Laser-Based Cancer Diagnostics and Therapy - Tissue Optics considerations

Abstract: Near-IR light penetrates deeply into tissue, but as the scattering coefficient is much larger than the absorption coefficient in this wavelength region, direct viewing through tissue is very difficult. In order to detect and locate inhomogeneities located inside tissue a time-resolved technique was developed, based on gated viewing, i.e. detecting the first arriving photons with the shortest path through the tissue. The focus was on developing a technique to substitute X-rays for finding breast tumours. Mode-locked lasers and diode lasers were used as light sources. As detection techniques time-correlated single photon counting was used as well as a streak camera. Tumours were detected in in vitro breast specimens. Tissue phantoms were used to study the influence of different optical properties on the gated viewing results and to explore the limitations and benefits of the technique. It was found that the scattering properties influence the results more than the absorption properties of the tissue. A computer model was developed to simulate the photon propagation in a highly scattering environment. The model solves the time-dependent diffusion equation in three dimensions and with inhomogeneities implanted into the tissue model. The model was further developed to multiple sources and Fourier transformation of the result for comparison with frequencydomain studies. The time-dependent curves obtained in the near-IR region contain information on the optical properties of the tissue under investigation. A multi-spectral technique was developed to be able to extract the optical properties over a large wavelength region simultaneously. Sub-ps white light, generated with a high power laser, was used as light source and a streak camera in combination with a spectrometer as detector. The technique was applied to in vivo tissue investigations as well as to tissue phantoms. Laser-induced fluorescence (LIF) studies were performed in order to detect superficial tumours. As excitation source a nitrogen pumped dye laser was used and an optical multi-channel analyser as well as a multi-colour imaging technique was used for detection. Investigations were performed on skin lesions as well as in the bladder. Tumour demarcation was observed using exogenous chromophores as well as the endogenous tissue fluorescence. A computer program was developed for convenient evaluation of fluorescence spectra. Photodynamic therapy (PDT) were performed on superficial skin lesions using oamino levulinic acid as photosensitiser. A frequency-doubled Nd:YAG pumping a dye laser was used as light source. The PDT treatments showed good results. PDT was combined with LIF studies as well as with measurements of optical properties using an integrating sphere technique. It was found that the absorption coefficient increased due to the PDT treatment.