Thus, new complementary breast cancer imaging technologies are desired. Microwave-induced thermoacoustic imaging (TAI),combining the high contrast of microwaves and high resolution of ultrasound, is a promising candidate [7]. As a noninvasive imaging modality, TAI applies a short pulsed microwave signal to irradiate a breast. Some of the microwave energy is absorbed by the breast tissues and acoustic waves, generally referred to as thermoacoustic waves, are then generated from the breast tissues due to thermoelastic expansion. Malignant tissues, with higher dielectric loss than normal tissues, absorb more energy and emanate stronger acoustic waves. An acoustic transducer or transducer array measures the generated acoustic signals, which are collected to form an image. Locations, dimensions, and morphologies of tumors can be determined from the image. Besides higher image resolution, the acoustic heterogeneity is relatively benign compared to the microwave heterogeneity in human breast [8]. Therefore, TAI is relatively immune to the backscattering problem and image distortion in MI and thus endowed with much simpler signal processing.