What is a PET scan?

A PET scan or positron emission tomography scan is a type of imaging test that uses minute doses of a radiotracer, a radioactive chemical to view the anatomy (structure) and physiology (functionality) of body tissues and organs. It also helps in visualising biological changes taking place in body.

PET scans are generally recommended to detect cancers, central nervous system disorders and heart and brain disorders.

  1. Why is PET scan performed?
  2. How do you prepare for a PET scan?
  3. How is PET scan performed?
  4. What do PET scan results mean?

PET scan is generally performed to evaluate tissues and organs and detect the presence of disease. However, the most common use of PET scan is in detecting cancer and monitoring the efficacy of cancer treatment. Some other reasons for which a PET scan is performed are:

  • To diagnose conditions like:
    • Epilepsy (A disorder of the brain that causes recurrent seizures)
    • Parkinson’s disease (A progressive disease that affects the nervous system causing changes in normal gait, fine tremors and muscle weakness)
    • Stroke
    • Huntington’s disease (A genetic condition that affects the nervous system, causing uncharacteristic involuntary movements, dementia and an abnormal posture)
  • To assess a mass or lesion in lungs that was detected in chest computed tomography (CT) scan and chest x-rays
  • To locate the appropriate site for surgery
  • To understand the flow of blood and oxygen to heart muscles to help improve blood flow
  • To identify the spread of cancer from the site of its origin to other parts of the body
  • To assess brain function after detecting perfusion (oxygen and blood flow) in brain tissues, hematoma (blood clots) and trauma
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The following precautions are necessary before a PET scan:

  • The individual should not eat food or drink beverages like tea for about 4 to 6 hours before undergoing the scan. They will be asked to drink only water.
  • Diabetic individuals will be advised to stop taking diabetes medications before undergoing the test as they can alter test results. The healthcare provider should be informed of any prescription or non-prescription medications or supplements before undergoing the scan as these might alter results.
  • It is important to tell the doctor if the person suffers from claustrophobia (fear of closed spaces); in such cases, medicines to induce sleepiness or reduce anxiety may be prescribed.
  • Women who are pregnant or suspected to be pregnant should inform the doctor about it.
  • Also, the physician should be informed if the individual has allergies to injectable contrast dyes.

The following preparations are needed right before the scan:

  • The individual will be asked to wear comfortable clothes to the appointment and will be asked to change into a hospital gown during the scan.
  • They will also be asked to remove pieces of jewellery and other objects that might interfere with scan results.

A radiotracer is injected into the vein about one hour before the test. The tracer passes through body along with blood and is absorbed by tissues and organs that need to be examined. A scan is then performed, which lasts for about half an hour to 1 hour. Scans performed on the brain and heart take lesser time. The person will be asked to remain still during the examination as body movements can affect the quality of images generated. Chemicals or dyes used for the test may vary according to the organ tested. In some cases, other tests might be required with this scan.

The tracers used in PET scan are short-lived, so they are eliminated from the body within 2-10 hours. Repetitive PET scans may increase the risk of cancer; however, the risk is small. 

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Normal results:

A normal PET scan indicates that the organ examined has a normal shape and size and is in the right position.

Abnormal results:

An abnormal PET scan results might indicate the following conditions:

  • Problems in the functioning of an organ
  • Cancer
  • Infection

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor. 


  1. National Cancer Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; NCI Dictionary of Cancer Terms: PET test
  2. Radiological Society of North America (RSNA) [internet]; Positron Emission Tomography - Computed Tomography (PET/CT)
  3. National Health Service [internet]. UK; PET scan
  4. Johns Hopkins Medicine [Internet]. The Johns Hopkins University, The Johns Hopkins Hospital, and Johns Hopkins Health System; What is positron emission tomography (PET)?
  5. Hutton BF, Segerman D, Miles KA. Radionuclide and hybrid imaging. In: Adam A, Dixon AK, Gillard JH, Schaefer-Prokop CM, eds. Grainger & Allison's Diagnostic Radiology. 6th ed. New York, NY: Elsevier Churchill Livingstone; 2015:chap 6.
  6. Khuri FR. Lung cancer and other pulmonary neoplasms. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 25th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 191
  7. Meyer PT, Rijintjes M, Hellwig S, Kloppel S, Weiller C. Functional neuroimaging: functional magnetic resonance imaging, positron emission tomography, and single-photon emission computed tomography. In: Daroff RB, Jankovic J, Mazziotta JC, Pomeroy SL, eds.

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