Improved thermoacoustic imaging is provided by ensuring directional uniformity of the microwave excitation provided to the target being imaged. This directional uniformity can be quantified in terms of the eccentricity e of the polarization ellipse of the microwave excitation. We have e≤0.87, preferably e≤0.71, and more preferably e≤0.32. Optical excitation can be provided in addition to the microwave excitation. Excitation can be performed at multiple optical wavelengths and/or microwave frequencies to improve depth uniformity. In addition, the employment of excitation cells with optimized spacing and geometry provides the uniformity in another two degrees of freedom. One potential application is to detect blood vessel in user's finger for biometric authentication.