[Show abstract][Hide abstract] ABSTRACT: To compare thermal dosimetry metrics for specified diameters of coagulation achieved using three different ablation energy sources.
204 ablations measuring 20, 30, or 40 +/- 2 mm were created in an ex-vivo bovine liver model using 1) 2.5 cm cluster RF electrodes (n = 114), 2) 3 cm microwave antennas (n = 45), and 3) 3 cm laser diffusing fibers (n = 45). Continuous temperature monitoring was performed 5-20 mm from the applicators to calculate: a) the area under the curve (AUC), b) cumulative equivalent minutes at 43 degrees C (CEM43), and c) Arrhenius damage integral (Omega) for the critical ablation margin (DOC), with results compared by multivariate analysis of variance and regression analysis.
The end temperatures at the margin of coagulation varied, and was lowest for the RF cluster electrode (33-58 degrees C), higher for laser (52-72 degrees C), and covered the widest range for microwave (42-95 degrees C). These end temperatures correlated with applied energy, as linear functions (r(2) = 0.74-0.96). The total heat needed to achieve ablation (AUC) varied with applied energy and coagulation diameter as negative exponential (RF and laser) or negative power (microwave) functions (r(2) = 0.82-0.98). Similarly, CEM43 values varied exponentially with energy and distance (r(2) = 0.52-0.76) over a wide range of values (10(12)). Likewise, Omega varied not only based upon energy source and DOC, but also as a positive linear correlation to applied energy and with sigmoid correlation to duration of ablation (r(2) = 0.85-0.97).
Our study demonstrates that the thermal dosimetry of ablation is not based solely on a fixed end temperature at the margin of the coagulation zone. Thermal dosimetry is not constant, but dependent on the type and amount of energy applied and distance suggesting the need to take into account the rate of heat transfer for ablation dosimetry.
Full-text · Article · Dec 2009 · Academic radiology
[Show abstract][Hide abstract] ABSTRACT: To determine the effects of applied current, distance from an RF electrode and baseline tissue temperature upon thermal dosimetry requirements to induce coagulation in ex vivo bovine liver and in vivo porcine muscle models.
RF ablation was performed in ex vivo liver at varying baseline temperatures-19-21 degrees C (n = 114), 8-10 degrees C (n = 27), and 27-28 degrees C (n = 27)-using a 3-cm tip electrode and systematically varied current 400-1,300 mA, to achieve defined diameters of coagulation (20, 30 and 40 +/- 2 mm), and in in vivo muscle (n = 18) to achieve 35 mm +/- 2 mm of coagulation. Thermal dose required for coagulation was calculated as the area under the curve and cumulative equivalent minutes at 43 degrees C.
Thermal dose correlated with current in a negative exponential fashion for all three diameters of coagulation in ex vivo experiments (p < 0.001). The temperatures at the end of RF heating at the ablation margin were not reproducible, but varied 38 degrees C-74.7 degrees C, for 30 mm coagulation in ex vivo liver, and 59.8 degrees C-68.4 degrees C in the in vivo experiment. CEM(43) correlated with current as a family of positive exponential functions (r(2) = 0.76). However, a very wide range of CEM(43) values (on the order of 10(15)) was noted. Although baseline temperatures in the ex vivo experiment did not change required thermal dose, the relationships between end temperature at the ablation margin and RF current were statistically different (p < 0.001) as analysed at the 400 mA intercept.
In both models, thermal dosimetry required to achieve coagulation was not constant, but current and distance dependent. Hence, other formulas for thermal dose equivalence may be needed to predict conditions for thermal ablation.
No preview · Article · Jul 2008 · International Journal of Hyperthermia
[Show abstract][Hide abstract] ABSTRACT: To characterize the thermal dosimetry (ie, heating profile) of radiofrequency ablation (RFA) in multiple ex vivo tissues and in vivo tumor models.
