MRI of Acute Vascular Injury and Hemorrhagic Transformation in Ischemic Stroke

大多数血浆携带的物质进入大脑通常被血脑屏障（BBB）阻断，血脑屏障保护脑细胞免受这些物质的不良影响。缺血性中风中对BBB的损伤通常导致离子、水、氨基酸和血浆蛋白渗漏到脑中，并且因此在大中风中，在开始治疗之前可能发生显著的脑肿胀。随着时间的推移，屏障功能的这种衰减可能会恶化，使得红细胞外渗并形成血管，这一过程称为出血性转化（HT）。使用组织纤溶酶原激活剂（tPA）的溶栓疗法用于增加流向缺血组织的血流量，并且是急性缺血性卒中的唯一批准的治疗。但它的使用会使症状性HT的风险增加十倍。目前，tPA治疗缺血性卒中的唯一标准是发作后3小时和CT检查出血阴性，因为没有诊断性成像指标可用于排除高危卒中患者。我们以前的中风研究表明，急性血脑屏障损伤的区域，显示磁共振造影剂（MRCA）泄漏和升高的T1 sat，磁化传递参数，往往在以后的时间发展HT。从这项工作中，两种定量磁共振成像（MRI）方法已被确定为BBB损伤和HT的可能预测因子：1）磁化传递MRI（MT-MRI），特别是T1 sat参数;和2）MRCA增强MRI的钆-二亚乙基三胺五乙酸（Gd-DTPA）的血脑分布，通过Patlak图测定流入率。急性血脑屏障开放的程度，预测严重的血管源性水肿和HT，也不知道的生理变化，在血脑屏障成分，有助于这些改变的MRI信号。该提案旨在测试在卒中早期MT和/或MRCA-MRI参数变化的大小是否可用于建立预示随后HT的BBB损伤阈值，以及这种幅度或更大的变化是否可用于识别溶栓治疗后可能发生HT的受试者。将使用两种大鼠大脑中动脉（MCA）闭塞模型，即管腔内细丝和栓塞模型。目标1是一个转化目标;其目的是开发一种预测模型，将缺血和再灌注最初几个小时期间MT和/或MRCA-MRI参数的异常程度与tPA治疗后发生后续HT的可能性联系起来。如果成功，这种预测模型将为临床试验奠定基础，并可能有助于选择或拒绝接受tPA治疗的患者。目的2是机械的意义上，我们将调查可能有助于这些变化的MT和MRCA-MRI参数和急性BBB损伤的生理因素。该提案的长期目标是开发MRI特征，可以识别HT风险组织，为选择tPA治疗患者提供标准，并研究卒中中BBB开放和水肿形成的基本机制。 修改特定章节 具体目标1：量化：1）缺血发作后约2、5和48小时的MT-MRI参数; 2）Gd-DTPA穿过BBB的内流（渗透性的测量）;和3）通过组织学评估和组织血红蛋白含量确定的48小时的缺血性损伤（HT的定量测定），其中一半动物在MCA闭塞后3小时接受tPA，另一半接受安慰剂（即在急性成像研究和再灌注后不久;每组n=20）。这些数据将用于开发预测模型，该模型将缺血数小时后MRI异常程度与48小时HT发生率和tPA治疗诱导HT的概率联系起来。 假设：本研究旨在验证以下假设：在脑缺血和再灌注的最初几个小时内，可以通过MRI测量T1 sat和MRCA内流来评估tPA诱导HT的可能性。 具体目标二：量化与目标1相同的MRI和组织学参数，加上通过MRI和组织学估计的脑肿胀，以及通过免疫组织化学（IHC）在MCA闭塞3小时的动物中选定的感兴趣区域（ROI）中各种BBB相关蛋白的表达，其中一半动物接受tPA治疗，另一半动物未接受tPA治疗。将在缺血期间的时间点（MCA闭塞后约2小时）和闭塞后约7、11和24小时进行MRI研究，之后将采集大脑进行组织学和IHC分析（每个时间和治疗组n=10）。 假设：该目的将检验以下假设：1）MT参数的变化将与Gd-DTPA流入成比例（即，BBB开放），并且两者都将是BBB相关蛋白和/或脑肿胀的变化的函数;以及2）这些变化的大小与时间、ROI和治疗相关。

The entry of most plasma-borne materials into the brain is normally blocked by a blood-brain barrier (BBB) that protects the brain cells from the untoward effects of such substances. Injury to the BBB in ischemic stroke often leads to the leakage of ions, water, amino acids, and plasma proteins into the brain and as a result in large strokes significant brain swelling may occur before treatment is initiated. Over time this decay in barrier function can worsen such that red blood cells extravasate and form hemorrhages, a process referred to as hemorrhagic transformation (HT). Thrombolytic therapy with tissue plasminogen activator (tPA) acts to increase blood flow to the ischemic tissue and is the only approved treatment for acute ischemic stroke. But its usage increases the risk of symptomatic HT ten-fold. At present, the only criterion for tPA treatment of ischemic stroke is time 3 hr post-ictus and CT exam negative for bleeding, as no diagnostic imaging indicators are available for excluding high-risk stroke patients. Our previous stroke studies indicate that regions with acute BBB injury that show leakage of magnetic resonance contrast agents (MRCAs) and elevated T1sat, a magnetization transfer parameter, often develop HT at later times. From this work, two quantitative magnetic resonance imaging (MRI) methods have been identified as possible predictors of BBB injury and HT: 1) magnetization transfer MRI (MT-MRI), particularly the T1sat parameter; and 2) MRCA enhanced MRI of the blood-to-brain distribution of gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA), with the influx rate assayed by Patlak plots. Neither the degree of acute BBB opening, predictive of severe vasogenic edema and HT, nor the physiological