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Positron Emission Tomography (PET) Scan

A PET (positron emission tomography) scan is a test that shows how organs and tissues function inside the body. These tests help doctors diagnose and treat cancer because they can see and track the activity of cancer and other cells. The processes that can be measured include blood flow, tumor growth rate, and the action of chemotherapy drugs.

What is a PET-CT scan?

Sometimes a PET scan is performed at the same time as a computed tomography (CT) scan. A PET-CT scan collects images from both PET and CT scans and combines them, giving doctors information about the function, size, shape, and location of cancer and surrounding structures. CT is done first to create anatomic pictures of the organs and structures in the body, and then PET is done to create colored pictures that show metabolic or other functional changes in tissues.

PET CT Scan of a pediatric non-Hodgkin lymphoma patient
PET CT Scan of the lateral plane of a pediatric non-Hodgkin lymphoma patient
PET CT Scan of the axial plane of a pediatric non-Hodgkin lymphoma patient

A PET-CT scan collects images from both PET and CT scans and combines them.

How does a PET scan work?

PET scans can measure the level of glucose, or sugar, in cells. Before the PET scan, patients are injected with a radioactive tracer — a form of glucose with a small amount of radioactive material attached. Because cancer cells grow and divide so quickly, they absorb much more glucose than normal cells. The radioactive material attached to the glucose molecule causes the glucose to light up on the PET scan. 

Young male cancer patient receives radioactive tracer while in position for PET scan.

Patients are injected with a radioactive tracer, a form of glucose with a small amount of radioactive material attached.

The radioactive tracer emits invisible energy called gamma rays, which can be seen by special cameras called gamma cameras or PET scanners.

One of the most frequently used tracers is a material called FDG (fluorodeoxyglucose). It is manufactured in a laboratory at the pediatric center and transported directly to the PET clinic for the test.

What can the patient expect during a PET scan appointment?

  • Patients will first check in at the registration desk and go to a waiting area. Then the patient and parent will be called back to the area with the PET scanner.
  •  A nurse or technologist will check the patient’s glucose level. If the level is acceptable, the nurse or technologist will inject the radioactive tracer (called FDG) into a vein.
  • The patient will wait about 45 minutes for the tracer to circulate to the tissues to be scanned.
  • The PET machine is large and round and has an opening in the center with a bed that slides through the opening. When it’s time for the scan, the technologist will help the patient onto the bed, assist the patient in getting into the proper position, and will then go into an adjoining room where the technologist can see, hear, and talk to the patient while operating the PET scanner.
  • The test will not hurt, but it may be uncomfortable for the patient to lie still for 30-45 minutes. Patients may receive sedation medicine to help them sleep through the process. A child life specialist may work with patients on relaxation techniques to help them stay still.
Young male cancer patient is beginning PET scan, red lights visible on his face.

When the machine is turned on the patient will see the red laser lights of the scan, but will not feel them.

  • When the machine is turned on the patient will see the red laser lights of the scan but won’t feel them. The PET scanner does not make a lot of noise, and it will not touch the patient. Usually patients can listen to music or watch a movie.
  • The table will slide slowly through the large hole in the center of the machine a few times during the test.
  • If no sedation was used, patients can usually return to normal activity right away. If sedation was needed, patients will need to go to recovery first and wait until the effects of the anesthesia have worn off.

What should patients do before the test?

  • To prepare for the test, patients should not have certain foods and drinks or exercise strenuously several hours before the test because it can affect the scan readings. The care team will provide specific information about how to prepare.  
  • Patients should wear loose, comfortable clothes. PET scanners need to be kept cool, so the room may be chilly.
  • Arrive a few minutes before appointment time to allow time for check-in. 

Is the radioactive material safe?

The level of radiation is very small, and the doctor will discuss the risks and benefits involved. Patients and families should talk to their care team if they have any concerns. The radioactive material is taken up by the cells and does not stay in the body long. Through the natural process of radioactive decay, it will lose its radioactivity over time. It may also pass out of the body through urine or stool during the first few hours following the test. The medical team will likely tell patients to drink plenty of water to help flush out the radioactive material.

Even though the amount of radioactivity is very low, it is recommended that parents follow these precautions when caring for their child after the scan:

  • Always wash hands after changing patient diapers or handling body fluids.
  • Hold soiled diapers in a separate trash can for 2 days before placing them in the regular trash.
  • Pregnant women should not cuddle with the patient for at least 24 hours after the scan. Also, the patient should avoid direct contact with infants and toddlers until the next day.

How will patients find out the test results?

A radiologist who has specialized training in nuclear medicine will analyze the images and send a report to the referring physician. The doctor will go over the information during the patient’s next appointment.


Reviewed: June 2018