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What is a 'PET scan'?

PET Scans

Hossein Jadvar (Doctor) gives expert video advice on: How does a PET scan work?; How should I prepare for a PET scan?; Why do I have to fast before a PET scan? and more...

What is a 'PET scan'?

PET stands for positron emission tomography. It is an imaging system that is very similar to a CT scan from outside, but the patient is administered with a positron labelled radiotracer. The most important positron radio labeled tracer today is F18 fluorodeoxyglucose, or FDG, which is a glucose analogue. When the patient is administered FDG, we take images through a PET scan to see a general look of where FDG distributes.

How does a PET scan work?

A PET scan works in this way, that when a radio tracer is distributed in the body, the positron, which is actually an antielectron, travels a small amount before it hits a tissue electron in the body of the patient. When an anti-electron and an electron meet each other they annihilate each other, just like what Einstein stated in his E=mc² equation. That annihilation basically produces two gamma rays which are emitted from that point of annihilation at approximately 180 degrees from each other, and they travel out of the body. The PET camera system detects these two gamma rays that are emitted opposite to each other in what we call a "coincidence mode". In other words, if those two gamma rays are seen to occur at approximately a very short amount of time from each other, it is considered to be an event, and that is recorded by the PET camera system. That is what the PET camera system basically does. It records all of these events, and by recording those events, uses a computer to form an image.

What is a PET scan most commonly used for?

A PET scan is most commonly used in oncology patients with cancer. We use it for the initial diagnosis of cancer and also for the initial staging of cancer. That basically means how extensive is cancer at the time of initial diagnosis. Then it can be used for treatment evaluation. If a patient receives some sort of treatment, for example, surgery, chemotherapy or radiation therapy, and wonder is that therapy working or not, we can use FPG PET, for example, to answer that question. And also in patients who have already been treated, it can be used to see if the cancer is returning. That's what we call recurrence of cancer and also extent of the metastasis. And finally it can be used for prognostication to evaluate how well the patient is going to do specifically after treatments.

Besides its application in oncology, what other uses does a PET scan have?

The major thrust of positron emission tomography (PET) scan in clinical applications have been oncology. However, it has applications also in neurology, and especially more recently in Alzheimer's disease. In cardiology, for looking at what we call myocardial viability. Also, in an up and running clinical application in friction and inflammation energy.

Why do I have to fast before a PET scan?

The reason that a patient needs to fast for a FDG PET (Positron Emission Tomography) scan is because FDG is a glucose analog. So, when the patient eats before a procedure, the glucose level in the blood is going to interfere with the uptake of the FDG, for example, tumor. So to make sure that the tumor receives the most amount of FDG so that we can see it well on our images, we ask the patient to fast so that there's no competition with glucose in the blood.

What does the equipment used in a PET scan look like?

A PET scanner, as I mentioned, is very similar from the outside to an MRI scanner or a CT scanner. It basically has a bore and a bed. The patient lies on the bed and the bed moves through that hole, or that bore. The patient is then positioned appropriately in that bore which is surrounded by detectors which detect the gamma rays coming off from the inside of the patient. And therefore from outside a PET scanner this doesn't really look that much different from what you may have seen with an MRI scanner or a CT scanner.

How is a PET scan performed?

When the patient arrives to the clinic here, they are interviewed, and after the interview, making sure that it is appropriate for the study to be done and making sure the patient was prepared appropriately by fasting 46 hours before the injection of FDG, then the tracer, which in this case is the FDG, is administered intravenously. Then the patient waits anywhere between 30 minutes to an hour after the administration of the FDG for the FDG to be distributed in the body. Then somewhere between 45 to 60 minutes after the administration, the patient is positioned in the PET scanner and the technology starts actually acquiring images. The images, depending upon how or what part of the body, or the whole body, you're doing, take anywhere between 30 minutes to about 45 minutes for the entire study to be done. Therefore, that's basically the routine that the patient can expect to go through for a PET scan.

What will I experience during a PET scan?

There may be a small sensation of pain when the needle enters the skin at the time of trace administration to the vein, but other than that a patient doesn't really feel any different before, during or after the Positron Emission Tomography (PET) Scan. When they lie down in the bed of a PET Scanner basically they just relax. They should not move or talk during that time but there is really no specific sensation or pain of any sort during that procedure and the PET Scan by itself doesn't do anything to the patient. So it's a very relaxing or actually a very safe procedure to do.

What are the benefits and risks of a PET scan?

There are many benefits to a PET scan. Today the most common trace is FDG PET. Right now, FDG is changing the way that we approach a patient with cancer. This is also supported all the way by the National Institute of Health who is really interested in having PET scans, FDG PET scans, inserted into the algorithm of patient care and diagnosis. We are able to detect cancer more accurately, to stage it more accurately, and to more accurately determine if a treatment is working or not and subsequently change treatment if necessary. Also, we are more accurate in stating if the patient is going to do well or not do well with their specific type of cancer or treatment. Those are the major benefits of a PET scan. As far as their risks are concerned, their risks are very similar to general nuclear medicine techniques. They are very, very small. In fact, they are smaller probably because the radio tracers that are used in PET have a very, very short half life. F18, which is used in FDG, actually has one of the longer half lives which is only two hours, compared to general nuclear medicine radio traced radionuclide's, for example Technetium-99m, which has a six hour half life. The risk of PET is essentially summarized by the radioactivity in the tracer, and that is very small.

What procedures can be performed in conjunction with a PET scan?

More recently, PET has been combined with structural imaging systems such as CT. We call it a PET/CT. And the combined PET/CT, the development of a combined PET/CT, has been quite important because now we have the ability to have information, not only on the structure or anatomy, but also the physiology that goes hand in hand with that anatomy and that structure. So, for example, if we see some abnormality on a PET scan, we know exactly where that is in the body. Or if we have a structure on CT, for example, we can say if that structure has normal or abnormal metabolism. There are more recent strides in developing even more combined imaging systems, including PET/MRI, for example, combined systems. There is already SPEC/CT systems which is single photon imaging computer tomography CT systems. That is used for general nuclear medicine. So the combined imaging systems that has the diagnostic information both from anatomy and physiology are basically the wave of the future in nuclear medicine.