Isaac Chang

Thermal modeling of lesion growth with radiofrequency ablation devices

BackgroundTemperature is a frequently used parameter to describe the predicted size of lesions computed by computational models. In many cases, however, temperature correlates poorly with lesion size. Although many studies have been conducted to characterize the relationship between time-temperature exposure of tissue heating to cell damage, to date these relationships have not been employed in a finite element model.MethodsWe present an axisymmetric two-dimensional finite element model that calculates cell damage in tissues and compare lesion sizes using common tissue damage and iso-temperature contour definitions. The model accounts for both temperature-dependent changes in the electrical conductivity of tissue as well as tissue damage-dependent changes in local tissue perfusion. The data is validated using excised porcine liver tissues.ResultsThe data demonstrate the size of thermal lesions is grossly overestimated when calculated using traditional temperature isocontours of 42°C and 47°C. The computational model results predicted lesion dimensions that were within 5% of the experimental measurements.ConclusionWhen modeling radiofrequency ablation problems, temperature isotherms may not be representative of actual tissue damage patterns.

Finite Element Analysis of Hepatic Radiofrequency Ablation Probes using Temperature-Dependent Electrical Conductivity

BackgroundFew finite element models (FEM) have been developed to describe the electric field, specific absorption rate (SAR), and the temperature distribution surrounding hepatic radiofrequency ablation probes. To date, a coupled finite element model that accounts for the temperature-dependent electrical conductivity changes has not been developed for ablation type devices. While it is widely acknowledged that accounting for temperature dependent phenomena may affect the outcome of these models, the effect has not been assessed.MethodsThe results of four finite element models are compared: constant electrical conductivity without tissue perfusion, temperature-dependent conductivity without tissue perfusion, constant electrical conductivity with tissue perfusion, and temperature-dependent conductivity with tissue perfusion.ResultsThe data demonstrate that significant errors are generated when constant electrical conductivity is assumed in coupled electrical-heat transfer problems that operate at high temperatures. These errors appear to be closely related to the temperature at which the ablation device operates and not to the amount of power applied by the device or the state of tissue perfusion.ConclusionAccounting for temperature-dependent phenomena may be critically important in the safe operation of radiofrequency ablation device that operate near 100°C.

Radio-Frequency Ablation in a Realistic Reconstructed Hepatic Tissue

This study uses a reconstructed vascular geometry to evaluate the thermal response of tissue during a three-dimensional radiofrequency (rf) tumor ablation. MRI images of a sectioned liver tissue containing arterial vessels are processed and converted into a finite-element mesh. A rf heat source in the form of a spherically symmetric Gaussian distribution, fit from a previously computed profile, is employed. Convective cooling within large blood vessels is treated using direct physical modeling of the heat and momentum transfer within the vessel. Calculations of temperature rise and thermal dose are performed for transient rf procedures in cases where the tumor is located at three different locations near the bifurcation point of a reconstructed artery. Results demonstrate a significant dependence of tissue temperature profile on the reconstructed vasculature and the tumor location. Heat convection through the arteries reduced the steady-state temperature rise, relative to the no-flow case, by up to 70% in the targeted volume. Blood flow also reduced the thermal dose value, which quantifies the extent of cell damage, from approximately 3600 min, for the no-flow condition, to 10 min for basal flow (13.8 cms). Reduction of thermal dose below the threshold value of 240 min indicates ablation procedures that may inadequately elevate the temperature in some regions, thereby permitting possible tumor recursion. These variations are caused by vasculature tortuosity that are patient specific and can be captured only by the reconstruction of the realistic geometry.

Radiofrequency Ablation With a Gaussian Heat Source in a Realistic Reconstructed Hepatic Geometry

This study describes a detailed methodology for modeling a three-dimensional radiofrequency ablation procedure using reconstructed porous tissue geometries. In this study, MRI images of a sectioned liver tissue containing arterial vessels are converted into a finite element mesh. An rf heat source in the form of a spherically symmetric Gaussian distribution, fit from a previously computed profile, is employed Computations of temperature rise were performed for transient rf procedures in the case where the tumor is located near the bifurcation point of a hepatic artery. Results demonstrate a significant effect due to convective cooling by the large vessels. Substantial asymmetries in the temperature profiles indicate ablation procedures that may achieve adequate tumor destruction in some regions, but that elevate the temperature only minimally in other regions, thereby permitting possible tumor recursion. These critical features of the temperature field are due to the directional nature of the arterial flow and are difficult to capture with models that treat perfusion with a scalar source term in the bioheat equation.Copyright

FINITE ELEMENT ANALYSIS OF RADIO-FREQUENCY ABLATION IN A RECONSTRUCTED REALISTIC HEPATIC GEOMETRY

Radio-frequency (RF) ablation is a minimally invasive procedure that has the potential for widespread use in hepatic cancer therapy. In the procedure, RF current is applied to the tissue, resulting in the conversion of electrical to heat energy and thus, a rise in temperature, with the goal of eventual tumor necrosis. Potential complications from the procedure include insufficient heating of large tumors, resulting in tumor recursion, as well as excessive thermal damage to healthy tissue. Mathematical models are valuable in predicting the temperature rise within the organ during RF ablation, thereby enhancing the success rate of the procedure. Eventually, models can be used to guide ablation procedures, by predicting the optimal set of operational parameters e.g., catheter probe geometry and placement, given patient-specific information. The present study focuses on the analysis of temperature rise within a reconstructed model of a realistic three-dimensional (3D) section of a porcine liver during RF ablation. This study calculates the effect of blood flow through arteries as well as perfusion through the liver on the time-dependent temperature distribution near the RF ablation probe (Figure 1). For a time duration of 30 min of an ablation procedure, a temperature of about 80°C could be achieved over a diameter of about 4 cm with the present RF probe. As an initial step, the present study includes isotropic hepatic tissue and blood properties.Copyright

Comparison Between Isotropic and Anisotropic Electrical Properties of a DTI-Based Cardiac Model

Cardiac function is influenced by the three-dimensional organization of the myocardial fibers. Cardiac fibers are arranged in a circumferential, longitudinal, and a sheet-like fashion, forming counter-wound helices from the base to the apex of the heart. This fiber organization is responsible for the delicate balance between mechanical and electrical functioning of the heart. When electrical disruption of this coordinated function occurs, this is associated with cardiac arrhythmias which may lead to more serious conditions like ventricular fibrillation.Copyright

Numerical evaluation of heating in the human head due to magnetic resonance imaging (MRI)

In this paper we present a numerical model for evaluating tissue heating during magnetic resonance imaging (MRI). Our method, which included a detailed anatomical model of a human head, calculated both the electromagnetic power deposition and the associated temperature elevations during a MRI head examination. Numerical studies were conducted using a realistic birdcage coil excited at frequencies ranging from 63 MHz to 500 MHz. The model was validated both experimentally and analytically. The experimental validation was performed at the MR test facility located at the FDAs Center for Devices and Radiological Health (CDRH).