Comments:
This set of illustrations were created for Scientific American for inclusion in an
article about current medical imaging techniques. The artwork was completed in 7
days.
Description:
Positron emission tomography (PET) is a nuclear medicine imaging technique which
produces a three-dimensional image or map of functional processes in the body. The
system detects pairs of gamma rays emitted indirectly by a positron-emitting radioisotope,
which is introduced into the body on a metabolically active molecule. Images of metabolic
activity in space are then reconstructed by computer analysis, often in modern scanners
aided by results from a CT X-ray scan performed on the patient at the same time,
in the same machine.
Operation:
To conduct the scan, a short-lived radioactive tracer isotope, which decays by emitting
a positron, which also has been chemically incorporated into a metabolically active
molecule, is injected into the living subject (usually into blood circulation). There
is a waiting period while the metabolically active molecule becomes concentrated
in tissues of interest; then the research subject or patient is placed in the imaging
scanner. The molecule most commonly used for this purpose is fluorodeoxyglucose (FDG),
a sugar, for which the waiting period is typically an hour.
As the radioisotope undergoes positron emission decay (also known as positive beta
decay), it emits a positron, the antimatter counterpart of an electron. After travelling
up to a few millimetres the positron encounters and annihilates with an electron,
producing a pair of annihilation (gamma) photons moving in opposite directions. These
are detected when they reach a scintillator material in the scanning device, creating
a burst of light which is detected by photomultiplier tubes or silicon avalanche
photodiodes (Si APD). The technique depends on simultaneous or coincident detection
of the pair of photons; photons which do not arrive in pairs (i.e., within a few
nanoseconds) are ignored.
