Vanni Lopresto

Changes in the dielectric properties of ex vivo bovine liver during microwave thermal ablation at 2.45 GHz

In microwave thermal ablation (MTA) therapy, the dielectric properties of the target tissue play an important role in determining the radiation properties of the microwave ablation antenna. In this work, the ex vivo dielectric properties of bovine liver were experimentally characterized as a function of the temperature during MTA at the frequency of 2.45 GHz. The obtained data were compared with measurements performed at the end of the MTA treatment, and considering the heating achieved with a temperature-controlled water bath. Finally, measured data were used to perform a numerical study evaluating the effects of changes in tissues dielectric properties during the MTA treatment on the radiation properties of a microwave interstitial ablation antenna, as well as on the obtained thermal lesion. Results evidenced a significant decrease of both relative permittivity (about 38%) and electric conductivity (about 33%) in the tissue during treatment as the temperature increased to over 60 °C, with a dramatic drop when the temperature approached 100 °C. Moreover, the numerical study evidenced that changes in tissues dielectric properties during the MTA treatment affect the distribution of the power absorbed by the tissue (specific absorption rate-SAR, W kg(-1)) surrounding the microwave interstitial ablation antenna, leading to a peak SAR up to 20% lower, as well as to a thermal lesion up to 8% longer. This work may represent a preliminary step towards the future development of a procedure for MTA treatment planning.

A radio-frequency system for in vivo pilot experiments aimed at the studies on biological effects of electromagnetic fields

An exposure system consisting of two long transversal electromagnetic (TEM) cells, operating at a frequency of 900 MHz, is presented and discussed. The set-up allows simultaneous exposure of a significant number of animals (up to 12 mice per cell) in a blind way to a uniform plane wave at a frequency of 900 MHz, for investigating possible biological effects of exposure to electromagnetic fields produced by wireless communication systems. A heating/refrigerating system has also been designed for maintaining comfortable environmental conditions within the TEM cells during experiments. An accurate dosimetric study has been performed both numerically and by means of direct measurements on phantoms and living mice. The results have shown that good homogeneity of exposure and adequate power efficiency, in terms of whole-body specific absorption rate (SAR) per 1 W of input power, are achievable for the biological target.

Characterisation of tissue shrinkage during microwave thermal ablation.

Abstract Purpose: The aim of this study was to characterise changes in tissue volume during image-guided microwave ablation in order to arrive at a more precise determination of the true ablation zone. Materials and methods: The effect of power (20–80 W) and time (1–10 min) on microwave-induced tissue contraction was experimentally evaluated in various-sized cubes of ex vivo liver (10–40 mm ± 2 mm) and muscle (20 and 40 mm ± 2 mm) embedded in agar phantoms (N = 119). Post-ablation linear and volumetric dimensions of the tissue cubes were measured and compared with pre-ablation dimensions. Subsequently, the process of tissue contraction was investigated dynamically during the ablation procedure through real-time X-ray CT scanning. Results: Overall, substantial shrinkage of 52–74% of initial tissue volume was noted. The shrinkage was non-uniform over time and space, with observed asymmetry favouring the radial (23–43 % range) over the longitudinal (21–29%) direction. Algorithmic relationships for the shrinkage as a function of time were demonstrated. Furthermore, the smallest cubes showed more substantial and faster contraction (28–40% after 1 min), with more considerable volumetric shrinkage (>10%) in muscle than in liver tissue. Additionally, CT imaging demonstrated initial expansion of the tissue volume, lasting in some cases up to 3 min during the microwave ablation procedure, prior to the contraction phenomenon. Conclusions: In addition to an asymmetric substantial shrinkage of the ablated tissue volume, an initial expansion phenomenon occurs during MW ablation. Thus, complex modifications of the tissue close to a radiating antenna will likely need to be taken into account for future methods of real-time ablation monitoring.