RFA was performed for 3-24 minutes in ex vivo bovine livers (n=20), porcine kidneys (n=20), and turkey muscles (n=20) and in vivo canine venereal sarcomas (n=8). RFA was performed by using 1 and 3-cm long tips internally cooled electrodes. In addition, RFA was performed in in vivo R3220 rat mammary adenocarcinomas (n=36) and human renal cell carcinomas in nude mice (n=6) by using 1-cm monopolar electrodes. Continuous temperature monitoring was performed at multiple depths to calculate thermal dosimetry, reported as the area under the curve (AUC). Cumulative equivalent minutes at 43 degrees C (CEM43) were used for the critical ablation margin. Data were compared with analysis of variance and regression analysis.
For each tissue and/or tumor type, statistically significant temperature differences (up to 14 degrees) were observed at the ablation margin (P<.01). Temperature was dependent on the procedure duration. For 10-minute treatments, temperatures were significantly higher in the kidney compared with the R3230 tumor (72 degrees C+/-2.2) (P<.01) and lower in R3230 tumor (41.6 degrees C+/-1.4) (P<.05) but were similar for liver and muscle (51.6 degrees C+/-1.6 and 54.1 degrees C+/-1.8, respectively). Thus, a wide range of ablative temperatures were observed (41.0 degrees C+/-0.7 to 76.7 degrees C+/-1.9), with coagulation diameter correlating logarithmically with radiofrequency duration and AUC (R2=0.85-0.95). The CEM43 demonstrated an extreme range of values (10(11)).
The results of the study demonstrate a wide range of thermal sensitivity to RFA among commonly investigated tissues and tumor models, suggesting that further characterization of tissue-specific end points (ie, the duration and end temperature of ablation) is likely warranted. The AUC showed good correlation with ablation sizes, but the CEM43 proved unworkable given an extreme range of values for RFA.
No preview · Article · May 2007 · Journal of Vascular and Interventional Radiology
[Show abstract][Hide abstract] ABSTRACT: This study determined the effects of thermal conductivity on RF ablation tissue heating using mathematical modelling and computer simulations of RF heating coupled to thermal transport. Computer simulation of the Bio-Heat equation coupled with temperature-dependent solutions for RF electric fields (ETherm) was used to generate temperature profiles 2 cm away from a 3 cm internally-cooled electrode. Multiple conditions of clinically relevant electrical conductivities (0.07-12 S m-1) and 'tumour' radius (5-30 mm) at a given background electrical conductivity (0.12 S m-1) were studied. Temperature response surfaces were plotted for six thermal conductivities, ranging from 0.3-2 W m-1 degrees C (the range of anticipated clinical and experimental systems). A temperature response surface was obtained for each thermal conductivity at 25 electrical conductivities and 17 radii (n=425 temperature data points). The simulated temperature response was fit to a mathematical model derived from prior phantom data. This mathematical model is of the form (T=a+bRc exp(dR) s(f) exp(g)(s)) for RF generator-energy dependent situations and (T=h+k exp(mR)+n?exp(p)(s)) for RF generator-current limited situations, where T is the temperature (degrees C) 2 cm from the electrode and a, b, c, d, f, g, h, k, m, n and p are fitting parameters. For each of the thermal conductivity temperature profiles generated, the mathematical model fit the response surface to an r2 of 0.97-0.99. Parameters a, b, c, d, f, k and m were highly correlated to thermal conductivity (r2=0.96-0.99). The monotonic progression of fitting parameters permitted their mathematical expression using simple functions. Additionally, the effect of thermal conductivity simplified the above equation to the extent that g, h, n and p were found to be invariant. Thus, representation of the temperature response surface could be accurately expressed as a function of electrical conductivity, radius and thermal conductivity. As a result, the non-linear temperature response of RF induced heating can be adequately expressed mathematically as a function of electrical conductivity, radius and thermal conductivity. Hence, thermal conductivity accounts for some of the previously unexplained variance. Furthermore, the addition of this variable into the mathematical model substantially simplifies the equations and, as such, it is expected that this will permit improved prediction of RF ablation induced temperatures in clinical practice.
No preview · Article · Jun 2005 · International Journal of Hyperthermia