Local Brain Haemodynamics and Oxygen Metabolism in Cerebrovascular Disease

Abstract

Using position emission tomography (PET) and equilibrium imaging with oxygen-15 labelled gases, the cerebral blood flow (CBF), blood volume (CBV), oxygen extraction fraction (OEF) and oxygen consumption rate (CMRO2) were measured in multiple regions of interest over the cerebral cortex of 5 control subjects, 4 patients with strictly unilateral longstanding carotid artery occlusion, 1 patient with middle cerebral artery embolic stroke in the acute stage, and 4 patients with subcortical stroke and no cervical arterial disease. In each control subject, the regional CBV was linearly and positively correlated with both CBF and CMRO2, while the local mean transit time (t = CBV/CBF) was uniformly distributed, reflecting the local adaptation of both the vascular tone and the capillary density to the metabolic demand at constant cerebral perfusion pressure that characterizes the normal brain. In patients with subcortical stroke, cortical blood volume was reduced in proportion to the matched reduction in CBF and CMRO2, suggesting that the metabolic depression resulting from cortical deafferentation increases the resting tone of pial vessels. Unilateral carotid occlusion induced larger CBV and t, and steeper slopes of the CBV-CBF relationship, particularly on the occluded but also on the patent side. The assessment of the local cerebral perfusion pressure (CPP) as judged by the ratio CBF/CBV in 3 patients with focally raised OEF and preserved or reduced CMRO2, allowed the demonstration in multiple cerebral regions of single patients of two well-known physiological phenomena: the autoregulation of CBF, followed by the rise of the OEF as local CPP falls further. In addition, the depression of CMRO2 in the ischaemic cortex was associated with a trend for CBV to return towards normal values, compared with the maximally elevated CBV found in oligaemic but metabolically normal areas. This suggests that a process of metabolic vasoconstriction may participate, among other factors, in the vascular collapse that occurs, and would serve to regenerate some haemodynamic reserve, at very low CPP levels.

Preserved Acetazolamide Reactivity in Lacunar Patients with Severe White-Matter Lesions: 15O-Labeled Gas and H2O Positron Emission Tomography Studies

“…Postmortem neuropathologic studies have shown decreased neuronal connectivity in the white matter in progressive subcortical vascular encephalopathy of Binswanger type (Yamanouchi et al, 1989(Yamanouchi et al, , 1990. Functional reduction in cortical neuronal activity due to disruption of connections between the cortex and subcortex, as indicated previously (Pozzilli et al, 1987;Sette et al, 1989), is likely to be associated with a reduction of CMRO 2 in the centrum semiovale Acetazolamide reactivity in lacunar patients T Nezu et al in the patients with severe WMLs. Furthermore, the cerebral vessels would not dilate during fluctuations in systemic arterial pressure in daily life in these conditions of disruption of connections.…”

Sickle Cell Disease: Reference Values and Interhemispheric Differences of Nonimaging Transcranial Doppler Blood Flow Parameters

“…1,9,[31][32][33] The statistical average group difference cannot be applied to an individual, in whom the exact configuration of the circle of Willis, 34 including the presence, caliber, and course of each artery, 35,36 and the degree of interhemispheric anatomic, [37][38][39] physiologic, and pathophysiologic differences are, a priori, unknown. 31,35,43,44 Large variability in side-to-side impedance indices and modest correlation coefficients for vessels that supposedly have no stenosis and hemispheres that are "seeing" the same circulating oxygen content and Hgb indicate that there is not such a tight agreement between sides in blood flow redistributions in response to a chronic oxygen deficit. [6][7][8]43 Also a minor arterial stenosis, which could have remained undetected on MRA, potentially contributed to the variability in our study because the analysis of the Hagen-Poiseuille equation indicates that even small changes in the artery radius may result in extremely drastic changes in flow velocity.…”

Cerebral Vascular Mean Transit Time in Healthy Humans: A Comparative Study with PET and Dynamic Susceptibility Contrast-Enhanced MRI

“…The usefulness of quantitative MTT (absolute value) in DSC-MRI has been reported in patients with acute ischemic stroke as a threshold for hypoperfusion (Thijs et al, 2001) and in patients with chronic occlusive cerebrovascular disease as an indicator of perfusion reserve impairment (Kikuchi et al, 2002). Although normal MTT values have been reported by DSC-MRI (e.g., Østergaard et al, 1996a;Schreiber et al, 1998;Vonken et al, 1999;Helenius et al, 2003), there is a large discrepancy among reports, and MTT values measured by PET differ (e.g., Ito et al, 2003;Sette et al, 1989).…”