Computerized Axial Tomography
Computerized Axial Tomography
Computerized axial tomography (CAT) is a diagnostic procedure that employs x rays in a unique manner. The CAT scan machine is computer controlled to assure accuracy in placement of the x-ray beam. Axial refers to the fact that the x-ray tubes are arranged in an arc about an axis. Tomography is a combination of tomo, from the Greek meaning to cut, and graph, to draw, a reference to the fact that the CAT scan image reveals a crosssection of the body or body part. Frequent uses for CAT scans include: sinuses (including fractures of bones and sinusitis), body and extremities (various procedures involving the abdomen, spine, chest, and other areas), brain (such as detection of tumors, strokes, and aneurysms),
heart and aorta, and chest (such as infections within the cavity).
The tomograph was developed in England in 1972. After a number of years of developing the apparatus, it is now a critical clinical medical device. Prior to the development of the CAT, x rays were done on the familiar table by a single x-ray tube that passed the rays through a given part of the body and exposed a plate of x-ray film. That film had to be developed and, then, viewed by a physician. This form of x ray was displayed on a film plate that offered a one-dimensional view of the body part under the x-ray tube. If a different angle was needed, the patient had to be repositioned and another x-ray picture taken. The CAT offers a number of improvements over the old method.
The CAT scan machine, often referred to as the CT machine, consists of a horizontal pad on which the patient lies. Sandbags are placed around the patient to insure a motionless patient. At one end of the pad is a circular structure that contains an array of x-ray tubes. The patient lies on the pad that is advanced into the circle until the desired area of the body is under the x-ray tubes. The x-ray tubes are focused to provide a very narrow angle of exposure, approximately 0.4 in (1 cm). The first x rays are made after which the array of tubes rotates and another exposure is made. Then, the tubes rotate again, and so on, until x rays have been made from all angles around the body.
Besides the convention CT scan, the helical CT and electron beam CT scans are available. The helical CT scan, also called a spiral CT scan, uses constant highspeed motion in order to produce precise slices at very small intervals within the body. The electron beam CT scan is used primarily to focus on the buildup of calcium within the body. It is still a new procedure—one that is still quite expensive to use. However, when coronary heart disease, or other critical problem, is involved, the electron beam CT scan is worth the expense.
Each x-ray tube is connected to a controlling computer. As the x rays pass through the patient’s body they fall upon a sensitive window. The image from each tube is fed into the computer, and this is repeated whenever the x-ray tube fires, which is from a different angle each time.
In this way, the x-ray image is projected into the computer from different angles. The computer constructs a cross-sectional image of the body each time the array of x-ray tubes has completed a revolution around the patient.
Following each x ray exposure the patient is advanced another centimeter into the machine and the process repeats. X rays are made and the patient is advanced until exposures have been made in 0.4-in (1-cm) increments for the length of the organ or part of the body being examined.
The images from each x-ray tube are fed through a computer, giving numerical values to the density of tissue through which the beam passed. The computer uses the numerical values to reconstruct an image of the cross-section of the body at the level the x rays passed through. The image is printed onto a screen for the physician to see and on a panel of x-ray film.
The differences in tissue density give the CT scan its definition. The liver is more dense than the pancreas, bone is more dense than liver, and so forth. The structures appear in different shades of gray on the screen and the film. The film is printed as a series of cross sectional images showing, for example, the liver from top to bottom. Any of the images can be called up on the computer screen for closer evaluation if the physician needs to do so.
A CT scan is described is a noninvasive procedure; that is, nothing is inserted into the body. At times, the
KEY TERMS
Contrast medium— A liquid that can be injected into a given part of the body to give shape to an organ. The contrast medium resists the passage of x rays, so it shows on the film as a dense area shaped like the cavity in which it has settled. It can show imperfections or blockages not seen on the unaided x ray.
Radiologist— A physician who specializes in imaging techniques such as x rays, CAT scans, MRI scans, and certain scans using radioactive isotopes.
Roentgenogram— A technical term for the x-ray image, named after the discoverer of x rays.