Experimental characterisation of the thermal lesion induced by microwave ablation

Abstract Purpose: This work focuses on the characterisation of the ablated area induced by a microwave thermal ablation (MTA) procedure. An experimental methodology for establishing a straightforward correlation between the temperature gradient and the changes in the dielectric properties of the tissue is presented and discussed. Materials and methods: Temperature measurements were performed during an ablation procedure in ex vivo bovine liver, at different distances from the antenna, whereas measurements of complex permittivity were conducted in sagittal sections of the ablated samples. The measured temperatures and dielectric properties were then correlated to obtain the dependence of the dielectric properties’ spatial variation on the temperature gradient. The obtained correlation has been validated through comparison with previously obtained experimental data. A weighted cubic polynomial function and a weighted sigmoid function have been tested for best-fit interpolation of the measured data. Results: Temperatures in the range 23–105 °C were measured during the MTA procedure, while, after the end of the MTA trials, relative permittivities in the range 7–43 and electric conductivities in the range 0.3–1.8 S/m were measured according to the distance from the antenna’s axis. The polynomial function showed better regression coefficients than the sigmoid one for both the relative permittivity (R2 = 0.9947 versus R2 = 0.9912, respectively) and the conductivity (R2 = 0.9919 versus R2 = 0.9866, respectively). However, the weighted cubic function showed an unrealistic behaviour for the relative permittivity at temperatures lower than 40 °C. Conclusions: According to the results obtained, information on the changes in the dielectric properties of the tissue under MTA treatment could be inferred from measured temperature data. Once validated by in vivo studies, the proposed methodology could be exploited to develop predictive tools for treatment planning.

A procedure to develop realistic numerical models of cellular phones for an accurate evaluation of SAR distribution in the human head

This paper presents an optimization procedure based on the minimization of a cost function and devoted to implement realistic numerical models of cellular phones to be used inside a finite-difference time-domain code. The adopted cost function depends on geometrical and electrical parameters of the phone and quantifies the accuracy of the model by comparing the simulation results with experimental measurements of the near electric and magnetic fields in free space, and of the specific absorption rate (SAR) in a homogeneous cubic phantom. As an example of the application of the proposed optimization method, a numerical model of a commercial phone has been implemented and the power deposition in an anatomically based model of the human head has been computed for various phone positions. The obtained results show that the use of inaccurate phone models can lead to large errors on local SAR evaluation.

Numerical models to evaluate the temperature increase induced by ex vivo microwave thermal ablation

Microwave thermal ablation (MTA) therapies exploit the local absorption of an electromagnetic field at microwave (MW) frequencies to destroy unhealthy tissue, by way of a very high temperature increase (about 60 °C or higher). To develop reliable interventional protocols, numerical tools able to correctly foresee the temperature increase obtained in the tissue would be very useful. In this work, different numerical models of the dielectric and thermal property changes with temperature were investigated, looking at the simulated temperature increments and at the size of the achievable zone of ablation. To assess the numerical data, measurement of the temperature increases close to a MTA antenna were performed in correspondence with the antenna feed-point and the antenna cooling system, for increasing values of the radiated power. Results show that models not including the changes of the dielectric and thermal properties can be used only for very low values of the power radiated by the antenna, whereas a good agreement with the experimental values can be obtained up to 20 W if water vaporization is included in the numerical model. Finally, for higher power values, a simulation that dynamically includes the tissues dielectric and thermal property changes with the temperature should be performed.

Design and realisation of tissue-equivalent dielectric simulators for dosimetric studies on microwave antennas for interstitial ablation

Thermal ablation therapies, based on electromagnetic field sources (interstitial or intracavitary antennas) at radio and microwave frequencies, are increasingly used in medicine due to their proven efficacy in the treatment of many diseases (tumours, stenosis, etc). Such techniques need standardized procedures, still not completely consolidated, as to analyze the behaviour of antennas for treatment optimisation. Several tissue-equivalent dielectric simulators (also named phantoms) have been developed to represent human head tissues, and extensively used in the analysis of human exposure to the electromagnetic emissions from hand-held devices; yet, very few studies have considered other tissues, as those met in ablation therapies. The objective of this study was to develop phantoms of liver and kidney tissue to experimentally characterise interstitial microwave antennas in reference conditions. Phantom properties depend on the simulated target tissue (liver or kidney) and the considered frequency (2.45 GHz in this work), addressing the need for a transparent liquid to easily control the positioning of the probe with respect to the antenna under test. An experimental set-up was also developed and used to characterise microwave ablation antenna performances. Finally, a comparison between measurements and numerical simulations was performed for the cross-validation of the experimental set-up and the numerical model. The obtained results highlight the fundamental role played by dielectric simulators in the development of microwave ablation devices, representing the first step towards the definition of a procedure for the ablation treatment planning.

