Thermoacoustic imaging
From Wikipedia, the free encyclopedia
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]
Sound, which propagates as a pressure wave, can be induced in virtually any material, including biologic tissue, whenever time-varying electromagnetic energy is absorbed. The stimulating radiation that induces these thermally generated acoustic waves may lie anywhere in the electromagnetic spectrum, from high-energy ionizing particles to low-energy radio waves. The term "photoacoustic" (see photoacoustic imaging in biomedicine) applies to this phenomenon when the stimulating radiation is optical, while "thermoacoustic" is the more general term and refers to all radiating sources, including optical.
The process by which thermoacoustic waves are generated is depicted in the Figure 3. It can be understood as a four-step process:
- Biologic tissue is irradiated by an energy source that is absorbed by the body. The source of energy is non-specific, but typically consists of visible light, near infrared, radio waves or microwaves.
- The absorbed energy is converted to heat, which raises the temperature of the tissue, typically by less than 0.001 degree Celsius.
- The increase in the temperature of the tissue causes the tissue to expand in volume, however slightly.
- This mechanical expansion produces an acoustic wave that propagates outward in all directions from the site of energy absorption at the velocity of sound in biologic tissue, approximately 1.5 mm per microsecond.
When the tissue is irradiated with a pulse, the acoustic frequencies that characterize the acoustic wave span a range from zero to 1/(pulse width). E.g., a 1 microsecond pulse produces acoustic frequencies from zero to approximately 1 megahertz (MHz). Shorter pulses produce a wider range of acoustic frequencies. Frequencies greater than 1 MHz are referred to as ultrasonic, and are also associated with medical ultrasound applications.
