Positron Emission Tomography, also called PET/CT, is a diagnostic examination where a radioactive substance is administered to the patient which results in a picture of function (PET), a picture of anatomy (CT), and a merged picture of both: the body’s metabolism and structure. This substance localizes to different areas of the body depending on changes in tissue metabolism, and is detected by the PET/CT scanner. The PET/CT scan allows doctors to evaluate a variety of diseases anywhere in the body. This technique is mostly used assess tumors of the brain, lung, colon, breast, head and neck, esophagus, and melanoma. In addition, PET/CT is used to evaluate neurological and cardiovascular disorders.
PET images begin with an injection of FDG, an analog of glucose that is tagged to the radionuclide F18. Metabolically active organs or tumors consume sugar at high rates, and as the tagged sugar starts to decay, it emits positrons. These positrons then collide with electrons, giving off gamma rays, and a computer converts the gamma rays into images. The CT image portion begins by send x-rays through the body, which are then measured by detectors in the CT scanner. A computer algorithm then processes those measurements to produce pictures of the body’s internal structures. Combined PET/CT joins the two imaging areas and is more accurate than PET and CT alone.
- The information provided by nuclear medicine examinations is unique and often unattainable using other imaging procedures.
- By identifying changes in the body at the cellular level, PET imaging may detect the early onset of disease before it is evident on other imaging tests such as CT or MRI.
The benefits of a combined PET/CT scanner include:
- greater detail with a higher level of accuracy; because both scans are performed at one time without the patient having to change positions, there is less room for error.
- greater convenience for the patient who undergoes two exams (CT & PET) at one sitting, rather than at two different times.
- Because the doses of radiotracer administered are small, diagnostic nuclear medicine procedures result in low radiation exposure, acceptable for diagnostic exams. Thus, the radiation risk is very low compared with the potential benefits.
- Nuclear medicine diagnostic procedures have been used for more than five decades, and there are no known long-term adverse effects from such low-dose exposure.
- Allergic reactions to radiopharmaceuticals may occur but are extremely rare and are usually mild. Nevertheless, you should inform the nuclear medicine personnel of any allergies you may have or other problems that may have occurred during a previous nuclear medicine exam.
- Women should always inform their physician or radiology technologist if there is any possibility that they are pregnant or if they are breastfeeding their baby.
How do I prepare for the PET-CT Scan?
The Scheduling Department will advise the patient about any specific preparation required for the procedure. The following is a list of the general preparations to be followed by all patients:
- Do not exercise heavily for 24-hours prior to the scan.
- Arrive at the appointment without eating except for water for at least four hours. It is especially important that no sugar be ingested, because the glucose in foods competes with the uptake of the radioactive glucose in the PET scan.
- Medications should be taken as scheduled, however, if food is required, the patient should check with the PET Scheduling department for any special instructions.
- Diabetic patients often are scheduled in the early afternoon, peak control time after morning insulin.
How is the procedure performed?
The PET/CT procedure typically lasts 2-3 hours. The amount of time involved to complete your scan will depend on the type of procedure your physician has requested. It begins with an intravenous injection of a small amount of radioactive glucose (18FDG). There are no known side effects to this injection. The patient will be asked to wait for 30 – 60 minutes for the 18FDG to distribute in the body. Movement, reading, or any other activity during that time may be restricted, as these will affect the pattern of 18FDG uptake. After that, the patient will be escorted to the imaging camera. The brief scan will be taken using the CT part of the scanner to provide a picture of the patient’s anatomy, which will take about 5 minutes. Then the PET scan pictures will be taken, which will last anywhere from 20 minutes to 65 minutes, depending on the exam ordered.
The PET/CT scanner is very similar in size to a CT scanner and looks like a large doughnut. The patient is asked to lie comfortably on the imaging table while the scanner detects the concentration of the tracer within the body, and generates CT, PET, and fusion images by use of computer software.