PHOTOACOUSTIC IMAGING

LITRON LASERS FOR PHOTOACOUSTIC IMAGING

Photoacoustic imaging is an emerging technique with much promise in the field of medical imaging. A pulsed laser, typically with nanosecond pulse duration and a repetition frequency of tens of hertz, is used to irradiate a region of tissue, whereupon the radiation is absorbed and rapid thermal expansion gives rise to high frequency (megahertz) ultrasonic acoustic waves.

Different tissue types absorb laser radiation to differing degrees, depending on the laser wavelength and amount of the tissue type of interest and thus acoustic waves of differing intensities are generated. Ultrasonic transducers detect these waves and a three dimensional computer-generated image is formed of the differing acoustic intensities. Depth resolution is achieved by computing the time-of-flight of the acoustic wave from its generation to reaching the detector. This is used in the related technique of photoacoustic tomography.

Examples of the application of photoacoustic imaging include the in vivo detection and imaging of cancerous tumors such as skin melanomas and associated blood vessel development, as well as oxygenation levels in blood vessels.

Photoacoustic imaging using laser wavelengths in the visible and near infrared region is non-invasive, with penetration depths of up to a few millimetres while giving resolution in the region of a few microns.

Laser sources typically used in photoacoustic imaging comprise Q-switched, pulsed Nd:YAG lasers providing several tens or hundreds of millijoules at 1064nm, 532nm or 355nm, pumping broad band optical parametric oscillators (OPOs) to give tunable nanosecond pulses across the visible and near infrared spectral region.