Thermoacoustic Imaging - Thermoacoustic Wave Production

Thermoacoustic Wave Production

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”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:

1. 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.
2. The absorbed energy is converted to heat, which raises the temperature of the tissue, typically by less than 0.001 degree Celsius.
3. The increase in the temperature of the tissue causes the tissue to expand in volume, however slightly.
4. This mechanical expansion produces an acoustic wave that propagates outward in all directions from the sight 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.