Positron emission tomography (PET) is a sophisticated medical imaging technique. It uses a radioactive tracer to pinpoint differences in tissues on the molecular level. PET scans can detect differences in body functions, such as blood flow, use of oxygen, and uptake of sugar (glucose) molecules.
Other imaging scans, like computed tomography (CT), or magnetic resonance imaging (MRI), provide three-dimensional views of internal organs.
PET scans make use of this information to reveal how certain tissues within organs are functioning. A lung PET scan, often in combination with a lung CT, can be used to detect lung cancer. The computer combines images from the two scans to provide a three-dimensional image, which highlights any areas of especially rapid metabolic activity. This process is known as image fusion.
Some newer instruments allow technicians to run both scans at the same time. Lung PET/CT scans can help doctors distinguish between benign (noncancerous) and malignant (cancerous) masses that may appear on chest X-rays or CT scans.
If you need a lung PET scan, you will be injected with a small amount of a radioactive tracer substance about one hour before the scan. Most often, an isotope of the element fluorine will be used. The needle may sting temporarily, but otherwise the procedure is painless.
Once in the bloodstream, the tracer substance will accumulate in the organ or area that is being examined and begin to give off energy in the form of gamma rays. These rays are then detected by the PET scanner to create detailed images, which can help your doctor determine the structure and functioning of the organ or area that is being examined.
You will need to wait for about an hour to allow the tracer to accumulate, after which you will be asked to lie on a narrow table. The table slides inside a tunnel-shaped scanner. You will be able to talk to technicians while the scan takes place, but it’s important to lie still while the scan runs—too much movement could cause blurry images. The scan will take about 30 minutes.
There is no need for special preparation. You will be asked not to eat or drink anything besides water for several hours before the scan. It is very important to follow these instructions. A PET scan often depends on monitoring slight differences in how cells metabolize sugars. If you have eaten, or had a sugary drink, this could interfere with interpretation of the results.
Upon arrival, you may be asked to change into a hospital gown, or you may be allowed to wear your own clothes. You will need to remove any metallic objects from your body, including jewelry. If you are taking any medications or over-the-counter drugs or supplements, it’s important to tell your doctor. In some cases, diabetes medications can interfere with the results of a PET scan.
People who are uncomfortable in enclosed spaces may be given a medication to help them relax. This drug may cause drowsiness. The radioactive tracer will become inactive within your body in a few hours or days. It will eventually pass out of the body through the urine or stool. The radioactivity you will be exposed to is about the same you would receive from a CT scan, which uses X-rays.
Although radiation exposure is minimal, women who are pregnant or breastfeeding should always notify their doctors before undergoing any procedures that involve the use of radiation. PET scan uses a small amount of radiation within the body. CT scan exposes the body to X-rays from outside the body.
Lung PET scan is also used to “stage” lung cancer. Staging refers to how advanced a particular cancer is. Cancer is typically assigned a number between I and IV. For instance, stage IV cancer is more advanced, has spread farther, and is usually more difficult to treat than stage II cancer. Staging is also used to predict survival. A person with stage I lung cancer is much more likely to survive longer than someone with stage IV cancer is, for example.
Fast-growing tissues, such as lung cancer tumors, take up more of the tracer substance than other tissues. These areas stand out on the PET scan. A computer generates a three-dimensional image based on the results of the scan. Doctors use these images to detect growing tumors. Images may also help doctors determine the best course of treatment.