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Caitlin O'Brien


DPhil Researcher

  • EPSRC & MRC Oxford Nottingham Biomedical Imaging DTC

My research focusses on the development of a novel method for quantifying brain tissue oxygen extraction fraction, using MRI. 

Oxygen extraction fraction (OEF) is the relative difference in oxygen concentration between arterial and venous blood. Abnormal changes in OEF reflect underlying changes in oxygen metabolism of the tissue which occur in a variety of cerebrovascular diseases and conditions.

Positron emission tomography (PET) is considered the current gold standard for cerebral OEF mapping, although the requirement of short-lived O-15 radiotracers leaves it undesirable for research and limits accessibility for clinical application. The focus has therefore shifted towards MRI based methods for quantifying brain OEF. 

MRI-based methods include susceptibility-based oximetry, velocity selective gradients, and quantitative BOLD imaging. The approach I am focusing on is one which exploits the relationship between blood oxygen concentration and the MR relaxation time T2. There exists an MR method, T2-Relaxation-Under-Spin-Tagging (TRUST) which uses this relationship to successfully quantify whole brain OEF through measurement of venous blood oxygenation in a key vessel in the brain. The TRUST method has been highly influential, and has been validated in a number of studies and cerebrovascular conditions, however a key limitation of the method is its lack of spatial specificity. 

I have adapted the TRUST method in order to achieve spatially specific measures of venous blood T2, and thus quantify OEF across different regions of the brain. We call this sequence Selective Localised TRUST (SL-TRUST). 

Thus far I have devised and tested SL-TRUST in healthy controls, verifying the measurement stability and reproducibility. I am able to acquire OEF measurements from contralateral hemispheres, in under 4 minutes. The method can localise up to a 70x70x80 mmtissue region in the brain. 

Current work focuses on applying the method in patient groups as part of an ongoing stroke study at the Acute Vascular Imaging Centre at the John Radcliffe Hospital. 


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