Broadband Dielectric Spectroscopy with a Microwave Ablation Antenna

Microwave ablation is a technique used to treat tumorous tissue. Its clinical use has been greatly expanding in the last few years. Because the design of the ablation antenna and the success of the treatment greatly depend on the accurate knowledge of the dielectric properties of the tissue being treated, it is highly valuable to have a microwave ablation antenna that is also able to perform in-situ dielectric spectroscopy. In this work, an open-ended coaxial slot ablation antenna design operating at 5.8 GHz is adopted from previous work, and its sensing abilities and limitations are investigated in respect of the dimensions of the material under test. Numerical simulations were performed to investigate the functionality of the floating sleeve of the antenna and to find the optimal de-embedding model and calibration option for obtaining accurate dielectric properties of the area of interest. Results show that, as in the case of the open-ended coaxial probe, the accuracy of the measurement greatly depends on the likeness between the calibration standards' dielectric properties and the material under test. Finally, the results of this paper clarify to which extent the antenna can be used to measure dielectric properties and paves the way to future improvements and the introduction of this functionality into microwave thermal ablation treatments

Evaluation of the Electromagnetic Power Absorption in Humans Exposed to Plane Waves: The Effect of Breathing Activity

The safety aspects of the exposure of people to uniform plane waves in the frequency range from 900 MHz to 5 GHz are analyzed. Starting from a human body model available in the literature, representing a man in resting state, two new anatomical models are considered, representing different phases of the respiratory activity: tidal breath and deep breath. These models have been used to evaluate the whole body Specific Absorption Rate (SAR) and the 10-g averaged and 1-g averaged SAR. The analysis is performed using a parallel implementation of the finite difference time domain method. A uniform plane wave, with vertical polarization, is used as an incident field since this is the canonical exposure situation used in safety guidelines. Results show that if the incident electromagnetic field is compliant with the reference levels promulgated by the International Commission on Non-Ionizing Radiation Protection and by IEEE, the computed SAR values are lower than the corresponding basic restrictions, as expected. On the other side, when the Federal Communications Commission reference levels are considered, 1-g SAR values exceeding the basic restrictions for exposure at 4 GHz and above are obtained. Furthermore, results show that the whole body SAR values increase passing from the resting state model to the deep breath model, for all the considered frequencies

safety aspects of people exposed to ultra wideband radar fields

The safety aspects of people exposed to the field emitted by ultra wideband (UWB) radar, operating both in the spatial environment and on ground, for breath activity monitoring are analyzed. The basic restrictions and reference levels reported in the ICNIRP safety guideline are considered, and the compliance of electromagnetic fields radiated by a UWB radar with these limits is evaluated. First, simplified analytical approaches are used; then, both a 3-dimensional multilayered body model and an anatomical model of the head have been used to better evaluate the electromagnetic absorption when a UWB antenna is placed in front of the head. The obtained results show that if the field emitted by the UWB radar is compliant with spatial and/or ground emission masks, then both reference levels and basic restrictions are largely satisfied

Hyperthermia Treatment Monitoring via Deep Learning Enhanced Microwave Imaging: A Numerical Assessment

Simple Summary Non-invasive temperature monitoring during hyperthermia cancer treatment is of paramount importance. It allows physicians to verify the therapeutic temperature is reached in the treated area. Currently, only superficial or invasive thermometry is performed on a clinical level. Magnetic resonance thermometry has been proposed as a a non-invasive alternative but its applicability is limited. Conversely, microwave imaging based thermometry is a potential low cost candidate for non-invasive temperature monitoring. This works presents a computational study in which the use of deep learning is proposed to face the challenges related to the use of microwave imaging in hyperthermia monitoring. The paper deals with the problem of monitoring temperature during hyperthermia treatments in the whole domain of interest. In particular, a physics-assisted deep learning computational framework is proposed to provide an objective assessment of the temperature in the target tissue to be treated and in the healthy one to be preserved, based on the measurements performed by a microwave imaging device. The proposed concept is assessed in-silico for the case of neck tumors achieving an accuracy above 90%. The paper results show the potential of the proposed approach and support further studies aimed at its experimental validation

Dielectric Properties of Healthy Ex-Vivo Ovine Lung Tissue at Microwave Frequencies

