Arterial spin labeling perfusion MRI in cerebral ischaemia

Abstract

Cerebral perfusion is the basis for the delivery of oxygen and nutrients to the brain. Brain tissue can become damaged when there is a shortage in the blood supply. Basic physiological functions such as synaptic transmission, the membrane ion pump and energy metabolism are disrupted and within minutes can lead to irreversible damage and neurological symptoms. The aim of this thesis was to investigate the ability of arterial spin labeling (ASL) MRI to measure cerebral perfusion in patients with acute stroke and symptomatic large vessel disease of the brain feeding arteries. ASL perfusion imaging is an alternative non-invasive magnetic resonance imaging (MRI) technique for visualizing brain perfusion and quantifying cerebral blood flow that does not require injection of contrast agents. The blood flowing into the brain is used as an endogenous contrast agent by magnetically labeling the inflowing blood with radiofrequency pulses. This thesis consists of two parts. The first part focuses on the application of different ASL-MRI techniques to assess brain perfusion. In patients with acute stroke it is shown that ASL can detect perfusion deficits and perfusion-diffusion mismatch prior to treatment comparable to gadolinium based DSC perfusion imaging (chapter 2). By acquiring a series of perfusion-weighted images at increasing delay times after the initial labeling, ASL is able to measure the inflow of blood into the brain. Although, when compared to H215O PET there was a relative systematic overestimation of cerebral blood flow (chapter 3), ASL can depict the presence and extent of regions with hypoperfusion and increased transit times in patients with carotid artery disease (chapter 4 and 5). The second part of this thesis focuses on assessing the cerebrovascular reactivity by combining ASL perfusion imaging with a vascular challenge. This is a measure of the brain’s capacity to sustain blood flow when perfusion pressure drops. Impairment is associated with increased stroke occurrence. Cerebrovascular reactivity is impaired in patients with a carotid artery stenosis on the side of the stenosis (chapter 6). By using a selective ASL technique that can visualize the flow territories of the brain feeding arteries, it was shown that cerebrovascular reactivity impairment varies throughout the brain (chapter7). Tissue areas fed via collaterals are the most impaired. Both large vessel disease and the occurrence of white matter lesions were furthermore found to have an effect upon white matter hemodynamics (chapter 8). ASL perfusion imaging can be used to visualize the cerebral blood flow and cerebrovascular reactivity. It can be used in both patients with acute stroke and large vessel disease to detect impaired hemodynamics and be used to assess a patient’s risk for future stroke. In the future, ASL may potentially be used to customize medicinal and surgical treatment to individual patient needs