Regional cerebral blood flow (CBF), oxygen extraction ratio (OER), oxygen utilization (CMRO2) and blood volume (CBV) were measured in a group of 34 healthy volunteers (age range 22-82 yrs) using the 15O steady-state inhalation method and positron emission tomography. Between subjects CBF correlated positively with CMRO2, although the interindividual variability of the measured values was large. OER was not dependent on CMRO2, but highly negatively correlated with CBF. CBV correlated positively with CBF. When considering the values of all the regions of interest within a single subject, a strict coupling between CMRO2 and CBF, and between CBF and CBV was found, while OER was constant and independent of CBF and CMRO2. In 'pure' grey and white matter regions CMRO2, CBF and CBV decreased with age approximately 0.50% per year. In other regions the decline was less evident, most likely due to partial volume effects. OER did not change or showed a slight increase with age (maximum in the grey matter region 0.35%/yr). The results suggest diminished neuronal firing or decreased dendritic synaptic density with age.

SUMMARY Regional cerebral blood flow, blood volume, fractional oxygen extraction and oxygen consumption were measured by positron emission tomography in six patients with sickle cell disease to see how oxygen delivery to the brain is maintain**! in the presence of both anemia and a low oxygen affinity hemoglobin. Both regional cerebral Mood flow and blood volume were found to be markedly increased compared to values obtained from 14 normal subjects in the same age range. The mean fractional oxygen extraction was not significantly different in the two groups. Mean oxygen consumption in the two groups was also not significantly different but low values in individual patients with sickle cell disease and the presence of atrophy on the CT-scans of three of them were suggestive of some neuronal loss in patients without any history of nervous system involvement. In view of the known high values of cerebral blood flow and metabolism in childhood, it b suggested that when compounded by anemia and abnormal red cells, a hyperdrculatory state may make patients In this age-group particularly prone to ischemlc infarction.

Cerebral oxygen and glucose metabolism have been studied in 8 patients with biochemically defined mitochondrial myopathies and 7 normal control subjects using positron emission tomography. Four patients had myopathy alone and 4 had predominantly central nervous system (CNS) disease. Cerebral oxygen and glucose metabolism were measured concurrently in the two groups of patients with 15O, 11C and 18F labelled tracers and positron emission tomography (PET). Patients with major CNS disease showed an uncoupling of glucose and oxygen metabolism when compared with patients without cerebral disease and normal subjects. The mean ratio between oxygen and glucose utilization was 3.8 moles of oxygen per mole of glucose in patients with CNS disease, compared with 5.6 for controls and 6.4 for patients with myopathy alone. The altered stoichiometry in CNS cases indicates aerobic glycolysis to lactate and/or other intermediate metabolites. Patients with major CNS disease showed a 50% reduction in cerebral oxygen utilization compared with cerebrally unaffected patients and normal subjects. These findings indicate that in patients with a mitochondrial encephalopathy the defect identified in skeletal muscle is also expressed in the brain.

SUMMARY Regional cerebral blood flow (CBF), oxygen utilisation, fractional oxygen extraction (OER) and cerebral blood volume (CBV) were measured by positron emission tomography (PET) in 21 patients with occlusive carotid artery disease. In the same patients, measurements of cerebral CO2 reactivity were performed using the intravenous xenon-1 33 technique. A significant correlation was found in symptomatic hemispheres between the CBF/CBV ratio and CO2 reactivity. Four patients had significant increases in OER and this was associated with a reduction in CBF/CBV ratio implying exhaustion of haemodynamic reserve. CO2 reactivity was reduced below 1-5% mm Hg in all four cases with raised OER but only in two cases with normal OER. In patients with C02 reactivities above 1 5% mm Hg, OER was normal in all cases. It is concluded that measurements of CO2 reactivity provide a satisfactory method for assessing cerebral haemodynamic reserve.The international EC-IC bypass study recently showed that EC-IC bypass does not reduce the subsequent risk of stroke in patients with carotid artery occlusion.' The results of this well designed and conducted trial appear to provide convincing evidence against the further use of this procedure. Patients were selected for the study on the basis of clinical criteria and a description of the anatomy of the carotid arteries obtained by angiography.2 However, some commentators have pointed out that there remains a possibility that the operation might be successful in preventing stroke if a subgroup of patients with impaired cerebral haemodynamics could be identified.3'4 A quantitative assessment of the haemodynamic compensatory responses to a fall in perfusion pressure can be obtained by the combined measurement of regional cerebral blood flow (CBF), cerebral blood volume (CBV) and fractional oxygen extraction