[unreadable] This proposal describes a research program to develop an experimental prototype ultrawideband stepped frequency microwave-induced thermoacoustic imaging (TAI) system to image a breast phantom. Recent studies have shown that imaging the human breast for cancer detection with microwave-induced thermoacoustic waves is a viable technology. This new technology has many desirable properties of an ideal early breast cancer screener including being noninvasive, safe, comfortable, inexpensive, sensitive (to tumors), and specific (to cancers). Indeed, microwave-induced TAI combines the merits of both microwave stimulation, which provides excellent contrast between cancerous and normal breast tissue, and acoustic imaging, which has the advantage of very fine millimeter range spatial resolution. However, so far the experimental imaging results of real human breast are much poorer than those obtained under rather idealistic experimental setup. The goal of this study is to identify the key problems encountered by the microwaveinduced TAI under the realistic conditions of human breast imaging and propose innovative techniques to mitigate these problems, during Phase I of the fast-track application: 1. develop a prototype ultrawideband stepped-frequency microwave-induced TAI system to image a breast phantom, 2. demonstrate the benefits of multiple frequency stimulation for breast phantom anomaly detection, and 3. demonstrate the merits of using adaptive signal processing algorithms for thermoacoustic image formation and breast phantom anomaly detection. By the completion of this study, we will have developed and evaluated an experimental prototype ultrawideband stepped frequency microwave-induced TAI system to image a breast phantom. We will have obtained sufficient supporting evidence and experience for the Phase II efforts. We finally anticipate that the results of this and subsequent studies will provide new opportunities in a wide range of multidisciplinary studies ranging from electromagnetics and acoustics to adaptive beamforming for thermoacoustic image formation and pattern recognition for early cancer detection. [unreadable] [unreadable]