PET Scan

A PET scan (Positron Emission Tomography, a type of nuclear imaging that maps how your body's cells are using energy) is a diagnostic test that produces detailed pictures of activity inside the body by detecting a small amount of radioactive tracer introduced into the bloodstream.

Before the scan, you receive an injection of a radioactive sugar called a tracer. Because cancer cells and other abnormally active cells consume sugar much faster than normal cells, they absorb more of the tracer and appear as bright spots on the images. The scanner then detects the energy released by the tracer and converts it into a three-dimensional map of cell activity throughout the body. This helps doctors see not just where a problem is located, but how biologically active it is — information that a CT or MRI alone cannot always provide.

PET scans are used most often in oncology (cancer medicine) and neurology (the study of the brain and nervous system). Doctors request them when they need to understand not just the structure of a tissue, but how actively it is functioning. Common situations include:

• Detecting cancer — finding tumours that may be too small or too early to show clearly on a CT or MRI.
• Staging cancer — determining how far a cancer has spread to lymph nodes (small glands that are part of the immune system) or distant organs.
• Evaluating treatment response — checking whether chemotherapy or radiotherapy is working by seeing if previously active areas have quietened down.
• Detecting cancer recurrence (return of cancer after treatment) — identifying new activity before it is visible on other scans.
• Investigating suspected brain conditions such as Alzheimer's disease (a progressive memory disorder), epilepsy (a condition causing repeated seizures), or brain tumours.
• Assessing heart tissue — distinguishing living but damaged heart muscle from tissue that has died after a heart attack, to guide decisions about heart procedures.

A PET scan may not be suitable in certain situations. Your doctor will weigh up these factors before recommending the test:

• Pregnancy — the radioactive tracer poses a potential risk to a developing baby, so the scan is generally avoided unless the benefit clearly outweighs the risk.
• Breastfeeding — mothers are usually advised to pause breastfeeding for a period after the scan to avoid passing the tracer to the baby through breast milk.
• Poorly controlled blood sugar (diabetes) — high blood sugar levels interfere with how the tracer is absorbed, making the images harder to interpret; blood sugar is usually stabilised before the scan.
• Severe claustrophobia (intense fear of enclosed spaces) — the scanner is a large ring-shaped machine, and some patients find lying inside it difficult without prior anxiety management.
• Very recent surgery or infection — inflammation from healing tissue can mimic the bright spots of cancer, reducing the reliability of results.

A PET scan is a diagnostic procedure that carries a low overall risk; the main considerations are the small radiation dose and the injected tracer, rather than any surgical risk.

• Radiation exposure — the tracer emits a small amount of ionising radiation (energy waves that can affect cells). The dose is considered low and the tracer loses most of its radioactivity within hours, but it is still a factor your doctor weighs against the benefit of the scan.
• Allergic reaction to the tracer — mild reactions such as skin flushing, itching, or a rash are uncommon; severe allergic reactions are very rare.
• Discomfort at the injection site — bruising, mild swelling, or brief pain where the needle was inserted.
• Anxiety or discomfort during the scan — lying still inside the scanner for an extended period can be challenging, particularly for patients with claustrophobia or chronic pain.
• Misleading results — areas of normal healing, recent infection, or high blood sugar can appear bright on the scan and may require additional tests to distinguish from cancer.
• Risks specific to combination PET-CT scans — many centres perform PET alongside a CT scan in the same session, which adds a second, separate dose of radiation. Your doctor will explain if this applies to your scan.

Careful preparation improves the quality of your PET scan images significantly. Your care team will give you specific instructions, but the following describes what is typically involved.

In the hours before the scan, you will usually be asked to fast (eat and drink nothing except plain water) for at least four to six hours. This keeps your blood sugar at a stable, low level so that the tracer is absorbed evenly rather than competing with dietary sugar. Sugary drinks, juice, and chewing gum are all excluded. Plain water is encouraged because staying well-hydrated helps flush the tracer from your body after the scan.

Regarding medications and lifestyle: your doctor or nuclear medicine team will advise whether any of your regular medications need to be paused. Diabetic patients typically receive special instructions to adjust their blood-sugar management on the day. You will usually be asked to avoid strenuous physical exercise for at least twenty-four hours before the scan, because exercise causes muscles to absorb more tracer and can create confusing bright spots on the images. Alcohol should also be avoided in the day or two before the scan for similar reasons.

Before the scan, certain tests and checks are usually carried out:

• Blood sugar measurement — a finger-prick or small blood draw to confirm your glucose level is within the range needed for reliable imaging.
• Review of your medical history and current medications — to check for allergies, kidney function, and any conditions that might affect the scan.
• Pregnancy test — for women of childbearing age, in most centres.
• Weight and height measurement — used to calculate the correct tracer dose for your body.

On the day, the procedure itself typically follows these steps:

• 1. Arrival and check-in — you are welcomed by the nuclear medicine team, your identity is confirmed, and consent is reviewed.
• 2. Preparation of the tracer — the radioactive tracer is prepared in a shielded facility and drawn into a syringe ready for injection.
• 3. Injection of the tracer — a small cannula (a thin flexible tube) is placed in a vein, usually in your arm, and the tracer is injected through it. The injection itself is brief.
• 4. Uptake period — you rest quietly in a low-stimulus room for usually around forty-five minutes to an hour while the tracer travels through your bloodstream and is absorbed by active cells. During this time you are asked to stay still, avoid talking excessively, and stay warm, as activity and cold can affect tracer distribution.
• 5. Positioning on the scanner — you lie flat on a motorised table that moves slowly through the centre of the large, ring-shaped PET scanner. The room is quiet and the machine itself makes little noise.
• 6. Scanning — the scanner records energy signals from the tracer. You must lie as still as possible. The scanning portion may take from around twenty minutes to over an hour depending on the area being imaged and whether CT images are being taken at the same time.
• 7. Completion — the table moves out, the cannula is removed, and you are free to move. You will be encouraged to drink plenty of water to help clear the tracer from your body.

A PET scan does not require surgery or sedation, so most patients leave the imaging centre the same day and recover quickly. However, because a small amount of radioactive tracer remains in your body for a short time afterward, some specific precautions are usually recommended.

• Hydration — drinking plenty of water in the hours after the scan is strongly encouraged to help your kidneys flush the tracer out of your body through urine.
• Temporary distance from vulnerable people — for a period of several hours after the scan, most centres advise keeping a comfortable distance from pregnant women, young children, and infants, as a precaution against the small residual radiation. Your team will tell you exactly how long this applies.
• Breastfeeding pause — if you are breastfeeding, your team will advise you on how long to pause and how to manage this safely.
• Returning to normal activities — for most patients, normal daily activities can be resumed promptly, as no general anaesthetic (medication that puts you fully to sleep) or incision was involved.
• Eating and drinking normally — you may eat and drink as usual as soon as the scan is finished.
• Driving — if you were not given any sedative medication to manage anxiety, you can usually drive yourself home. Confirm this with your team beforehand.
• Follow-up appointment — the PET scan images are analysed by a specialist in nuclear medicine or radiology (medical imaging). Results are not usually available immediately. Your referring doctor will discuss the findings with you and explain what, if any, next steps are recommended — such as a biopsy (a small tissue sample), further imaging, or a treatment plan.
• Monitoring for unusual symptoms — if you experience any unexpected symptoms such as persistent skin rash, swelling, or pain at the injection site, contact your care team.