An improved nonlinear ultrasonic technique for detecting and monitoring impact induced damage in composite plates.

An improved technique for sensing damage initiation and progression in thermoplastic resin composite plate specimens is presented in this study. The composite plate specimens are investigated by using a nonlinear ultrasonic (NLU) technique called Sideband Peak Count Index or SPC-I. The technique presented in this paper is an improvement from the previous SPC-I technique. This improved technique provides more reliable and consistent results and can monitor the damage progression over a wide range. In this paper the narrow band SPC-I technique is introduced to replace the conventional wide band SPC-I technique. The method implemented here is improved in three ways. First and foremost the narrow band SPC-I technique is introduced. Secondly, the non-permanently adhered gel coupled Lead-Zirconate-Titanate (PZT) transducers are used to reduce inconsistency in transducer adhesion and manufacturing. Lastly, higher sampling rate equipment is used for better signal resolution and peak counting. The experiments are performed on 4 sets of composite plate specimens fabricated using two composite fiber materials (Glass and Basalt) that have increasing levels of damage. The composite plate specimens were damaged by a falling weight impact machine with increasing impact energy (0 J, 10 J, 20 J and 30 J). The composite plate specimens were examined by propagating a narrow band chirp signal through the specimens using gel coupled transducers in a transmission mode setup. The received signals were recorded and analyzed using the NLU SPC-I technique. The modified SPC-I technique proposed in this paper can reliably and consistently detect both initiation and progression of damage in the composite plate specimens.

Temperature dependence of thermal properties of ex vivo liver tissue up to ablative temperatures.

Thermal properties of ex vivo bovine liver were measured as a function of temperature, by heating tissue samples in a temperature-controlled oil bath over a temperature range from about 21 °C to about 113 °C. Results evidenced temperature-dependent non-linear changes of the thermal properties, with the temperature of 100 °C representing a break point: the thermal properties increased with temperature up to 99 °C and then decreased above 100 °C. The rate of increase appeared dramatic between 90 °C and 99 °C, owing to the onset of vaporisation of water contained in the tissue. In particular, at 99 °C, the thermal conductivity reported an increase of about four times with respect to the value measured at 90 °C, whilst about a two-fold increase was reported for both the volumetric heat capacity and the thermal diffusivity. Temperatures higher than 100 °C were reached only after complete vaporisation of water contained in the tissue, resulting in about 70% loss of weight from the tissue. An overall decrease of about 71% and 63% was reported for the thermal conductivity and volumetric heat capacity, respectively, in the temperature range 101 °C-113 °C. A decrease of about 25% was reported in the measured values of the thermal diffusivity in the temperature range 101 °C-108 °C, whilst a slight increase of measured values, not statistically significant, was observed in the temperature range 108 °C-113 °C. The temperature dependent changes of the thermal parameters were modelled with non-linear regression analysis to calculate the best-fit curves interpolating measured data. The proposed regression models could be used to numerically assess the changes in the thermal properties of biological tissues at supra-physiological temperatures relevant in thermal ablation procedures, as well as their effect on the prediction of the ablation zone dimensions in computational models for treatment planning.

Microwave thermal ablation using CT-scanner for predicting the variation of ablated region over time: advantages and limitations.

This study aims at investigating in real-time the structural and dynamical changes occurring in an ex vivo tissue during a microwave thermal ablation (MTA) procedure. The experimental set-up was based on ex vivo liver tissue inserted in a dedicated box, in which 3 fibre-optic (FO) temperature probes were introduced to measure the temperature increase over time. Computed tomography (CT) imaging technique was exploited to experimentally study in real-time the Hounsfield Units (HU) modification occurring during MTA. The collected image data were processed with a dedicated MATLAB tool, developed to analyse the FO positions and HU modifications from the CT images acquired over time before and during the ablation procedures. The radial position of a FO temperature probe (rFO) and the value of HU in the region of interest (ROI) containing the probe (HUo), along with the corresponding value of HU in the contralateral ROI with respect to the MTA antenna applicator (HUc), were determined and registered over time during and after the MTA procedure. Six experiments were conducted to confirm results. The correlation between temperature and the above listed predictors was investigated using univariate and multivariate analysis. At the multivariate analysis, the time, rFO and HUc resulted significant predictive factors of the logarithm of measured temperature. The correlation between predicted and measured temperatures was 0.934 (p  < 0.001). The developed tool allows identifying and registering the image-based parameters useful for predicting the temperature variation over time in each investigated voxel by taking into consideration the HU variation.

