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S Melvyn Lobo

Publications and source records attributed to S Melvyn Lobo.

5 recordsLinked to original sources
PubMed2005

Radiofrequency ablation: importance of background tissue electrical conductivity--an agar phantom and computer modeling study.

PURPOSE: To determine whether radiofrequency (RF)-induced heating can be correlated with background electrical conductivity in a controlled experimental phantom environment mimicking different background tissue electrical conductivities and to determine the potential electrical and physical basis for such a correlation by using computer modeling. MATERIALS AND METHODS: The effect of background tissue electrical conductivity on RF-induced heating was studied in a controlled system of 80 two-compartment agar phantoms (with inner wells of 0.3%, 1.0%, or 36.0% NaCl) with background conductivity that varied from 0.6% to 5.0% NaCl. Mathematical modeling of the relationship between electrical conductivity and temperatures 2 cm from the electrode (T2cm) was performed. Next, computer simulation of RF heating by using two-dimensional finite-element analysis (ETherm) was performed with parameters selected to approximate the agar phantoms. Resultant heating, in terms of both the T2cm and the distance of defined thermal isotherms from the electrode surface, was calculated and compared with the phantom data. Additionally, electrical and thermal profiles were determined by using the computer modeling data and correlated by using linear regression analysis. RESULTS: For each inner compartment NaCl concentration, a negative exponential relationship was established between increased background NaCl concentration and the T2cm (R2= 0.64-0.78). Similar negative exponential relationships (r2 > 0.97%) were observed for the computer modeling. Correlation values (R2) between the computer and experimental data were 0.9, 0.9, and 0.55 for the 0.3%, 1.0%, and 36.0% inner NaCl concentrations, respectively. Plotting of the electrical field generated around the RF electrode identified the potential for a dramatic local change in electrical field distribution (ie, a second electrical peak ["E-peak"]) occurring at the interface between the two compartments of varied electrical background conductivity. Linear correlations between the E-peak and heating at T2cm (R2= 0.98-1.00) and the 50 degrees C isotherm (R2= 0.99-1.00) were established. CONCLUSION: These results demonstrate the strong relationship between background tissue conductivity and RF heating and further explain electrical phenomena that occur in a two-compartment system.

Agar↗
PubMed2004

Radiofrequency ablation: modeling the enhanced temperature response to adjuvant NaCl pretreatment.

PURPOSE: To characterize the effects of volume and concentration of adjuvant NaCl pretreatment on radiofrequency (RF) ablation and to model these results to determine their applicability to in vivo systems. MATERIALS AND METHODS: Standardized 1-L 5% agar phantoms were constructed with central wells of varying volume that were filled with a protein-based polymer gel of varying NaCl concentration (0%-35%). RF ablation to the maximum system current output (2000 mA) was applied to internally cooled 2-cm electrodes placed in the center of the gel wells. Remote thermometry was performed 20 mm from the electrode. Temperatures generated within the phantom were then used to model the response surface by using regression analysis. The generated model was then applied to previously published in vivo data to determine its applicability to a porcine liver tissue model. Statistical analyses included one-way analysis of variance to compare the temperatures reached with different NaCl concentrations and volumes with those reached without NaCl. In addition, modeled functions were evaluated for goodness of fit and the statistical significance of their coefficients. RESULTS: NaCl volume and concentration had significant effects on RF-generated heating of the agar phantoms. The mean maximum temperature, 91.4 degree C +/- 0.8 (SD), was reached with 3.5 mL of 10% NaCl gel. This was significantly higher than the mean temperature reached in phantoms containing 0% NaCl gel, 40.3 degree C +/- 4.9 (P <.001). Heat increases to the maximum temperature correlated strongly with the deposited RF energy, with maximum temperatures limited by the current output of the RF generator. The response surface was defined by a generator energy-dependent region and a generator current-limited region, which were best modeled by a modified gamma-variate function and an exponential function, respectively (r2 = 0.92). This model correlated well with previously published in vivo data (r2 = 0.86). CONCLUSION: Modulation of electrical conductivity has different effects on RF ablation response that are dependent on generator capabilities and the volume and concentration of NaCl pretreatment.

Animals↗
PubMed2004

Radiofrequency ablation: effect of surrounding tissue composition on coagulation necrosis in a canine tumor model.

