Thermoacoustic imaging

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.^[1] But Alexander Graham Bell first reported the physical principle upon which thermoacoustic imaging is based a century earlier.^[2] 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.^[3] 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 (2D).^[4] 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 (TCT).^[5] By 1998 researchers at Indiana University Medical Center^[6] extended TCT to 3D and employed pulsed microwaves to produce the first fully three-dimensional (3D) thermoacoustic images of biologic tissue [an excised lamb kidney (Fig. 1)].^[7] The following year they created the first fully 3D thermoacoustic images of cancer in the human breast, again using pulsed microwaves (Fig. 2).^[8] Since that time, thermoacoustic imaging has gained widespread popularity in research institutions worldwide.^{[9][10][11][12][13][14][15]} As of 2008, three companies were developing commercial thermoacoustic imaging systems – Seno Medical,^[16] Endra, Inc.^[17] and OptoSonics, Inc.^[18]