When used to measure blood flow in the brain, water leaves a residue in the vascular bed that influences the estimation of blood flow by current methods. To assess the magnitude of this influence, we developed a two-compartment model of blood flow with separate parameters for transport and vascular distribution of brain water. Maps of the water clearance, K1 into brain tissue, separated from the circulation by a measurably resistant blood-brain barrier (BBB), were generated by time-weighted integration. Depending on the validity of the assumptions underlying the two-compartment model presented here, the maps revealed a significant overestimation of the clearance of water when the vascular residue was ignored. Maps of Vo, the estimate of the apparent vascular distribution volume of tracer H2(15)O, clearly revealed major cerebral arteries. Thus, we claim that the accumulation of radioactive water in brain tissue also reflects the volume of the arterial vascular bed of the brain.

The coupling of oxidative metabolism to the blood flow of the sensory motor hand area is uncertain. The authors tested the hypothesis that continued vibrotactile stimulation ultimately must lead to increased oxygen consumption consumption. Twenty-two healthy right-handed young volunteers underwent positron emission tomography (PET) with the [(15)O]water bolus injection method to measure water clearance (K1H2O an index of blood flow (CBF), and with the [(15)O]oxygen bolus inhalation method to measure CMR(O2). The CMR(O2) was measured 30 seconds and 20 minutes after onset of intermittent (1 second on, 1 second off) vibrotactile stimulation (110 Hz) and compared with baseline measurements without stimulation. The K1H2O and CMR(O2) changes (delta K1H2O and delta CMR(O2)) were determined using intersubject averaging, together with magnetic resonance imaging based stereotaxic registration technique. The K1H2O increase was 21 +/- 4% and 12 +/- 4% at 30 seconds and 20 minutes after onset of stimulation, respectively. No significant increase of CMR(O2) was found until 30 minutes after the onset of stimulation. The authors conclude that blood flow and oxidative metabolism undergo uncoupling during sustained phasic stimulation of the sensory hand area. Therefore, neuronal activity stimulated in this manner does not rely on significantly increased oxidative phosphorylation.

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Although a number of studies have demonstrated the neuroprotective effects of antagonists of postsynaptic N-methyl-D-aspartate (NMDA) and non-NMDA receptors in cerebral ischemia, little is known about the treatment of cerebral infarction through presynaptic blocking of extracellular glutamate release. We evaluated the effects of a presynaptic selective N-type calcium channel antagonist (SNX-111, given intravenously by continuous infusion at 5 mg/kg/h from 20 min prior to occlusion until 2 h postocclusion) on blood flow, extracellular glutamate, and infarct volume in rats with permanent occlusions of the right middle cerebral and right common carotid arteries plus 1-h transient occlusion of the left common carotid artery. There was no significant difference in CBF in the occluded cortex during the experiment between the treated and vehicle groups. SNX-111 significantly reduced total amount of extracellular glutamate during the experiment and the peak value of the glutamate after occlusion from 44.2 +/- 15.8 microM (mean +/- SD) to 21.4 +/- 11.4 microM (p < 0.01). Infusion of SNX-111 also significantly reduced the cortical volume of infarction from 47.2 +/- 5.8 to 19.9 +/- 7.3% (p < 0.0001). These results suggest that SNX-111 has a protective effect against focal ischemia through the inhibition of glutamate release from presynaptic sites, although SNX-111 may also affect the release of other neurotransmitters.

When used to measure blood flow, water leaves a residue in the vascular bed, which may contribute to the calculation of increased blood flow during functional activation of brain tissue. To assess the magnitude of this contribution with the two-compartment positron emission tomography (PET) method, we mapped the water clearance (K1) of the brain as an index of cerebral blood flow (CBF) and the apparent vascular distribution of nonextracted H2 15O (Vo). The latter map represented mainly the cerebral arterial and arteriolar volume. We also prepared subtraction maps (delta K1, delta Vo) of the response to vibrotactile stimulation of the fingertips of the right hand of six normal volunteers. Using magnetic resonance (MR) images of all subjects, the data were rendered into Talairach's stereotaxic coordinates and the averaged subtraction images (activation minus baseline) merged with the corresponding averaged MRI image. The delta K1 map revealed the expected response in the primary sensory hand area; the delta Vo response was located about 13 mm more anteriorly, close to the central fissure, most likely reflecting changes of the arteries feeding the primary sensory hand area. We conclude that cerebral perfusion and cerebrovascular responses to vibrotactile stimulation may occur in disparate locations that can be identified separately by using the two-compartment method.

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URL: http://www.clinicaltrials.gov. Unique identifier: NCT01994720.