Microwave Imaging and Emerging Applications

Active microwave imaging for biomedical has been proposed, studied, and implemented to varying degrees by a plethora of worldwide institutions for the last 30 years. Much of the interest has been based on the fact that tissue dielectric properties embody important information about physiological state and function. Early efforts concentrated on techniques involving classic techniques such as diffraction imaging and the Born and Rytov approximations which ultimately proved too limited for the high degree of field scattering involved with electromagnetic fields. At a time of rapidly increasing computational power and speed, the electromagnetics field quickly adopted these capabilities for developing improved forward and inverse modeling tools. This was especially convenient in that hardware implementations could be rapidly simulated and validated without the considerable time and expense of fabrication and testing. The evolution of numerical capabilities has been considerable to the point where fully 3D software packages, both customized and commercial, can represent complex geometries in reasonable time frames with exquisite resolution. Microwave imaging will always be a hybrid between numerical modeling and hardware. The two principle approaches today involve some form of tomography or 3D inversion and radar-based techniques. The former primarily utilizes transmission data, in combination with an inverse algorithm, produces maps of the interrogated dielectric properties. The algorithms fall under the category of classical parameter estimation problems and stem from a basic mathematical statement to minimize the differences between the measured and computed fields. A wide range of optimization problems have been implemented including, but not limited to, Gauss-Newton iterative schemes, genetic algorithms and backprojection techniques. These have often been 2D and 3D implementations and involved single and multifre-quency modes. While this discipline has been dominated by simulation experiments, several efforts have expanded to at least the level of phantom validation investigations and even more advanced clinical implementations for breast cancer imaging. As with any technology, these realizations have involved important design trade-offs with respect to signal attenuation and detection strength, operating frequency, resolution, antenna selection, mechanical versus electrical array scanning, and many others. Early results have been encouraging and in an important development it is clear that the technique can recover accurate property maps that, as earlier hypothesized, can discern objects at levels well below the λ/2, Rayleigh criteria. Radar approaches have also progressed substantially over the last decade or two. While much of the earlier work remained in the simulation realm, more recent efforts have included …