Imaging studies are tests performed with a variety of techniques that produce pictures of the inside of a patient's body. They have become indispensable tools in cancer screening and detection.
Imaging tests are performed using sound waves, radioactive particles, magnetic fields, or x rays that are detected and converted into images after passing through body tissues. Dyes are sometimes used as contrasting agents with x-ray tests so that organs or tissues not seen with conventional x rays can be enhanced. The operating principle of the various techniques is based on the fact that rays and particles interact differently with various types of tissues, especially when cancerous growths are present. In this way, the interior of the body can be visualized and pictures are provided of normal structure and function as well as of abnormalities.
Imaging tests differ from endoscopic tests, which are carried out with a flexible, lighted piece of tubing connected to a viewing lens or camera.
Imaging studies are used to detect cancer in its early stages in a procedure called screening. Screening is performed in patients who have no obvious cancer symptoms. Imaging studies are also used to locate tumors in patients who have symptoms which the physician may wish to investigate further so as to distinguish between benign growths or cancerous tumors. They are also used to determine the extent of a cancer and indicate how a given treatment is unfolding. As such, they represent crucial tools for cancer diagnosis and management.
Major imaging techniques
Computed tomography scan (CT scan)
Computed tomography scans show a cross-section of a part of the body. In this technique, a thin beam is used to produce a series of exposures detected at different angles. The exposures are fed into a computer which overlaps them so as to yield a single image analogous to a slice of the organ or body part being scanned. A dye is often injected into the patient so as to improve contrast and obtain images that are clearer than images obtained with x rays.
Magnetic resonance imaging (MRI)
Magnetic resonance imaging also produces cross-sectional images of the body using powerful magnetic fields instead of radiation. MRI is especially useful to detect and locate cancers of the liver and the central nervous system, which occur in the brain or the spinal cord. It uses a cylinder housing a magnet which will induce the required magnetic field. The patient lies on a platform inside the scanner. The magnetic field aligns the hydrogen atoms present in the tissue being scanned in a given direction. Following a burst of radio-frequency radiation, the atoms flip back to their original orientation while emitting signals which a fed into a computer for conversion into a two-or three-dimensional image. Dyes can also be injected into patients to produce clearer images.
Mammography is an x-ray examination of the breast. It is often used as a screening tool to detect breast abnormalities and cancers before they can be felt. Mammograms (the image produced) are acquired using an xray machine working at lower radiation levels than conventional x ray. The breast is compressed between two plates so as to allow the low-level x-ray radiation to produce a film.
Nuclear scans, also called radionuclide imaging or scintigraphy, use substances called tracers or radionuclides that release low levels of radioactivity. The test is based on the principle that the tracers will be absorbed to a different degree by different tissues, thus allowing to distinguish between normal and cancerous tissues. Common nuclear medicine scans for cancer patients to receive are bone scans; liver, spleen, and thyroid scans are also frequently performed.
Position emission tomography (PET)
Positron emission tomography uses a form of sugar that contains a radioactive atom which emits particles called positrons. The positrons are absorbed to a different extent by cells varying in their metabolic rate. PET scans are especially useful for brain imaging studies and are widely applied to the assessment of the spread of cancers in the lungs. PET scans are also being used experimentally in the assessment of breast, colon, rectum, and ovarian cancers.
X rays produce shallow images of certain specificorgans or tissues. X rays are a form of high-energy radiation and tissues of the body can absorb it to varying degrees. For example, bones absorb less x rays than soft tissue. After passing through the body, the x rays are directed on a film, where the dense tissue appears as a
Chest x rays are used to detect lung and bronchial cancers, and also to evaluate a patient's symptoms, such as shortness of breath. Other types of x rays, such as abdominal x rays, may also be ordered to assess a patient's symptoms, but are not used as cancer screening tools as chest x rays may be used.
X rays with dye studies
Dye studies are usually performed by injecting the contrasting agent in the patient's circulatory system or in the target organ. These studies are used to produce angiograms, cystograms, myelograms, lymphangiograms and fistulograms.
An angiogram is an examination of the blood vessels using x rays. It is usually performed with intravenous injection of fluorescein dye followed by multiframe photography. The doctor inserts a small tube (catheter) into the blood vessel and then injects the dye that makes the vessels visible when the x-ray pictures are acquired.