X ray— Electromagnetic radiation of very short wavelength, and very high energy.
X-ray tube— Evacuated tube in which electrons moving at high velocities are made to hit a metal target producing x rays.
physician may want more contrast or definition to a given organ and may inject a contrast medium to accomplish this procedure. A contrast medium is a substance that is visible on x rays. The medium, injected into the blood, will concentrate in an organ and will outline the organ or a cavity within it. In this way, the size of a kidney tumor may be determined, for example, as may other forms of pathology.
Obviously, the CAT scan is a specialized form of diagnosis and is not practical for such cases as bone fractures. The procedure requires more time to complete than does the ordinary, one-dimensional x ray and is not cost-effective for simpler procedures.
For diagnosis of soft-tissue tumors, which are difficult to print on an ordinary x ray, the CAT scan is superior.
All x rays rely on differences in tissue density to form the x-ray image. Bone resists the passage of the x-ray beam more than muscle, which resists more than a softer tissue, such as liver. Thus, the x-ray image from a single x-ray beam is a plate somewhat like a film negative showing various tones of gray. Small differences in tissue density, as would be seen with a tumor in the liver, where both tissues are nearly the same density, would not be seen as two separate structures. The liver would appear as a uniformly dense organ.
The CAT scan, however, takes x rays from different angles and the machine is capable after several exposures to determine the slight difference in densities of nearly similar tissues. The liver will appear as an organ of a certain shade of gray, and a tumor within it will be discernable as a spot of slightly lighter gray because of the minute variation in density. Also, by finding the panel on which the tumor first appears and following it through to the panel on which it disappears, the radiologist can determine the size of the tumor.
See also Radioactive tracers.
Resources
BOOKS
Haaga, John R. et. al., eds. CT and MR Imaging of the Whole Body. St. Louis, MO: Mosby, 2003.
Hsieh, Jiang. Computer Tomography: Principles, Design, Artifacts, and Recent Advances. Bellingham, WA: SPIE Press, 2003.
Webb, Wayne Richard. Fundamentals of Body CT. Philadelphia, PA: Saunders, 2006.
Larry Blaser
Computerized Axial Tomography
Computerized axial tomography
Computerized axial tomography (CAT) is a diagnostic procedure that employs x rays in a unique manner. The CAT scan machine is computer controlled to assure accuracy in placement of the x-ray beam. Axial refers to the fact that the x-ray tubes are arranged in an arc about an axis. Tomography is a combination of tomo, from the Greek meaning "to cut," and graph, "to draw," a reference to the fact that the CAT scan image reveals a crosssection of the body or body part.
The tomograph was developed in England in 1972. After a number of years of fine tuning the apparatus, it became a part of clinical medicine that is widely relied on now. Prior to the development of the CAT, x rays were done on the familiar table by a single x-ray tube that passed the rays through a given part of the body and exposed a plate of x-ray film. That film had to be developed and then viewed by a physician. This form of x ray was displayed on a film plate that offered a one-dimensional view of the body part under the x-ray tube. If a different angle was needed, the patient had to be turned over. The CAT offers a number of improvements over the old method.
The CAT scan machine, often referred to as the CT machine, consists of a horizontal pad on which the patient lies. Sandbags are placed around him to insure that he lies motionless. At one end of the pad is a circular structure that contains an array of x-ray tubes. The patient lies on the pad which is advanced into the circle until the desired area of the body is under the x-ray tubes. The x-ray tubes are focused to provide a very narrow angle of exposure, approximately 0.4 in (1 cm). The first x rays are made after which the array of tubes rotates and another exposure is made, the tubes rotate again, and so on until x rays have been made from all angles around the body.
Each x-ray tube is connected to the controlling computer. As the x rays pass through the patient's body they fall upon a sensitive window. The image from each tube is fed into the computer, and this is repeated whenever the xray tube fires, which is from a different angle each time.
In this way, the x-ray image is projected into the computer from different angles. The computer constructs a cross-sectional image of the body each time the array of xray tubes has completed a revolution around the patient.
