Mapping of blood vessels for biometric authentication

Apparatus for thermoacoustic imaging, the apparatus comprising:

• a microwave source configured to provide a microwave output;
• a microwave emitter array including two or more unit cells, wherein the unit cells each comprise two or more near-field excitation elements;
• wherein the microwave emitter array is configured to receive the microwave output from the microwave source, and wherein the microwave emitter array is configured to provide microwave excitation to a target via the near-field excitation elements of the unit cells, wherein a spacing between the microwave emitter array and the target is 10 wavelengths or less, wherein the wavelength is a wavelength of the microwave emitter array within the target;
• wherein the microwave output is provided to the near-field excitation elements of the unit cells with relative phases selected to provide polarization eccentricity of 0.87 or less within the target;
• an acoustic sensor array having one or more sensor elements configured to receive acoustic signals from the target responsive to the microwave excitation;
• a processor configured to receive signals from the acoustic sensor array and to provide an acoustic image of the target.