Computer Axial Tomography) A series of X-rays that show the human body in slices. The X-ray mechanism, which surrounds the body, "inches" its way along the area being examined, taking multiple tomograms (slices). The computer is used to turn the tomograms into pictures. See tomography.

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A three-dimensional image of a cross section of the body made with x-rays that is useful in diagnosing disease (for example, in detecting tumors). CAT stands for computerized axial tomography, the name of the method used to produce the image.