Nuclear medicine is a specialized area of radiology that uses very small amounts of radioactive materials, or radiopharmaceuticals, to examine organ function and structure. Nuclear medicine imaging is a combination of many different disciplines, including chemistry, physics, mathematics, computer technology, and medicine. This branch of radiology is often used to help diagnose and treat abnormalities very early in the progression of a disease, such as thyroid cancer.
Because x-rays pass through soft tissue, such as intestines, muscles, and blood vessels, these tissues are difficult to visualize on a standard x-ray, unless a contrast agent is used to cause the tissue to be seen more clearly. Nuclear imaging enables visualization of organ and tissue structure as well as function. The extent to which a radiopharmaceutical is absorbed, or "taken up," by a particular organ or tissue may indicate the level of function of the organ or tissue being studied. Thus, diagnostic x-rays are used primarily to study anatomy, whereas nuclear imaging is used to study organ and tissue function.
A tiny amount of a radioactive substance is used during the procedure to assist in the examination. The radioactive substance, called a radionuclide (radiopharmaceutical or radioactive tracer), is absorbed by body tissue. Several different types of radionuclides are available, including forms of the elements technetium, thallium, gallium, iodine, and xenon. The type of radionuclide used will depend on the type of study and the body part being studied.
After the patient has taken in the radionuclide and it has collected in the body tissue under study, radiation will be given off. This radiation is detected by a radiation detector. The most common type of detector is the gamma camera. Digital signals are produced and stored by a computer when the gamma camera detects the radiation.
By measuring the behavior of the radionuclide in the body during a nuclear scan, the physician can assess and diagnose various conditions, such as tumors, abscesses, hematomas, organ enlargement, or cysts. A nuclear scan may also be used to assess organ function and blood circulation.
The areas where the radionuclide collects in greater amounts are called "hot spots." The areas that do not absorb the radionuclide and appear less bright on the scan image are referred to as "cold spots."
In planar imaging, the gamma camera remains stationary. The resulting images are two-dimensional (2D) images of the part or organ being studied. Single photon emission computed tomography, or SPECT, produces three-dimensional (3D) images because the gamma camera rotates around the patient.
Scans are used to diagnose many medical conditions and diseases. Some of the more common tests include the following:
- renal scans - used to examine the kidneys and to detect any abnormalities, such as tumors or obstruction of the renal blood flow.
- thyroid scans - used to evaluate thyroid function.
- bone scans - used to evaluate any degenerative and/or arthritic changes in the joints, to detect bone diseases and tumors, and/or to determine the cause of bone pain or inflammation.
- gallium scans - used to diagnose active infectious and/or inflammatory diseases, tumors, and abscesses.
- heart scans - used to identify abnormal blood flow to the heart, to determine the extent of the damage of the heart muscle after a heart attack, and/or to measure heart function.
- brain scans - used to investigate problems within the brain and/or in the blood circulation to the brain.
- breast scans - often used in conjunction with mammograms to locate cancerous tissue in the breast.
As stated above, nuclear medicine scans may be performed on many organs and tissues of the body. Each type of scan employs certain technology, radionuclides, and procedures.
A nuclear medicine scan consists of three phases: tracer (radionuclide) administration, taking images, and image interpretation. The amount of time between administration of the tracer and the taking of the images may range from a few moments to a few days, depending on the body tissue being examined and the tracer being used. The time required to obtain the images may also vary from minutes to hours to several days.
One of the most commonly performed nuclear medicine examinations is a heart scan. Myocardial perfusion scans and radionuclide angiography scans are the two primary heart scans. In order to give an example of how nuclear medicine scans are done, the process for a resting radionuclide angiogram (RNA) scan is presented below.
Although each facility may have specific protocols in place, generally, a resting RNA follows this process:
- The patient will be asked to remove any jewelry or other objects that may interfere with the procedure.
- If the patient is asked to remove clothing, he/she will be given a gown to wear.
- An intravenous (IV) line will be started in the hand or arm.
- The patient will be connected to an electrocardiogram (ECG) machine with electrodes (leads) and a blood pressure cuff will be attached to the arm.
- The patient will lie flat on a table in the procedure room.
- The radionuclide will be injected into the vein to "tag" the red blood cells. Alternatively, a small amount of blood will be withdrawn from the vein so that it can be tagged with the radionuclide. The radionuclide will be added to the blood and will be absorbed into the red blood cells.
- After the tagging procedure, the blood will be returned into the vein through the IV tube. The progress of the tagged red blood cells through the heart will be traced with a scanner.
- During the procedure, it will be very important to lie as still as possible, as any movement can adversely affect the quality of the scan.
- The gamma camera will be positioned over the patient as he/she lies on the table, and will obtain images of the heart as it pumps the blood through the body.
- The patient may be asked to change positions during the test; however, once the position has been changed, the patient will need to lie still without talking.
- After the scan is complete, the IV line will be discontinued, and the patient will be allowed to leave, unless the physician gives different instructions.
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