Evolution of brain imaging instrumentation.

Computed tomography (CT) and static magnetic resonance imaging (MRI) are now the most common imaging modalities used for anatomic evaluation of pathologic processes affecting the brain. By contrast, radionuclide-based methods, including planar imaging, single-photon emission computed tomography (SPECT), and positron emission tomography (PET), are the most widely used methods for evaluating brain function. SPECT and PET have been evolving for a longer time than CT and MRI and have made significant contributions to understanding brain function. The pioneering work on cerebral flow early in the last century laid the foundation of measurement with radioactive gases. This was initially performed with scintillation counters, which gave way to single, then multiple scintillation and multiprobe detectors. The invention of rectilinear scanners, MARK series, Anger cameras, and SPECT imaging further advanced nuclear medicine's role in brain imaging. Measurement of regional cerebral blood flow by SPECT provides pathophysiologic information that directs patient management in a variety of central nervous disorders (CNS), with the greatest clinical impact found in cerebrovascular disease and seizure disorder. In the former, SPECT not only provides means of early detection and localization of acute strokes but can also direct thrombolysis and determine prognosis in the postcerebrovascular accident period. With respect to the latter, ictal SPECT can localize seizure foci so that patients with refractory disease can potentially undergo surgical resection of the affected area. In contrast to brain SPECT, brain PET images reflect regional cerebral metabolism. Because of neurovascular coupling, findings on SPECT and PET images are often comparable. PET, however, still has improved spatial resolution and is therefore more sensitive than SPECT, particularly in the evaluation of dementias. Brain PET instrumentation has greatly evolved from its infancy, when it was used in regional localization, to currently providing excellent resolution with imaging characteristics that can greatly impact clinical management. In addition, although ictal SPECT remains more sensitive than interictal PET for detection of seizure foci, the stringent conditions required for SPECT can be difficult to achieve, making interictal PET a very reasonable alternative. The clinical utility of PET and SPECT in neuropsychiatric and addictive disorders has not yet been defined, though a plethora of data exits. This arena of CNS disease has been the impetus for development of neurotransmitter-receptor-specific radioligands, which have already led to better understanding of dopaminergic, GABAergic, and serotonergic pathways. Another functional brain imaging technique that has gained broad acceptance since its invention in the early 1990 s, is functional MRI, which indirectly measures CNS neuronal activity by evaluating oxygenation levels of cerebral vessels. Despite other recent related developments, such as MR spectroscopy, arterial spin labeling, and diffusion tensor imaging, nuclear medicine-based techniques remain clinically relevant and robust modalities, especially with the ever-expanding armamentarium of radiotracers and radioligands in conjunction with industry-driven improvements in image-analysis hardware and software.