6.3 How a gamma camera works

Positron emission tomography (PET) is different from other gamma camera techniques. In PET the radionuclides used produce positrons. When an emitted positron collide with an electron in, say tissue, two gamma rays are produced which travel in exactly opposite directions. These gamma rays have a much higher energy than those normally used in gamma imaging, but if they can be detected, tomographic reconstruction methods can used to produce an image.

Dedicated PET scanners have been around for several years, but most hospitals don’t have access to one. Recently dual headed gamma cameras with PET capability have been produced, and one of these is in use here.

In order to be able to detect the 511keV gamma rays produced in PET, the camera heads on this scanner contain thicker sodium iodide crystals than are normally used for gamma imaging.

Since the two gamma rays produced are emitted in opposite directions, the camera heads must be moved so that they are always opposite one another.

Another difference is that since we are looking for co-incident opposite events, the collimators are not used; this has the added advantage that sensitivity is increased.

This patient is having a brain scan. The radiophamaceutical used for this is a glucose analogue containing fluorine-18 which has a half life of about two hours. This substance is particularly useful in demonstrating metabolism.

Scanners like this promise to make PET scanning, which has a wide range of potential uses, far more widely available.