Perfusion imaging

A knowledge of the brain-blood partition coefficient (lambda) for water is usually required for the measurement of CBF with [15O]water. The currently accepted value for whole-brain lambda, 0.95-0.96 ml/g, calculated from brain and blood water content data, is incorrect because in the calculation, the blood water content was not adjusted for the density of blood. The correct value is 0.90 ml/g. Variations in brain or blood water content affect lambda. Thus, lambda changes during development of the brain and varies regionally in it, even among different gray matter structures, owing to variation in brain water content. In addition, lambda would be expected to vary with the hematocrit, owing to changes in blood water content. The impact of using an incorrect value for lambda will depend on the sensitivity of the CBF measurement technique used to errors in lambda.

The oxygen-15 steady-state inhalation technique provides quantitative values of regional cerebral blood flow, fractional oxygen extraction, and oxygen utilisation. These last two, however, have been found to be higher than those reported using arteriovenous difference techniques. On theoretical grounds, this overestimation is due to the signal from nonextracted intravascular oxygen-15. In this paper, a method is presented to correct for this intravascular component utilising a measurement of regional cerebral blood volume. The statistical penalty imposed as a result of this correction is defined.