What is a PET CT scan?
A PET CT scan combines a CT scan (x-ray) and a PET scan (Nuclear Medicine) into one image, to give more detailed information about your disease.
How does a PET CT scan work?
A small amount of radioactive glucose is administered, which highlights areas of your body where the cells are more active than normal (PET Scan) and 3D images are generated. At the same time, x-rays are passed through your body and 3D images of CT are generated. PET and CT images are fused together to get information about anatomy and function together. This allows your doctor to see any changes in the metabolic activity of cells and know exactly where the changes are happening.
What do doctors use PET CT for?
PET CT scans are used for many types of cancer and infections and are generally thought to be more accurate in diagnosing certain types of cancer.
PET CT scans can help to:
- Diagnose cancer
- Stage cancer
- Make decisions about whether you can have surgery to remove your cancer
- Make decisions about which is the best treatment for your cancer
- Show how well the treatment is working
- Find the place in the body where you cancer first started to grow (primary cancer)
- Check whether your cancer has come back
- Show the difference between scar tissue and active cancer tissue
- Show where an infection in your body is located
- Show inflammation of arteries
A CT scan may show that there are still some signs of the disease left. This may not be active cancer but scar tissue left over from cancer cells killed by your treatment. A PET CT scan can help show whether this tissue is active cancer or not.
What do you need to do when having a PET CT scan?
You will receive instructions on how to prepare for your scan, normally written in your appointment letter. Generally, you should not eat anything for six hours prior to your appointment, however you can drink water during this time.
Avoid eating the following items the day before your scan:
- Rice, bread, potatoes, pasta, noodles
- Processed foods, cookies, chocolate, candies, pizza
- Caffeine containing drinks such as tea, coffee, cola etc.
You can have non-carbohydrate or low carbohydrate containing foods such as:
- Milk, yogurt, cheese
- Nuts and seeds
You should not do any strenuous exercise for 24 hours before the scan. Unless you are told otherwise, you should carry on taking any medicines prescribed by your doctor. If you are diabetic, you should contact the department a couple of days before your appointment. You may need to adapt your diet for blood sugar control or consult an endocrinologist. When you arrive, you need to check in with the receptionist so the technologists will be aware of your arrival. You will take a seat in the waiting room until the technologist calls you for your appointment. You can take a friend or relative, but they will not be allowed to stay with you once you are administered the radiopharmaceutical. The nurse may ask you to change into a hospital gown before the injection. You will need to take off all your jewellery and any other metallic objects. The technologist will fill in a questionnaire with you and ask you to sign a consent form. After this, the nurse will check your blood pressure, pulse and blood sugar level and you will be given a medication (Inderal) to relax your muscles, if not contraindicated. Then you will have a small tube (cannula) put into one of the veins in the back of your hand or arm. The radioactive glucose (tracer) will be given to you as an injection through this tube. You will be asked to take a seat in a specially prepared room. In this room you need to rest completely for about an hour after the injection (no music, no reading and no mobile phones) and you have to drink few glasses of water during this resting period. This allows the drug to spread through your body and into your tissues. Before your scan begins, you will be asked to go to the toilet to empty your bladder.
In the scanning room, you will lie on your back on a narrow bed. The technologist will help you to get comfortable and make sure you are in the right position. The bed will gently move through the scanner. The scan takes between 10 to 15 minutes, depending on which part of your body is scanned. The scan is not particularly noisy but the computers and air conditioning make a constant background noise. You need to stay as still as you can during the scan. You can talk to the scan operator through an intercom if you need to. Some people feel a bit claustrophobic (‘closed in’) when they are having a scan. If you think you are likely to feel this way, tell the technologist before fixing the appointment.
After the scan
Once the scan is finished you will be asked to wait in a separate room until the doctor checks your images. Sometimes a delayed scan is required by the doctor. If required, you may have to wait up to one hour more, for the delayed scan. Once the scan is completed you will be able to go home straight away. You can eat and drink what you like and go back to your normal activities.
Although the amount of radiation from the radioactive drug is very small, it is best not to have long periods of close contact with pregnant women, babies and young children for the rest of the day. If you have had any sedatives for the purpose of scanning (which may be required on some occasions, depending on the area of interest), you must not drive for the rest of the day, as it makes you drowsy. You will need someone to take you home from the hospital.
Possible risks of a PET CT scan
During a PET-CT scan you are exposed to radiation from the x-rays and the radioactive tracer drug. The radiation in the radioactive tracer drug is very small, and goes away (decays) very quickly. It does not make you feel unwell. Drinking plenty of water after the scan helps flush the drug out of your system more quickly. The radiation from the CT part of the scan is kept to the minimum necessary for this exam. The risk of the radiation causing any problems in the future is very small. Doctors only do these scans if they are necessary. They make sure the benefit of having the scan outweighs any possible risks.
Please inform the staff if you are or could be pregnant, as there is a risk that the radiation could harm the baby. If your doctors think it is essential for you to have the scan during pregnancy (very rare and is normally contra indicated), they will tell the staff in the scanning department and the dose of radiation will be reduced. You should not bring children (younger than 16 years old) or anyone who is pregnant to the scanning department.
If you are breastfeeding, let the department know a few days before your appointment. They will let you know the duration you need to stop breast feeding after having the radioactive substance. You may need to store enough expressed milk for at least one feed.
It can take time for test results to come through. Usually, a specialist in Nuclear Medicine examines the scan and writes the report. The scanning department sends the report to your referring doctor, who will discuss the results with you.
Understandably, waiting for results can make you anxious. It may take up to two working days for the results to become available and the same will be forwarded to your referring physician. If your doctor needs the results urgently, they make a note of this on the scan request form. You can collect the report and CDs on the day of the appointment with your referring physician.