Ahmad Safaai-Jazi’s research while affiliated with Virginia Tech and other places

A systematic approach for solving linear time‐invariant electric circuits with periodic input signals is presented. The method involves the following steps: (1) The governing differential equation for the desired output is solved for each time segment of the input signal in one period, leaving the constant coefficients of the homogeneous parts of the solution as unknowns. For an nth‐order circuit with a source consisting of q segments in one period, there are n×q$n\times q$ such unknown coefficients. (2) Using the solution obtained in step 1, the voltage of each capacitor and the current of each inductor are determined. (3) Transition conditions for the voltages and currents found in step 2 at the transition points of successive segments and also periodicity conditions for the beginning and end points of the period are applied. (4) Implementation of step 3 results in a system of n×q$n\times q$ equations in terms of the n×q$n\times q$ unknown coefficients described in step 1. Solving this system of equations, the circuit response in one period becomes readily available.

Objective: The use of high voltage, high frequency bipolar pulses (HFBPs) is an emerging electroporation-based therapy for the treatment of solid tumors. In this study, we quantify the extent of nonlinearity and dispersion during HFBP treatment. Methods: We utilize flat-plate electrodes to capture the impedance of porcine liver tissue during the delivery of a burst of HFBPs of widths 1μs and 2μs at different pulse amplitudes. Next, we fit the impedance data to a frequency dependent parallel RC network to determine the conductivity and permittivity of the tissue as a function of frequency, for different applied electric fields. Finally, we present a simple model to approximate the field distribution in tissue using the conductivity function at a frequency that could minimize the errors due to approximation with a nondispersive model. Results: The conductivity/permittivity of the tissue was plotted as a function of frequency for different electric fields. It was found that the extent of dispersion reduces with higher applied electric field magnitudes. Conclusion: This is the first study to quantify dispersion and nonlinearity in tissue during HFBP treatment. The data has been used to predict the field distribution in a numerical model of liver tissue utilizing two needle electrodes. Significance: The data and technique developed in this study to monitor the electrical properties of tissue during treatment can be used to generate treatment-planning models for future high frequency electroporation therapies as well as provide insights regarding treatment effect.

Uniformly excited, equally spaced linear arrays have a side lobe level larger than −13.3dB which is too high for many applications. This limitation can be remedied by nonuniform excitation of array elements. We present an efficient method for side lobe reduction in equally‐spaced linear arrays with low penalty on the directivity. The method involves the following steps: construction of a periodic function containing only the side lobes of the uniformly excited array, calculation of the Fourier series of this periodic function, subtracting the series from the array factor of the original uniformly excited array after it is truncated, and finally mitigating the truncation effects which yields significant increase in side lobe level reduction. A side lobe reduction factor is incorporated into element currents that makes much larger side lobe reductions possible and also allows varying the side lobe level incrementally. It is shown that such newly formed arrays can provide side lobe levels that are at least 22.7 dB below those of the uniformly excited arrays with the same size and number of elements. Analytical expressions for element currents are presented. Radiation characteristics of the side‐lobe‐reduced arrays introduced here are examined and numerical results for directivity, side lobe level and half‐power beamwidth are presented for example cases. Performance improvements over popular conventional array synthesis methods, such as Chebyshev and linear current tapered arrays, are obtained with the new method. This paper presents a novel side lobe reduction method for linear arrays that is fully analytical, allowing for rapid calculation of their radiation properties. The penalty on the array directivity is smaller than or nearly equal to those of other side‐lobe‐reduced arrays such as Chebyshev array. Analytical expressions for element currents are presented and numerical results for radiation properties of arrays with 3 to 100 elements are provided.

For irreversible electroporation (IRE) based therapies, the underlying electric field distribution in the target tissue is influenced by the electroporation-induced conductivity changes and is important for predicting the treatment zone. Objective: In this study, we characterized the liver tissue conductivity changes during high-frequency irreversible electroporation (H-FIRE) treatments of widths, 5 s and 10 s, and proposed a method for predicting the ablation zones. Methods: To achieve this, we cre-ated a finite element model (FEM) of the tissue treated with H-FIRE and IRE pulses based on experiments conducted in an in vivo rabbit liver study. We performed a parametric sweep on a Heaviside function that captured the tissue conductivity versus electric field behavior to yield a model current close to the ex-perimental current during the first burst/pulse. A temperature module was added to account for the current increase in subse-quent bursts/pulses. The evolution of the electric field at the end of the treatment was overlaid on the experimental ablation zones determined from Hematoxylin and Eosin staining to find the field thresholds of ablation. Results: Dynamic conductivity curves that provided a statistically significant relation between the model and experimental results were determined for H-FIRE. In addition, the field thresholds of ablation were obtained for the tested H-FIRE parameters. Conclusion: The proposed numerical model can simulate the electroporation process during H-FIRE. Signif-icance: The treatment planning method developed in this study can be translated to H-FIRE treatments of different widths and for different tissue-types.

A new wideband antenna made of a helical wire wrapped around a larger helix is introduced. This antenna thus has a doubly helical structure with two pitch angles and two radii. Compared to a conventional helix with comparable radiation properties, the doubly helical antenna occupies about 50% less volume. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:2351–2355, 2015

The application of a recently introduced microwave imaging technique based on the Huygens principle (HP), has been extended to multilayered objects with inclusions in this paper. The methodology of HP permits the capture of contrast such that different material properties within the region of interest can be discriminated in the final image, and its simplicity removes the need to solve inverse problems when forward propagating the waves. Therefore the procedure can identify and localize significant scatterers inside a multilayered volume, without having apriori knowledge on the dielectric properties of the target object. Additionally, an analytically-based approach for analyzing UltraWide Bandwidth (UWB) body propagation is presented, where the body is modeled as a 3-layer stratified cylinder with an eccentric inclusion. Validation of the technique through both simulations and measurements on multilayered cylindrical objects with inclusions has been performed.