Positron Emission Tomography/Computed Tomography (PET/CT) is a diagnostic tool used mainly in oncology, neurology, cardiology, infection and inflammation, and in planning surgery and treatment.
PET/CT combines two different techniques in one scanner, giving clinicians two sets of related information about the body – functional and structural – from one examination.
If you’ve never had a PET/CT scan you might not know what to expect and this brief guide is designed to answer the questions that might be in your mind.
PET/CT combines two scanning techniques: positron emission tomography (PET) and computerised tomography (CT).
CT images provide anatomical information while the PET study gives an indication of activity and function. Overlaying the PET image onto the CT image produces a fusion of anatomy with activity.
Technically speaking, this term is applied when an unstable nuclide, in this case fluorine 18 attached to a glucose-like compound (FDG), emits a positively charged particle called a positron. The positron travels only 1 mm in tissue before reacting with an electron, a negatively charged particle, to produce two 511keV gamma rays (photons) at 180° to each other. These two photons are emitted simultaneously from within the patient. Using a sophisticated PET/CT "camera", we're able to detect these events.
The scanner surrounds the area of the patient being imaged. As the photons are emitted from the patient, they interact with a sensitive camera. The computer in the camera calculates where the positron originated.
By collecting millions of events, the PET scanner is able to create an image of the body that demonstrates where the greatest accumulation of labelled glucose is. This image is then superimposed onto the CT image.
The PET/CT scan requires an injection of a radioactive form of sugar. This injection is given via a small needle, typically into an arm vein.
The pharmaceutical is called FDG, an abbreviation for fluorodeoxyglucose which is taken up by cells like normal glucose. More active cells, such as some inflammatory and cancer cells will take up more FDG than the surrounding tissue.
These areas will produce greater numbers of photons, acting like a “beacon”, to show up abnormal sites. The injection is radioactive, but only for a short time.
The radioactive component of the PET/CT scan is called Fluorine–18. Radioactivity is often described in terms of its half-life which is defined as the time taken for radioactivity to fall to half its initial level. Fluorine has a half-life of 110 minutes. This means that the level of radioactivity remaining in your system halves every 110 minutes. Therefore it's safe to say that 8 hours after a scan there would be an insignificant amount of radioactivity left in the patient.