Tumor boundary estimation through time-domain peaks monitoring: numerical predictions and experimental results in tissue-mimicking phantoms.

A method to estimate the boundary of a tumor using an interstitial microwave probe was evaluated in numerical and phantom models. This method utilizes time-domain signal reflection from the tumor/liver interface to provide information about tumor boundary in both radial and axial directions. Using computational experiments, tumors with radial diameters up to 25 mm were estimated with less than 1 mm error. Axial diameters were estimated with at most 5 mm error. Accuracy seemed to increase with radial diameter but decreased with axial diameter. Phantom experiments confirmed the computational results. These early results indicate that the proposed method may be used to estimate tumor boundary in both radial and axial dimensions without imaging. The technique may also be applicable in other situations that contain dielectric contrast between a volume of tissue and its background, such as monitoring tumor ablation growth. Additional work is needed to validate and optimize this method in tumor models.

An optimal sliding choke antenna for hepatic microwave ablation.

Microwave ablation (MWA) is a minimally invasive technique increasingly used for thermal therapy of liver tumors. Effective MWA requires efficient interstitial antennas that destroy tumors and a margin of healthy tissue, in situ, while minimizing damage to the rest of the organ. Previously, we presented a method for optimizing MWA antenna designs by coupling finite element method models of antennas with a real-coded, multiobjective genetic algorithm. We utilized this procedure to optimize the design of a minimally invasive choke antenna that can be used to create near-spherical ablation zones of adjustable size (radius 1-2 cm) by adjusting treatment durations and a sliding structure of the antenna. Computational results were validated with experiments in ex vivo bovine liver. The optimization procedure yielded antennas with reflection coefficients below -30 dB, which were capable of creating spherical ablation zones up to 2 cm in radius using 100 W input power at 2.45 GHz with treatment durations under 2 min.

Feasibility study of tumor size estimation through time domain peak monitoring.

A new ultrawideband (UWB) microwave method to estimate tumor size based upon detection of the tumor/liver interface is proposed. This method involves monitoring the response of a broadband pulse launched down a coaxial treatment antenna and radiated into the tumor. By monitoring the peak in the returned signal, and estimating the propagation velocity within the tumor, the location of the tumor/liver interface can be determined and the size of a spherical lesion estimated. The feasibility of this technique is demonstrated by finite element (FE) electromagnetic simulations of a spherical tumor in the liver. Robustness to noise is also investigated as well as the effects of insertion depth. The promising outcome of this feasibility study suggests that further development of this technique should be pursued.

Design optimization of a robust sleeve antenna for hepatic microwave ablation.

We describe the application of a Bayesian variable-number sample-path (VNSP) optimization algorithm to yield a robust design for a floating sleeve antenna for hepatic microwave ablation. Finite element models are used to generate the electromagnetic (EM) field and thermal distribution in liver given a particular design. Dielectric properties of the tissue are assumed to vary within +/- 10% of average properties to simulate the variation among individuals. The Bayesian VNSP algorithm yields an optimal design that is a 14.3% improvement over the original design and is more robust in terms of lesion size, shape and efficiency. Moreover, the Bayesian VNSP algorithm finds an optimal solution saving 68.2% simulation of the evaluations compared to the standard sample-path optimization method.

Expanding the bioheat equation to include tissue internal water evaporation during heating.

We propose a new method to study high temperature tissue ablation using an expanded bioheat diffusion equation. An extra term added to the bioheat equation is combined with the specific heat into an effective (temperature dependent) specific heat. It replaces the normal specific heat term in the modified bioheat equation, which can then be used at temperatures where water evaporation is expected to occur. This new equation is used to numerically simulate the microwave ablation of bovine liver and is compared to experimental ex vivo results.

Dielectric properties of human normal, malignant and cirrhotic liver tissue: in vivo and ex vivo measurements from 0.5 to 20 GHz using a precision open-ended coaxial probe.

Hepatic malignancies have historically been treated with surgical resection. Due to the shortcomings of this technique, there is interest in other, less invasive, treatment modalities, such as microwave hepatic ablation. Crucial to the development of this technique is the accurate knowledge of the dielectric properties of human liver tissue at microwave frequencies. To this end, we characterized the dielectric properties of in vivo and ex vivo normal, malignant and cirrhotic human liver tissues from 0.5 to 20 GHz. Analysis of our data at 915 MHz and 2.45 GHz indicates that the dielectric properties of ex vivo malignant liver tissue are 19 to 30% higher than normal tissue. The differences in the dielectric properties of in vivo malignant and normal liver tissue are not statistically significant (with the exception of effective conductivity at 915 MHz, where malignant tissue properties are 16% higher than normal). Also, the dielectric properties of in vivo normal liver tissue at 915 MHz and 2.45 GHz are 16 to 43% higher than ex vivo. No statistically significant differences were found between the dielectric properties of in vivo and ex vivo malignant tissue (with the exception of effective conductivity at 915 MHz, where malignant tissue properties are 28% higher than normal). We report the one-pole Cole-Cole parameters for ex vivo normal, malignant and cirrhotic liver tissue in this frequency range. We observe that wideband dielectric properties of in vivo liver tissue are different from the wideband dielectric properties of ex vivo liver tissue, and that the in vivo data cannot be represented in terms of a Cole-Cole model. Further work is needed to uncover the mechanisms responsible for the observed wideband trends in the in vivo liver data.

