Mark Haynes

Real-time Microwave Imaging of Differential Temperature for Thermal Therapy Monitoring

A microwave imaging system for real-time 3-D imaging of differential temperature has been developed for the monitoring and feedback of thermal therapy systems. Design parameters are constrained by features of a prototype-focused microwave thermal therapy system for the breast, operating at 915 MHz. Real-time imaging is accomplished with a precomputed linear inverse scattering solution combined with continuous vector network analyzer (VNA) measurements of a 36-antenna, HFSS-modeled, cylindrical cavity. Volumetric images of differential change of dielectric constant due to temperature are formed with a refresh rate as fast as 1 frame/s and 1 °C resolution. Procedures for data segmentation and postprocessed S-parameter error-correction are developed. Antenna pair VNA calibration is accelerated by using the cavity as the unknown thru standard. The device is tested on water targets and a simple breast phantom. Differentially heated targets are successfully imaged in cluttered environments. The rate of change of scattering contrast magnitude correlates 1:1 with target temperature.

Experimental Verification of the Recursive T-Matrix Method Solutions at Microwave Frequencies

We present an experimental verification of the recursive T-matrix method (RTM), which is a common method for solving electromagnetic scattering from multiple scatterers. This is done using an antenna and propagation model uniquely suited for T-matrices and network analyzer measurements. In the experiments, we use a multistatic system to measure the scattering from collections of objects consisting of conducting or dielectric spheres, as well as conducting cylinders. In simulation, we calculate the scattering from the objects using the RTM and further predict the transition parameters expected between the receivers and the transmitter in the measurement setup using a propagation model. The predicted and measured values of the transmission parameters are compared, and thereby used to verify the recursive T-matrix algorithm. Good agreement observed in these results provides experimental validation of the RTM.

Self-characterization of commercial ultrasound probes in transmission acoustic inverse scattering: transducer model and volume integral formulation

A self-contained source characterization method for commercial ultrasound probes in transmission acoustic inverse scattering is derived and experimentally tested. The method is based on modified scattered field volume integral equations that are linked to the source-scattering transducer model. The source-scattering parameters are estimated via pair-wise transducer measurements and the nonlinear inversion of an acoustic propagation model that is derived. This combination creates a formal link between the transducer characterization and the inverse scattering algorithm. The method is tested with two commercial ultrasound probes in a transmission geometry including provisions for estimating the probe locations and aligning a robotic rotator. The transducer characterization results show that the nonlinear inversion fit the measured data well. The transducer calibration and inverse scattering algorithm are tested on simple targets. Initial images show that the recovered contrasts are physically consistent with expected values.

Advances in real-time non-contact monitoring of medical thermal treatment through multistatic array microwave imaging

In this paper, we present some recent advances in real-time non-contact monitoring of thermal treatment through multistatic array imaging. The work presented was primarily motivated by the need to improve the flexibility and computational efficiency of the forward modeling of microwave imaging systems in general and of our real-time microwave thermal monitoring system in particular. Specifically, we have developed a conformal finite-difference time domain (CFDTD) solver that addresses several limitations inherent to our previously reported forward modeling methods. In particular, this CFDTD solver has enabled the implementation of a fully integrated numerical vector Greens function that addresses the task of linking the objects scattered field to the measured scattered voltage in a more general and flexible way than the waveport vector Greens function method used in our earlier work. Both the CFDTD solver and the integrated numerical Greens function are validated for a prototype microwave imaging cavity through comparison with CST Microwave Studio. These validated methods are currently being used in an ongoing experimental characterization of microwave imaging for both dielectric reconstruction and real-time thermal monitoring.

An optimized GPU-accelerated FDTD method for microwave imaging using a fast nonlinear inverse scattering algorithm

Summary form only given: Microwave imaging has received considerable attention as a low cost, non-invasive, non-ionizing method for breast cancer detection. In previous work, we have presented a time-domain nonlinear inverse scattering algorithm with multiparameter optimization for microwave imaging. In order to apply this algorithm to an experimental system that we have developed, it is crucial to have an accurate forward model of the imaging cavity. In this presentation, the modeling of the cavity using an in-house GPU accelerated finite-difference-time-domain (FDTD) method will be introduced, demonstrating several optimizations for increased computational efficiency and accuracy. S-parameter simulations of the cavity antennas comparing the results with the commercial software packages Ansys HFSS and CST MWS will be shown. Finally, results from microwave imaging tests of our GPU accelerated inversion algorithm using this fast forward model for both breast cancer detection and for real-time thermal monitoring of focused hyperthermia will be presented.The imaging cavity is a dodecagon consisting of 12 panels. Each panel has three dual-band bow-tie patch antennas operating at 915MHZ and 2.1GHz. In order to accurately capture the fine geometry of the cavity, we have utilized a nonuniform orthogonal mesh. The electrical field grid distance varies slowly in each direction, while the magnetic field resides in the middle of two adjacent electrical field. Though in this scenario the electrical field no long resides in the middle of two adjacent magnetic field points, which may result in first-order error locally, it has been shown by Monk1 that second-order error can still be achieved globally. In addition, we exploit the fact that within one Yee cell, the electrical field and magnetic field in each direction are half grids away to create an anisotropically filled Yee grid. This implementation maintains the accuracy of the cavity model with reduced grids and thus reduced cost of computation.

A calibrated 35 GHz airborne scatterometer for NASA's surface water and ocean topography mission

In this paper, an airborne Ka-band (35 GHz) FMCW scatterometer and its preliminary results are presented. The paper describes the system, the calibration and the data processing needed to study the response of different targets to the scatterometer. The results obtained in the first flights include normalized radar cross-section measurements of inland water bodies and a tree height estimation algorithm.

A preclinical system for focused microwave thermal therapy with integrated real-time 3D microwave thermal monitoring

The use of focused microwave thermal therapy as an adjuvant to radiatior and chemotherapy for breast cancer treatment continues to be an active a of research with the aim of reducing local recurrence, as well as reducing harmful side effects and cosmetic harm of traditional treatments. With it, there is a clinical need for real-time, non-invasive monitoring of subcutaneous heat deposition.

Series Reversion Solution for MIMO Microwave Feed Networks

A solution based on series reversion is derived and tested to estimate an N-port device-under-test (DUT) scattering matrix through a multiple-in/multiple-out microwave feed network. The feed network uses power dividers in place of fan-in/fan-out stages of traditional double-pole N-throw (2PNT) switching matrices. A fast recursive algorithm is derived to simultaneously estimate the DUT scattering matrix and solve the multiple scattering problem for weakly interacting networks. In experiment, the method was able to recover the scattering matrix of a passive DUT to within 0.1 dB in magnitude and 2° in phase.