Maria Christopoulou

Age-dependent tissue-specific exposure of cell phone users

The peak spatial specific absorption rate (SAR) assessed with the standardized specific anthropometric mannequin head phantom has been shown to yield a conservative exposure estimate for both adults and children using mobile phones. There are, however, questions remaining concerning the impact of age-dependent dielectric tissue properties and age-dependent proportions of the skull, face and ear on the global and local absorption, in particular in the brain tissues. In this study, we compare the absorption in various parts of the cortex for different magnetic resonance imaging-based head phantoms of adults and children exposed to different models of mobile phones. The results show that the locally induced fields in children can be significantly higher (>3 dB) in subregions of the brain (cortex, hippocampus and hypothalamus) and the eye due to the closer proximity of the phone to these tissues. The increase is even larger for bone marrow (>10 dB) as a result of its significantly high conductivity. Tissues such as the pineal gland show no increase since their distances to the phone are not a function of age. This study, however, confirms previous findings saying that there are no age-dependent changes of the peak spatial SAR when averaged over the entire head.

Performance of a novel miniature antenna implanted in the human head for wireless biotelemetry

In this study, we present a novel, miniaturized, biocompatible antenna at the medical implant communications service (MICS) band (402–405 MHz) for integration in wireless biotelemetry devices implanted in the human head. To reduce simulation time, the antenna is designed while in the center of a skin tissue simulating box and subsequently implanted inside the skin tissue of an anatomical human head model. The resonance, radiation and specific absorption rate (SAR) performance of the antenna is evaluated and design modifications are suggested to overcome the inherent detuning effect.

PARAMETRIC STUDY OF POWER ABSORPTION PATTERNS INDUCED IN ADULT AND CHILD HEAD MODELS BY SMALL HELICAL ANTENNAS

A comparative assessment of power absorption in adult and child heads exposed to a small helical antenna at 1710MHz, is presented, emphasizing the efiect of age related parameters. Finite Difierence Time Domain simulations are employed to study the interaction between MRI-based head models and a mobile communication terminal equipped with a small helical monopole. A semi-analytical method, based on Greens function theory and the Method of Moments, is used to study the absorption in three- layer spherical head models exposed to a small helical dipole. SAR patterns in child head models derived by non-uniform scaling of adult ones were assessed against SAR patterns computed in child heads derived by uniform downscaling procedures. In both realistic and canonical exposure scenarios, comparable levels of absorbed power

Design of a Novel Miniaturized Implantable PIFA for Biomedical Telemetry

A broadband, circular, double-stacked, implantable planar inverted-F antenna (PIFA) is proposed for biomedical telemetry at f0= 402MHz. Both patches are meandered and a high permittivity substrate material is used to limit the radius and height of the antenna to 3.6 mm and 0.7 mm, respectively. The tuning and radiation characteristics as well as the specific absorption rate (SAR) distribution induced by the proposed antenna implanted inside a skin-tissue simulating box and inside the skin layer of a three-layer spherical human head model are evaluated. Simulations based on both finite-difference time-domain (FDTD) method and finite-element-method (FEM) are carried out. The feasibility of the communication link between the proposed antenna implanted in the spherical head model and an exterior λ 0/2 dipole antenna is also examined.

Exposure system to study hypotheses of ELF and RF electromagnetic field interactions of mobile phones with the central nervous system

A novel exposure system for double-blind human electromagnetic provocation studies has been developed that satisfies the precision, control of fields and potential artifacts, and provides the flexibility to investigate the response of hypotheses-driven electromagnetic field exposure schemes on brain function, ranging from extremely low frequency (ELF) to radio frequency (RF) fields. The system can provide the same exposure of the lateral cerebral cortex at two different RF frequencies (900 and 2140 MHz) but with different exposure levels at subcortical structures, and also allows uniform ELF magnetic field exposure of the brain. The RF modulation and ELF signal are obtained by a freely programmable arbitrary signal generator allowing a wide range of worst-case exposure scenarios to be simulated, including those caused by wireless devices. The maximum achievable RF exposure is larger than 60 W/kg peak spatial specific absorption rate averaged over 10 g of tissue. The maximum ELF magnetic field exposure of the brain is 800 A/m at 50 Hz with a deviation from uniformity of 8% (SD).