Current status of liver tumor ablation devices.

The liver is a common site of disease for both primary and metastatic cancer. Since most patients have a disease that is not amenable to surgical resection, tumor ablation modalities are increasingly being used for treatment of liver cancer. This review describes the current status of ablative technologies used as alternatives for resection, clinical experience with these technologies, currently available devices and design rules for the development of new devices and the improvement of existing ones. It focuses on probe design for radiofrequency ablation, microwave ablation and cryoablation, and compares the advantages and disadvantages of each ablation modality.

Measurement and analysis of tissue temperature during microwave liver ablation.

We measured tissue temperature changes during ex vivo microwave ablation (MWA) procedures for bovine liver tissue. Tissue temperature increased rapidly at the beginning of the MW power application. It came to a plateau at 100 degrees C to 104 degrees C before it increased again. We split the changes of tissue temperature versus time into four phases. This suggests that tissue temperature changes may be directly related to tissue water related phenomena during MWA, including evaporation, diffusion, condensation and tissue water composition. An additional analysis indicated the lesion boundary at approximately 50 degres C to 60 degrees C temperature. We also measured the water content of ablated tissue lesions and examined the relationship of tissue water content and tissue temperature by mapping temperature to remaining tissue water after ablation. The results demonstrate significant tissue water content changes and lead to a better understanding of tissue water movement.

Measurement of the specific heat capacity of liver phantom.

Phantoms are often used to simulate tissue during the development, testing and calibration of medical devices. In order to infer the specific absorption rate (SAR) and resistive heating in phantoms from temperature measurements, the specific heat capacity and density of the phantom are needed. Stauffer et al (2003 Int. J. Hyperth. 19 89-101) developed several phantoms that mimic dielectric properties of liver tissue at 915 MHz. However, thermal properties of the phantoms were not presented. We have measured specific heat capacities and densities for these phantoms. We also present dielectric properties for these phantoms measured from 0.7 to 20 GHz, including 2.45 GHz--a commonly used frequency for microwave hyperthermia and ablation.

A review of coaxial-based interstitial antennas for hepatic microwave ablation.

Although surgical resection remains the gold standard for treatment of liver cancer, there is a growing need for alternative therapies. Microwave ablation (MWA) is an experimental procedure that has shown great promise for the treatment of unresectable tumors and exhibits many advantages over other alternatives to resection, such as radiofrequency ablation and cryoablation. However, the antennas used to deliver microwave power largely govern the effectiveness of MWA. Research has focused on coaxial-based interstitial antennas that can be classified as one of three types (dipole, slot, or monopole). Choked versions of these antennas have also been developed, which can produce localized power deposition in tissue and are ideal for the treatment of deep-seated hepatic tumors.

A floating sleeve antenna yields localized hepatic microwave ablation.

We report a novel coaxial antenna for hepatic microwave ablation. This device uses a floating sleeve, that is, a metal conductor electrically isolated from the outer connector of the antenna coaxial body, to achieve a highly localized specific absorption rate pattern that is independent of insertion depth. This floating sleeve coaxial dipole antenna has low power reflection in the 2.4-GHz IMS band. Ex vivo experiments confirm our numerical simulation results. Index Terms-Ablation, coaxial aperture antennas, finite element methods, floating sleeve, microwave heating.

Ultrawideband temperature-dependent dielectric properties of animal liver tissue in the microwave frequency range.

The development of ultrawideband (UWB) microwave diagnostic and therapeutic technologies, such as UWB microwave breast cancer detection and hyperthermia treatment, is facilitated by accurate knowledge of the temperature- and frequency-dependent dielectric properties of biological tissues. To this end, we characterize the temperature-dependent dielectric properties of a representative tissue type-animal liver-from 0.5 to 20 GHz. Since discrete-frequency linear temperature coefficients are impractical and inappropriate for applications spanning wide frequency and temperature ranges, we propose a novel and compact data representation technique. A single-pole Cole-Cole model is used to fit the dielectric properties data as a function of frequency, and a second-order polynomial is used to fit the Cole-Cole parameters as a function of temperature. This approach permits rapid estimation of tissue dielectric properties at any temperature and frequency.

Antenna design for microwave hepatic ablation using an axisymmetric electromagnetic model.

BACKGROUND: An axisymmetric finite element method (FEM) model was employed to demonstrate important techniques used in the design of antennas for hepatic microwave ablation (MWA). To effectively treat deep-seated hepatic tumors, these antennas should produce a highly localized specific absorption rate (SAR) pattern and be efficient radiators at approved generator frequencies. METHODS AND RESULTS: As an example, a double slot choked antenna for hepatic MWA was designed and implemented using FEMLABtrade mark 3.0. DISCUSSION: This paper emphasizes the importance of factors that can affect simulation accuracy, which include boundary conditions, the dielectric properties of liver tissue, and mesh resolution.