Q. I'm in treatment for colorectal cancer. My doctor has scheduled a PET scan to see how well my treatment is working. What will happen during this test?
A. A positron emission tomography, or PET, scan is an imaging technique. Unlike most imaging techniques, a PET scan primarily shows how different parts of the inside of the body are working, rather than just how they are shaped.
As in your case, PET scans are often used to determine how well cancer treatment is working. They can also be used to detect cancerous tumors and to determine how much cancer has spread. In addition, doctors use PET scans to evaluate neurological illnesses, especially epilepsy and dementia. In patients with coronary artery disease, PET scans may be used to evaluate how well the heart is functioning.
A PET scan involves radioactivity. Tell your doctor if you are pregnant or if there is a possibility that you might be pregnant.
A PET scan usually is done as an outpatient test in a major medical center. The PET scanner is a ring-shaped device with an attached table. You will lie on the scanning table.
During the PET scan, either you will inhale a substance called a tracer, or it will be injected into one of your veins, usually in your arm. Once the tracer is given, the PET scan must be done immediately because the tracer decays rather quickly. The tracer will travel through your bloodstream to the organ being targeted for imaging.
Once there, the tracer will produce radioactive particles that interact with other particles in your body to produce gamma rays (similar to X-rays). The PET scanner will detect these gamma rays. A computer analyzes the scans to form an image.
You must lie very still during the PET scan. The scanning table will slide slowly through the opening in the scanner ring, so you won't need to move. The entire scan should take 30 minutes to two hours. Afterward, you can go home and resume your normal activities.
The radioactive tracers used in PET scans are considered to be safe. They are short-lived and are quickly cleared from the body.
Before the discovery of X-rays about a century ago, we had no way of looking inside the body without cutting the skin. X-rays were a revolutionary advance, honored with the Nobel Prize. They have helped greatly to diagnose a variety of diseases, but the pictures they produce are fuzzy. For example, you can barely see anything in the brain -- like a brain tumor. However, until about 50 years ago, X-rays were all we had.
Since then, the discovery of computerized tomography (CT) scans, magnetic resonance imaging (MRI), functional MRI scans and PET scans allow really clear pictures of our insides. The invention of flexible tubes called endoscopes has also made it possible to see inside the body. All of these technologies have helped doctors solve and fix problems inside the body.
• Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com.