Diagnostics: PET-CT
(Positron Emission Tomography and Computed Tomography)
What is a PET-CT scan?
A PET-CT scan, also known as positron emission tomography-computed tomography, is a diagnostic imaging test utilized to identify and characterize anatomic areas of increased energy use (metabolism). The PET component of the scan evaluates cellular metabolic activity, identifying regions with increased or decreased metabolic rates that could indicate underlying medical conditions, such as cancer. inflammation, heart disease, or brain conditions. Conversely, the CT aspect generates detailed anatomical images through cross-sectional views of the body. Combining these two scanning methods, and superimposing the PET images onto the CT images, metabolic activity may be mapped to specific anatomic structures accurately, which then allow a physician to interpret the study with greater diagnostic power.
Coronal CT (left) and combined PET-CT scan, also coronal (right). The CT information shows bone as white, air as black, and soft tissues as gray. The PET information shows metabolic activity in the other colors. Note the normal areas of high metabolic activity, such as the brain, heart, and other recently active muscles and the elimination of the radioactivity by the kidneys and bladder.
Axial PET-CT scan from the base of the skull to mid-thighs. The CT information shows bone as white, air as black, and soft tissues as gray. The PET information shows metabolic activity in the other colors. Note the normal areas of high metabolic activity, such as the brain, heart and other recently active muscles, and the elimination of the radioactivity by the kidneys and bladder.
How does the PET-CT scan work?
The PET portion of a PET-CT scan works on a clever principle. At the beginning of a PET-CT scan, a solution including a radioactive analogue of glucose (called 18F-FDG) is given intravenously. Tissues that are using significant energy transport glucose (and the chemically similar 18F-FDG) from circulating blood at a higher rate than metabolically dormant tissues. After administration of 18F-FDG, this radioactive substance is concentrated in metabolically active tissues. 18F-FDG emits gamma rays (electrons) which are detected by the PET scanner and mapped anatomically in a series of images. The total amount of radioactivity exposure is small and considered safe. Shortly after the scan, the radioactivity is eliminated through the urine.
During a PET-CT study, a CT scan is obtained simultaneously to the PET scan. The corresponding images from the CT and the PET may be superimposed to allow the interpreting physician good anatomic resolution (from the CT scan) and the metabolic activity (from the PET scan).
Why would high blood glucose interfere with the PET scan?
When blood glucose is already high, even tissues with high metabolic rates may not uptake and concentrate much of the 18-FDG, as it can be out-competed by the high concentration of blood glucose molecules. This could lead to a poor quality study with loss of accuracy.
How is cancer identified and not confused with inflammation or muscle use?
While there is no perfect cancer scan that shows all cancer and nothing but cancer, PET-CT imaging is a very helpful tool in many cancer situations. Areas that are using a lot of energy are not only identified by a PET-CT with anatomic accuracy, the intensity of metabolic activity (called the SUV) is also quantified. As such, the context of the clinical scenario is used by the interpreting physician. Some structures demonstrate high in SUV not because of cancer but because of normal usage of energy—examples include the heart muscle, and the brain. Other muscles that are used shortly before the imaging will also “light up” on the PET. Eye muscles used in looking aroundand muscles that move the vocal cords for talking on the way to the test often have some activity. If a person were to get a PET scan shortly after exercise, the muscles used during that activity would also take up the radioactive glucose and show up with hypermetabolism on the PET. Practically speaking, when a patient is getting a PET-CT scan for a cancer in the mouth or throat, it is advisable to rest the muscles in this area prior to the scan in order to prevent muscle activity from obscuring activity from a cancer in the region. In other words, such a patient would be best off not to talk, eat, drink, or otherwise use nearby muscles for a few hours prior to the scan.
When is a PET-CT scan used?
A PET-CT scan is not a screening test to determine if a patient has cancer. Rather, use of this test is reserved for special situations, including the following:
A diagnosis of cancer has already been established, and now one needs to determine the extent of cancer (including the site where the cancer originated and areas to which it may have spread)
A diagnosis of cancer has already been established, and a baseline study is needed for comparison with future PET-CT scan(s)
Cancer has been treated and now one needs to assess for the response to treatment (persistence vs. eradication of cancer)
A single lung nodule that is suspicious for cancer, and where risk of biopsy is high.
Advanced papillary or follicular thyroid cancers where a radioactive iodine scan is not conclusive
What is done when a PET-CT is not definitive for the presence or absence of cancer?
While a PET-CT scan may be quite definitive for either the presence or the absence of cancer, occasionally the pattern and intensity of the metabolic activity does not provide a clear-cut and definitive indication on the presence of cancer. Decision making in this circumstance is in the hands of the treating physician and the patient. Available options are specific to an individual situation, but may include performing a biopsy of the tissue in question, waiting for a period of time (such as 3 months) and obtaining a repeat PET-CT scan, or in some cases, proceeding with treatment as if the area in question were cancer. Of course, decision-making is based on the entire clinical picture, not just one piece of data.