A Noninvasive Method for Quantifying Cerebral Blood Flow by Hybrid PET/MRI

Abstract

Although PET with ¹⁵O-water is the gold standard for imaging cerebral blood flow (CBF), quantification requires measuring the arterial input function (AIF), which is an invasive and noisy procedure. To circumvent this problem, we propose a noninvasive PET/MRI approach that eliminates the need to measure AIF by using global CBF determined by phase-contrast (PC) MRI as a reference region. This approach not only is noninvasive but also involves no additional imaging time, because PC MRI and ¹⁵O-water PET are acquired simultaneously. The purpose of this study was to test the accuracy of this hybrid method in an animal model in which AIF was measured directly and CBF was varied by changing the arterial CO₂ tension. <b>Methods:</b> PET and MRI data were simultaneously acquired in juvenile pigs at hypocapnia (<i>n</i> = 5), normocapnia (<i>n</i> = 5), and hypercapnia (<i>n</i> = 4). CBF was measured by the MRI reference method and by PET alone using an MRI-compatible blood sampling system to measure AIF. <b>Results:</b> Global CBF estimates from PC MRI and ¹⁵O-water PET agreed well, with a correlation coefficient of 0.9 and a slope of 0.88. Strong positive correlations (<i>R</i>² &gt; 0.96) were also found between regional CBF generated by the PET-only and the MRI-reference methods. <b>Conclusion:</b> These findings demonstrate the accuracy of this hybrid PET/MRI approach, which might prove useful in patients for whom obtaining accurate CBF measurements is challenging.