Neuroreceptor Imaging in Psychiatry: Theory and Applications

Brain single-photon emission CT physics principles

The basic principles of scintigraphy are reviewed and extended to 3D imaging. Single-photon emission computed tomography (SPECT) is a sensitive and specific 3D technique to monitor in vivo functional processes in both clinical and preclinical studies. SPECT/CT systems are becoming increasingly common and can provide accurately registered anatomic information as well. In general, SPECT is affected by low photon-collection efficiency, but in brain imaging, not all of the large FOV of clinical gamma cameras is needed: The use of fan- and cone-beam collimation trades off the unused FOV for increased sensitivity and resolution. The design of dedicated cameras aims at increased angular coverage and resolution by minimizing the distance from the patient. The corrections needed for quantitative imaging are challenging but can take advantage of the relative spatial uniformity of attenuation and scatter. Preclinical systems can provide submillimeter resolution in small animal brain imaging with workable sensitivity.

Using positron emission tomography to facilitate CNS drug development

Positron emission tomography (PET) is a non-invasive technology of nuclear medicine that has sensitivity for tracing low picomolar concentrations of radiolabeled molecules in the human body. Radiolabeling a new drug to high specific radioactivity facilitates a detailed mapping of its distribution to crucial organs in humans after the administration of a "microdose" (< 1 microg), for which limited toxicology documentation is required. For drugs directed at the CNS, this method is particularly useful for confirming exposure to the brain. A different approach is to develop suitable radioligands for quantitative PET studies of drug binding to target proteins and subsequently to correlate receptor occupancy with pharmacodynamic responses. To follow disease progression and to monitor the outcome of new treatments, PEt also facilitates longitudinal studies of biomarkers of pathophysiology such as amyloid plaque load in Alzheimer's disease. Finally, combining genomic knowledge with PET neuroreceptor imaging is expected to facilitate the search for genetic predictors of drug response.