Ahmed Muneeb, Liu Zhengjun, Humphries Stanley, Goldberg S Nahum
Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02215, USA.
Int J Hyperthermia. 2008 Nov;24(7):577-88. doi: 10.1080/02656730802192661.
To use an established computer simulation model of radiofrequency (RF) ablation to characterize the combined effects of varying perfusion, and electrical and thermal conductivity on RF heating.
Two-compartment computer simulation of RF heating using 2-D and 3-D finite element analysis (ETherm) was performed in three phases (n = 88 matrices, 144 data points each). In each phase, RF application was systematically modeled on a clinically relevant template of application parameters (i.e., varying tumor and surrounding tissue perfusion: 0-5 kg/m(3)-s) for internally cooled 3 cm single and 2.5 cm cluster electrodes for tumor diameters ranging from 2-5 cm, and RF application times (6-20 min). In the first phase, outer thermal conductivity was changed to reflect three common clinical scenarios: soft tissue, fat, and ascites (0.5, 0.23, and 0.7 W/m- degrees C, respectively). In the second phase, electrical conductivity was changed to reflect different tumor electrical conductivities (0.5 and 4.0 S/m, representing soft tissue and adjuvant saline injection, respectively) and background electrical conductivity representing soft tissue, lung, and kidney (0.5, 0.1, and 3.3 S/m, respectively). In the third phase, the best and worst combinations of electrical and thermal conductivity characteristics were modeled in combination. Tissue heating patterns and the time required to heat the entire tumor +/-a 5 mm margin to >50 degrees C were assessed.
Increasing background tissue thermal conductivity increases the time required to achieve a 50 degrees C isotherm for all tumor sizes and electrode types, but enabled ablation of a given tumor size at higher tissue perfusions. An inner thermal conductivity equivalent to soft tissue (0.5 W/m- degrees C) surrounded by fat (0.23 W/m- degrees C) permitted the greatest degree of tumor heating in the shortest time, while soft tissue surrounded by ascites (0.7 W/m- degrees C) took longer to achieve the 50 degrees C isotherm, and complete ablation could not be achieved at higher inner/outer perfusions (>4 kg/m(3)-s). For varied electrical conductivities in the setting of varied perfusion, greatest RF heating occurred for inner electrical conductivities simulating injection of saline around the electrode with an outer electrical conductivity of soft tissue, and the least amount of heating occurring while simulating renal cell carcinoma in normal kidney. Characterization of these scenarios demonstrated the role of electrical and thermal conductivity interactions, with the greatest differences in effect seen in the 3-4 cm tumor range, as almost all 2 cm tumors and almost no 5 cm tumors could be treated.
Optimal combinations of thermal and electrical conductivity can partially negate the effect of perfusion. For clinically relevant tumor sizes, thermal and electrical conductivity impact which tumors can be successfully ablated even in the setting of almost non-existent perfusion.
使用已建立的射频（RF）消融计算机模拟模型，来描述不同灌注、电导率和热导率对射频加热的综合影响。
使用二维和三维有限元分析（ETherm）进行RF加热的双室计算机模拟，分三个阶段进行（共88个模型，每个阶段144个数据点）。在每个阶段，针对内部冷却的3 cm单电极和2.5 cm簇状电极，在临床上相关的应用参数模板上进行系统的RF应用建模（即不同的肿瘤和周围组织灌注：0 - 5 kg/m³·s），肿瘤直径范围为2 - 5 cm，以及RF应用时间（6 - 20分钟）。在第一阶段，改变外部热导率以反映三种常见的临床情况：软组织、脂肪和腹水（分别为0.5、0.23和0.7 W/m·℃）。在第二阶段，改变电导率以反映不同的肿瘤电导率（0.5和4.0 S/m，分别代表软组织和辅助盐水注射）以及代表软组织、肺和肾的背景电导率（分别为0.5、0.1和3.3 S/m）。在第三阶段，对电导率和热导率特征的最佳和最差组合进行联合建模。评估组织加热模式以及将整个肿瘤±5 mm边缘加热至>50℃所需的时间。
对于所有肿瘤大小和电极类型，增加背景组织热导率会增加达到50℃等温线所需的时间，但在较高的组织灌注下能够消融给定大小的肿瘤。内部热导率相当于软组织（0.5 W/m·℃）且被脂肪（0.23 W/m·℃）包围时，能在最短时间内实现最大程度的肿瘤加热，而被腹水（0.7 W/m·℃）包围的软组织达到50℃等温线所需时间更长，在较高的内部/外部灌注（>4 kg/m³·s）下无法实现完全消融。对于不同灌注情况下的不同电导率，在模拟电极周围注射盐水且外部电导率为软组织时，内部电导率会产生最大的RF加热，而在模拟正常肾中的肾细胞癌时加热量最少。对这些情况的特征描述证明了电导率和热导率相互作用的作用，在3 - 4 cm肿瘤范围内效果差异最大，因为几乎所有2 cm肿瘤和几乎没有5 cm肿瘤能够得到治疗。
热导率和电导率的最佳组合可以部分抵消灌注的影响。对于临床相关的肿瘤大小，热导率和电导率会影响哪些肿瘤即使在几乎不存在灌注的情况下也能成功消融。
