page_title: Thermoacoustic imaging
section_title: Image formation principles
hierarchical_section_title: Thermoacoustic imaging / Image formation principles
caption_attribution_description: English: Cartoon illustrating generic instrumentation for acquiring thermoacoustic imaging data
context_page_description: Thermoacoustic imaging was originally proposed by Theodore Bowen in 1981 as a strategy for studying the absorption properties of human tissue using virtually any kind of electromagnetic radiation. But Alexander Graham Bell first reported the physical principle upon which thermoacoustic imaging is based a century earlier. He observed that audible sound could be created by illuminating an intermittent beam of sunlight onto a rubber sheet. Shortly after Bowen's work was published, other researchers proposed methodology for thermoacoustic imaging using microwaves. In 1994 researchers used an infrared laser to produce the first thermoacoustic images of near-infrared optical absorption in a tissue-mimicking phantom, albeit in two dimensions. In 1995 other researchers formulated a general reconstruction algorithm by which 2D thermoacoustic images could be computed from their "projections," i.e. thermoacoustic computed tomography. By 1998 researchers at Indiana University Medical Center extended TCT to 3D and employed pulsed microwaves to produce the first fully three-dimensional thermoacoustic images of biologic tissue [an excised lamb kidney ].
context_section_description: 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 absorption site with a point transducer.  A variety of strategies have been employed to mitigate this ambiguity.
caption: Fig. 4: Generic thermoacoustic imaging instrumentation
exif: {}