Nuclear Medicine

I had a PET scan last week and was curious about how it worked. Members of the team of specialists explained how this powerful tool elegantly brings together a diverse combination of scientific knowledge from physics, chemistry, computing and mathematics, all working in close harmony. (Note: the acronym PET stands for Positron Emission Tomography.)

There is a greater take-up of glucose in tumors, areas of infected tissue and active areas of the brain. In order to locate these areas and construct an image of them, the basic idea is to inject radioactively-tagged glucose into a vein and watch where it goes. It works better if sugar levels aren’t too high at the outset, and so patients are restricted to “water only” for at least 6 hours before the scan.

How to make glucose radioactive. This is where the chemistry comes in. An oxygen atom is attached to the glucose molecule in such a way that it still functions like glucose. Then a suitable radioactive atom – in this case, an atom of the unstable isotope fluorine-18 -- can hook up to the modified glucose molecule and tag along for the ride.

How to see where the glucose goes. There are two steps in the process. The first occurs when one of the unstable fluorine atoms attached to a glucose molecule decays and releases a positron (the antimatter version of an electron). When this positron encounters a nearby electron, typically within one millimeter, the two annihilate each other, releasing their energy in the form two gamma-ray photons that fly off in diametrically opposite directions – the physics comes in here when the combined mass m of the positron and the electron is converted into energy via the formula e = mc². This pair of energetic gamma-ray photons shoot straight through the body, mostly undeterred by bone or flesh, and are detected by the scanner that surrounds the patient lying in the scanner tunnel. The two photons arrive together at opposite ends of a straight line passing through the point where the positron-electron annihilation took place.

The second step is to locate the gamma-ray pairs and use the data to create a detailed 3-D image of the areas of high metabolic (glucose) activity. The detector is a layer of so-called scintillation crystals surrounding the body in the tunnel; the crystals light up when a gamma ray arrives, and a camera records the positions of just those pairs of flashes that occur simultaneously at opposite ends of their shared straight-line path. Finally, a geometric transformation from applied mathematics is implemented in a computer program to convert the scintillation data into a high-resolution three-dimensional picture of the areas of high metabolic activity.

How to ensure the patients are not radioactive for the rest of their lives. Any radioactive isotope has a measure of persistence called its half-life. That is the time it takes for half of its atoms to decay into a more stable atomic form. Fluorine-18, which has 9 protons and 9 neutrons in its nucleus, decays into the stable isotope oxygen-18 which has 8 protons and 10 neutrons. The by-product of the decay is usually a positron and a neutrino. The half-life of fluorine-18 is just under 110 minutes. So every two hours the amount of fluorine-18 in the body drops by more than a half and this means that after 24 hours the level drops to less that one eightieth of one per cent of its initial value.