Some medical and biological applications of ultrafast lasers

Ultrafast lasers are finding many applications in medicine and biology. Pulses from such systems can be used directly or after non-linear modification. Direct utilisation includes time-resolved fluorescence spectroscopy and imaging. Such studies can be performed for tissue diagnosis in the field of malignant diseases as well as in the cardiovascular sector. Two-photon induced fluorescence provides a basis for high-quality optical microscopy, and optical coherence tomography with ultra-short and thus broadband pulses allow a radar-like depth-ranging into tissue. Photon propagation in scattering media also has important applications in optical mammography, dosimetry for photodynamic therapy and species concentration assessment. Intense continua of electromagnetic radiation of very brief duration are formed in the interaction of focused ultra-short and intense laser pulses with matter. Focusing into water, or better, a photonic band-gap fibre, leads to the generation of a light continuum through self-phase modulation. The propagation of white light through tissue was studied addressing questions related to tissue chromophore concentration measurements employing their absorptive imprint in the light. Even free gas enclosures in solids and liquids give rise to an absorptive signature, which, however, is typically 10(4) times narrower than spectral structures in the matrix material. Such sharp features are best detected by wavelength modulation of a single-mode laser; and many new applications of gas-in-scattering-media absorption spectroscopy (GASMAS) are developing. When terawatt laser pulses are focused onto a solid target with high nuclear charge Z, intense X-ray radiation of few ps duration and with energies exceeding hundreds of keV is emitted. Biomedical applications of this radiation are described, including differential absorption and gated-viewing imaging.