Busch Martin H J, Vollmann Wolfgang, Schnorr Jörg, Grönemeyer Dietrich H W
Research and Development Center for Microtherapy (EFMT), D-44799 Bochum, Germany.
Biomed Eng Online. 2005 Apr 8;4:25. doi: 10.1186/1475-925X-4-25.
Active Magnetic Resonance Imaging implants are constructed as resonators tuned to the Larmor frequency of a magnetic resonance system with a specific field strength. The resonating circuit may be embedded into or added to the normal metallic implant structure. The resonators build inductively coupled wireless transmit and receive coils and can amplify the signal, normally decreased by eddy currents, inside metallic structures without affecting the rest of the spin ensemble. During magnetic resonance imaging the resonators generate heat, which is additional to the usual one described by the specific absorption rate. This induces temperature increases of the tissue around the circuit paths and inside the lumen of an active implant and may negatively influence patient safety.
This investigation provides an overview of the supplementary power absorbed by active implants with a cylindrical geometry, corresponding to vessel implants such as stents, stent grafts or vena cava filters. The knowledge of the overall absorbed power is used in a finite volume analysis to estimate temperature maps around different implant structures inside homogeneous tissue under worst-case assumptions. The "worst-case scenario" assumes thermal heat conduction without blood perfusion inside the tissue around the implant and mostly without any cooling due to blood flow inside vessels.
The additional power loss of a resonator is proportional to the volume and the quality factor, as well as the field strength of the MRI system and the specific absorption rate of the applied sequence. For properly working devices the finite volume analysis showed only tolerable heating during MRI investigations in most cases. Only resonators transforming a few hundred mW into heat may reach temperature increases over 5 K. This requires resonators with volumes of several ten cubic centimeters, short inductor circuit paths with only a few 10 cm and a quality factor above ten. Using MR sequences, for which the MRI system manufacturer declares the highest specific absorption rate of 4 W/kg, vascular implants with a realistic construction, size and quality factor do not show temperature increases over a critical value of 5 K.
The results show dangerous heating for the assumed "worst-case scenario" only for constructions not acceptable for vascular implants. Realistic devices are safe with respect to temperature increases. However, this investigation discusses only properly working devices. Ruptures or partial ruptures of the wires carrying the electric current of the resonance circuits or other defects can set up a power source inside an extremely small volume. The temperature maps around such possible "hot spots" should be analyzed in an additional investigation.
主动式磁共振成像植入物被构建为谐振器，调谐至具有特定场强的磁共振系统的拉莫尔频率。谐振电路可嵌入正常金属植入物结构或添加到该结构中。这些谐振器构成电感耦合的无线发射和接收线圈，能够放大通常会因涡流而减弱的金属结构内部的信号，而不影响其余的自旋系综。在磁共振成像期间，谐振器会产生热量，这是除特定吸收率所描述的通常热量之外的额外热量。这会导致主动植入物的电路路径周围组织以及管腔内温度升高，可能对患者安全产生负面影响。
本研究概述了具有圆柱形几何形状的主动植入物（对应于血管植入物，如支架、支架移植物或腔静脉滤器）吸收的补充功率。在最坏情况假设下，利用总吸收功率的知识进行有限体积分析，以估计均匀组织内不同植入物结构周围的温度图。“最坏情况”假设植入物周围组织内无血液灌注的热传导，且血管内血流大多无任何冷却作用。
谐振器的额外功率损耗与体积、品质因数、磁共振成像系统的场强以及所应用序列的特定吸收率成正比。对于正常工作的设备，有限体积分析表明在大多数磁共振成像研究中仅会出现可耐受的发热情况。只有将几百毫瓦转化为热量的谐振器可能会使温度升高超过5K。这需要体积为几十立方厘米、电感电路路径短至仅几10厘米且品质因数高于10的谐振器。使用磁共振成像系统制造商宣称特定吸收率最高为4W/kg的磁共振序列时，具有实际构造、尺寸和品质因数的血管植入物不会出现超过临界值5K的温度升高。
结果表明，仅对于血管植入物不可接受的构造，在假设的“最坏情况”下会出现危险的发热。实际设备在温度升高方面是安全的。然而，本研究仅讨论了正常工作的设备。承载谐振电路电流的导线发生破裂或部分破裂或其他缺陷可能会在极小体积内形成电源。应在进一步研究中分析此类可能的“热点”周围的温度图。
