Noninvasive and Three-Dimensional Imaging of CMRO2 in Rats at 9.4 T: Reproducibility Test and Normothermia/Hypothermia Comparison Study

Abstract

Ability to image cerebral metabolic rate of oxygen (CMRO2) is essential for studying the fundamental role of oxidative metabolism in brain function and disease. We have demonstrated recently that three-dimensional (3D) CMRO2 images can be obtained in the rat brain during a 2-min 17O2 inhalation using the 17O MR spectroscopic imaging (MRSI) approach at high field. The feasibility for establishing a completely noninvasive approach for imaging CMRO2 has also been demonstrated. In this study, we further explored the feasibility of 17O MRSI approach for performing repeated CMRO2 measurements within a short period of time and evaluated the reproducibility of the repeated measurements. Subsequently, we applied the 17O MRSI approach to measure CMRO2 and cerebral blood flow (CBF) values at two brain temperatures in the α-chloralose anesthetized rat brain at 9.4 T. Finally, we tested the validity of simplified model for noninvasively determining CMRO2 in normothermic and hypothermic rat brain. The results show (i) an excellent reproducibility among repeated measurements of 3D CMRO2 images under the same physiologic condition; (ii) a 44% decrease of CMRO2 across the rat brain at mild hypothermic (32°C) condition as compared with normothermic (37°C) condition; and (iii) a close correlation between CMRO2 and CBF within a relatively wide physiologic range. This study demonstrates the capability of 17O MRSI approach for noninvasively imaging CMRO2 and its changes caused by physiologic perturbation. This approach, thus, should provide a promising neuroimaging modality for studying oxidative metabolism and bioenergetics associated with brain functions and diseases.