Positron emission tomography (PET)
Positron Emission Tomography (PET) is a medical imaging technique that uses small amounts of radioactive substances, typically glucose or amino acids, injected into the bloodstream to create detailed images of the body's internal structures. This method is particularly valuable in oncology, as cancer cells often have higher metabolic rates and thus absorb more of these substances, making them more visible on scans. PET scans are used for various purposes, including the detection of tumors, staging of cancer, assessing treatment responses, and identifying infections and cardiovascular issues.
Before a PET scan, patients must prepare by limiting sugar and caffeine intake, fasting for a minimum of six hours, and staying well-hydrated. The scanning process generally lasts about thirty minutes, during which the patient remains still as the scanner captures images of metabolic activity. While PET scans are generally safe and painless, they involve minimal radiation exposure, akin to that of a standard X-ray, and precautions are advised for pregnant individuals.
The results from PET scans are interpreted by trained radiologists and can provide insights into abnormal metabolic activity indicative of cancer. PET scans are often combined with other imaging techniques like CT scans to enhance diagnostic accuracy. However, interpreting PET data requires careful consideration of various factors, as some noncancerous conditions may mimic cancerous lesions, leading to potential false positives.
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Positron emission tomography (PET)
ALSO KNOWN AS: PET scan, 18F-FDG PET, 18F-FET PET
DEFINITION: Positron emission tomography (PET) scanning is a procedure in which a small amount of radioactive glucose or amino acid is injected into a vein, and a scanner is used to make detailed, computerized pictures of areas inside the body. Since cancer cells often utilize more glucose and amino acids than normal cells, the images can be used to find cancer cells in the body by looking for areas with higher uptake of these nutrients.
Cancers diagnosed: A wide range of cancers
Why performed: PET scans screen for tumors in cancer diagnosis and aid in tumor staging, locating metastases, and assessing treatment response, such as tumor recurrence. In addition to cancers, PET scans effectively determine the presence of infections, heart disease, brain disorders, abnormal blood flow, and bone disorders.
Patient preparation: Patients should restrict the sugar and caffeine consumed the day before the scan. On the day of the scan, patients should not ingest anything except water for a minimum of six hours before the scan. Before the PET scan, blood glucose levels should be less than 120 milligrams per deciliter in nondiabetics and should fall between 150 and 200 milligrams per deciliter in people with diabetes. Patients are asked to drink 500 milliliters of water after injecting a radioactive compound approximately one hour before the PET scan. Twenty to forty milligrams of furosemide for renal or pelvic imaging ten to fifteen minutes after injecting radioactive glucose or amino acid. Patients should also refrain from strenuous exercise for twenty-four hours before PET to minimize the uptake of the radioactive compound by the muscles during the test. Before the scan, patients are asked to remove any dentures, jewelry, or metal objects that may interfere with imaging.
Steps of the procedure: Patients are injected with glucose (FDG) or tyrosine (FET) labeled with radioactive fluorine (18F). The compounds are given about an hour to distribute throughout the body. The patient then empties the bladder and lies still on the scanner bed. The PET scan takes approximately thirty minutes, depending on the scan type and scanner model used. Commonly, an initial whole-body scan is conducted about an hour after injection of the radioactive compound—this scan typically samples from the angle of the jaw to the mid-thigh level. A later or delayed scan occurs approximately two hours after the injection, focusing on the organ or tissue of interest.
A significant complication in interpreting PET data is the presence of multiple primary and metastatic lesions throughout the body. The uptake of radioactive glucose or amino acids into these tumors and the surrounding normal tissues is a dynamic process that peaks at different times, depending on the specific tissues examined, patient preparation, and other factors. Thus, if a patient undergoes serial PET scanning, the time between injection of the radioactive compound and PET imaging should be the same in the baseline and subsequent studies. The National Cancer Institute consensus recommendations for FDG-PET scanning in clinical trials state that serial PET scans should be conducted at the same institution, using the same type of camera, the same dose of radioactive compound, the exact imaging times, and the same acquisition and reconstruction parameters.
After the procedure: The patient is asked to drink plenty of fluids for a day after the scan to flush the radioactive compound from the body. The PET scans are interpreted by a trained radiologist, who sends the results to the referring physician.
Risks: The procedure is completely painless, with no side effects. The amount of radiation exposure is very small and similar to that from a standard X-ray. Like any other radiation procedure, however, PET is accompanied by a small risk of tissue damage. Some patients may also experience soreness in the arm where the intravenous (IV) line was placed. Pregnant patients should inform the doctor of their condition, as PET scans may be harmful to the fetus.
Results: PET scans can identify regions in the body with abnormal metabolism of nutrients, which may indicate the presence of cancer cells. These areas can be examined more closely by computed tomography (CT) or magnetic resonance imaging (MRI) to confirm that tumors are present and, if so, where they are located. PET scans may be useful in determining the extent of the spread of certain cancers, assessing how the cancer responds to treatment, and determining if the cancer has recurred. Cancers may use more energy than surrounding tissues and appear brighter on the PET scan.
PET and CT scanning can produce anatomical pictures of organs and tissues showing abnormal metabolism regions. PET/CT scanning machines are available at many medical centers. This combination method can be more potent than either technique used alone. R. A. Kuker and colleagues showed that a combination of delayed-phase FDG-PET and CT could identify liver tumors and metastases that could not be detected by CT alone.
Several important factors must be considered when analyzing PET scans. A crucial factor in analyzing PET data is the time of measurement. For example, whole-body images are composites of static images obtained at various predetermined times after the injection of the radioactive compound. These scans must be corrected for signal attenuation over time. Other factors impacting data analysis are the volume of distribution, body weight, lean body mass, body surface area, and average serum glucose or amino acid concentrations. After correcting these factors, the standardized uptake value (SUV) is the most commonly used assessment. When suspected lesions are identified on a PET scan, both maximum and mean SUV are determined for regions in the tumor and normal tissue in the same organ. The tumor-to-normal-tissue SUV (T/N) ratio can be calculated. For example, a maximum SUV T/N ratio value can be set as a threshold above which a suspected lesion is considered positive.
When using PET to determine tumor response to treatment, experts recommend conducting PET studies six to eight weeks after radiation therapy and two weeks after chemotherapy. The SUVs of the target tumors are determined before and after treatment. The SUVs of normal tissue in the organs where the tumors occur are also obtained as a reference, and T/N ratios are obtained. This ensures that the SUV changes are attributable to either tumor progression or treatment response and not to regular cell changes over time. The shape and size of tumors often change during treatment. These changes should be recorded along with the changes in tumor uptake values. The interpretation of PET data is key to obtaining accurate and reliable results. Many types of cancer do not appear on PET scans and may only be detected using high-resolution imaging techniques like PET/CT or MRI. Some noncancerous tissues can also appear similar to tumors on a PET scan, resulting in false positives.
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