Knowledge of dielectric properties of lung tissue is fundamental for the improvement of lung disease diagnostics and therapeutic solutions (e.g. microwave imaging and microwave thermal ablation treatment). Although lung disease rates are increasing, lung tissue remains one of the least characterized tissues due to its heterogeneity, variability in air content, and handling difficulties. In this work, dielectric properties of ex-vivo ovine lung tissue samples were measured in the frequency range 500 MHz – 8 GHz, together with measurements of sample density (air content). Different Cole-Cole models were applied to the measured dielectric properties values. The best fitting model was chosen, and results were compared with available literature. Furthermore, the dielectric property measurements were correlated with the air content of the samples. Updated Cole-Cole models for lung tissue of different density is provided in the 500 MHz – 8 GHz range. The existence of air content threshold in lung is shown. Below this limit, the properties begin to change drastically with the change in densit

Safety Aspects of People Exposed to Ultra Wideband Radar Fields

The safety aspects of people exposed to the field emitted by ultra wideband (UWB) radar, operating both in the spatial environment and on ground, for breath activity monitoring are analyzed. The basic restrictions and reference levels reported in the ICNIRP safety guideline are considered, and the compliance of electromagnetic fields radiated by a UWB radar with these limits is evaluated. First, simplified analytical approaches are used; then, both a 3-dimensional multilayered body model and an anatomical model of the head have been used to better evaluate the electromagnetic absorption when a UWB antenna is placed in front of the head. The obtained results show that if the field emitted by the UWB radar is compliant with spatial and/or ground emission masks, then both reference levels and basic restrictions are largely satisfied

Broadband Dielectric Spectroscopy with a Microwave Ablation Antenna

Microwave ablation is a technique used to treat tumorous tissue. Its clinical use has been greatly expanding in the last few years. Because the design of the ablation antenna and the success of the treatment greatly depend on the accurate knowledge of the dielectric properties of the tissue being treated, it is highly valuable to have a microwave ablation antenna that is also able to perform in-situ dielectric spectroscopy. In this work, an open-ended coaxial slot ablation antenna design operating at 5.8 GHz is adopted from previous work, and its sensing abilities and limitations are investigated in respect of the dimensions of the material under test. Numerical simulations were performed to investigate the functionality of the floating sleeve of the antenna and to find the optimal de-embedding model and calibration option for obtaining accurate dielectric properties of the area of interest. Results show that, as in the case of the open-ended coaxial probe, the accuracy of the measurement greatly depends on the likeness between the calibration standards’ dielectric properties and the material under test. Finally, the results of this paper clarify to which extent the antenna can be used to measure dielectric properties and paves the way to future improvements and the introduction of this functionality into microwave thermal ablation treatments

uwb pulse propagation into human tissues

In this paper the propagation of a UWB pulse into a layered model of the human body is studied to characterize absorption and reflection of the UWB signal due to the different body tissues. Several time behaviours for the incident UWB pulse are considered and compared with reference to the feasibility of breath and heartbeat activity monitoring. Results show that if the UWB source is placed far from the human body, the reflection coming from the interface between air and skin can be used to detect the respiratory activity. On the contrary, if the UWB source is placed close to the human body, a small reflection due to the interface between the posterior lung wall and the bone, which is well distanced in time from the reflections due to the first layers of the body model, can be used to detect lung and heart changes associated with the cardio-respiratory activity

Tissue shrinkage in microwave ablation of liver: an ex vivo predictive model

The aim of this study was to develop a predictive model of the shrinkage of liver tissues in microwave ablation.Thirty-seven cuboid specimens of ex vivo bovine liver of size ranging from 2 cm to 8 cm were heated exploiting different techniques: 1) using a microwave oven (2.45 GHz) operated at 420 W, 500 W and 700 W for 8 to 20 min, achieving complete carbonisation of the specimens, 2) using a radiofrequency ablation apparatus (450 kHz) operated at 70 W for a time ranging from 6 to 7.5 min obtaining white coagulation of the specimens, and 3) using a microwave (2.45 GHz) ablation apparatus operated at 60 W for 10 min. Measurements of specimen dimensions, carbonised and coagulated regions were performed using a ruler with an accuracy of 1 mm. Based on the results of the first two experiments a predictive model for the contraction of liver tissue from microwave ablation was constructed and compared to the result of the third experiment.For carbonised tissue, a linear contraction of 31 ± 6% was obtained independently of the heating source, power and operation time. Radiofrequency experiments determined that the average percentage linear contraction of white coagulated tissue was 12 ± 5%. The average accuracy of our model was determined to be 3 mm (5%).The proposed model allows the prediction of the shrinkage of liver tissues upon microwave ablation given the extension of the carbonised and coagulated zones. This may be useful in helping to predict whether sufficient tissue volume is ablated in clinical practice