Quick guide to positron emission tomography (PET)
What is a PET scan and how does it work? Nancy Wilkinson finds out
Positron emission tomography (PET) is a form of nuclear imaging (nuclear because it uses radioactive chemicals) that produces a 3D image of the inside of the body.
PET can be used to detect brain abnormalities such as tumours, Alzheimer’s disease and epilepsy, as well as to investigate conditions such as heart disease or atherosclerosis. One of the main uses of PET is to diagnose cancer and see how far it has spread or how well it is responding to treatment.
To have a PET scan, the patient either swallows or is injected with a radiotracer, a common chemical that has been altered by replacing one part of it with a radioactive atom. One common radiotracer is FDG (fludeoxyglucose), which is glucose with one oxygen atom replaced by a radioactive isotope of fluorine, fluorine-18.
FDG is processed in the body just as glucose would be: it gets broken down within cells for energy. Cancer cells use glucose much faster than normal cells, meaning the FDG is taken up by the cancer cells faster and, therefore, collects in the cancerous areas.
Throughout the 1950s and the 1960s, different kinds of tomography were being developed to study human diseases. In 1973 the first PET scanner was built, but without a radiotracer the technology couldn’t be used to look at disease in humans. In 1976, when FDG had been developed, the first humans scans took place.
FDG works so well because fluorine-18 is a radioactive isotope that decays (breaks down) at a suitable rate. If it decayed too quickly we wouldn’t be able to measure the energy, but if it took too long it would stay in our bodies for longer than we wanted. FDG decays to form positrons. Positrons are ‘antielectrons’ – particles similar to electrons but with the opposite charge. Once formed, they travel through the tissue until they collide with an electron and give out energy in the form of gamma rays.
From energy to image
To collect and measure the gamma ray energy, you have to lie down inside a machine that surrounds your body. Within it are multiple rings of detectors that record how many gamma rays are being emitted and from where.
Once a reading has been taken, the detectors convert the energy into an image of the inside of the brain (or other part of the body). If there were a tumour in the brain, the scan would detect the excess of FDG there, and so more gamma rays would be emitted from the tumour than from the healthy tissue.
The scan would show the location and, to some extent, the size of the cancer. But PET is less commonly used than magnetic resonance imaging (MRI) and computed tomography (CT), as there are only a handful of PET scanners in the UK.
On average, 40,000 PET scans take place each year, compared to more than 3.5 million CT scans and almost 2 million MRI scans. This is due to both the lack of availability of scanners and the difficulty of transporting the radiotracer long distances (in case it decays before it reaches the patient).
PET has many advantages over other types of imaging such as MRI or CT, including the ability to reveal the functional changes occurring in an organ or tissue on a cellular level. This is important because the early stages of many diseases involve functional changes in cells rather than structural changes. MRI and CT scans measure structural changes, which means that PET scans are able to diagnose certain diseases earlier than MRI or CT scans.
They can also detect smaller areas of disease that don’t necessarily show up with other imaging techniques. In particular, they can be useful for finding out whether an unknown mass left over after treatment is scar tissue or an active tumour.
What’s the problem?
A downside of measuring the metabolic changes in cells is that other differences in the body can give false results. For example, a patient with diabetes may have a different rate of processing glucose than a normal patient and this could skew the results slightly. In addition, PET scans are not so good at providing information about the size, shape and structure of tumours.
PET scanners are extremely expensive, which is why they are in limited supply. Even more expensive are the cyclotrons that produce the radioactive atoms used in the radiotracer. There are only a few in the UK, but more on-site cyclotrons are being built, hopefully reducing the cost and waiting time of getting a PET scan.Lead image:
Wellcome Photo Library, Wellcome Images
- NHS Choices: PET scans
- Medical News Today: What is a PET scan? How does a PET scan work?
- RadiologyInfo: PET/CT
- Wikipedia: Radioactive tracers
Questions for discussion
- Can you list some differences between PET, MRI and CT?