Positron Emission Tomography – PET Scan

What is Positron Emission Tomography or PET Scan?

Positron emission tomography (PET) scan is an imaging test. It is conducted in order to visualize your internal organs and to ascertain how well your organs and tissues are functioning. A PET scan takes the help of a radiotracer to yield 3-dimensional images of the organs. It can detect changes at the cellular level and thus can help diagnose a disease before other imaging tests can do so.

A PET scan is a specialized provision in radiology and uses nuclear medicine. It is often used alongside a CT scan (PET/CT scan) or an MRI scan (PET/MRI scan) to yield better diagnostic images and thus results. PET scan was invented by Edward J. Hoffman and Michael E. Phelps in 1974.

What is Nuclear Medicine?

Nuclear medicine is a branch of medicine that employs radioactive substances known as radiotracers to evaluate bodily functions, diagnose diseases at an early stage and as a therapeutic modality.

What is a radiotracer?

A radioactive tracer is a carrier molecule that binds tightly to a radioactive particle. The tracers may be injected, swallowed, inhaled or injected directly into a specific organ depending on the organ or tissue to be analyzed.

A radiotracer is also known as a radiopharmaceutical as it needs to be approved by the regulatory agencies like FDA before being used for diagnostic or therapeutic purposes.

Many PET scans use fluorodeoxyglucose (FDG) as a radiotracer. It is chemically similar to the naturally occurring sugar-glucose. Your body tends to treat it in a similar fashion. Your own cells can also be used as a tracer. For example- when an internal bleed is suspected, your own blood can be used to trace the site of bleeding.

What are the indications for a PET scan?

A PET scan can be used in the following scenarios-

Oncology – Cancer detection and treatment
– Detect cancerous cells at an early stage.
– Detect the extent of the spread of a malignancy in the body.
– Define a treatment plan for specific cancer.
– Evaluate the response of cancer to therapy.
– Detect the recurrence of a tumor (cancer cells appear as bright spots on PET scans as their metabolic rates are higher than the normal cells)

Brain lesions
– PET scans can be used to detect and analyze brain tumors, epilepsy, and dementia (including Alzheimer’s disease).

Cardiac diseases
– PET scans can identify those areas of the heart where the blood supply is compromised. It also helps in deciding whether an angioplasty (a surgery which opens the lumen of a clogged blood vessel) or a bypass (coronary artery bypass) surgery will suit the patient.

How should I prepare for a PET scan?

If you are about to undergo a PET scan, keep the following things in mind –

– Have a frank discussion with your physician regarding your medical history, any medicines or supplements (including herbal/ayurvedic) that you may be taking, any allergy that you suffer from and if you are pregnant or breastfeeding your child.

– Inform your physician if you are taking any blood-thinners. During a PET scan, a radiotracer may be injected in a vein on your fore-arm. This procedure runs the risk of bleeding if you are taking blood thinners.

– Inform your physician if you suffer from claustrophobia (fear of closed spaces). It can make you very uncomfortable when you are moved inside the body of the scanner during the procedure. You may be prescribed a mild sedative to calm your nerves just before the scan.

– Remove any jewelry or metallic objects that you may be wearing before you go for a PET scan as these may hinder the process. Remove any dentures, hearing aids and spectacles before a PET scan.

– Wear loose-fitting comfortable clothes on the day of the scan.

– Avoid vigorous exercise for a couple of days before a PET scan is due.

Do I need to fast before a PET scan?

Yes. You will be instructed to fast for at least 6 hours prior to a PET scan. You can consume water but need to avoid sugary drinks and drinks containing caffeine or alcohol. Moreover, you need to consume a low-carbohydrate, no sugar diet for 24 hours before the test. In case you are a diabetic, your physician will give specific instructions for the same.

Can I undergo a PET scan if I am pregnant?

Any exposure to radiation, however, small carries a risk to the unborn child. Therefore, PET scan should preferably be deferred until the pregnancy is complete. However, if the indication is urgent and justified, a PET scan can be carried out on a pregnant female because the benefit/risk ratio is high.

Is it safe to get a PET scan done if I am breastfeeding?

Exposure to radiation during a PET scan can lead to the presence of the radiotracer in your milk until the time radioactivity is eliminated from your body. Therefore, the scan should only be performed once you have stopped breastfeeding your child. But in cases, where a PET scan is required urgently, it is advisable to collect some breast milk before the procedure. You can start breastfeeding your child after a lag period when your body has rid itself of the radiotracer. This period can wary from a couple of hours to a few days, depending on the tracer used.

Are there any risks involved in a PET scan?

– A PET scan is a non-invasive procedure (intravenous access is established in some cases to inject the radiotracer). You may feel the pain of a needle puncture and as the dye is injected, a cold sensation in your arm.

– An allergic reaction to the radiotracer used is a rare complication.

– You may feel claustrophobic inside the scanning machine.

– There is always a theoretical risk of cancer whenever a person is exposed to radiation.

How should I prepare for a PET scan?

Before the procedure-

– You will be asked to change into a hospital gown.

– You will be instructed to empty your bladder and bowel.

– You will be administered a radiotracer (the route varies, as described above).

– There will be a waiting period of half an hour to 60 minutes as your organs and tissues absorb the radioactive substance.

What happens during a PET scan?

– You will be made to lie on an examination table and moved into the center of a doughnut-shaped scanning machine.

– You will be instructed to stay still during the procedure so that the images do not get hazy.

– The radiotracer absorbed by your body will give off gamma rays which are detected by a special camera and conveyed to a computer screen. Three-dimensional images of the internal organs are then generated.

– The areas in your tissues where the radiotracer collects will appear as bright spots on the images. These are the regions where the chemical activity is higher than the surrounding tissues. These are called ‘hot spots’. Areas with a lesser degree of activity are labeled as ‘cold spots’.

What happens after the PET scan is done?

After the procedure-

-No specific precautions are required. Your body will eliminate the radioactive material from your body via feces and urine.

– Drink plenty of fluids to help your body flush out the radiotracers. It can take a couple of days for your body to be completely free of any radioactive material.

– In case you experience bleeding or infection at the site of intravenous access, consult your physician.

How long does it take to perform a PET scan?

The entire procedure of performing a PET scan can take about 2 hours.

What is the cost of a PET scan in India?

The cost of PET scan in India in government hospitals can lie between Rs. 2000 to Rs. 10,000, depending on the area scanned. In private facilities, a full-body PET scan can cost anywhere between Rs. 15,000 to Rs 27,000.

What are the advantages and limitations of a PET scan?