Both PET, also called dual photon emission tomography, and SPECT are sensitive methods of measuring in vivo neurochemistry (4,5). The choice of imaging modality is ultimately determined by the specific study questions and study design. Generally, PET cameras have better resolution than SPECT cameras; however, SPECT studies may be technologically and clinically more feasible, particularly for large clinical studies and in clinical practice. PET studies may benefit from greater flexibility in the range of radiopharmaceuticals that can be tested, but SPECT studies have the advantage of longer half-life radiopharmaceuticals necessary for some studies.
The strengths and limitations of in vivo neuroreceptor imaging studies depend on the imaging technology utilized to measure brain neurochemistry and the ligand or biochemical marker used to tag a specific brain neurochemical system. The properties of the radiopharmaceutical are the most crucial issue in developing a useful imaging tool for PD. Some of the key steps in development of a potential radioligand include assessment of its brain penetration, its selectivity for the target site, its binding properties to the site, and its metabolic fate. These properties help to determine the signal-to-noise ratio of the ligand and the ease of quantitation of the imaging signal. Although ligands targeting neuronal metabolism have been used successfully to study PD patients, this review will focus on dopaminergic ligands (6). Specific markers for the dopaminergic system, including 18F-DOPA (7-12), 11C-VMAT2 (13-15), and dopamine transporter (DAT) ligands (16-22), have been widely used to evaluate patients with PD.
Dopamine ligands are useful to assess PD insofar as they reflect the ongoing dopaminergic degeneration in PD. In the study most directly correlating changes in
TABLE 1 Comparison of Dopamine Presynaptic Ligands in Parkinson's Disease Studies
Bilateral reduction in hemi-PD Correlates with UPDRS
in cross section Annual reduction change with aging (% loss from baseline) Annual progression (% loss from baseline)
DA transporter Yes Yes
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