MRI quantitative hemodynamic evaluation of the brain

Abstract

The cerebral blood flow (CBF) or the delivery of nutrients to the brain tissue is essential for the viability of brain cells and is a necessity for the human body to perform physical and mental activities. Both under-and overperfusion of the brain tissue can cause substantial harm wherefore the CBF is very tightly regulated. Dilatation and constriction of arterioles contribute to this regulation which can be expressed by the cerebrovascular reactivity (CVR). Through the cerebral blood flow, essential nutrients such as glucose and oxygen are delivered to the brain tissue. The consumption of these nutrients can either be expressed by the cerebral metabolic rate of glucose (CMRGlc), the oxygen extraction fraction (OEF) or the cerebral metabolic rate of oxygen (CMRO2). The CBF, the CVR, the CMRGlc, the OEF, and the CMRO2, or the brain hemodynamic parameters, provide important information of the brain tissue’s health. For instance, in patients with steno-occlusive disease, they provide insight into the amount of stress the brain tissue is opposed to, and, they are reflective of maturation and aging processes. Fluorodeoxyglucose positron emission tomography (PET) and oxygen-15 PET are the gold standards to evaluate these parameters. But, the invasiveness of these techniques and the requirement of an onsite cyclotron in case of oxygen-15 PET have limited their application in clinical practice. This thesis focused on the development and application of alternative non-invasive techniques to evaluate the brain hemodynamic parameters. We demonstrated that arterial spin labeling (ASL) obtained CBF values in neonates reflect maturation and can predict adverse outcome in case of a perinatal ischemic event. Similar, T2-TRIR obtained OEF values were lower in these latter infants and could thus be indicative of adverse outcome. In the adult population we applied calibrated MRI, which combines ASL and blood oxygenation level-dependent (BOLD) measurements performed at hypercapnic and hyperoxic breathing, to obtain a full evaluation of all brain hemodynamic parameters. Although some technical issues still prohibit its translation to clinical practice, the technique seems to be promising in the evaluation of patients with steno-occlusive disease and in the investigation of the aging process. At last, as the amount of brain tissue in adults also provides important information regarding (pathological) aging, a fast volumetric CSF mapping technique was developed. This technique was consequently shown to be able to act as a surrogate for time-and post-processing intensive imaging (tools) which are now commonly used to evaluate brain atrophy. In conclusion, non-invasive imaging techniques to evaluate brain hemodynamics show great promise. The application of these techniques in clinical practice will be possible when some of the limitations, encountered in our clinical studies, are dealt with. Once optimized they will be of great value in the investigation of maturation and ageing processes, in therapeutic decision making in patients with ischemic or cerebrovascular disease, and in the prediction of outcome in patients with ischemic disease.