A tracer is a radiopharmaceutical that is used to investigate a
particular physiologic process. A radiopharmaceutical consists of a
radioactive label (the radionuclide) and a ligand, a molecule whose
chemical and physical properties define the uptake or biodistribution
of the tracer once labeled with the radionuclide. Upon administration
of the tracer, investigation of a physiologic process is made possible
because its presence in a particular stage or location in the
physiologic process is recorded by detection of the traces or signs of
its spatial location in the form of gamma rays emitted from the
decaying radionuclide. The behavior of the tracer in the brain determines
what type of information the resultant image will yield. In positron
emission tomography (PET) using F-labeled 2-deoxyglucose
(FDG) for example, since glucose is the primary metabolic substrate of
neurons, the regional concentration of
FDG in the resultant
images reflects the local cerebral metabolic rates for glucose
(LCMRglc). Similarly, CBF tracers in SPECT which have ligands that
allow the patterning of blood perfusion in the brain will produce
images which attempt to reflect cerebral perfusion. The main
imaging radionuclides used in brain SPECT
imaging of CBF are
Tc,
I,
Xe, and
Tl [SP87][PSS92][CE91].