Optoacoustic imaging

Optoacoustic imaging (Photoacoustic Imaging) is an imaging technology based on the photothermal effect, and can be used to obtain images of structures in turbid environments. The optoacoustic technique combines the accuracy of spectroscopy with the depth resolution of ultrasound.

Background
The oldest technical application of the photothermal effect is believed to be the photophone, a communication device invented by Bell (1880). Bell observed the emanation of audible sounds from a transparent tube containing material in suspension, when a modulated light source is focused on the tube. Modulation of the light impinging on an absorbing substance will produce a similar modulation in temperature via the photothermal effect.

Optoacoustic wave generation
When a short laser pulse irradiates an absorbing medium there is local absorption →local heating → local expansion. This local expansion leads to ultrasonic pressure waves that travel through the medium at the speed of sound, and can be recorded using high frequency pressure sensors (such as piezoelectric sensors or optical sensors). The slow speed of sound in tissue (~1500 m/s) in comparison to the speed of light allows for the time resolved detection of these pressure waves and the determination of depth from where these pressure waves originated. By using an array of sensors the temporal delay of these incoming pressure wavefronts can be combined into an ultrasound image.

Proposed applications
Although the technology is still in its infancy, optoacoustic imaging is being employed in the development of various devices. Such devices include breast cancer detection equipment, as well as equipment used for measuring blood oxygenation levels. In both cases, the change in the optical properties of blood in respect to oxygen saturation and the strong optical contrast between hemoglobin and surrounding tissue is utilized.