Radiofrequency ablation combined with conductive fluid-based dopants (saline normal and colloidal gold): computer modeling and ex vivo experiments.

BACKGROUND: The volume of the coagulation zones created during radiofrequency ablation (RFA) is limited by the appearance of roll-off. Doping the tissue with conductive fluids, e.g., gold nanoparticles (AuNPs) could enlarge these zones by delaying roll-off. Our goal was to characterize the electrical conductivity of a substrate doped with AuNPs in a computer modeling study and ex vivo experiments to investigate their effect on coagulation zone volumes. METHODS: The electrical conductivity of substrates doped with normal saline or AuNPs was assessed experimentally on agar phantoms. The computer models, built and solved on COMSOL Multiphysics, consisted of a cylindrical domain mimicking liver tissue and a spherical domain mimicking a doped zone with 2, 3 and 4 cm diameters. Ex vivo experiments were conducted on bovine liver fragments under three different conditions: non-doped tissue (ND Group), 2 mL of 0.9% NaCl (NaCl Group), and 2 mL of AuNPs 0.1 wt% (AuNPs Group). RESULTS: The theoretical analysis showed that adding normal saline or colloidal gold in concentrations lower than 10% only modifies the electrical conductivity of the doped substrate with practically no change in the thermal characteristics. The computer results showed a relationship between doped zone size and electrode length regarding the created coagulation zone. There was good agreement between the ex vivo and computational results in terms of transverse diameter of the coagulation zone. CONCLUSIONS: Both the computer and ex vivo experiments showed that doping with AuNPs can enlarge the coagulation zone, especially the transverse diameter and hence enhance sphericity.

Two-compartment mathematical modeling in RF tumor ablation: New insight when irreversible changes in electrical conductivity are considered.

The objective was to explore variations of temperature distribution and coagulation zone size computed by a two-compartment radiofrequency ablation (RFA) model when including simultaneously reversible changes in the tissue electrical conductivity (σ) due to temperature and irreversible changes due to thermal coagulation. Two-compartment (tumor and healthy tissue) models were built and simulated. Reversible change of σ was modeled by a piecewise function characterized by increments of +1.5%/℃ up to 100 ℃, and a 100 times smaller value from 100 ℃ onwards. Irreversible changes of σ were modeled using an Arrhenius model. We assumed that both tumor and healthy tissue had a different initial σ value (as suggested by the experimental data in the literature) and tended towards a common value as thermal damage progressed (necrotized tissue). We modeled a constant impedance protocol based on 90 V pulses voltage and three tumor diameters (2, 3 and 4 cm). Computer simulations showed that the differences between both models were only 0.1 and 0.2 cm for axial and transverse diameters, respectively, and this small difference was reflected in the similar temperature distributions computed by both models. In view of the available experimental data on changes of electrical conductivity in tumors and healthy tissue during heating, our results suggest that irreversible changes in electrical conductivity do not have a significant impact on coagulation zone size in two-compartment RFA models.

Passive cooling of cutaneous and subcutaneous tissues using phase changing materials: feasibility study using a numerical model.

In many dermatological applications, lowering the temperature of skin and maintaining specific temperatures for extended periods of time are fundamental requirements for treatment; for example, in targeting adipose tissue and managing cutaneous pain. In this work, we investigate the feasibility of using phase changing materials (PCMs) as an alternative passive, open-loop, heat extraction method for cooling cutaneous and subcutaneous tissues. We used a finite difference parametric approach to model the spatial and temporal progression of the heat transferred from the skin to a PCM in contact with the skin surface. We modelled the thermal performance of different PCMs, including different thicknesses. In addition, we used our model to propose application strategies. Numerical simulations demonstrate the feasibility of using PCMs for extracting heat from the skin and upper fat layers, inducing and maintaining similar temperatures as those induced by active closed-loop cooling with a cold plate. In terms of development, the critical design parameters are the temperature range of solidification of the material, the thickness of the material, and the rate of melting. Our study suggests that PCM-based devices may offer an alternative skin and adipose tissue cooling method that is simple to implement and use.

Study of the effect introduced by an integrating sphere on the temporal profile characterization of short laser pulses propagating through a turbid medium.

When a nanosecond laser pulse is transmitted through a highly scattering material, its irradiance decreases as it propagates; this is because of the spatial and temporal pulse profile stretching owing to multiple scattering events. Although the effect of temporal distortion is much less significant than that of the spatial distortion for applications where the laser beam is focused on a subsurface target (writing of waveguides, for example), it becomes significant for applications where the laser pulse must attain certain temporal width after the beam propagated is collimated through a turbid medium (photoacoustic tomography, for example). The objective of this work is to determine the transfer function associated to an integrating sphere measurement of the temporal intensity profile involving turbid media samples. The transfer function is found to be related to the geometrical characteristics of the integrating sphere and the optical properties of the turbid media. This procedure opens a new possibility for optical property characterization and enables the use of an integrating sphere for time-dependent intensity measurements.

Electrical-thermal performance of a cooled RF applicator for hepatic ablation with additional distant infusion of hypertonic saline: in vivo study and preliminary computer modeling.

