Imaging tests for cancer

ALSO KNOWN AS: Radiology tests

DEFINITION: Imaging tests are procedures that use a source of energy to evaluate specific internal structures of the body and obtain a visual representation of such structures using special equipment. The energy sources used for these procedures may be magnetic fields, radio waves, radioactive particles, sound waves, or X-rays. Imaging tests include computed tomography (CT) scans, magnetic resonance imaging (MRI), radiography (X-rays) with and without contrast studies, mammography, radionuclide imaging, and ultrasonography.

Imaging tests are distinguished based on how the bodily structure is viewed. CT scans use a concentrated beam of X-rays to obtain multiple views of such structures; the images are then processed in a computer to provide a three-dimensional composite for easier analysis and interpretation.

MRI uses magnetic force to align the body's hydrogen atoms in one direction. A burst of radiofrequency waves is then applied, causing a change in alignment. The hydrogen atoms emit signals as they return to their original orientation. A computerized scanner detects and converts these signals into two- or three-dimensional images for analysis and interpretation.

Radiography utilizes small doses of X-rays, or high-energy electromagnetic waves, to obtain images of body structures. Modified X-ray studies employ dyes as contrast agents to view structures that regular X-ray techniques do not visualize. The contrast agents can outline or fill in structures and provide better radiographic imaging. These modified studies include upper gastrointestinal (GI) series (barium swallow), examining the lining of the esophagus, stomach, and upper part of the small intestines; lower GI series (barium enema), analyzing the colon and rectum linings; intravenous pyelography (IVP), viewing the urinary tract (kidney, ureters, and bladder); angiography inspecting arteries such as those in the heart and brain; and lymphangiography, investigating the lymph vessels and lymph nodes.

Mammography employs X-rays for examining the breasts to screen for breast disease in the absence of symptoms and for diagnosis when breast abnormalities are detected by screening. The images are available either on X-ray photographic films or in digital form electronically stored in a computer.

Radionuclide imaging (nuclear scan) uses radioactive tracers to obtain images from within the body. A computerized external detector picks up radioactive emissions, and the processing of these signals provides images of the internal body structures. Examples of radiopharmaceuticals include gallium 67, used to detect bone marrow, lung, and lymph node tumors; technetium 99, employed in whole-body scans to evaluate metastasis of various primary cancers; thallium 201, utilized in detecting breast cancers, thyroid tumors, and lymphomas; and iodine 123 and iodine 131, used to detect thyroid cancers and neuroendocrine tumors such as carcinoid tumors. Depending on the disease, the tissues affected absorb more or less of the radioactive tracers; the nuclear scans can locate tumors and identify cancer spread.

Ultrasonography employs high-frequency sound waves to produce images of internal body structures. The sonographer uses a small handheld transducer to generate sound waves during the procedure. Internal body organs reflect these sound waves to the transducer, and a computer processes these signals into images. Ultrasound scans show the size and shape of organs and, in some cases, blood flow. Abnormalities in the organ structures and blood flow can be observed if present.

Cancers diagnosed or treated: Almost all types of solid cancers, including those of the breasts, brain and spinal cord, thyroid, bones, gastrointestinal organs (esophagus, stomach, pancreas, liver, colon, and rectum), reproductive organs (ovaries, prostate, and testes), soft tissues (muscles, tendon, and fat), and urinary tract organs (bladder, ureters, and kidneys); hematological cancers such as Hodgkin disease and non-Hodgkin lymphoma

Why performed: Imaging tests are performed in cancer management for many reasons, including screening for the presence or absence of cancer in people who have no symptoms, diagnosing cancer, obtaining an image-guided biopsy, staging cancer or evaluating tumor spread, planning a cancer treatment (such as assessing the specific site of a tumor before radiation treatment), and evaluating cancer treatment to determine if it is working or whether a tumor has recurred.

Patient preparation: The type of imaging test and contrast material will dictate the patient preparation. For example, patients may be instructed not to eat or drink several hours before a CT scan, radionuclide imaging, special X-ray procedures, or ultrasound is performed. If a contrast dye is used—depending on what part of the body will be evaluated—the patient may need to drink the liquid contrast, may need an intravenous (IV) catheter for injection, or may need an enema. The radioactive material for nuclear scans is given by mouth or by injection through an IV line. Standard X-rays and mammograms do not require special preparation. The best time for a mammogram is about one week following a menstrual period, when the breasts are least likely to be sensitive. For a pediatric patient, the parents need to be counseled on proper preparations for the imaging tests so that the patient’s anxiety may be reduced. In emergency cases, preparation for imaging procedures may not be possible.

Steps of the procedure: Imaging tests are often done on an outpatient basis in a hospital or clinic radiology department. The radiology test type and contrast material used will dictate the procedure. Generally, the patient will be asked to use a medical examination gown or to undress to expose the body part that needs to be studied. Metallic objects that can interfere with imaging tests will be removed, if possible. The patient may be asked to sit, stand, or lie down depending on the body structure that needs to be evaluated. A certified radiology technologist will guide patients through the steps of the procedure. A radiologist may be present if there is a need to perform a biopsy or provide an intervention. If so, the radiologist will provide the final radiological report with the evaluation and analysis of the images.

Patients undergoing CT scans lie on a flat table attached to a CT scanner that looks like a large doughnut-shaped machine. The table can slide in and out of the scanner's hole. The X-ray tube and detector are rotated around the patient while images are obtained. The detector takes numerous snapshots of the X-ray beam, and a computer processes these image slices to provide the three-dimensional CT scan image.

