PET Scans

What is it?

A Positron Emission Tomography (PET) scan is an imaging technique that uses radioactive molecules to create a dynamic image of internal tissues and organs. PET scans are very useful in detecting diseases like cancer because tumors will be a different color than surrounding tissue activity. This is in contrast to techniques that reveal structure but not activity, such as a CT or X-ray.

How it works

PET scans use radioactively labeled tracers (radiotracers) that are injected into the bloodstream. Radiotracers are small molecules that are designed to be very similar to compounds normally found in the body. These specially marked compounds are used in normal body processes, but emit positrons as they break down. PET scanners contain special cameras that detect these positrons and create images with the information. The image distinguishes areas that have little radiotracer concentration from areas with increased concentrations of radiotracer. On the image, the differences are represented by color changes.

Below is a list of the information included in this section:

• Equipment
• Radiotracers
• The Exam
• PET Imaging by Cancer Type

Equipment

What instruments are used?

A PET scanner looks very similar to a CT scanner and in many cases it is the same machine. There are 4 main components a PET scan machine, the gantry (frame) that houses the detection apparatus with a large patient port (opening), the subject table that moves in and out of the patient port allowing for a complete scan, the detector/camera system that captures the image, and a computer system that processes and presents the images.

Radiotracers

18F-FDG

The most common tracer used for PET scans is 18F-fluoro-2-deoxy-glucose (18F-FDG), which is a radioactively labeled form of glucose. Glucose is a simple sugar that is the body's main source of energy and it is carried throughout the body in the bloodstream. Because all cells need energy to properly function and divide, glucose is found in every tissue and organ. When a cell needs energy, it collects glucose and processes it (via glycolysis and respiration) into a usable form of energy. PET scans take advantage of this process to create images. Cancer cells tend to use much more glucose than normal cells. Therefore, cancer cells (and tumors) collect more of the radiotracer and appear as a different color on the resulting images. The radiotracer 18F-FDG is frequently used because it is readily transported into cells. Once in cells, the chemical is phosphorylated into FDG-6-phosphate. The newly created molecule (FDG-6) is trapped within the cell and will no longer participate in metabolism.1

18F-FDG is an unstable molecule because it contains excess protons and once inside the body it begins to break down. As it breaks down it emits positrons, a positive electron, which break down further, releasing 2 equivalent photons in opposite directions. These photons are detected by a camera inside the PET unit, producing a high-resolution 3 dimensional image.2 3

Disadvantages to 18F-FDG Use

18F-FDG is readily absorbed by macrophages, neutrophils, and active muscle cells. This may result in false positive test results, particularly in areas of inflammation and infection. Younger patients often give false negatives because of increased uptake in areas such as the brain, urinary system, thymus, and in smooth or striated muscle. False negatives can also occur when tumors are slow growing and absorb less glucose.3 Watch a video explaining false positives and negatives in medical testing.

Other Radiotracers

The energy pathway identified by FDG is used by all cells, making this radiotracer useful in many cancer types. There are many different radiotracers used in PET scans that take advantage of other cellular processes. These are used to detect specific cancer types and to study other disease processes, like heart disease and Alzheimer's disease. Information on other radiotracers and their use can be found in the Molecular Imaging and Contrast Agent Database. New tracers are under development and promise to be more specific and produce more effective images with increased specificity, sensitivity, and overall accuracy.4

The Exam

Scheduling
When you are scheduling your exam be sure to mention if you:

• are diabetic
• have asthma
• are breast feeding
• are pregnant or think you are pregnant
• have a fear of enclosed spaces (claustrophobia)
• have previously had an allergic reaction to the tracer used in the exam
• NOTE:You should not schedule any other tests that use radioactive material for the same day

Day of exam
There are special instructions to follow the day of your PET exam:

• Drink plenty of water, but do not eat or drink anything (except water) for 6 hours before your exam
• Take your medications normally and drink them with ample water
• Make sure you arrive at the imaging center on time, the chemicals in the tracer break down rapidly and if you are late, the images may not be as good
• Wear loose fitting clothing
• Do not wear any jewelry; watches, chains, rings, piercings, etc.
• Prepare to be at the center for around 2 ½ hours
• NOTE: If you are a diabetic there are special rules you need to follow before the exam. You should follow your normal daily routine, eat small meals and take your insulin as usual. Your blood sugar should be between 100 and 200 mg/dL and your blood may be drawn to ensure this.

During
A nurse will start an IV and the tracer compound will be injected into a vein. You will be moved to a quiet, dark room where you will lie still for a specified amount of time, usually about 1 hour. During this period your body will completely absorb and distribute the tracer compound. In many cases you will be asked not to speak to prevent the compound from collecting in your tongue and vocal chords. You will then lie down on a padded table that will slide into the large opening in the PET machine. A technologist will position you (most likely with your arms placed above your head) to obtain the most accurate image and you will be asked to lie completely still during the entire scan, any movement may distort the image. The scan may last anywhere from 30 minutes to 1 ½ hours, if you have trouble staying still for long periods of time you may be sedated.

After
You should not be bothered by any side effects after the exam. You will be encouraged to drink plenty of fluids to help flush any remaining tracer from your body. If you were sedated you will need to arrange for someone to drive you home.

PET Imaging by Cancer Type 

Below is a table describing the usefulness of ¹⁸F-FDG (¹⁸F-fluorodeoxyglucose) PET scanning with repect to diagnosis, staging, prognosis, treatment planning and response to therapy for several cancer types.

All the information in this table was obtained from a review published in Clinical Oncology in 2007.3

Learn more about sensitivity and specificity of medical tests

Learn more about cancer staging

• 1ME Phelps. Molecular imaging with positron emission tomography. Annual Review of Nuclear and Particle Science. 2002; 52: 303-338.
• 2ME Phelps. Molecular imaging with positron emission tomography. Annual Review of Nuclear and Particle Science. 2002; 52: 303-338.
• 3 a b c KA Wood, PJ Hoskin, MI Saunders. Positron Emission Tomography in Oncology: A Review. Clinical Oncology. 2007 19: 237-255. [PUBMED]
• 4JF Vansteenkiste. Imaging in lung cancer: positron emission tomography scan. European Respiratory Journal. 2002; 19: Suppl. 35, 49s-60s. [PUBMED]
• 5Dehdashti, F., Wu, N., Ma, C.X. et al. Association of PET-based estradiol-challenge test for breast cancer progesterone receptors with response to endocrine therapy. Nat Commun 12, 733 (2021). https://doi.org/10.1038/s41467-020-20814-9