PURPOSE: To determine the effect of surrounding tissue type on coagulation necrosis from radiofrequency (RF) ablation in a homogeneous animal tumor model. MATERIALS AND METHODS: Thirty canine venereal sarcomas were implanted in three tissue sites (subcutaneous, kidney, and lung) in 13 mildly immunosuppressed dogs. Five of 25 tumors, which were 19 mm +/- 3 (mean +/- SD) in diameter, were allocated to each of five groups: (a) subcutaneous tumors, (b) kidney tumors, (c) lung tumors with blood flow, and (d) subcutaneous and (e) renal tumors without blood flow, which was achieved by sacrificing the animal to eliminate tumor perfusion. A sixth group comprised larger subcutaneous tumors (mean diameter, 46 mm +/- 4) that were also treated. RF ablation was performed with a 1-cm tip and 5 minutes of ablation at 90 degrees C +/- 1. Impedance, temperature, and resultant coagulation diameter were recorded and compared. Data were analyzed statistically, including one-way analysis of variance to determine the effect of tissue conductivity (ie, systemic impedance) on necrosis size and tissue temperatures. Linear regression analysis was used to compare changes in impedance between the control and experimental groups. RESULTS: Increasing linear correlation was observed between tumor coagulation diameter and overall baseline system impedance (R(2) = 0.65). RF ablation of lung tumors resulted in the greatest coagulation diameter (13.0 mm +/- 3.5) compared with that in the other groups (P <.01). The smallest coagulation diameter was observed in kidney tumors in the presence of blood flow (7.3 mm +/- 0.6) compared with that in the other groups (P <.01). Elimination of blood flow in kidney tumors increased coagulation diameter to 10.3 mm +/- 0.6 (P <.01). After RF ablation, coagulation diameter in the subcutaneous tumor groups was the same (mean, 9.8 mm +/- 1.0) (difference not significant), regardless of tumor size or presence of blood flow. CONCLUSION: The characteristics of tissue that surrounds tumor, including vascularity and electric conductivity, affect ablation outcome. Predominance of tissue-specific characteristics will likely result in site-specific differences in RF-induced coagulation necrosis.

Animals↗
PubMed2002

Improved coagulation with saline solution pretreatment during radiofrequency tumor ablation in a canine model.

PURPOSE: To determine whether pretreatment with local NaCl injection can increase radiofrequency (RF)-induced coagulation in a large animal model. MATERIAL AND METHODS: Multiple canine venereal sarcomas (n = 25) were implanted subcutaneously in eight mildly immunosuppressed dogs (25 mg/kg cyclosporin A twice daily). Tumors were incubated for 8-12 weeks to a diameter of 4.2-6.3 cm (5.1 cm +/- 0.7). Internally cooled RF ablation (1-cm tip; 12 min; pulsed technique; 2,000-mA maximum) was performed. Tumors were pretreated with 6 mL of 18%, 24%, or 36% NaCl injected intratumorally under direct ultrasound guidance after RF electrode insertion, and this treatment was compared to RF treatment without NaCl injection and to 36% NaCl injection without RF ablation. Impedance measurements and remote thermometry were performed. These measurements and resultant coagulation were compared. RESULTS: Significantly greater RF heating (73 degrees C +/- 11 degrees C at 20 mm) was observed when the tumors were treated with 24% or 36% NaCl pretreatment, compared to the 47 degrees C +/- 5 degrees C observed when 18% or no NaCl was injected (P <.02). In the 36% NaCl group, the entire tumor (5.2 cm +/- 0.8 diameter) was completely ablated in every case, with coagulation extending several centimeters into the surrounding tissues. By comparison, control tumors (without NaCl injection) contained coagulation measuring 3.1 cm +/- 0.2, surrounded by viable, well-perfused tumor (P <.01), and 36% NaCl alone produced 2.7 cm +/- 0.6 of patchy necrosis. CONCLUSIONS: Pretreatment with intratumoral injection of small volumes of highly concentrated NaCl markedly increases RF heating and coagulation in a large animal tumor model. The complete destruction of tumors 5 cm in diameter or larger suggests that this substantial increase may be achieved for tumor ablation in clinical practice.

Animals↗