Quick Harald H, Kuehl Hilmar, Kaiser Gernot, Bosk Silke, Debatin Jörg F, Ladd Mark E
MR Center, Department of Diagnostic Radiology, University Hospital Essen, Essen, Germany.
Magn Reson Med. 2002 Nov;48(5):781-90. doi: 10.1002/mrm.10269.
The development of intimal hyperplasia following stent deployment can lead to narrowing or even occlusion of the stent lumen. The underlying mechanisms leading to neointimal proliferation within stents remain largely unknown. Long-term evaluation of stent patency requires a noninvasive means for assessing the stent lumen. MR angiography (MRA) has shown potential to provide noninvasive assessment of the vascular system. However, a detailed assessment of the stent lumen with MRI is often hampered by material-dependent susceptibility artifacts, as well as by radiofrequency (RF) eddy currents generated inside the electrically conducting stent mesh. In this study, stent prototypes were designed to act as active resonant structures at the Larmor frequency of the MR system. Employing the principle of inductive coupling, the B(1) fields of the stents were coupled to that of an outside surface coil. The stents thus acted as local RF signal amplifiers. Various stent designs were investigated regarding their coupling to an external coil, signal homogeneity, and suitability for mechanical expansion for implantation purposes. The dependency of flip angle amplification on the quality factor Q of the stents was systematically investigated. Phantom experiments revealed signal amplification in all stent prototypes. Signal enhancement inside and close to the surface of the stents enabled their localization with high contrast in MR images. In vivo imaging experiments in the iliac, renal, and splenic arteries of two pigs confirmed the in vitro findings. Wireless active visualization of stents allows for detailed analysis of the stent lumen with high contrast and spatial resolution. The proposed method could thus provide a powerful diagnostic means for the noninvasive long-term follow-up of stent patency, thereby enhancing our understanding of the mechanisms of restenosis.
支架植入后内膜增生的发展可导致支架管腔狭窄甚至闭塞。导致支架内新生内膜增殖的潜在机制在很大程度上仍不清楚。对支架通畅性的长期评估需要一种非侵入性方法来评估支架管腔。磁共振血管造影（MRA）已显示出对血管系统进行非侵入性评估的潜力。然而，利用MRI对支架管腔进行详细评估常常受到材料相关的敏感性伪影以及导电支架网内部产生的射频（RF）涡流的阻碍。在本研究中，支架原型被设计为在MR系统的拉莫尔频率下作为有源共振结构。利用感应耦合原理，将支架的B(1)场与外表面线圈的B(1)场耦合。因此，这些支架起到了局部RF信号放大器的作用。研究了各种支架设计在与外部线圈的耦合、信号均匀性以及用于植入目的的机械扩张适用性方面的情况。系统地研究了翻转角放大对支架品质因数Q的依赖性。体模实验显示所有支架原型都有信号放大。支架内部和表面附近的信号增强使得它们在MR图像中能够以高对比度定位。在两只猪的髂动脉、肾动脉和脾动脉中进行的体内成像实验证实了体外实验结果。支架的无线主动可视化能够以高对比度和空间分辨率对支架管腔进行详细分析。因此，所提出的方法可为支架通畅性的非侵入性长期随访提供一种强大的诊断手段，从而增进我们对再狭窄机制的理解。