Treatment planning in microwave thermal ablation: clinical gaps and recent research advances.

Microwave thermal ablation (MTA) is a minimally invasive therapeutic technique aimed at destroying pathologic tissues through a very high temperature increase induced by the absorption of an electromagnetic field at microwave (MW) frequencies. Open problems, which are delaying MTA applications in clinical practice, are mainly linked to the extremely high temperatures, up to 120 °C, reached by the tissue close to the antenna applicator, as well as to the ability of foreseeing and controlling the shape and dimension of the thermally ablated area. Recent research was devoted to the characterisation of dielectric, thermal and physical properties of tissue looking at their changes with the increasing temperature, looking for possible developments of reliable, automatic and personalised treatment planning. In this paper, a review of the recently obtained results as well as new unpublished data will be presented and discussed.

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 tissue's 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.

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.

Exposure setup to study potential adverse effects at GSM 1800 and UMTS frequencies on the auditory systems of rats.

To investigate possible biological effects of exposure to electromagnetic (EM) fields at the frequencies of global system for mobile communication (GSM) 1800 system and universal mobile telecommunication system (UMTS) on the auditory system of rats, an exposure setup for in vivo experiments is presented. The study was carried out in the framework of two European research projects. The target of the investigation was the cochlea. A dosimetric study was performed, both numerically and through direct measurements, to assess the interaction of the radiated fields and the dose distribution in the biological target. For the local exposure of rats, a loop antenna operating at the frequency bands of interest was designed, realised and characterised through numerical and experimental dosimetric procedures. Moreover, an exposure apparatus was set up, consisting of three arrays of four loop antennas, placed on three levels, thus allowing simultaneous exposure of 12 rats to give statistical power to the experiments. To isolate the exposure arrays, the setup was assembled by a wooden rack with EM field absorbing panels, inserted among the levels and at the four sides of the rack. Isolation was verified by direct measurements. Two exposure arrays were simultaneously supplied, whereas the third one was used for sham exposure. Blind exposure was achieved through a black box, hiding physical connections to the microwave power supply. During exposure sessions, rats were restrained in special plastic jigs for repeatable positioning, thus assuring the fixed level of dose in the target.

1800 MHz in vitro exposure device for experimental studies on the effects of mobile communication systems.

A wire patch cell (WPC) operating at the uplink frequency band of GSM 1800 MHz has been designed for in vitro experiments with the aim of investigating the possible biological effects of electromagnetic radiation associated with cellular phones. The 1800 MHz WPC design is a direct descendant of the original 900 MHz WPC introduced by Laval et al. This system provides a homogeneous specific absorption rate distribution, using four 3.5 cm petri dishes simultaneously. Numerical dosimetry has been performed using a commercial code (CST Microwave Studio), in order to evaluate accurately the efficiency of the structure (in terms of W kg(-1) per 1 W input power) and the distribution in the chosen biological target. The numerical results have been confirmed by experimental measurements performed by measuring thermal increase due to a high power impulse. The efficiency of the structure is 1.25 +/- 25% W kg(-1) per 1 W input power higher than the efficiency of the 900 MHz WPC. A few adjustments have been made in order to use the WPC in a standard incubator and to avoid thermal increases related to the radio frequency exposure. This exposure system has been adopted for the experiments scheduled in the RAMP and GUARD projects (VFPE).