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Caudate nuclei

HIV encephalopathy

Progressive palsy

(Steel-Richardson syndrome)

Rare

Global with superimposed cortical and subcortical heterogenity

Global but most supranuclear pronounced in frontal areas and basal ganglia

Normal aging (benign Forgetfulness of the elderly)

Normal

Table summarizes data from multiple studies, usually small case series.

Table summarizes data from multiple studies, usually small case series.

stroke. Since reduced blood flow is the primary event, SPECT rCBF studies become abnormal immediately and are only limited by the size and location of the affected area.

The clinical usefulness of SPECT rCBF to demonstrate early interruption in blood flow to the brain is still unclear as no obvious impact in terms of prognostic information or patient management has emerged. This situation may change if it can be shown to help guide patient selection for therapies such as thrombolysis. Successful lysis of blood clots obstructing cerebral blood vessels can dramatically improve post-stroke functional status if it is achieved in the first few hours. This benefit is partially offset by a significant risk of intracranial hemorrhage. This complication might be predicted by SPECT rCBF studies: the presence of large areas of profoundly decreased perfusion reveals tissue that has been submitted to severe ischemia, and in which vascular cells may have been irreversibly damaged. Reflow into such vessels is prone to intraparenchymal bleeding with a high rate of complications related to acutely increased intracranial pressure. Inclusion of SPECT rCBF studies in stroke protocols to evaluate suitability for thrombolysis will depend on clinical confirmation that it reliably predicts which patients are at increased risk of bleeding (Fig. 6). In theory, it should be easy to detect the large, severe decrease in tracer uptake that would portend a poor outcome.

Energy Metabolism and Neurotransmission Studies

Although still largely limited to research applications, direct measurement of energy metabolism and of specific binding of radioligands to a variety of molecular targets in the brain are emerging as potential clinical applications in neuropsychiatric nuclear medicine. As more agents aimed at specific molecular targets become available, nuclear medicine will be able to characterise brain metabolism and neurotransmission with ever greater precision, thereby shedding light not only on the mechanisms underlying the diseases studied but also contributing to the development of new therapies and allowing disease diagnosis through molecular characterisation.

Energy Metabolism Studies

Glucose Consumption

Cerebral metabolism is mainly assessed with the most frequently used radiopharmaceutical in PET, 18F- fluorodeoxyglucose (FDG). This glucose analogue is transferred across the BBB and taken up by cellular glucose transporters (largely on the cell membranes of astrocytes). Once inside the cell, 18F-FDG is phosphorylated to 18F-FDG-6-PO4 but cannot be further metabolised by the enzymes involved in glycolysis. As there is also very little phosphatase activity in the brain, the 18F-FDG-6-PO4 compound is "trapped" where it was initially taken up. Mathematical modeling of the combined imaging and arterial blood measurements allows calculation of the regional cerebral metabolic rate for glucose (rCMRGlu).

As with blood flow, 18F-FDG uptake most closely parallels the distribution of glutamatergic transmission. 18F-FDG PET studies also benefit from the much greater spatial resolution and quantitative capacities of PET as compared to SPECT. On the other hand, 18F-FDG PET studies have low temporal resolution, since accumulation of the tracer in brain tissue must proceed for up to 45 minutes before acquisition can begin: the resulting data reflect the integrated activity of the brain over that period of time. This is not a problem when studying a disease or a

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