Thermoacoustic Imaging - Image Formation Principles

Image Formation Principles

Any thermoacoustic imaging device requires a source of electromagnetic radiation, be it a laser or a microwave antenna, to deliver energy to the anatomy being studied, and one or more acoustic detectors coupled acoustically to the outside surface of the anatomy, as is illustrated in Fig. 4.

The typical acoustic detector is an ultrasound transducer, which is commonly made of a piezo-electric material that converts detected pressure to an electrical signal. Thermoacoustic waves are induced within the anatomy wherever absorption takes place, and the strength of these thermoacoustic waves is proportional to the energy absorbed within the tissue. Some of these waves propagate through the anatomy over some time interval (time-of-flight) before being detected by one or more of the acoustic transducers. The exact time-of-flight is proportional to the distance between an absorption site and a transducer, assuming for the moment that each transducer is a point detector. For any given time-of-flight, each transducer will receive the sum of the thermoacoustic waves originating at the same distance from the detector in question as is illustrated in Fig. 5. For this reason, ambiguity arises when attempting to localize an aborption site with a point transducer. A variety of stratgies have been employed to mitigate this ambiguity.