Improvements in IVUS Help Identify Vulnerable Plaque

　　应用微米及纳米气泡对比剂有助于显示滋养血管，能更好地诊断易损斑块。滋养血管与动脉粥样硬化斑块的进展，炎症以及斑块内出血及活动性有关。

　　直径约为2 μm 到6 μm 之间的微泡，有利于频率在2-5 MHz之间的超声显示。更高频率的超声需直径更小的纳米微泡，其直径小于1μm 。这些微泡可自由通过毛细血管并辅助显示斑块及滋养血管。
 
　　荷兰鹿特丹Erasmus 大学的Antonius F. W. van der Steen医生说：“谐振血管内超声（IVUS）导管可与在20-40MHz之间的谐振频率声波产生反应，而传统IVUS导管通常只对单一频率产生良好反应。还有次谐振频率波成像，但尚不清楚谐振波与次谐振波哪个成像更好。”

　　滋养血管的发生

　　应用对比剂可显著提高识别易损斑块的能力，芝加哥Rush大学医学中心Steven B. Feinstein医生说。

　　“当动脉壁因狼疮，糖尿病及动脉粥样硬化等各种炎性病变的进展而增厚时，需要从动脉外膜表面提供充分的血供，”他说。对提供血流的滋养血管的显示能力可显示血壁内异常区域的存在。“我们亦可在斑块内寻找新生血管。”

　　血管生成的成像与组织学测量有良好的相关性（CD-31表达）。Feinstein说，在15个患者中，“肉眼所见与组织学分级有较好的相关性。”组织学分为0-3级，其中0表示无或极少的血管生成，3表示存在血管生成，相关系数r=0.68。
 
　　影像学进展

　　Feinstein 报道对比剂IVUS用来显示停止应用他汀后滋养血管的消退情况（图）。这一影像技术可用以显示某一特定治疗方法是否有效。

　　“这一技术将最终用于何处呢？我们有希望将其最终能应用于管腔的容积分析，内膜的厚度以及滋养血管的3-D超声成像，”Feinstein说，目前，对比剂超声已能清楚确定中膜厚度（IMT）及确定管壁溃疡和软斑块破裂。

　　这些对比剂与先进的谐振及次谐振IVUS结合，将显著地增强显示易损斑块的能力。

　　（阜外心血管病医院　宋会军 高立建　编译）

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Improvements in IVUS Help Identify Vulnerable Plaque

Contrast agents provide better analysis of vessel walls and plaque formation.

The use of micro- and nanobubble contrast agents with IVUS can aid in imaging the vasa vasorum, allowing for better diagnosis of vulnerable plaque. The vasa vasorum has been linked to atherosclerotic plaque progression, inflammation, and intraplaque hemorrhage and instability.

Microbubbles, with diameters of approximately 2μm to 6μm , allow for ultrasound at 2 MHz to 5 MHz. Higher frequencies require smaller nanobubbles, with diameters smaller than 1μm . These bubbles pass easily through capillaries and assist in outlin¬ing of plaque and the vasa vasorum.

A harmonic IVUS catheter responds to two harmonic frequencies at 20 MHz and 40 MHz as opposed to a conventional IVUS catheter, which generally responds best to one frequency. There is a subharmonic frequency as well, but it is unclear whether harmonic or subharmonic imaging is more useful, said Antonius F. W. van der Steen, PhD, of Erasmus University in Rotterdam, The Netherlands.

Vasa vasorum angiogenesis

Using contrast agents could significantly improve the ability to recognize vulnerable plaque, said Steven B. Feinstein, MD, of Rush University Medical Center in Chicago.

“As artery walls thicken due to a variety of inflammatory disease processes such as lupus, diabetes, and atherosclerosis, there has to be an adequate blood supply from the adventitial surface,” he said. The ability to image the vasa vasorum that provides that blood supply can indicate the presence of problem areas within the vessels. “We also can look for angiogenesis within the plaque itself,” he said.

Imaging of angiogenesis correlates well with histologic measures (CD-31 expression). Among 15 patients, Feinstein said his group found “a relatively nice correlation between what we saw with our eyes and the histology grade.” On a scale of 0 to 3, with 0 indicating limited or no angiogenesis and 3 indicating arterial genesis, the correlation was r = 0.68.
 
Imaging improvement

Feinstein also reported that contrast IVUS has been used to show the withdrawal of the vasa vasorum after statin use (Figure). This type of imaging can indicate whether a specific therapy is effective.

“Where are we ultimately going with this? We are going to hopefully end up with a volumetric analysis of the lumen, the intimal medial thickness, and the vasa vasorum using 3-D ultrasound imaging,” Feinstein said.

Already, the use of contrast agents with IVUS can clearly define the IMT and identify ulcers and soft plaque tears.
These agents, along with advances in harmonic and subharmonic IVUS, will greatly enhance the ability to identify vulnerable plaque, he added.