Busch Martin, Vollmann Wolfgang, Bertsch Thomas, Wetzler Rainer, Bornstedt Axel, Schnackenburg Bernhard, Schnorr Jörg, Kivelitz Dietmar, Taupitz Matthias, Grönemeyer Dietrich
Grönemeyer Institut für Mikrotherapie, Universität Witten/Herdecke, Bochum, Germany.
Magn Reson Med. 2005 Oct;54(4):775-82. doi: 10.1002/mrm.20618.
Stents that have been implanted to preserve the results of vascular dilatation are frequently affected by in-stent restenosis, which ideally should be followed up by a noninvasive diagnostic modality. Active MRI stents can enable this kind of follow-up, while normal metallic stents can not. The prototype stents investigated in this study were designed as electric resonating circuits without a direct connection to the MR imager, and function as inductively coupled transmit coils. The model of a long solenoid coil is used to describe the additional power loss caused by such resonators. The theoretically estimated temperature increase is verified by measurements for different resonators and discussed for worst-case conditions. The RF power absorption of an active resonator is negligible compared to the total power absorbed during MRI. The local temperature increase observed for prototypes embedded in phantoms is in a range that excludes direct tissue damage. However, ruptures in the conducting structure of a resonator can cause hot spots, which may establish a high local temperature. This hazard can be reduced by designing resonators with a low quality (Q) factor or by setting the circuit slightly off resonance; however, this would lower the nominal amplification for which the resonator was designed.
为维持血管扩张效果而植入的支架常常会受到支架内再狭窄的影响，理想情况下，这需要通过一种非侵入性诊断方式进行随访。有源MRI支架能够实现此类随访，而普通金属支架则无法做到。本研究中所研究的原型支架被设计成无需直接连接到MR成像仪的电谐振电路，并用作电感耦合发射线圈。长螺线管线圈模型用于描述此类谐振器所导致的额外功率损耗。通过对不同谐振器进行测量验证了理论估计的温度升高，并针对最坏情况进行了讨论。与MRI期间吸收的总功率相比，有源谐振器的射频功率吸收可忽略不计。在体模中嵌入的原型所观察到的局部温度升高处于排除直接组织损伤的范围内。然而，谐振器导电结构中的破裂可能会导致热点，从而可能产生较高的局部温度。通过设计具有低品质因数（Q）的谐振器或使电路略微失谐，可以降低这种风险；然而，这会降低谐振器设计的标称放大倍数。
