What is the correct value for the brain: blood partition coefficient for water

A knowledge of the brain: blood partition coefficient (λ) for water is usually required for the measurement of cerebral blood flow (CBF) with positron emission tomography (PET) and 0-15 labelled water. The correct calculation of this important parameter from the ratio of brain and blood water contents is reviewed, and the effect of physiological variations in these water contents on λ is demonstrated. The currently accepted value for whole brain λ is 0.95-0.96 ml/g, calculated from brain and blood water contents of 77g/100g and 80.5g/100g, respectively. However, this value for λ is incorrect, because in the calculation the blood water content value was not adjusted for the density of blood. The correct value is 0.91 ml/g. Variations in brain or blood water content affect λ. Over an hematocrit range of 25% to 55%, λ varies from 0.86 to 0.93 ml/g, due to a decrease in blood water content. λ changes with age, and varies regionally in the brain, as brain water content is inversely related to lipid and myelin content. The λ of the human newborn brain, 1.10 ml/g, is considerably higher than in the adult. Differences in λ between gray and white matter are well known. However, because of variations in water content, the λ's of thalamus (0.88 ml/g) and caudate nucleus (0.96 ml/g) are less than that of cerebral cortex (0.99 ml/g), while the λ of corpus callosum (0.89 ml/g) is greater than that of centrum semiovale (0.83 ml/g). These regional variations in λ will assume more importance as PET resolution improves. The impact of using an incorrect λ will depend upon the sensitivity of the particular CBF measurement technique to errors in λ