Following each x ray exposure the patient is advanced another centimeter into the machine and the process repeats. X rays are made and the patient is advanced until exposures have been made in 0.4-in (1-cm) increments for the length of the organ being examined.
The images from each x-ray tube are fed through a computer, giving numerical values to the density of tissue through which the beam passed. The computer uses the numerical values to reconstruct an image of the cross-section of the body at the level the x rays passed through. The image is printed onto a screen for the physician to see and on a panel of x-ray film.
The differences in tissue density give the CT scan its definition. The liver is more dense than the pancreas, bone is more dense than liver, and so forth. The structures appear in different shades of gray on the screen and the film. The film is printed as a series of cross sectional images showing, for example, the liver from top to bottom. Any of the images can be called up on the computer screen for closer evaluation if the physician needs to do so.
A CT scan is described is a noninvasive procedure; that is, nothing is inserted into the body. At times the physician may want more contrast or definition to a given organ and may inject a contrast medium to accomplish this. A contrast medium is a substance that is visible on x rays. The medium, injected into the blood , will concentrate in an organ and will outline the organ or a cavity within it. In this way, the size of a kidney tumor may be determined, for example, as may other forms of pathology .
Obviously, the CAT scan is a specialized form of diagnosis and is not practical for such cases as bone fractures. The procedure requires more time to complete than does the ordinary, one-dimensional x ray and is not cost-effective for simpler procedures.
For diagnosis of soft-tissue tumors, which are difficult to print on an ordinary x ray, the CAT scan is superior.
All x rays rely on differences in tissue density to form the x-ray image. Bone resists the passage of the xray beam more than muscle, which resists more than a softer tissue, such as liver. Thus, the x-ray image from a single x-ray beam is a plate somewhat like a film negative showing various tones of gray. Small differences in tissue density, as would be seen with a tumor in the liver, where both tissues are nearly the same density, would not be seen as two separate structures. The liver would appear as a uniformly dense organ.
The CAT scan, however, takes x rays from different angles and the machine is capable after several exposures to determine the slight difference in densities of nearly similar tissues. The liver will appear as an organ of a certain shade of gray, and a tumor within it will be discernable as a spot of slightly lighter gray because of the minute variation in density. Also, by finding the panel on which the tumor first appears and following it through to the panel on which it disappears, the radiologist can determine the size of the tumor.
See also Radioactive tracers.
Resources
books
Cukier, Daniel, and Virginia E. McCullough. Coping With Radiation Therapy. Los Angeles: Lowell House, 1993.
periodicals
"Finding the Brain's Autopilot." USA Today 122 (April 1994): 13.
Nadis, S.J. "Kid's Brainpower: Use It or Lose It." Technology Review 96 (November-December 1993): 19-20.
Larry Blaser
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Contrast medium
—A liquid that can be injected into a given part of the body to give shape to an organ. The contrast medium resists the passage of x rays, so it shows on the film as a dense area shaped like the cavity in which it has settled. It can show imperfections or blockages not seen on the unaided x ray.
- Radiologist
—A physician who specializes in imaging techniques such as x rays, CAT scans, MRI scans, and certain scans using radioactive isotopes.
- Roentgenogram
—A technical term for the x-ray image, named after the discoverer of x rays.
- X ray
—Electromagnetic radiation of very short wavelength, and very high energy.
- X-ray tube
—Evacuated tube in which electrons moving at high velocities are made to hit a metal target producing x rays.
Computerized Axial Tomography
Computerized Axial Tomography
A computerized axial tomography (CAT) scan is a special type of X-ray used for viewing the internal organs of patients. Although a regular chest X-ray can show the heart and lungs, the CAT scan can show the same organs but with detail 100 times greater and with little or no additional irradiation (exposure to radiation).
In the late 1960s Alan Cormack, an American physicist, and Godfrey Hounsfield, an English electrical engineer, independently developed a way to produce a three-dimensional image of the parts of the body, (called tomogram) by taking many different X-ray cross sections and combining them. Eventually, the two would share a 1979 Nobel Prize for medicine for their work, research which led to development of the CAT scanner.