PURPOSE: The Cool-tip electrode is one of the most widely employed applicators in radiofrequency (RF) hepatic ablation. Previous research demonstrated that it is possible to enlarge coagulation volume when the single cooled electrode is associated with distant infusion of saline (hybrid applicator). The aim of this study was to compare the electrical-thermal behaviour of the Cool-tip electrode with that of the hybrid applicator. MATERIALS AND METHODS: Forty-two RF ablations were performed on a total of 10 pigs: 22 with the Cool-tip electrode and 20 with the hybrid applicator (low infused saline volumetric flow rate of 6 mL/h at 2 mm distance). We compared both electrical performance (delivered power and number of roll-offs, i.e. sudden rises in impedance that interrupt the power delivery) and coagulation zone characteristics. In addition, we built a one-dimensional model to provide a basic physical explanation of the difference in performance between the different applicators. RESULTS: The experimental results showed that the number of roll-offs with the Cool-tip electrode was higher (24.3 ± 3.1 versus 6.7 ± 7.0). The hybrid applicator created larger coagulation volumes (19.7 ± 9.5 cm(3) versus 9.5 ± 5.8 cm(3)) with larger transverse diameters (2.5 ± 0.6 versus 1.9 ± 0.5 cm). The one-dimensional model confirmed the delay in the incidence of the first roll-off, but not the heterogeneity of the hybrid applicator's electrical performance in the experiments. CONCLUSIONS: The hybrid applicator produces fewer roll-off episodes than the Cool-tip electrode and creates larger coagulation volumes with larger transverse diameters.

Procedure to estimate thermophysical and geometrical parameters of embedded cancerous lesions using thermography.

Based on the fact that malignant cancerous lesions (neoplasms) develop high metabolism and use more blood supply than normal tissue, infrared thermography (IR) has become a reliable clinical technique used to indicate noninvasively the presence of cancerous diseases, e.g., skin and breast cancer. However, to diagnose cancerous diseases by IR, the technique requires procedures that explore the relationship between the neoplasm characteristics (size, blood perfusion rate and heat generated) and the resulting temperature distribution on the skin surface. In this research work the dual reciprocity boundary element method (DRBEM) has been coupled with the simulated annealing technique (SA) in a new inverse procedure, which coupled to the IR technique, is capable of estimating simultaneously geometrical and thermophysical parameters of the neoplasm. The method is of an evolutionary type, requiring random initial values for the unknown parameters and no calculations of sensitivities or search directions. In addition, the DRBEM does not require any re-meshing at each proposed solution to solve the bioheat model. The inverse procedure has been tested considering input data for simulated neoplasms of different sizes and positions in relation to the skin surface. The successful estimation of unknown neoplasm parameters validates the idea of using the SA technique and the DRBEM in the estimation of parameters. Other estimation techniques, based on genetic algorithms or sensitivity coefficients, have not been capable of obtaining a solution because the skin surface temperature difference is very small.

Analytical solution of the Pennes equation for burn-depth determination from infrared thermographs.

A serious problem in emergency medicine is the correct evaluation of skin burn depth to make the appropriate choice of treatment. In clinical practice, there is no difficulty in classifying first- and third-degree burns correctly. However, differentiation between the IIa (superficial dermal) and IIb (deep dermal) wounds is problematic even for experienced practitioners. In this work, the use of surface skin temperature for the determination of the depth of second-degree burns is explored. An analytical solution of the 3D Pennes steady-state equation is obtained assuming that the ratio between burn depth and the burn size is small. The inverse problem is posed in a search space consisting of geometrical parameters associated with the burned region. This space is searched to minimize the error between the analytical and experimental skin surface temperatures. The technique is greatly improved by using local one-dimensionality to provide the shape of the burned region. The feasibility of using this technique and thermography to determine skin burn depth is discussed.

Extent of lateral epidermal protection afforded by a cryogen spray against laser irradiation.

BACKGROUND AND OBJECTIVES: Cryogen spray cooling (CSC) has become an integral part of dermatologic laser surgery because of its ability to remove selectively large amounts of heat from human skin in short periods of time, thereby protecting the epidermis from unintended thermal injury. The objective of the present study is to investigate the extent of lateral epidermal protection afforded by a cryogen spray during laser irradiation. MATERIALS AND METHODS: CSC experiments on skin phantoms are conducted using a commercial nozzle (GentleLase, Candela) to characterize epidermal cooling in time and space; namely, surface temperatures and heat fluxes during a 60 milliseconds spurt and 30 milliseconds delay. Numerical methods are used to model the light distribution (755 nm), heat diffusion and thermal injury within the epidermis and dermis. A 755 nm laser (GentleLase, Candela) was used to assess in vivo the extent of lateral epidermal protection against irradiation in human skin. RESULTS: The commercial nozzle produced an uneven deposition and spread of liquid cryogen, thereby creating zones of high and low heat extraction on the surface. Numerical and in vivo studies show that 18 mm diameter laser beams may induce skin injury at the periphery of the irradiated areas. However, a 10 mm diameter beam provides the safest therapy because only the zone of highest heat extraction is exposed to laser irradiation. Beyond 10 mm, heat extraction is no more than a third of the maximum heat extraction within this diameter. CONCLUSIONS: Accumulation of heat within the epidermis is always greater at the laser beam periphery, away from the CSC nozzle tip, where heat extraction is lowest. Therefore, there is risk of thermal injury at the beam periphery when there is a mismatch between the skin protected by CSC and that exposed to laser irradiation. For the cooling and irradiation sequences considered herein, heat extraction provided by a 60 milliseconds spurt/30 milliseconds delay correctly matches the heating profile of a 10 mm diameter beam.

