Liu Zhengjun, Ahmed Muneeb, Weinstein Yehuda, Yi Ming, Mahajan Roop L, Goldberg S Nahum
Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02215, USA.
Int J Hyperthermia. 2006 Jun;22(4):327-42. doi: 10.1080/02656730600609122.
To determine the effect of background tissue thermal conductivity on RF ablation heating using ex vivo agar phantoms and computer modelling.
Two-compartment cylindrical agar phantom models (5% agar, 5% NaCl, 3% sucrose) were constructed. These included a standardized inner compartment (2 cm diameter, 4 cm length, 0.25% agar) representing a tumour, surrounded by an outer compartment representing background tissue. The thermal conductivity of the outer compartment was varied from 0.48 W m-1 degrees Celsius (normal liver) to 0.23 W m-1 degrees Celsius (fat) by adding a fat-saturated oil-based solute (10-90%) to the agar. RF ablation was applied at 2000 mA current for 2 min. Temperatures were recorded up to 4 cm from the electrode tip at 1 cm intervals. Subsequently, a 2-D finite element computer model was used to simulate RF ablation of 2-24 min duration for tumours measuring 2-4 cm in diameter surrounded by tissues of different thermal conductivity with the presence or absence of perfusion (0-5 kg m-3 s-1) (n = 44). A comparison of results was performed.
In agar phantoms, the amount of fat in the background tissue correlated with thermal conductivity as a negative exponential function (r2 = 0.98). Significantly increased temperatures were observed at the edge of the inner compartment (1 cm from the electrode tip) as the fat content of the outer compartment increased (p < 0.01). Thus, temperatures at 2 min measured 31.5 +/- 2.2 degrees Celsius vs 45.1 +/- 3.1 degrees Celsius for thermal conductivities of 0.46 W m-1 degrees Celsius (10% fat) and 0.23 W m-1 degrees Celsius (90% fat), respectively. On the other hand, higher levels of fat led to lower temperature increases in the background compartment (0.2 +/- 0.3 degrees Celsius for 90% fat vs. 1.1 +/- 0.05 degrees Celsius for 10% fat, p < 0.05). Phantom thermal heating patterns correlated extremely well with computer modelling (r2 = 0.93), demonstrating that background tissues with low thermal conductivity increase heating within the central tumour, particularly for longer durations of RF ablation and in smaller tumours. Furthermore, computer modelling demonstrated that increases in temperature at the tumour margin for background tissues of lower thermal conductivity persisted in the presence of perfusion, with a clinically relevant 4.5 degrees Celsius difference between background thermal conductivities of fat and soft tissue for a 3 cm tumour with perfusion of 2 kg m-3 s-1, treated for 12 min.
Lower thermal conductivity of background tissues significantly increases temperatures within a defined ablation target. These findings provide insight into the 'oven effect' (i.e. increased heating efficacy for tumours surrounded by cirrhotic liver or fat) and highlight the importance of both the tumour and the surrounding tissue characteristics when contemplating RF ablation efficacy.
使用离体琼脂模型和计算机模拟来确定背景组织热导率对射频消融加热的影响。
构建双室圆柱形琼脂模型（5%琼脂、5%氯化钠、3%蔗糖）。这些模型包括一个标准化的内室（直径2 cm、长度4 cm、0.25%琼脂），代表肿瘤，周围是代表背景组织的外室。通过向琼脂中添加脂肪饱和油基溶质（10 - 90%），使外室的热导率从0.48 W m⁻¹℃（正常肝脏）变化到0.23 W m⁻¹℃（脂肪）。以2000 mA电流进行2分钟的射频消融。在距电极尖端4 cm范围内，每隔1 cm记录温度。随后，使用二维有限元计算机模型模拟直径2 - 4 cm的肿瘤在不同热导率组织包围下，有无灌注（0 - 5 kg m⁻³ s⁻¹）时持续2 - 24分钟的射频消融情况（n = 44）。对结果进行比较。
在琼脂模型中，背景组织中的脂肪量与热导率呈负指数函数相关（r² = 0.98）。随着外室脂肪含量增加，在内室边缘（距电极尖端1 cm处）观察到温度显著升高（p < 0.01）。因此，对于热导率分别为0.46 W m⁻¹℃（10%脂肪）和0.23 W m⁻¹℃（90%脂肪）的情况，2分钟时测量的温度分别为± 2.2℃和45.1 ± 3.1℃。另一方面，较高的脂肪水平导致背景室温度升高幅度较低（90%脂肪时为0.2 ± 0.3℃，10%脂肪时为1.1 ± 0.05℃，p < 0.05）。模型的热加热模式与计算机模拟结果高度相关（r² = 0.93），表明热导率低的背景组织会增加中央肿瘤内的加热，特别是对于较长时间的射频消融和较小的肿瘤。此外，计算机模拟表明，在有灌注的情况下，热导率较低的背景组织在肿瘤边缘的温度升高仍然存在，对于灌注为2 kg m⁻³ s⁻¹、治疗12分钟的3 cm肿瘤，脂肪和软组织背景热导率之间在临床上有4.5℃的差异。
背景组织较低的热导率会显著提高限定消融靶点内的温度。这些发现为“烤箱效应”（即被肝硬化肝脏或脂肪包围的肿瘤加热效果增强）提供了见解，并突出了在考虑射频消融疗效时肿瘤和周围组织特征的重要性。
