Novel MRI Approaches for Assessing Cerebral Hemodynamics in Ischemic Cerebrovascular Disease

Abstract

Abstract-Changes in cerebral hemodynamics underlie a broad spectrum of ischemic cerebrovascular disorders. An ability to accurately and quantitatively measure hemodynamic (cerebral blood flow and cerebral blood volume) and related metabolic (cerebral metabolic rate of oxygen) parameters is important for understanding healthy brain function and comparative dysfunction in ischemia. Although positron emission tomography, single-photon emission tomography, and gadolinium-MRI approaches are common, more recently MRI approaches that do not require exogenous contrast have been introduced with variable sensitivity for hemodynamic parameters. The ability to obtain hemodynamic measurements with these new approaches is particularly appealing in clinical and research scenarios in which follow-up and longitudinal studies are necessary. The purpose of this review is to outline current state-of-the-art MRI methods for measuring cerebral blood flow, cerebral blood volume, and cerebral metabolic rate of oxygen and provide practical tips to avoid imaging pitfalls. MRI studies of cerebrovascular disease performed without exogenous contrast are synopsized in the context of clinical relevance and methodological strengths and limitations. (Stroke. 2012;43:903-915.)Key Words: cerebral blood flow Ⅲ cerebral blood volume Ⅲ cerebral hemodynamics Ⅲ stroke Ⅲ stenosis Ⅲ cerebrovascular disease Ⅲ MRI T he aim of this review is to discuss noninvasive (without contrast injection) MRI methods that are available to assess cerebral blood flow (CBF), cerebral blood volume (CBV), cerebral metabolic rate of oxygen (CMRO 2 ), and oxygen extraction fraction (OEF) in the setting of cerebrovascular disease. Many such methods have recently been shown to provide comparable contrasts to more established invasive techniques and have been used with appropriate modifications to characterize tissuelevel hemodynamics in patients with acute and chronic ischemia.Physiologically, as cerebral perfusion pressure (CPP) reduces, the extent of hemodynamic compromise at the tissue level reflects the autoregulatory capacity of vasculature to increase CBV and/or develop collaterals to supplement CBF. CBF collateralization, as well as regional variability in CBV and OEF, have been hypothesized to correlate uniquely with stroke risk, with evidence suggesting that risk positively correlates with elevated CBV and OEF. 1 Detailed measurements of CBF, CBV, OEF, and CMRO 2 have provided important clues regarding progression of cerebrovascular disease, 2,3 and accurate measurements of hemodynamic compromise have the potential to improve diagnosis and risk stratification. 1 Yet, hemodynamic measures are not standardized for the acute or long-term assessment of cerebrovascular disease. The critical barrier to achieving this rests with a lack of (1) methodology for measuring tissue hemodynamics with high specificity and availability and (2) noninvasive approaches capable of monitoring longitudinal progression of impairment.Recently, additional MRI methods have been developed that cl...

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