PET scan is the abbreviation for Positron Emission Tomography. It is an imaging procedure, similar to CT scan. However, in PET scan a special drug is used, that emits radiation (a radiotracer).
PET scan is used to identify
- Cancer spread
- Heart disease
- Brain disorder
PET scan may be useful in
- Finding cancer
- Mapping the spread of cancer in the concerned organ, surrounding lymph nodes and other organs like lung, liver and bones etc.
- Checking whether treatments like radiation and chemotherapy are showing effect on tumor
- Detecting if the cancer has come back after successful completion of treatment
To understand how a PET scan works, one has to understand how cells function. There are varieties of cells in human body which make up the organs. These cells are constantly recycled. The old cells die and are removed from the body while new cells are generated. Production of new cells requires energy and certain other elements.
Cancerous cells multiply much faster than normal cells in the body. Hence they compete for energy and nutrients with the normal cells in the body. This differential rate of multiplication is utilized in PET scan.
In FDG PET scan – which is one of the most commonly used PET scan, patient is injected FDG (fluro-deoxy-glucose). FDG is a radioactive tracer which contains glucose that is bound to a tracer element. Glucose is the most commonly used energy source and cells take up this FDG. And since cancer cells are multiplying at a much faster rate, they take up the majority of this tracer injected into the body.
The radiotracer emits radiation. And the radiation emitted by cancerous cells will be much higher than normal cells. This differential output is identified by gamma camera during PET scan and the sites of cancerous activity appear like hotspots (bright yellow) when compared to other normal body parts.
Similarly, for thyroid cancers – radioactive iodine is used (I-123 and I-131)
For prostate cancer –PSMA PET scan is used.
The PET scan is not 100% accurate.
Infections and trauma can produce results similar to cancer on PET scan.
Hence the findings of the PET scan have to be interpreted in clinical context.
Sometimes you will be advised other tests to confirm or refute the findings of the PET scan.
- You will need to have a prior appointment for PET scan. Typically, you can get an appointment 2-3 days from the date of your request.
- For female patients, it is essentially to confirm that they are not pregnant. In case of doubt , please do a pregnancy test
- Your blood creatinine level needs to be measured before the scan. You need to inform the scan center about the value and carry the printed report of “creatinine level” on the day of the scan. We recommend that you do this blood test 2-3 days prior to your PET scan.
- If you are breast feeding, pump and store the breast milk as you will not be allowed to breastfeed your child for 24hours after the scan.
- If you are diabetic, it is essential that your blood sugar levels are under control. For a PET scan the your fasting blood glucose needs to be less than 180mg/dL. If it is more than 200, the scan has to be rescheduled. We recommend that you get your fasting blood sugars measured 1-2 days before the scheduled scan date.
- Avoid strenuous physical activity and exercise for 24 hours before the scan.
- You need minimum of 6 hours of fasting before the scan. However, you can take sips of plain water during this period
PET scan is something similar to CT scan. The PET scan machine is also similar to a CT scan machine.
When you arrive at the PET scan center, you will be required to complete the official formalities.
You also have to declare if you have any allergies and pre-existing health conditions like hypertension and diabetes. Please carry all the relevant medical documents for your scan.
You will have to empty your bladder before the procedure. A nurse will place a cannula in your vein and then you will be given the radiotracer drug. Usually the drug is given by injection. Depending on the indication sometimes the drug is given orally or via inhalation.
After the drug injection you will be asked to wait for 60-90 minutes for the tracer to be absorbed by the cells. Then a PET scan is performed.
Nowadays combining a CT scan with PET scan is done routinely. If you are undergoing PET CT scan, then the CT scan is performed before the injection of the radiotracer drug. During the scanning also, you may be given another contrast dye injection. A delayed scan may also be required.
The entire process of scanning takes about 3-4 hours. (Waiting time after injection is 1-1.5 hours; scan duration is about 15-20 min)
But depending on the waiting list and logistics you may have to spend more time in the scanning center. Hence it is better to keep half a day reserved for your PET scan.
You will be asked to change into a medical gown before the scan.
You will be asked to lie down on a flat table that slides into a donut shaped scanning machine.
You will be hearing instructions on a speaker and the technician performing the scan will be able to hear you.
You will be asked to hold your breath sometimes to prevent images from getting blurry.
You will be kept under observation for 30-60 minutes after the scan. This is to confirm that you are not going to develop any adverse reactions.
You can eat and drink immediately after the scan.
The radiotracer injected into the body is eliminated from the body via urine and stool in the next 10-12 hours. So, avoid children and pregnant women in that time to prevent them from radiation exposure.
Most often the diagnosis of cancer is already confirmed by biopsy, before you are advised to get a PET scan done.
Less commonly, when there is a suspicion of cancer and biopsy is not straight forward (e.g. lesion in lung) you may be asked to get the PET scan done before the biopsy. This is for 2 reasons-
PET scan will help in
- deciding if the lump in question is showing sufficient activity to raise suspicion of a cancer.
- deciding the most suitable site for biopsy.
A nuclear medicine specialist and a radiologist will interpret the images obtained from your scan.
You will be provided with
- A detailed printed report
Allergy to the drug that needs to be injected is an important risk while performing PET scan.
If you have any allergy, it is necessary that you disclose them before the scan.
The scanning centers are required to have all the essential lifesaving drugs, adequate medical equipment and personnel to handle drug reactions.
However, if you are not really confident about getting a PET scan done at particular center, it is better you talk to your treating doctor and the scanning center in charge before undergoing the scan.
Since the PET scan technology uses radioactive drugs there is a small risk of radiation exposure. However the benefits far outweigh the risks.
Yes, PET scan will be able to tell reliably if the cancer has spread to other parts of your body. However the findings of the scan need to be interpreted in the context of clinical scenario and findings of other investigations.
PET scan is a functional scan. The scan images do not give accurate structural details. If you have been advised to undergo surgery then the surgeon may ask you to undergo combined PET +CT scan which give better structural details to plan surgery.
Even if you are undergoing PET CT scan which is getting popular nowadays, you may still have to undergo other scans.
e.g.: PET scan is a not a sensitive imaging to diagnose brain metastasis, where a MRI is needed.
For cancers of pancreas and liver, a triphasic CT or contrast enhanced MRI is needed for accurate surgical planning.
Organ preserving surgeries for kidney tumours require 3 D details of blood vessels. A triphasic CT scan with angiography and 3D reconstruction may be required for the same.