MRI of Acute Vascular Injury and Hemorrhagic Transformation in Ischemic Stroke

大多数血浆物质进入大脑通常会被血脑屏障(BBB)阻挡，血脑屏障保护脑细胞免受此类物质的不良影响。缺血性中风时血脑屏障的损伤通常会导致离子、水、氨基酸和血浆蛋白渗入大脑，导致大面积中风，在开始治疗之前可能会出现明显的脑肿胀。随着时间的推移，这种屏障功能的衰退可能会恶化，导致红细胞渗出并形成出血，这一过程被称为出血性转化(HT)。组织纤溶酶原激活剂(TPA)溶栓治疗可增加缺血组织的血流量，是唯一被批准的急性缺血性中风的治疗方法。但它的使用使患症状性高血压的风险增加了十倍。目前，对缺血性中风患者进行tPA治疗的唯一标准是发病后3小时且CT检查为阴性的出血，因为目前尚无诊断影像指标可用于排除高危中风患者。我们以前的卒中研究表明，急性BBB损伤的区域表现为磁共振造影剂(MRCA)的泄漏和磁化转移参数T1sat的升高，通常在以后发生HT。从这项工作中，两种定量磁共振成像(MRI)方法被确定为可能的预测BBB损伤和HT的方法：1)磁化转移MRI(MT-MRI)，特别是T1sat参数；2)MRCA增强MRI，显示Gd-二乙三胺五乙酸(Gd-DTPA)的血液到脑的分布，通过Patlak曲线图分析流入速率。急性血脑屏障开放的程度，预示着严重的血管源性水肿和高血压，以及导致这些MRI信号改变的血脑屏障成分的生理变化都是未知的。这项建议旨在测试卒中早期MT-和/或MRCA-MRI参数的大小是否可以用来建立预示随后的HT的BBB损害的阈值，以及这种或更大的变化是否可以用来识别在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：管腔内细丝模型和栓塞法。目的1是一个翻译目标；它的目的是开发一个预测模型，将缺血和再灌流后最初几个小时MT和/或MRCA-MRI参数的异常程度与tPA治疗后发生随后的HT的可能性联系起来。如果成功，这个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。AIM 2的机制在于，我们将研究可能导致MT和MRCA-MRI参数变化以及急性BBB损伤的生理因素。这项建议的长期目标是开发能够识别高血压风险组织的MRI征象，为选择接受tPA治疗的患者提供标准，并研究中风患者血脑屏障开放和水肿形成的基本机制。 修改后的特定部分 具体目的1：量化：1)MT-MRI参数在缺血后约2小时、5小时和48小时；2)Gd-DTPA在缺血后约5小时和48小时跨血脑屏障流入；以及3)通过组织学评估和血管内缝合和栓塞性卒中模型中组织血红蛋白含量(羟色胺定量测定)确定的48小时缺血性损伤，其中一半动物接受tPA治疗，另一半接受安慰剂治疗(即急性成像研究和再灌注后不久；每组20只)。这些数据将被用来开发一个预测模型，将缺血几小时后MRI异常的程度与48小时时高血压的发生率以及tPA治疗引发高血压的可能性联系起来。 假设：这个目的是为了验证这样一个假设，即tPA诱发高血压的可能性可以从脑缺血和再灌流最初几个小时的T1sat和MRCA内流的MRI测量中获得。 具体目标2：量化与目标1相同的一套MRI和组织学参数，加上MRI和组织学对脑肿胀的估计，以及免疫组织化学(IHC)对选定感兴趣区(ROI)中各种BBB相关蛋白的表达，其中一半动物接受tPA治疗，另一半动物不接受tPA治疗。在缺血时(大脑中动脉闭塞后约2小时)和闭塞后约7、11和24小时进行MRI检查，然后取脑组织进行组织学和IHC分析(n=10)。 假设：这一目的将检验假设：1)MT参数的变化将与Gd-DTPA流入(即BBB开放)成正比，两者都将是BBB相关蛋白和/或脑肿胀变化的函数；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.