Modeling of EEG electrode artifacts and thermal ripples in human radiofrequency exposure studies

The effects of radiofrequency (RF) exposure on wake and sleep electroencephalogram (EEG) have been in focus since mobile phone usage became pervasive. It has been hypothesized that effects may be explained by (1) enhanced induced fields due to RF coupling with the electrode assembly, (2) the subsequent temperature increase around the electrodes, or (3) RF induced thermal pulsing caused by localized exposure in the head. We evaluated these three hypotheses by means of both numerical and experimental assessments made with appropriate phantoms and anatomical human models. Typical and worst-case electrode placements were examined at 900 and 2140 MHz. Our results indicate that hypothesis 1 can be rejected, as the induced fields cause <20% increase in the 10 g-averaged specific absorption rate (SAR). Simulations with an anatomical model indicate that hypothesis 2 is also not supported, as the realistic worst-case electrode placement results in a maximum skin temperature increase of 0.31 °C while brain temperature elevations remained <0.1 °C. These local short-term temperature elevations are unlikely to change brain physiology during the time period from minutes to several hours after exposure. The maximum observed temperature ripple due to RF pulses is <0.001 °C for GSM-like signals and <0.004 °C for 20-fold higher pulse energy, and offers no support for hypothesis 3. Thus, the mechanism of interaction between RF and changes in the EEG power spectrum remains unknown.

Novel methodology to characterize electromagnetic exposure of the brain

Due to the greatly non-uniform field distribution induced in brain tissues by radio frequency electromagnetic sources, the exposure of anatomical and functional regions of the brain may be a key issue in interpreting laboratory findings and epidemiological studies concerning endpoints related to the central nervous system. This paper introduces the Talairach atlas in characterization of the electromagnetic exposure of the brain. A hierarchical labeling scheme is mapped onto high-resolution human models. This procedure is fully automatic and allows identification of over a thousand different sites all over the brain. The electromagnetic absorption can then be extracted and interpreted in every region or combination of regions in the brain, depending on the characterization goals. The application examples show how this methodology enhances the dosimetry assessment of the brain based on results obtained by either finite difference time domain simulations or measurements delivered by test compliance dosimetry systems. Applications include, among others, the detailed dosimetric analysis of the exposure of the brain during cell phone use, improved design of exposure setups for human studies or medical diagnostic and therapeutic devices using electromagnetic fields or ultrasound.

Inter-Subject Variability Evaluation towards a Robust Microwave Sensor for Pneumothorax Diagnosis

Pneumothorax is the medical condition caused by the air concentration inside the pleural cavity, the space between the lung and the chest wall. Apart from traditional diagnostic methods, it can be detected by using microwave sensors that capture variations in reflected electromagnetic field (EMF). Sex and obesity, related to the internal composition of the biological tissues, can influence the reflected EMF and therefore the sensor diagnostic ability. This paper investigates the effect on the performance of a proposed on-body dual-patch antenna sensor for pneumothorax diagnosis, due to inter-subject variability in underlying tissue structure. The sensor operates at frequency range of 1-4 GHz. The challenge of the paper is to propose frequency bands for robust and safe sensor operation. S12 parameter alternation versus frequency is assessed for healthy and pathological cases. Implemented thorax numerical models include modified (i) closed rectangular multilayered and (ii) MRI-based anatomical ones. In rectangular models, thickness and configuration of muscle, fat and bone tissues are varied, according to literature. Additionally, sex-related anatomical differences are taken into account in MRI-based models. All scenarios are solved using Finite Difference Time Domain method. Results revealed that the proposed frequency bands lie within 1-2.7 and 2.9-3.5 GHz, for muscle, 1.4-3.5 GHz for fat and 1-2.2 and 2.8-3.5 GHz, for bone variations. Numerical evaluations for accurate anatomical models verify the findings.

Dual Patch Antenna Sensor for Pneumothorax Diagnosis: Sensitivity and Performance Study

Pneumothorax may cause serious health problems and often death if medical and surgical treatment is delayed. The absence of reliable, safe, portable and easy-to-use equipment in the ambulance is the primary clinical motivation of this work. We investigate pneumothorax diagnostic performance and sensitivity of a dual patch antenna system (sensor). The operation frequency range is set to 1-4 GHz. Parametric study is conducted using simplified rectangular tissue numerical models. Variation of S12 parameter, related to frequency, is compared in order to distinguish healthy and pneumothorax cases, reaching a difference of 20.1 dB, at 1.87 GHz. MRI-based anatomic models are also modified in order to simulate pneumothorax incident, in realistic clinical case. The best performance configuration scenario is applied onto the modified anatomic models, revealing satisfactory sensor performance (7.1 dB, at 2.3 GHz). Sensor diagnostic ability reaches 1 cm of air thickness. The paper concludes with proposed design specifications for thorax experimental phantom.

Design requirements of microwave sensor for pneumothorax diagnosis

We investigate the design of a low-power, non-invasive device for detection of air cavities in the space between chest and lung (i.e. pneumothorax). To this end, we study the operational frequency band and sensitivity capabilities of such device. Simplified scenarios of parameterized layered biological models in the exposure of electromagnetic plane wave and coupled radiating antennas are assessed, via an analytical solution and Finite Difference Time Domain (FDTD) method. The frequency band under study is 1-10 GHz. Differential comparative calculations of reflection coefficient and reflected electric field indicate the frequency band and resolution of the sensor. Results reveal that the existence of air alters considerably the reflection coefficient in the frequency band of 1-3 GHz. The resolution of the sensor is predicted to reach 5 mm of air layer thickness.