PET/CT scan facts

PET/CT – a brief technical explanation

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.

Positron emission

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. 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 must have originated from.

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 injection and how it works

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. FDG is taken up by cells like normal glucose. More active cells, such as some inflammatory and cancer cells will take up more 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 injection contains a radioactive component

The radioactive component of the PET/CT scan is called Fluorine–18. Radioactivity is often described in terms of its half-life – 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. It's safe to say that 8 hours after the scan there would be an insignificant amount of radioactivity left in the patient.

It’s necessary to rest between having the injection and being scanned

This resting phase is vital to obtaining high quality PET/CT images. Pathology is demonstrated because areas of disease use sugar more rapidly than the surrounding tissue. This means that the FDG injection will travel to areas of disease and demonstrate them on the scan. Working muscles also use this form of sugar as an energy supply. The FDG injection will therefore travel to these muscles if they're being used.

By completely resting for a period of one hour after the injection, there's more chance of the FDG going to areas of disease, rather than normal tissues.