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.
