Imaging of cerebral hemodynamics with arterial spin labeling

Abstract

A knowledge of the cerebral circulation is of great importance in the understanding cerebrovascular disease. The ability to visualize the brain vasculature has become key in the diagnosis and treatment of cerebrovascular disease. Modern imaging modalities, such as digital subtraction angiography, computed tomography (CT) angiography and magnetic resonance (MR) angiography, allow the in vivo examination of the patient in great detail. A novel completely noninvasive method to visualize cerebral perfusion is arterial spin labeling (ASL) MR imaging. Using ASL the water protons in the blood are magnetically labeled by radiofrequency pulses and, in this manner, are used as an endogenous tracer instead of contrast agents. In this thesis the possibilities of territorial and perfusion ASL MR imaging are described as well as its importance in studying the cerebral hemodynamics. Examination of the cerebral circulation in patients with ischemic stroke may be of crucial importance to differentiate between a thromboembolic and hemodynamic etiology. In patients with steno-occlusive carotid artery disease, collateralization is stimulated to maintain adequate oxygenation and cell function. Variability in the cerebral perfusion territories of the major brain feeding arteries requires therefore special consideration in these patients. Planning-free vessel encoded pseudo-continuous ASL (VE p-CASL) is a suitable general purpose territorial ASL (T-ASL) strategy for mapping cerebral perfusion territories in clinical MRI exams. It is a highly efficient method, since multiple vessels are labeled simultaneously, and requires a minimum amount of planning. VE p-CASL may be used to determine the possible contribution of collateral arteries, identify border zones, and, in combination with appropriate image processing, to detect areas with mixed perfusion. Cerebral hemodynamics may be further examined by combining perfusion ASL MR Imaging with a vasodilatory challenge. Cerebral hemodynamic impairment depends on the degree of stenosis in the afferent vessel, the presence of collateral vessels to provide additional blood flow, and the remaining autoregulatory function of the tissue. Autoregulative vasodilation maintains adequate cerebral blood flow during perfusion pressure fluctuations. Perfusion ASL measurements before and after a vasodilatory challenges may be used to evaluate the degree of hemodynamic impairment or more specifically to determine the remaining vasodilatory capacity of the terminal arterioles in afflicted brain parenchyma. Primary collateral pathways via the circle of Willis, as well as secondary collateral pathways such as leptomeningeal arteries, play an important role in the hemodynamic impairment of brain parenchyma in patients with steno-occlusive carotid artery disease. The influence of collateral pathways on cerebral hemodynamics can be investigated by relating the actual territory being supplied by these collaterals to the presence of hemodynamic impairment. Collateral blood flow is shown to be a sign of inadequate capacity of the major brain feeding arteries, especially in the case of secondary collateral blood flow, which signals sever hemodynamic impairment in patients with steno-occlusive carotid artery disease. The studies presented in this thesis investigated the use of ASL for imaging of cerebral hemodynamics. Research is still ongoing to establish its exact role in clinical practice, these studies however show ASL MR Imaging can visualize perfusion territories, determine primary and secondary collateral blood flow routes, and evaluate cerebral perfusion and hemodynamic impairment in patients with symptomatic cerebrovascular disease. ASL MR Imaging is still relatively uncommon outside of research groups, partly due to lack of support from vendors for clinical purposes. This may change due to increasing demands as ASL proves itself as a powerful non-invasive alternative to other modalities. Personalized medicine in the near future may possible include territorial and perfusion ASL MR Imaging to evaluate cerebral hemodynamic processes in individual patients.