MRI-­based quantification of cerebral oxygen extraction and oxygen metabolism using the relationship between phase shift and magnetic susceptibility

Abstract: Introduction: The main purpose of this study was to extract global values of oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO2) in healthy volunteers, using magnetic resonance imaging (MRI) phase maps. The repeatability of the method was evaluated, and the age dependences of OEF and CMRO2 were analyzed. Material and methods: Phase data were acquired using a 3T MRI scanner with an 8- channel head coil, and a total of 20 volunteers (10 male, 10 female, age 25-84 years) participated. Magnitude and phase data were acquired from each subject, on two different occasions separated by 7-20 days, using a 3D double gradient echo pulse sequence. The difference in magnetic susceptibility between venous blood and surrounding tissue was obtained for the superior sagittal sinus (SSS) and the vein of Galen, using MRI phase data, and estimates of OEF and CMRO2 were subsequently calculated. Results: OEF estimates were 0.40 0.11 for vein of Galen and 0.31 0.08 for the SSS, and CMRO2 was 159.8 and 116.7 27.5 for the vein of Galen and the SSS, respectively. The method showed promising repeatability, with intraclass correlation coefficients (ICCs) of 0.95 and 0.82 for OEF measured in the vein of Galen and the SSS, respectively, and similar repeatability for CMRO2. The estimates showed, however, relatively large spread between volunteers, with coefficients of variation (CoVs) of 0.25 and 0.26 for OEF measured in the vein of Galen and the SSS, respectively, and similar CoVs for CMRO2. Finally, CMRO2 showed the anticipated relationship with age. Conclusion: Population mean values of OEF and CMRO2 were in good agreement with literature values, and the method delivered high repeatability, indicating stable measurements. The spread between different volunteers, however, was somewhat larger than expected. This may suggest that the method is sensitive towards measuring in different anatomical locations between volunteers.