How PET/CT scans work

Dr Marguerite Morkel a Nuclear Physician based at Mediclinic Panorama explains how a PET/CT scan works – from preparation, to how it is used to identify disease and finally the surrounding safety concerns.

‘The amazing thing about the PET/CT scan is that you can see both physiology (metabolism) and anatomy in one go. This means we can diagnose diseases earlier because of what we see on the PET/CT even though the organ might still look anatomically normal,’ says Dr Marguerite Morkel, nuclear physician at the Cape PET/CT Centre, based at Mediclinic Panorama.

This is the only private PET/CT centre in the Western Cape and patients come from far afield. Referrals come mostly from oncologists, but other specialities can also refer patients.

Before the PET/CT scan

Prior to a PET/CT scan a patient should not eat or drink anything except plain water for at least four hours before the appointment. A patient is also required not to exercise from 48 hours before the scan. Breastfeeding or diabetic patients can undergo a scan, but they should discuss their unique situation with their doctor for correct preparation; however pregnancy is a contraindication.

How it works

A PET/CT scan is mostly painless. All that is required is an injection to administer the radioactive sugar F-18 FDG (Fluorine-18 fluoro-deoxyglucose). There are no side effects to the injection. The amount that gets injected depends on the patient’s height and weight. Dr Morkel says, ‘It is calculated according to international guidelines, which we follow to the letter.’

After the injection, the patient lies still in a quiet room for an hour to allow all the cells to take up the radioactive sugar, exactly like normal sugar is taken up. Cells that have a higher metabolic rate (like those affected by cancer or infection) will take up more of the sugar and show up as dark areas or ‘hot spots’ on the scan.

The intensity of the uptake differs from normal to abnormal cells and it is the nuclear physician’s job to know and understand when and where this uptake is not within the normal levels for a certain organ or cell.

During the scan

The patient is settled in on the adjustable bed that moves into and out of the scanner. The control room, adjacent to the camera room, has a window through which the radiographer or nuclear physician can monitor the patient and the scanner.

During the scan the patient moves slowly through the scanner so the entire body is not inside the ‘tunnel’ the whole time. ‘We usually do an “eyes to thighs” scan, as we call it,’ says Dr Morkel. ‘Unless we’re checking for skin cancer in which case we scan from the top of the head to the tips of the toes.’*

 ‘As the patient moves through the scanner the image starts forming on the screen,’ says radiographer Fatima de Coito. ‘It is then reconstructed into a three-dimensional image that is scrutinised by both the nuclear physician and the radiologist.’

The PET/CT scan takes about 30 minutes, but the duration depends on factors such as the height of the patient. A written report is then sent to the referring doctor, usually by noon the next day.

After the scan

The half-life of FDG is almost two hours. Dr Morkel says, ‘That means after two hours only 50% of the injected radioactivity is left in your body; after four hours there is 25% left and so on.’

The radioactivity is excreted via urine so drinking more fluids will speed up this process. The patient can continue with normal activities immediately after the scan but are advised to avoid prolonged close contact with small children, babies or pregnant women for a few hours afterwards as a precaution.