Patients with sickle cell anemia (SCA) have reduced oxygen carrying capacity, elevated cerebral blood flow and oxygen extraction fraction (OEF), and increased rate of stroke and vasculopathy. Half of adults with SCA will display a cerebral infarct by age 30; however, despite the prevalence of disability-inducing injury, no mechanism for primary stroke prevention has been established in adults with SCA. Difficulty in designing treatment protocols in these patients is partially due to difficulties associated with measuring cerebral hemo- metabolism. Recent work suggests that elevation in OEF is correlated with clinical indicators of disease, including cerebral vasculopathy and stroke history, however widespread implementation of OEF measures to clinical practice has been impeded due to technical difficulties associated with 15-O positron emission tomography measurements, which have historically been used to measure OEF. OEF can be measured non- invasively in magnetic resonance imaging (MRI) with the T2-relaxation-under-spin-tagging (TRUST) pulse sequence, however this method does not provide regional measures of OEF, so does not report regional impairment and stroke risk, which is common in patients with SCA associated cerebral vasculopathy. To address these gaps in treatment, the proposed research will focus on improving the TRUST pulse sequence to report regional TRUST (rTRUST) measures of OEF. A limited-functionality rTRUST pulse sequence is already operational in our lab, however this method is limited to unilateral, hemispheric measurements of OEF. In the first aim of this work, we will refine rTRUST to improve labeling efficiency and increase spatial information content further, using customized spin labeling approaches with radiofrequency developments that allow for 3D volumes to be excited with improved specificity. The refined rTRUST pulse sequence will be validated in ex vivo blood phantoms and optimized in healthy controls. In the second aim, rTRUST will be demonstrated in vivo whereby participants will be recruited from a cohort of more than 91 known, well-characterized, SCA patients for a translational component of this research. The SCA cohort with lateralizing vasculopathy will be scanned at a 3 Tesla MRI scanner, and using spatially specific rTRUST OEF measurements, we will validate that regions with vasculopathy and reduced CBF have elevated OEF, consistent with known relationships between CBF, OEF, and CMRO2. Development of rTRUST, and validation of MRI methods for acquiring spatial OEF information will help to introduce OEF measurement into clinical practice, improving our ability to design treatment protocols for aggressive surgical revascularization, transfusion, or medical management for patients with cerebral vasculopathy and sickle cell anemia. These methods will be generalizable to other diseases that cause hemo-metabolic dysfunction and stroke, particularly other cerebrovascular disease, such as atherosclerosis in which oxygen delivery is impaired by steno-occlusion.
Cerebrovascular disease is a leading cause of death and the leading cause of adult disability in the United States. Reducing cerebrovascular disease related morbidity will ultimately require improved imaging technologies that can be applied in patients with cerebrovascular disease to evaluate stroke risk, which could be used as biomarkers to titrate preventative therapies. In this work, we will extend a non-invasive tool to measure brain oxygen extraction fraction quickly throughout the brain, which could be used to identify tissue at risk of stroke with added sensitivity relative to existing methods.
