Fernando Bardati

Modeling the detectability of vesicoureteral reflux using microwave radiometry

We present the modeling efforts on antenna design, frequency selection and receiver sensitivity estimation to detect vesicoureteral reflux (VUR) using microwave (MW) radiometry as warm urine from the bladder maintained at fever range temperature using a MW hyperthermia device reflows into the kidneys. The radiometer center frequency (f(c)), frequency band (Deltaf) and aperture radius (r(a)) of the physical antenna for kidney temperature monitoring are determined using a simplified universal antenna model with a circular aperture. Anatomical information extracted from the computed tomography (CT) images of children aged 4-6 years is used to construct a layered 3D tissue model. Radiometric antenna efficiency is evaluated in terms of the ratio of the power collected from the target at depth to the total power received by the antenna (eta). The power ratio of the theoretical antenna is used to design a microstrip log spiral antenna with directional radiation pattern over f(c) +/- Deltaf/2. Power received by the log spiral from the deep target is enhanced using a thin low-loss dielectric matching layer. A cylindrical metal cup is proposed to shield the antenna from electromagnetic interference (EMI). Transient thermal simulations are carried out to determine the minimum detectable change in the antenna brightness temperature (deltaT(B)) for 15-25 mL urine refluxes at 40-42 degrees C located 35 mm from the skin surface. Theoretical antenna simulations indicate maximum eta over 1.1-1.6 GHz for r(a) = 30-40 mm. Simulations of the 35 mm radius tapered log spiral yielded a higher power ratio over f(c) +/- Deltaf/2 for the 35-40 mm deep targets in the presence of an optimal matching layer. Radiometric temperature calculations indicate deltaT(B) 0.1 K for the 15 mL urine at 40 degrees C and 35 mm depth. Higher eta and deltaT(B) were observed for the antenna and matching layer inside the metal cup. Reflection measurements of the log spiral in a saline phantom are in agreement with the simulation data. The numerical study suggests that a radiometer with f(c) = 1.35 GHz, Deltaf = 500 MHz and detector sensitivity better than 0.1 K would be the appropriate tool to noninvasively detect VUR using the log spiral antenna.

Toward Online Adaptive Hyperthermia Treatment Planning: Correlation Between Measured and Simulated Specific Absorption Rate Changes Caused by Phase Steering in Patients

PURPOSE Hyperthermia is the clinical application of heat, in which tumor temperatures are raised to 40°C to 45°C. This proven radiation and chemosensitizer significantly improves clinical outcome for several tumor sites. Earlier studies of the use of pre-treatment planning for hyperthermia showed good qualitative but disappointing quantitative reliability. The purpose of this study was to investigate whether hyperthermia treatment planning (HTP) can be used more reliably for online adaptive treatment planning during locoregional hyperthermia treatments. METHODS AND MATERIALS This study included 78 treatment sessions for 15 patients with non-muscle-invasive bladder cancer. At the start of treatments, temperature rise measurements were performed with 3 different antenna settings optimized for each patient, from which the absorbed power (specific absorption rate [SAR]) was derived. HTP was performed based on a computed tomography (CT) scan in treatment position with the bladder catheter in situ. The SAR along the thermocouple tracks was extracted from the simulated SAR distributions. Correlations between measured and simulated (average) SAR values were determined. To evaluate phase steering, correlations between the changes in simulated and measured SAR values averaged over the thermocouple probe were determined for all 3 combinations of antenna settings. RESULTS For 42% of the individual treatment sessions, the correlation coefficient between measured and simulated SAR profiles was higher than 0.5, whereas 58% showed a weak correlation (R of <0.5). The overall correlation coefficient between measured and simulated average SAR was weak (R=0.31; P<.001). The measured and simulated changes in average SAR after adapting antenna settings correlated much better (R=0.70; P<.001). The ratio between the measured and simulated quotients of maximum and average SARs was 1.03 ± 0.26 (mean ± SD), indicating that HTP can also correctly predict the relative amplitude of SAR peaks. CONCLUSIONS HTP can correctly predict SAR changes after adapting antenna settings during hyperthermia treatments. This allows online adaptive treatment planning, assisting the operator in determining antenna settings resulting in increased tumor temperatures.

Time-dependent microwave radiometry for the measurement of temperature in medical applications

Microwave radiometry has been considered for the noninvasive monitoring of internal temperature in biological bodies when the temperature is varied under the control of external sources and contacting fluid. The body temperature is modeled as a discrete-time controlled statistical process, whose estimate is cyclically updated exploiting radiometric measurements. The Kalman filter has been used, which is able, with the proper choice of parameters, to balance the temperature retrieval between a priori information and measurements. Prospective applications to medicine have been investigated for temperature monitoring within a neonatal head during a hypothermia treatment.

