Better late than never: the long journey for noncontrast arterial spin labeling perfusion imaging in acute stroke.
Better late than never: the long journey for noncontrast arterial spin labeling perfusion imaging in acute stroke.
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迟到总比不到好：急性中风非造影动脉自旋标记灌注成像的漫长旅程。
DOI：
10.1161/strokeaha.111.644344
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发表时间：
2012
期刊：
影响因子：
8.3
通讯作者：
Zaharchuk,Greg
中科院分区：
文献类型：
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作者：
Zaharchuk,Greg
(ASL)并将其与动态磁敏感对比MRI进行比较。后一种技术有时也简称为灌注加权成像（PWI），以强调其与弥散加权成像的互补性。作者发现PWI和ASL提供的成像信息之间有很好的重叠，感知信噪比或病变明显性没有可测量的差异。他们还发现，尽管存在慢血流对ASL信号的影响，但这并不是图像解读的不可逾越的障碍，因为它们与PWI达到最大残余函数（Tmax）的时间和非延迟校正脑血流图中观察到的变化相关。他们特别指出，ASL图像提供的实质微血管对比度在显示与再灌注相关的充血（有时称为过度灌注）方面特别有用。基本的ASL和PWI技术最初是在20年前描述的，2，3，此后不久，两者都被应用于中风患者。4-6 PWI已成为学术医疗中心MR中风检查的支柱。卒中成像领域的主要范例是PWI-弥散加权成像不匹配，其假定具有大灌注病变但小弥散病变的患者具有潜在可挽救的组织，可以通过及时再通“挽救”。7，8 ASL尽管具有非造影灌注技术的优势，但由于两个因素，尚未获得广泛的临床应用：第一，图像的信噪比通常较低;第二，ASL信号对标记和成像部位之间的延迟动脉通过时间的强烈依赖性。随着采用更高场MR扫描仪9和使用伪连续标记、背景抑制和优化图像读出的改进ASL脉冲序列，这些问题中的第一个问题正在得到缓解。10.第二个问题更为根本。由于标记水随血液T1时间衰减，T1时间大约为1至2秒，因此可能通过侧支通路延迟到达的血流可能被错误地解释为没有血流。这是急性缺血性卒中的关键区别，其中侧支血流已被证明是患者结局的关键因素。11，12虽然目前的方法似乎产生了关于侧支的信息，13，14更新的技术，如速度选择性ASL，15基于速度而不是位置标记血液，并且理论上对到达时间不敏感，有望缓解这个问题。最后，钆对比剂和肾源性系统性纤维化之间的相关性的认识，使推注PWI的禁忌症，在一些患者的中风，使非造影剂中风协议使用ASL临床上更可取的，因为它消除了需要确定肌酐清除率的MR扫描前。越来越多的MR供应商正在开发产品序列，临床医生在卒中中使用ASL获得了更多经验。由于标准ASL通常不会产生有关动脉到达时间的信息，因此未来的研究必须确定脑血流测量值与Tmax之间关于不匹配状态和患者结局的关系。另一个重要的重点是开发自动化方法来评估病变大小，16由于灰质和白色物质之间固有的脑血流差异，这对ASL具有挑战性。使用ASL和PWI的研究（如本研究），至少在短期内，可能会提供...
(ASL), in patients with acute ischemic stroke and compares this with dynamic susceptibility contrast MRI. This latter technique is also sometimes referred to simply as perfusionweighted imaging (PWI) to emphasize its complementary nature to diffusion-weighted imaging. The authors found excellent overlap between imaging information available with PWI and ASL with no measurable difference in perceived signal-to-noise ratio or lesion conspicuity. They also found that the effects of slow flow on the ASL signal, although present, were not an insurmountable impediment to image interpretation in that they correlated with the changes seen in the PWI time-to-maximum of the residue function (Tmax) and the nondelay-corrected cerebral blood flow maps. In particular, they point out that the parenchymal microvascular contrast afforded by the ASL images was particularly useful in visualizing hyperemia related to reperfusion, sometimes termed luxury perfusion. The basic ASL and PWI techniques were initially described 20 years ago, 2, 3 and both were applied to human patients with stroke relatively soon thereafter. 4–6 PWI has become a mainstay of academic medical center MR stroke examinations. The dominant paradigm in the stroke imaging community is the PWI–diffusion-weighted imaging mismatch, which posits that patients with large perfusion lesions but small diffusion lesions have potentially salvageable tissue that can be “rescued” by prompt recanalization. 7, 8 ASL, despite the advantage of being a noncontrast perfusion technique, has not enjoyed widespread clinical use, however, because of 2 factors: first, the generally lower signal-to-noise ratio of the images; and second, the strong dependence of the ASL signal on delayed arterial transit times between the site of labeling and imaging. The first of these concerns is becoming alleviated with the adoption of higher field MR scanners9 and improved ASL pulse sequences that use pseudocontinuous labeling, background suppression, and optimized image readout. 10 The second issue is more fundamental. Because the labeled water decays with the blood T1 time, which is on the order of 1 to 2 seconds, flow that arrives late, perhaps through collateral pathways, may be incorrectly interpreted as absence of flow. This is a critical distinction in acute ischemic stroke, in which collateral flow has been shown to be a key factor in patient outcome. 11, 12 Although current methods appear to yield information about collaterals, 13, 14 newer techniques such as velocity-selective ASL, 15 which labels blood based on velocity rather than by position, and is theoretically insensitive to arrival time, will hopefully mitigate this issue. Finally, the recognition of the association between gadolinium contrast agents and nephrogenic systemic fibrosis has made bolus PWI a contraindication in some patients with stroke, making a noncontrast stroke protocol using ASL more clinically desirable because it eliminates the need to determine creatinine clearance before MR scanning. More MR vendors are developing product sequences and clinicians are gaining more experience with ASL in stroke. Because standard ASL does not typically yield information about arterial arrival times, future studies must determine the relationship between cerebral blood flow measures and Tmax regarding mismatch status and patient outcome. Another important focus will be the development of automated methods to assess lesion size, 16 which is challenging for ASL due to the inherent cerebral blood flow differences between gray and white matter. Studies (such as this one) using both ASL and PWI, at least in the short term, will likely provide …