A photoacoustic photothermal optical fibre sensor for the minimally invasive detection of cancers and other tissue pathologies

We are developing a photoacoustic/photothermal optical fibre sensor for the minimally invasive, non-destructive detection of cancers and other tissue pathologies. It will make use of the optical, thermal, acoustic and spatial tissue properties to help make an immediate in vivo diagnosis. The sensor has the advantage that it can measure the photoacoustic and photothermal tissue responses simultaneously. Furthermore, the directions of optical excitation and of photoacoustic and photothermal detection are coaxial, thereby simplifying the measurements and their interpretation. The sensor is based on a thin transparent polymer film acting as a low-finesse Fabry-Perot interferometer that is mounted at the distal end of a multimode optical fibre and illuminated by a low power tuneable diode laser. Laser pulses below the tissue damage threshold are launched into the fibre, transmitted through the polymer film and absorbed in the target, where both an ultrasonic thermoelastic wave and a low frequency thermal wave are produced. The changes in optical thickness of the film due to the acoustically and thermally induced stresses are detected interferometrically. We demonstrate the ability of the sensor to make photothermal measurements from which differences in the optical properties of phantom tissues (mu(a) = 0.05 mm-1, mu(s)' = 0-2.5 mm(-1)) can be detected. An absorption coefficient of 0.05 mm(-1) was found to be the lowest detectable absorption in the absence of scattering. A Finite Element model of heat diffusion is being employed to interpret the photothermal measurements made using the optical fibre sensor.