A method for relative measurement of cerebral blood flow (CBF), oxygen extraction fraction (OEF), and metabolic rate of oxygen (CMRO2) using positron emission tomography (PET) without arterial sampling in patients with hyperacute ischemic stroke was presented.
Methods: The method requires two PET scans, one for H2(15)O injection and one for 15O2 inhalation, and calculates regional CBF, CMRO2, and OEF relative to those at the reference brain region by means of table-lookup method. In this study, we calculated "relative lookup-tables" which relate relative CBF to relative H2(15)O count, relative CMRO2 to relative 15O2 count, and relative OEF to relative 15O2/H2(15)O count. Two assumptions were applied to the lookup-table calculation: 1) In the reference region. CBF and OEF were assumed to be 50.0 ml/min/100 ml and 0.40, respectively, 2) Cerebral blood volume (CBV) was assumed to be constant at 4.0 ml/100 ml over the whole brain. Simulation studies were done to estimate the error of the present method derived from the assumptions.
Results: For relative CBF measurements, 20% variation in reference CBF gave about +/- 10% error for measured relative CBF at maximum. Changes in CBV caused relatively large errors in measured OEF and CMRO2 when relative CBF and OEF decreased. Errors for measured relative OEF caused by 50% variation in CBV were within +/- 8% at 0.8 of relative CBF and +/- 12% at 0.4 of relative CBF when relative OEF was greater than 1.0.
Conclusion: CBV effects caused larger errors in estimated OEF and CMRO2 in the region of the ischemic core with decreasing relative CBF and/or OEF but only slight errors in the region of "misery perfusion" with relative OEF values greater than 1.0. The present method makes PET measurements simpler than with the conventional method and increases understanding of the cerebral circulation and oxygen metabolism in patients with hyperacute stroke of several hours after onset.