Arterial Transit Awesomeness.
Arterial Transit Awesomeness.
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DOI：
10.1148/radiol.2020203838
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发表时间：
2020-10
期刊：
影响因子：
19.7
通讯作者：
G. Zaharchuk
中科院分区：
文献类型：
--
作者：
G. Zaharchuk
我们如何对近期有急性中风或短暂性脑缺血发作相关症状的患者进行成像？原因只有一个--帮助神经科医生和其他提供者适当地治疗病人，以尽量减少他们将来中风的风险。为此，已经提出了许多测试来提供对这个问题的洞察，包括颈动脉狭窄的血管成像，颈动脉血管壁和斑块（如果存在）的形态学特征的评估，弥散加权成像，灌注成像和脑血管储备（即，测量大脑增加脑血流量的能力，以响应血管扩张的挑战）。在本期《放射学》杂志中，Di Napoli等人（1）发现了一种可能的颈动脉狭窄患者的新体征：非造影剂动脉自旋标记（ASL）灌注图像上存在动脉传输伪影（ATA）。要理解ATA是什么，回顾一下ASL的工作原理是很有用的。血液中的水质子在通往大脑的颈动脉中被磁性标记，通常持续几秒钟。在此之后，序列中出现暂停（称为“标记后延迟”），以允许标记水从颈部移动到脑的毛细血管床，在那里它被提取到实质中。暂停的长度是一个折衷：它必须足够长，使标记水到达毛细血管，但不能太长，使它们的信号开始消失，这发生在由动脉血的T1确定的速率（3.0 T时为1.75秒）。出于这些原因，选择的标签后延迟通常在1到2秒之间（国际医学磁共振学会最近的一篇白色论文建议2秒作为临床应用的良好选择）（2）。这个时间安排对绝大多数患者都很有效，但对一些人来说，暂停的时间不够长。这包括老年患者，因为人类的脑血流量在青春期达到峰值，此后持续下降（这有点令人沮丧！）（三）、动脉狭窄的患者是另一组暂停时间不够长的患者;这可能有几个原因，包括简单地减慢通过狭窄的流速或血液通过侧支通路到达所需的时间（4）。不管是什么原因，如果标签后延迟不足以使患者的标记血液到达毛细血管床，则在受影响的血管区域中将出现高和低ASL信号强度的混合，通常呈点状模式。这是因为两种现象正在发生。高信号强度是因为在成像时供血动脉中存在标记的血液。周围的低信号强度反映了标记的血液最终将灌注的更远端的实质，但在信号采集时尚未到达。Alsop和Detre（5）以及Detre等人（6）在人体ASL评价的早期发现了这种混合模式，他们将其称为“动脉传输伪影”。然而，一个人的人工制品是另一个人的生物标志物。多年来，研究人员和临床医生已经意识到，这种对动脉到达时间的敏感性可能是一个有用的诊断标志。ASL是一种易于出现许多伪影的序列，包括来自不完整标记、到达时间变化、跨平面模糊和脑脊液脉动的伪影（7）。由于这个原因，当正常时它是最有用的，因为这是脑血流动力学未受损的证据。根据这种逻辑，血管区域中ATA的存在表明某些事情不太正确，应该促使上游循环的原因调查。例如，Yeom等人（8）发现，此类ASL异常在1型神经纤维瘤病患者中更为常见，这些患者通常存在大动脉血管病变。在另一个例子中，de Havenon等人（9）表明，急性缺血性卒中后立即出现ATA是更有利结局的预测因素，代表存在良好的侧支血管。出于这样的原因，我喜欢认为字母ATA实际上应该代表动脉运输真棒！Di Napoli等人再次证明了这一惊人之处。Di Napoli等人对来自两个登记研究的44例颈动脉狭窄≥ 50%的患者进行了3.0-T颈动脉斑块成像和ASL。他们试图了解哪些影像学标记物与近期症状相关，如短暂性脑缺血发作或卒中，并特别关注狭窄程度、颈动脉斑块形态结构、斑块内出血的存在和动脉通过的奇妙性。
W do we image patients with recent symptoms related to acute stroke or transient ischemic attack? There can be only one reason—to help neurologists and other providers treat the patient appropriately to minimize their risk of stroke in the future. To that end, many tests have been suggested to provide insight into this question, including vascular imaging for carotid stenosis, evaluation of the morphologic characteristics of the carotid artery vessel wall and plaque (if present), diffusion-weighted imaging, perfusion imaging, and cerebrovascular reserve (ie, a measurement of the ability of the brain to increase cerebral blood flow in response to a vasodilatory challenge). In this issue of Radiology, Di Napoli et al (1) identified a potentially new sign for patients with presumed carotid stenosis: the presence of arterial transit artifact (ATA) on noncontrast arterial spin labeling (ASL) perfusion images. To understand what ATA is, it is useful to review how ASL works. Water protons in the blood are labeled magnetically in the cervical arteries leading to the brain, usually for a duration of several seconds. After this, a pause (called the “postlabel delay”) in the sequence occurs to allow the labeled water to move from the neck to the capillary bed of the brain, where it is extracted into the parenchyma. The length of the pause is a compromise: it must be long enough for the labeled water to reach the capillaries but not so long that their signal starts to disappear, which occurs at a rate determined by the T1 of arterial blood (1.75 seconds at 3.0 T). For these reasons, the postlabel delay chosen is typically between 1 and 2 seconds (a recent International Society of Magnetic Resonance in Medicine white paper suggested 2 seconds as a good choice for clinical applications) (2). This timing works well for the vast majority of patients, but there are some people for whom the pause is not long enough. These include older patients, because in humans cerebral blood flow peaks in adolescence and declines relentlessly thereafter (a somewhat depressing thought!) (3). Patients with arterial stenosis are another group for whom this pause is not long enough; this can be for several reasons, including simply slowing of the flow rate through the stenosis or the time it takes for blood to arrive via collateral pathways (4). Regardless of the reason, if the postlabel delay is not long enough for the patient’s labeled blood to reach the capillary bed, a mix of high and low ASL signal intensity, often in a punctate pattern, will appear in the affected vascular territory. This is because two phenomena are occurring. The high signal intensity is because of the presence of the labeled blood in the feeding arteries at the time of imaging. The surrounding low signal intensity reflects more distal parenchyma that the labeled blood will eventually perfuse, but has not yet reached at the time of signal acquisition. This mixed pattern was identified early in the evaluation of ASL in humans by Alsop and Detre (5) and Detre et al (6), who called it “arterial transit artifact.” However, one person’s artifact is another person’s biomarker. Over the years, investigators and clinicians have realized that this sensitivity to arterial arrival times can be a helpful diagnostic sign. ASL is a sequence that can be prone to many artifacts, including those from incomplete labeling, arrival time variations, through-plane blurring, and cerebrospinal fluid pulsation (7). For this reason, it is most useful when normal because this is evidence that cerebral hemodynamics are not compromised. By this logic, the presence of ATA in a vascular territory suggests that something is not quite right and should prompt the investigation of the upstream circulation for a cause. For example, Yeom et al (8) identified such ASL abnormalities more commonly in patients with neurofibromatosis type 1, in whom large artery vasculopathy is commonly present. In another example, de Havenon et al (9) showed that the presence of ATA immediately after acute ischemic stroke was a predictor of more favorable outcome, representing the presence of good collateral vessels. For reasons such as this, I like to think that the letters ATA should in fact stand for arterial transit awesomeness! This awesomeness is again demonstrated by Di Napoli et al. Di Napoli et al performed 3.0-T carotid plaque imaging and ASL in 44 patients with 50% or greater carotid stenosis from two registries. They sought to understand what imaging markers were associated with recent symptoms, such as transient ischemic attack or stroke, and specifically looked at degree of stenosis, carotid plaque morphologic structure, presence of intraplaque hemorrhage, and Arterial Transit Awesomeness