Optical Wavelength Effects Observed in Photoacoustic Signals from Biological Tissue

Abstract

We have previously reported our observations that, at laser wavelengths of 1,064 and 532 nm, following repeated exposure to short pulses at power density levels well below those reported to cause permanent damage to biological tissue, the photoacoustic signal detected using an ultrasound probe diminishes significantly. Examination by electron microscopy of such tissue at the site at which repeated laser pulse exposures have been made reveals little evidence of structural changes to the tissue. The effect that we have observed remains unexplained.

The experiments referred to above were carried out as part of a programme to develop a new, combined form of diagnostic and therapeutic probe in which the laser power is delivered via an optical fibre which is concentric with a miniature forward and sideways looking ultrasound transducer. At the laser wavelengths mentioned previously signal decay on repetitive laser shots is of great concern since it reduces signal-to-noise ratios. We have carried out further experiments at a wavelength of 266 nm. At this wavelength, the diminution of the photoacoustic signal does not occur, although the energy level that we are currently able to operate at is not sufficient to give ultrasonic echoes from structure within samples of arterial wall.

This paper reports on these recent experiments and on the development of the new form of intra-arteriäl probe.