Numerical and experimental characterization of through-the-body UHF-RFID links for passive tags implanted into human limbs

Radio frequency identification (RFID) in the UHF band has been recently proposed as enabling technology to develop implanted radio-sensors to be integrated into orthopedic prosthesis because of the power autonomy and standardized communication protocols. This paper investigates the feasibility of direct and forward links for UHF-RFID (860-960 MHz) tags implanted into human limbs, that are interrogated by a noncontacting readers antenna, with the purpose to label and, in a near future, to collect data about the health status of an implanted orthopedic prosthesis. Performance gain indicators of the through-the-body RFID channel are estimated by electromagnetic simulations over an anthropomorphic phantom as well as by means of experimentation with a real RFID communication link involving a simplified in vitro setup. The achieved results suggest that, by exploiting the current potentialities of RFID technology, and for the specific tag (loop antenna) and reader antenna (SPIFA) herein considered, a stable communication link with tags implanted inside limbs might be already feasible up to 10-35 cm from the body in full compliance with the constrains over electromagnetic exposure. In the particular case of implanted tag into an elbow, the estimated power margin in the direct and inverse links could be even suitable to set up sensing-oriented systems based onto turn-on and backscattered power modulation.

DOSIMETRY OF A SET-UP FOR THE EXPOSURE OF NEWBORN MICE TO 2.45-GHZ WIFI FREQUENCIES

This work describes the dosimetry of a two waveguide cell system designed to expose newborn mice to electromagnetic fields associated with wireless fidelity signals in the frequency band of 2.45 GHz. The dosimetric characterisation of the exposure system was performed both numerically and experimentally. Specific measures were adopted with regard to the increase in both weight and size of the biological target during the exposure period. The specific absorption rate (SAR, W kg(-1)) for 1 W of input power vs. weight curve was assessed. The curve evidenced an SAR pattern varying from <1 W kg(-1) to >6 W kg(-1) during the first 5 weeks of the life of mice, with a peak resonance phenomenon at a weight around 5 g. This curve was used to set the appropriate level of input power during experimental sessions to expose the growing mice to a defined and constant dose.

Influence of the target tissue size on the shape of ex vivo microwave ablation zones

Abstract Purpose: The aim of this study was to numerically and experimentally characterise the influence of tissues dimensions on the size and shape of microwave-induced ablation zones. Materials and methods: A 2.45 GHz interstitial antenna was introduced into ex vivo bovine liver samples, delivering 60 W for 10 min; then the dimensions of the coagulated area were measured. Ablations were performed both in large samples (termed unrestricted tissue) for characterising the tissue response, and in thin samples, whose dimensions in the plane perpendicular to the antenna were smaller than the short axis of the ablated area obtained in unrestricted samples. In the numerical study the electromagnetic field emitted from the antenna and the corresponding temperature increase were evaluated in both unrestricted and thin tissue samples. Results: When the height of the tissue was smaller than the ablation diameter measured in unrestricted samples, a 7.5% increase in length of the ablated zone was experimentally observed. When both the height and width were lower than the diameter measured in unrestricted samples, an elongation of about 23.4% was experimentally obtained. The numerical study showed that the boundary conditions between the target tissue and the surrounding materials are critical. Conclusions: The ex vivo performances of microwave ablation devices are notably influenced by the shape and dimension of the tissues where the procedure takes place. Accordingly, dedicated interventional protocols should be developed for treatment planning on targets of different shape and size.