Quantitative imaging of cerebral oxygen metabolism using MRI

Thesis

Magnetic resonance imaging (MRI) is a non-invasive medical imaging technique that is sensitive to the level of oxygen in the blood. Calibrated MRI is capable of producing maps of absolute oxygen metabolism by using gas challenges to independently manipulate blood flow and blood oxygen content. In this thesis, several aspects of the signal model are investigated. It is confirmed that the commonly used Davis model is reassuringly insensitive to field strength, provided sufficient signal-to-noise (SNR) can be obtained. The effect in varying the experimental parameter of echo time is explored, and the results are shown to match the theory more closely when intravascular signal contribution is reduced by crusher gradients.

A direct comparison was performed between resting oxidative metabolism as measured by calibrated MRI and the gold standard method of positron emission tomography (PET). Good correlation was observed for resting blood flow, but no correlation was found for oxygen extraction fraction (OEF) or absolute cerebral metabolic rate of oxygen consumption (CMRO₂). A follow-up study was performed to further investigate some methodological aspects of the calibrated MRI procedure, including the application of background tissue suppression, different gas delivery methods, the effects of using measured respiratory timecourses as part of image analysis, and the impact of physiological noise correction.

The limiting factor in the quality of data obtained for calibrated MRI is the SNR of the arterial spin labelling (ASL) method, which is used to quantify blood flow to the brain. The alternative method of intravoxel incoherent motion (IVIM) was investigated, which is hypothesised to be sensitive to cerebral blood volume and perfusion without the signal limitations of ASL. However IVIM was shown to be primarily sensitive to the presence of cerebrospinal fluid within the brain, and thus is not a suitable alternative to ASL when quantification is of interest.

Authors: Hare, H

• DOI: 10.5287/ora-vmekr4jyo
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: ASL; arterial spin labeling; brain; positron emission tomography
• Subjects: Magnetic resonance imaging