Cormack was the first to construct a machine to "shoot" tomograms. His first model used a thin beam of X-rays aimed at one part of the body but taken from many different angles. Unfortunately he lacked a system that could process all of the information that one CAT scan produces. The solution to this problem was the computer. Hounsfield began working on his own CAT scanner in 1967. He used a system similar to Cormack's, but he also used a computer. The computer was able to sort all of the X-ray data into a picture. The first CAT scanner took nine days to complete a picture of a preserved human brain. Before it could be used for patients, Hounsfield had to improve its design. Later models required nine hours and eventually only nine minutes. His final apparatus took about five minutes to complete its scan. The first CAT machine of practical use was installed at Atkinson Morley's Hospital in Nimbledon, England, in 1971.
Today the CAT scanner finishes a scan in a few minutes. The pictures can be seen almost immediately on a television monitor, and within 30 minutes the entire scan can be copied to a computer disk. The scan can be stored on the computer or made into a picture that looks like a usual X-ray. The CAT scan data can be transmitted by modern (computer-phone hookup) to another hospital if, for example, a doctor needed to see the results of the CAT scan quickly.
Scanning Procedures
The CAT scanner consists of a flat table on which a patient lies. A tube-shaped device is on either side of the table and is fitted with many X-ray scanners. Before the scan begins, the part of the patient that is to be scanned is moved inside this tube. The CAT scanner rotates 180 degrees around the patient's body and sends out a very thin beam of X-rays. Each X-ray scanner has a detector placed exactly opposite to it. The scanner and detector are built to move together so that when the scanner moves, it is aimed directly at the detector.
During the CAT scan, the scanner sends out a short impulse that is picked up by the detector. After the detector picks up the first signal, the scanner and detector rotate slightly and the same picture is taken again but at a slightly different angle. As the beam travels from the scanner through the patient, it is changed as it goes through bones and organs. The beam that the detector picks up is different from that the scanner emitted. This difference is noted by the computer. It analyzes this data and converts it into electrical impulses that make a picture on a television screen. The amount of data is huge, since some CAT scanners use up to 300 X-ray scanners taking 300 pictures each. This results in almost 90,000 X-ray slices or tomograms that the computer must convert into a picture.
In the 1970s a more advanced tomography technique was developed by Michael Phelps and Edward Hoffman, biophysicists at the UCLA School of Medicine. The duo developed a PET scan (PET is an acronym for positron emission tomography). For this scan, the patient has a radioactive material injected into the bloodstream. This radioactive material is not harmful and it emits positrons, a kind of beta particle (high-speed electron), and gamma rays. The PET scanner reads gamma rays like the CAT scanner reads X-rays. Since certain radioactive material travels to certain parts of the body (for example, iodine goes to the thyroid), more detailed pictures of a specific organ can be made.
Scanner Enhancements
CAT scanners have revolutionized the way doctors diagnose and treat patients. Since doctors can now see inside the patients body, the patient is often spared surgery. If the patient has a cancer in a certain part of the body and surgery is needed, the surgeon will have a clear idea of how big the cancer is, the surrounding tissues it may be affecting, and how much to cut out. If a cancer cannot be cut out, but it can be treated with X-rays, the CAT scanner can precisely map out the exact borders of the cancer and direct the X-rays to only the cancer.
CAT scanners are being continually refined to take pictures faster and faster. This is important for taking pictures of moving things like blood in a vein or artery. Scanners can now detect blockages, called clots or thromboses, in the arteries or veins. Most recently, an ultrafast type of CAT scanning has been developed called EBCT, an acronym for electron beam computed tomography. With this technique a scan can now be taken of the arteries of the heart, called coronary arteries. The scan reveals a cross section of the coronary arteries and shows the amount of calcium in the walls of the arteries. If there is too much calcium in the artery walls, it can cause a blockage that will lead to a heart attack. If these blockages are detected early, a heart attack can be prevented by doing a simple operation.
[See also X-ray machine ]
computerized axial tomography
CAT scan
CAT scan • n. an X-ray image made using computerized axial tomography.DERIVATIVES: CAT scan·ner n.