Mikhail Andrew S, Negussie Ayele H, Graham Cole, Mathew Manoj, Wood Bradford J, Partanen Ari
Center for Interventional Oncology, Radiology and Imaging Sciences, Clinical Center, National Institutes of Health, Bethesda, Maryland 20892.
Center for Interventional Oncology, Radiology and Imaging Sciences, Clinical Center, National Institutes of Health, Bethesda, Maryland 20892 and Clinical Science MR Therapy, Philips, Andover, Massachusetts 01810.
Med Phys. 2016 Jul;43(7):4304. doi: 10.1118/1.4953394.
This work describes the characterization and evaluation of a tissue-mimicking thermochromic phantom (TMTCP) for direct visualization and quantitative determination of temperatures during radiofrequency ablation (RFA).
TMTCP material was prepared using polyacrylamide gel and thermochromic ink that permanently changes color from white to magenta when heated. Color vs temperature calibration was generated in matlab by extracting RGB color values from digital photographs of phantom standards heated in a water bath at 25-75 °C. RGB and temperature values were plotted prior to curve fitting in mathematica using logistic functions of form f(t) = a + b/(1 + e((c(t-d)))), where a, b, c, and d are coefficients and t denotes temperature. To quantify temperatures based on TMTCP color, phantom samples were heated to temperatures blinded to the investigators, and two methods were evaluated: (1) visual comparison of sample color to the calibration series and (2) in silico analysis using the inverse of the logistic functions to convert sample photograph RGB values to absolute temperatures. For evaluation of TMTCP performance with RFA, temperatures in phantom samples and in a bovine liver were measured radially from an RF electrode during heating using fiber-optic temperature probes. Heating and cooling rates as well as the area under the temperature vs time curves were compared. Finally, temperature isotherms were generated computationally based on color change in bisected phantoms following RFA and compared to temperature probe measurements.
TMTCP heating resulted in incremental, permanent color changes between 40 and 64 °C. Visual and computational temperature estimation methods were accurate to within 1.4 and 1.9 °C between 48 and 67 °C, respectively. Temperature estimates were most accurate between 52 and 62 °C, resulting in differences from actual temperatures of 0.6 and 1.6 °C for visual and computational methods, respectively. Temperature measurements during RFA using fiber-optic probes matched closely with maximum temperatures predicted by color changes in the TMTCP. Heating rate and cooling rate, as well as the area under the temperature vs time curve were similar for TMTCP and ex vivo liver.
The TMTCP formulated for use with RFA can be used to provide quantitative temperature information in mild hyperthermic (40-45 °C), subablative (45-50 °C), and ablative (>50 °C) temperature ranges. Accurate visual or computational estimates of absolute temperatures and ablation zone geometry can be made with high spatial resolution based on TMTCP color. As such, the TMTCP can be used to assess RFA heating characteristics in a controlled, predictable environment.
本研究描述了一种组织模拟热变色体模（TMTCP）的特性及评估，用于在射频消融（RFA）过程中直接可视化和定量测定温度。
使用聚丙烯酰胺凝胶和热变色墨水制备TMTCP材料，该墨水在加热时会从白色永久性变为品红色。通过从在25 - 75°C水浴中加热的体模标准品的数码照片中提取RGB颜色值，在Matlab中生成颜色与温度校准曲线。在Mathematica中，使用形式为f(t) = a + b/(1 + e((c(t - d))))的逻辑函数对RGB和温度值进行曲线拟合之前绘制它们，其中a、b、c和d是系数，t表示温度。为了基于TMTCP颜色定量温度，将体模样品加热到研究人员不知情的温度，并评估了两种方法：（1）将样品颜色与校准系列进行视觉比较；（2）使用逻辑函数的反函数进行计算机分析，将样品照片的RGB值转换为绝对温度。为了评估TMTCP在RFA中的性能，在加热过程中使用光纤温度探头从射频电极径向测量体模样品和牛肝中的温度。比较了加热和冷却速率以及温度与时间曲线下的面积。最后，根据RFA后对半切开的体模中的颜色变化通过计算生成温度等温线，并与温度探头测量结果进行比较。
TMTCP加热导致在40至64°C之间出现渐进性、永久性的颜色变化。视觉和计算机温度估计方法在48至67°C之间的准确度分别在1.4°C和1.9°C以内。温度估计在52至62°C之间最为准确，视觉和计算机方法与实际温度的差异分别为0.6°C和1.6°C。使用光纤探头在RFA期间进行的温度测量与TMTCP颜色变化预测的最高温度密切匹配。TMTCP和离体肝脏的加热速率、冷却速率以及温度与时间曲线下的面积相似。
为RFA使用而配制的TMTCP可用于在轻度热疗（40 - 45°C）、亚消融（45 - 50°C）和消融（>50°C）温度范围内提供定量温度信息。基于TMTCP颜色，可以以高空间分辨率对绝对温度和消融区几何形状进行准确视觉或计算机估计。因此，TMTCP可用于在可控、可预测的环境中评估RFA加热特性。