Patients undergoing MRI scans lie flat on a table that can slide back and forth in the opening of the cylindrical or tunnel-type MRI scanner. The cylinder encloses the patient, causing a feeling of claustrophobia in some patients. During the procedure, the machine makes loud thumping or banging noises that represent the magnetic fields turning on and off. Earplugs or headphones can be used during the procedure. Patients with metallic implants such as pacemakers, surgical clips or staples, implanted pumps, IV catheters, or ports for medications should thoroughly discuss the risks and benefits of MRI scans with their physicians. Tattoos and cosmetics can affect MRI images; thus, it is important to inform the radiology technologist about them before the procedure.

Standard radiographic studies are straightforward. The patient will be examined for the specific body structure that is being evaluated, such as the chest or abdomen. Modified X-ray studies using contrast agents such as barium swallow, barium enema, IVP, angiography, and lymphangiography will require provision of the contrast material via oral, injection, or rectal routes prior to the procedure. Angiography and lymphangiography will require diet restriction to only liquid oral intakes before the test; the contrast agent will be injected through an IV line just before the X-ray procedure. Restriction of food intake (no food or drink) for eight to twelve hours prior to barium swallow, barium enema, or IVP will be required; a series of X rays of the upper GI, lower GI, and urinary tract will be obtained during the procedure.

Mammography screening requires exposure of the breasts (undressing from the waist up to the breasts) and removal of jewelry from the neck. Deodorants, ointments, creams, perfumes, and powders should not be used on the day of the mammogram, as these substances can interfere with the breast imaging. The breasts are placed one at a time on a flat surface and compressed during the X-ray imaging; the patient has to hold her breath during this procedure. Additional views of the breasts will be obtained if there is suspicion of breast disease.

Radionuclide imaging requires a patient to lie on a table while the nuclear scanner moves back and forth examining the nuclear activity in the patient’s body. Prior to the procedure, a radioactive tracer will be provided to the patient through oral intake or IV injection (for example, two hours before a bone scan or a few days before a gallium scan). The radiopharmaceuticals emit gamma rays during the procedure; these signals are detected by a special gamma camera and transformed by a computer process into a two- or three- dimensional image showing where the tracers are absorbed by the body to a greater or lesser degree.

Ultrasonography is a straightforward examination in which the patient lies flat on a table. The ultrasonographer passes a transducer over the body structure (such as the liver, kidneys, gallbladder, or uterus) that is being evaluated. A gel is applied to the surface of the body above the structure. The gel's purpose is to lubricate the skin to prevent friction and discomfort and to improve the transmission of sound waves. In some cases, the transducer is shaped as a probe (as in vaginal ultrasound or prostate evaluation through the rectum). The transducer will be covered with gel before its insertion into the vaginal tract or anorectal passage. These procedures are not painful, but they can cause discomfort and a feeling of pressure.

After the procedure: Generally, no aftercare is required for the imaging tests discussed, and patients can return to normal activities immediately after the procedures. Patients who used contrast dyes may be monitored after the imaging tests to ensure that they have no adverse reactions to the agents used. Patients who ingested radioactive tracers should drink plenty of fluids to aid in their excretion; flushing of the toilet immediately after voiding can also decrease the risk of exposure to radioactivity. Removal of the gel used in ultrasonography is done by the ultrasound technician.

Risks: Strict guidelines are practiced by radiology departments to decrease the risk of radiation exposure in imaging tests. The risk of exposure to X-rays is generally exceeded by the benefits. The amount of radiation is low enough that adverse reactions are rare. Mammography also uses such low levels of X-rays that any risk from radiation exposure is minimal. CT scan exposure to radiation is higher than that of a standard X-ray, but the level of exposure is still low and should not cause adverse effects. The risks of nuclear medicine scans are also minimal. The amount of radiopharmaceuticals used exposes patients to an amount of radiation that is similar to or lower than that of a conventional X-ray. The radiotracers are passed out of the body quickly (within a few hours or days). However, increased fluid intake can facilitate more rapid excretion. Close proximity to children and sexual activity should be discussed with the patient’s healthcare team since depending on what radiotracer was used, these activities may be restricted for a short time. Ultrasound is considered free of risks. MRI also poses no risks and does not produce adverse physical effects. However, there are no known biological effects of MRIs on fetuses.

Although radiation exposure in imaging tests is minimal and should not produce adverse effects, pregnant patients and mothers who are breast-feeding should discuss the risks and benefits of the imaging tests with their physicians, especially patients who will require ingestion of contrast dyes and radioactive materials. The abdominal region of pregnant patients is protected with a radiation shield when imaging tests are performed during mammography and radiography. Patients who have renal failure or who are allergic to contrast agents and radiopharmaceuticals should also discuss the use of these substances and other imaging test options with their physicians. Patients who have surgically implanted devices such as cardiac pacemakers, surgical aneurysm clips, and other metallic implants in the eyes or ears should inform their physicians about these devices. Because of the strong magnetic field involved, MRI is absolutely contraindicated when these metallic devices are present. If a biopsy or other invasive technique is performed, then the risks will include infection and bleeding at the site of intervention.

Results: Normal results will show no structural abnormalities in the imaging tests. Abnormal findings will include masses, tumors, abnormal presence of blood or fluids, enlarged organs, and structural anomalies (such as unusual shape, size, borders, or function). The radiologist determines whether the structures are normal or abnormal and will send a formal report to the physician about these findings.

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