PET scans accurately image the body’s physiologic changes. A Computed Tomography (CT) scan shows the structure of the anatomy where the changes are taking place. Combining these two scans in one highly sophisticated PET/CT imaging technique provides, during a single outpatient exam, detailed information to physicians about the presence or spread of disease and accurately identifies its precise location. A PET scan, or Positron Emission Tomography scan, is an imaging technique that allows radiation oncologists to examine many organs of the body and is helpful in diagnosing many diseases, such as cancer. Other techniques, such as CT scan or MRI, only show organ structure, where as PET shows organ structure and function. PET is able to differentiate between malignant and benign tumors since it shows how the organ functions. PET can detect if the cancer has moved from one part of the body to another which is called metastasis. By uncovering abnormalities that might otherwise go undetected, a PET scan guides the radiation oncologist to the most appropriate treatment and helps to outline the area for treatment to lessen the amount of normal tissue that will be treated.
How does a PET scan work?
Before carrying out a PET scan, a radioactive medicine is produced in a cyclotron (a type of machine). The radioactive medicine is then tagged to a natural chemical called glucose or better known as sugar. The tagged natural chemical is known as a radionuclide or FDG (fluorodeoxyglucose – a radioactive drug) . The radionuclide is then injected into the patient. When it is inside the patient, the radionuclide will go to areas inside the body that use glucose. The glucose goes into those parts of the body that use glucose for energy. cancer uses glucose differently from normal tissue – so, FDG shows up cancer in the body.
Detecting positrons – A PET scan detects the energy emitted by positively-charge particles (positrons). As the radionuclide is broken down inside the patient’s body positrons are made. This energy appears as a 3-dimensional image on a computer monitor. And the image reveals how parts of the patients body function by the way they break down the radiotracer. A PET image will display different levels of positrons according to brightness and color. When the image is complete it will be examined by the radiation oncologist, who will then work with the radiation physicist or dosimetrist to plan out the treatment area.
How should I prepare for a PET scan?
You should inform your Radiation Oncologist and the therapist performing your scan of any medications you are taking, including vitamins and herbal supplements. You should also inform them if you have any allergies and about recent illnesses or other medical conditions. You will receive specific instructions based on the type of PET scan you are undergoing. Diabetic patients will receive special instructions to prepare for this exam. Usually, you will be asked not to eat anything for at least 8 hours prior to your PET scan since eating will alter the distribution of the PET tracer in your body and can lead to a suboptimal scan. This could require the scan to be repeated on another day, so following instructions regarding eating is very important. You should NOT drink any liquids except for plain water, it is good to drink plenty of water so that you are well hydrated. Please let your radiation therapist and radiation oncologist know if your are claustrophobic prior to your scan, the scanner is mainly open, however there are times when you may feel claustrophobic, so it is best to let your team know. There are medications that can be given to help ease your claustrophobia.
What does the equipment look like?
A positron emission tomography (PET) scanner is a large machine with a round, doughnut shaped hole in the middle, similar to a CT.
Will the PET scan hurt and what happens after the PET scan?
The actual scan itself does not hurt. The FDG is given in your vein, you will feel a slight pin prick when the needle is inserted into your vein for the intravenous line. When the FDG is injected into your arm, you may feel a cold sensation moving up your arm, but there are generally no other side effects. You will then need to sit for at least a half hour to allow the FDG to move through your body. It is important that you remain still during the scan. Though the scan itself causes no pain, there may be some discomfort from having to remain still or to stay in one particular position during imaging. Your radiation therapist is watching you at all times and listens to you through an intercom, if for any reason you need the therapist just call out and they will come and assist you. Through the natural process of radioactive decay, the small amount of FDG in your body will lose its radioactivity within about 2 hours. You may be instructed to take special precautions after urinating, to flush the toilet twice and to wash your hands thoroughly. You should also drink plenty of water to help flush the FDG out of your body as instructed by your team.
A PET scan is an important tool for your radiation team to do the planning for your treatments.