changes in the BBB components that contribute to these altered MRI signals are known. This proposal aims to test if the size of the MT- and/or MRCA-MRI parameter changes at an early stage of stroke can be used to establish a threshold of BBB damage that portends subsequent HT and if changes of this magnitude or larger can be used to identify subjects that are likely to develop HT after thrombolytic therapy. Two rat middle cerebral artery (MCA) occlusion models, intraluminal filament and embolic, will be used. Aim 1 is a translational aim; its purpose is to develop a predictive model that links the degree of abnormality of the MT- and/or MRCA-MRI parameters during the first several hours of ischemia and reperfusion to the probability of developing subsequent HT following tPA treatment. If successful, this predictive model would lay the groundwork for clinical trials and might be useful for selecting or rejecting patients for tPA treatment. Aim 2 is mechanistic in the sense that we will investigate physiological factors that may contribute to these changes in the MT and MRCA-MRI parameters and to acute BBB injury. The long-term goals of this proposal are to develop MRI signatures that can identify tissue at risk for HT, provide criteria for selecting patients for tPA treatment and to investigate the basic mechanisms of BBB opening and edema formation in stroke. Modified Specific Section Specific Aim 1: To quantify: 1) MT-MRI parameters at approximately 2, 5 and 48 hr after ischemia onset; 2) the influx of Gd-DTPA across the BBB (a measure of permeability) at approximately 5 and 48 hr after ischemia onset; and 3) ischemic injury at 48 hr as determined by histological assessment and tissue hemoglobin content (a quantitative assay of HT) in both intraluminal suture and embolic stroke model in the rat with half of the animals receiving tPA and the other half placebo 3 hr after MCA occlusion (i.e. shortly after the acute imaging studies and reperfusion; n=20 per group). These data will be used to develop a predictive model that links the degree of MRI abnormality after several hours of ischemia to the incidence of HT at 48 hr and the probability of tPA treatment inducing HT. Hypothesis: This aim tests the hypothesis that the probability of tPA inducing HT can be accessed from MRI measures of T1sat and MRCA influx during the first few hours of cerebral ischemia and reperfusion. Specific Aim 2: To quantify the same set of MRI and histological parameters as in Aim 1, plus estimates of brain swelling by MRI and histology and the expression of various BBB-associated proteins by immunohistochemistry (IHC), in selected regions of interest (ROIs) in animals subjected to 3 hrs of MCA occlusion with half of the animals treated with tPA and half without. MRI studies will be performed at time points during ischemia (~ 2 hr post MCA occlusion) and at approximately 7, 11 and 24 hrs post-occlusion after which the brains will be taken for histological and IHC analysis (n=10 per time and treatment group). Hypothesis: This aim will test the hypothesis that: 1) changes in the MT parameters will be proportional to Gd-DTPA influx (i.e., BBB opening) and both will be functions of the variations in BBB associated proteins and/or brain swelling; and 2) the size of these changes are correlated over time, ROI, and treatment.