Pet And Spect

Both SPECT and PET use radioactive labeled ligands and radio detection devices to quantitatively analyze brain metabolism or neurotransmitter receptors. The major difference between the two is in terms of spatial resolution and the ability for absolute quantification. In PET the radiolabel is incorporated in naturally occurring elements (carbon, oxygen, nitrogen), giving it versatility. PET cameras provide higher spatial resolution (~3 mm), and with the use of arterial sampling it is possible to achieve absolute quantification of metabolic/pharmacologic parameters. However, this comes at a cost. Positron emitters have relatively brief half-lives, necessitating the use of an on-site cyclotron to produce short-lived isotopes (~2 min for [15O] and ~20 min for [nC]), or sophisticated logistics in the case of longer acting ones (~109 min for [18F]). Additionally, a PET center requires the capacity for incorporating these isotopes into the desired tracer compound quickly, and immediately applying the tracer to a test subject.

SPECT, on the other hand, uses much longer lived isotopes such as [123I] (t1/2 ~ 13 hr), and for some purposes also [99mTc], which although short-lived itself, may easily be obtained from a longer lived precursor. The latter can be produced off-site and shipped to the location of use, thereby posing much lesser technical demands than PET. However, SPECT isotopes are not biological constituents of most molecules desirable as tracers, thus restricting the availability of useful tracer substances. Also, compared to PET, SPECT applications allow less accurate, sometimes called "semiquantitative," estimation of labeled structures and are restricted by the relatively low resolution of the procedure. Thus, in practice, PET has largely been used as a research tool, whereas SPECT has established itself as a clinical diagnostic modality.

In principal, emission-based neuroimaging studies can be broadly divided into two categories. The first and more commonly used category is constituted by studies attempting to visualize blood flow, glucose, or amino acid metabolic processes thought to represent the "activation" of specific brain regions. The tracers commonly used for this purpose, such as [99mTc]HMPAO for SPECT or [15O]H2O and [18F]FDG for PET, represent cerebral blood flow ([99mTc]HMPAO, [15O]H2O), an indirect measure of cerebral metabolism, or, more directly, neuronal glucose uptake ([18F]FDG). The second category, which may be termed "neurotransmitter imaging," employs receptor-specific ligands in order to investigate the concentration of available receptors (the "binding potential"), endogenous ligand concentration, or the pharmacodynamics and pharmacokinetics of psychotropic drugs.

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