Laser-assisted cryosurgery of prostate: numerical study.

A new methodology for preventing freezing damage beyond pre-specified boundaries during prostate cryosurgery is proposed herein. It consists of emitting controlled laser irradiation from the urethra, across the wall and into the prostate while conventional cryoprobes freeze the unwanted prostate tissue. The purpose of this methodology is to protect the urethral wall better and confine the desired cryoinjured region more accurately than the current cryosurgery approach. We also explore the potential use of light-absorbing dyes to further enhance the laser light absorption and corresponding heat generation to increase the thickness of the protected region. A finite difference heat diffusion model in polar coordinates with temperature-dependent thermophysical properties simulates the prostate freezing while laser irradiation across the urethral wall is emitted. This approach maintains the temperature of the urethral wall and the adjacent tissue above a pre-specified threshold temperature of -45 degrees C, independent of application time. Temperature contours resulting from prostate cryoablation with (a) conventional constant temperature heating; (b) laser irradiation heating; and (c) laser irradiation heating with pre-injected light-absorbing dye layers indicate that the thickness of the protected region increases in this order, and that the latter two methodologies may be more effective in limiting cryoinjury to a predefined region compared to constant temperature heating. An analysis of laser power requirements and sensibility of laser-assisted cryosurgery (LAC) of prostate is also presented. It is shown that tissue temperature may vary as much as +/-20 degrees C with variations of +/-10% in laser power relative to the nominal power required to maintain the tissue at 37 degrees C. This demonstrates the sensitivity to laser power and the need of an accurate laser power control algorithm.

Efecto en la hidrodinámica y transferencia de calor del desfasamiento entre placas de un intercambiador de calor placas onduladas / Effects on the hydrodynamics and heat transfer of dephasing between plates in wavy plate passages

Se utilizó un programa numérico para obtener información respecto a los campos de velocidad, presión y temperatura de un canal que asemeja la parte central de un intercambiador de calor de placas corrugadas. Los resultados buscan analizar el efecto de la diferencia de ángulo de fase entre placas en el desempeño global del intercambiador de calor. El problema requirió la solución de la hidrodinámica y transferencia de calor bajo condiciones bidimensionales de estado permanente con flujo laminar en el canal formado por un par de placas sinusoidales de igual amplitud y longitud de onda entre las cuales existe una diferencia de temperatura. Se consideró un dominio computacional suficientemente largo que incluye varias corrugaciones para poder asumir condiciones periódicas para una longitud de onda de una corrugación. Se presentan resultados de transferencia de calor local y global analizados para un rango de ángulos de defasamiento entre placas. Se obtuvo una configuración para la cual la relación de transferencia de calor adimensional global a caída de presión adimensional es máxima. Los resultados obtenidos son explicados por la relación de los campos de velocidad y temperatura obtenidos en la simulación numérica

Estudio de los parámetros que afectan la transferencia de calor conjugada en intercambiador de calor de tubos y placas-aleta / Study of the parameters affecting conjugated heat transfer in plate-fin and tube heat exchanger

Se utilizó un método numérico para analizar el efecto conjugado de la conducción de calor a través de las aletas y la convección de calor desde la superficie de las mismas en un intercambiador de calor de tubos y placas-aleta. Las simulaciones se desarrollaron con valores de parámetros similares a los encontrados en intercambiadores de calor comerciales. Se analizó el efecto de varios parámetros en la transferencia de calor conjugada. La superficie de la aleta es dividida en dos regiones: aguas arriba del tubo donde la transferencia de calor es elevada, y aguas debajo del tubo donde la transferencia es limitada. La región aguas arriba del tubo se ve más afectada por la conducción a través de las aletas, con disminución de la transferencia de calor cuando la conducción es considerable. Es posible identificar una región de transferencia de calor inversa aguas abajo del tubo. Los parámetros que afectan mayormente la transferencia de calor conjugada son la conductividad y espesor de la aleta, el número de Reynolds y la excentricidad del tubo respecto a la aleta. Existe la posibilidad de mejorar la transferencia de calor del intercambiador haciendo el tubo excéntrico respecto a la longitud de la aleta. Al mover el tubo más cerca del borde de salida de las aletas el área de baja transferencia de calor detrás de los tubos se reduce en tamaño y, al mismo tiempo, la mayor longitud de la parte frontal de la aleta causa un incremento del área frontal, con una reducción del valor local del coeficiente convectivo. Esto sugiere la existencia de una posición óptima del tubo respecto a la longitud de la aleta