A cystogram is a scan of the bladder and ureters. The ureters are passages that lead from the kidneys to the bladder. A catheter is inserted into the bladder or a radioactive material, called a radioisotope, is introduced into the bladder. An oral cholecystogram (OCG) is an x-ray examination of the gallbladder, the organ that helps release bile into the small intestine for the digestion of fats. The gallbladder is not seen well on conventional x-ray pictures and special tablets are ingested by mouth to enhance contrast.
A myelogram is an x ray of the spine and spinal cord. The spinal cord is the nerve tissue enclosed in the vertebral column that goes from the bottom of the brain to halfway down the back. During a myelogram, x-ray dye is injected into the spinal fluid and mixes with it, flowing around the spinal cord which can then be seen and recorded on x-ray film.
A lymphangiogram is an x ray of the lymphatic system, also carried out with dye injection for contrasting purposes. It is used to screen for lymph node involvement in cancer.
A fistula is an abnormal passage within body tissue. For example, a fistula may connect two organs inside the body that are not normally connected. A fistula may also lead from an internal organ inside the body to the surface outside. Examples are: between the skin and the bowel (enterocutaneous fistula), between the stomach and the colon (gastrocolic fistula). A fistulogram is an x-ray examination of this abnormal passage. The contrasting agent is injected directly into the fistula so that it will show up on x-ray pictures.
Fluoroscopy is one of the oldest areas of diagnostic radiology. It is similar to x ray in that a small dose of x rays is directed through a body part but the image obtained is displayed on a monitor rather than on the conventional x-ray film. The fluoroscope provides images of internal body parts as they move, similar to a movie. A continuous x-ray beam is passed through the body part being examined, and is transmitted to an image-intensifying tube, which is a TV-like monitor so that the body part and its motion can be seen in detail.
During fluoroscopy, the patient is placed between the x-ray source and the monitor. The live images generatedby the x-ray source strike the image-intensifying tube and allow doctors to see the size, shape, and structure of a patient's internal structures. Because the radiation is blocked more effectively by dense tissue, such as that of a tumor, the result is a dark shadow of the tumor on the screen, against a light background. Most fluoroscopy devices include television or video cameras attached to the image-intensifier tube. The camera output can be digitized and sent through a computer for image enhancement.
In fluoroscopic studies, the radiologist can either insert an intravenous (IV) catheter (hollow tube inserted into blood vessels or into an organ) to biopsy a tumor or he can use a contrast agent to visualize the organ or area of interest. The contrast agent allows the image to be viewed more clearly. Contrast agents may be introduced into the patient's body by injection, swallowing, or an enema. Fluoroscopic exams include the following types of tests: barium swallow, barium enema, and intravenous pyelography, also called intravenous urography.
Used for GI series. The patient drinks a chalky, milkshake-like concoction containing barium, which coats the esophagus and stomach. The barium absorbs the x rays so that the lining of the upper digestive tract can be clearly seen. In barium x rays, fluoroscopy allows the physician to see the movement of the intestines as the barium moves through them.
In a lower GI series, the patient receives a barium enema, which coats the intestines and rectum. A gap in the image in the stomach or small intestine could indicate an ulcer and bubbles in the normally smooth large intestinal lining may be abnormal growths.
INTRAVENOUS PYELOGRAPHY (IVP).
Pyelography, also called urography, consists of several x rays of all the
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National Cancer Information Center:1-800-ACS-2345
National Cancer Institute, Public Inquiries Office, Building 31, Room 10A31, 31 Center Drive, MSC 2580, Bethesda, MD 20892-2580. (301)435-2848. Imaging Information and data sheets: <http://search.nci.nih.gov/search97cgi/s97_cgi>.
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Monique Laberge, Ph.D.
—A procedure designed to detect cancer even though a person has no symptoms, usually performed using an imaging technique.
—An imaging technique that uses a computer to combine multiple x-ray images into a two-dimensional cross-sectional image
—An imaging technique producing x-ray pictures of the breast called mammograms.
—A special imaging technique used to image internal parts of the body, especially soft tissues.
—A highly specialized imaging technique using radioactive substances to identify active tumors.
—An imaging technique in which a radionuclide is injected through tissue and a display is obtained from a scanner device.