Astigmatic beam tracing for gtd/utd methods in 3-D complex environments

Asymptotic techniques are usually employed for electromagnetic analysis of large and complex scenarios. Determination of ray paths from source to observation region is the first stage of a computation. Beam tracing methods, formerly developed for computer graphics, ensure continuous scenario coverage overcoming drawbacks of usual ray tracing. In this paper these are extended for application to electromagnetic analysis and improved to solve for multiple order interactions with scatterers by resorting to the astigmatic beam scheme. Wedge diffraction is solved recursively by minimising an aberration error. A numerical analysis has been performed for comparisons with a backward ray tracer showing a good agreement of results.

Time-domain macromodel of planar microwave devices by FDTD and moment expansion

The microwave design of highly complex systems can be addressed by segmentation techniques. To this purpose, a subsystem macromodel, such as the impulse response matrix, needs to be computed in an accurate and efficient way. In this paper, we present a combined procedure, based on finite-difference time-domain and a moment-expansion deconvolution by which the impulse response matrix is obtained via time-domain processing only. The algorithm has been tested on microwave planar devices with satisfactory accuracy.

A PROJECTIVE APPROACH TO ELECTROMAGNETIC PROPAGATION IN COMPLEX ENVIRONMENTS

High frequency methods resort to numerical ray tracing for application to complex environments. A new method based on the geometrical projection performed by a ray-congruence has been developed as a preconditioning of the ray tracing procedure. It builds a visibility tree, i.e., a database, storing information on all possible ray paths inside a scenario. The method gives a solution to a class of open problems of ray tracing techniques: ray missing, double (multiple) counting, termination criterion, calculation upgrade. Other features of the method are the multipath map and the multipath classification that allow the user to know the relevance of multipath at any point of the scenario in advance, before ray-tracing calculation. The method can be systematically applied to scenarios pertaining to different applications provided that the objects belong to the class of polyhedrons. Reflected and diffracted contributions in a scene are modelled as secondary sources which are handled with an off-line electromagnetic field calculation. Numerical analysis is provided showing the efficiency of the method.

FDTD improvement by dielectric subgrid resolution

Material inhomogeneities are taken into account in the standard finite-difference time domain method by staircase modeling of medium boundaries. Resolution is, therefore, limited by Yees cell sizes. In this paper, a new scheme is proposed, which improves material resolution without increasing the demand of computer resources.

Preconditioned astigmatic beam tracing for urban propagation

In wireless communication network planning, deterministic and statistical propagation tools have been developed. Astigmatic Beam Tracing, a deterministic method based on ray tracing (GO/UTD), can be applied to urban propagation modeling. A numerical code has been applied to a model of urban scenario with increasing complexity. The results of a numerical analysis are discussed to evaluate the relevance of the test case size on the computational charge.

Field interpolation across discontinuities in FDTD

Primary field quantities and their derivatives, like power flux density or surface equivalent currents, are usually required in practical applications of finite-difference time domain (FDTD). The data obtained from the FDTD algorithm are therefore interpolated to provide the whole set of the electrical and magnetic field components at each grid node to evaluate them. In the presence of a material discontinuity, however, the linear interpolation, having the accuracy of the FDTD algorithm at points where the field is continuous, will fail for the components which are orthogonal to the interface. A dielectric interpolator is presented for these cases which retains accuracy by preserving the correct field discontinuities.

New-born-infant brain temperature measurement by microwave radiometry

Hypothermia of the new-born-infant brain has been proposed as a neuroprotective therapy after hypoxia-ischaemia. At a preliminary stage of the new therapy the knowledge of the achievable brain temperature is desirable as well as its variation with time during treatment. There is the need, therefore, for non-invasive temperature measurement during hypothermia treatments. Multifrequency microwave radiometry has been considered for the measurement of the baby-head temperature during therapeutic cooling. Thermal and electromagnetic models for the baby head are required. A thermal model of the cooling has been arranged from the Pennes equation and data found in literature and the model used to generate the synthetic data to be exploited in temperature reconstruction. The inverse problem has been formulated in two different ways: (a) a retrieval based on a thermal model fitting by matrix pencil and least square minimisation; (b) a finite state-space approach with Kalman filtering of data. A numerical analysis has been performed showing that the radiometric measurements can partially compensate for errors in the model provided the temperature profile belongs to the class of functions specified by the model.