Thermoacoustic tomography using integrating detectors

Thermoacoustic (optoacoustic, photoacoustic) tomography is an emerging technology for imaging semitransparent objects, like soft biological tissue. Thermoacoustics combines the advantages of pure optical imaging with those of ultrasound imaging. In optical imaging the contrast is usually high, but the image resolution is poor because of the diffuse nature of light propagation. In ultrasonic imaging, the contrast between structures with similar acoustic properties (e.g. different soft tissue constituents) is low, but the spatial resolution is high because of the relatively unhindered propagation of ultrasonic waves. Due to the conversion of absorbed electromagnetic radiation into sound waves, thermoacoustics is able to generate images that at the same time have optical contrast and ultrasonic resolution. A novel measurement setup using integrating detectors is proposed and demonstrated by reconstructing the cross section of a grape. The use of integrating detectors to obtain time-retarded projections of optical absorption along surfaces parallel to the detector surface is quite different to current approaches and has important advantages. It avoids the problem of piezoelectric detectors to obtain adequate detection sensitivity when using the acoustically small element sizes required for "conventional" imaging reconstruction algorithms. Another advantage is that the reconstruction can be achieved via the inverse Radon Transform. Thus, many of the existing reconstruction methods (e.g. those using sparse data sets) developed for computerized tomography are directly applicable enabling the progress in this area to be rapid and integrating detectors may become widely adopted.