Laser-Specific Effects

The work of many researchers into the fundamental mechanisms of light-based therapies focusses largely on photon-absorption by photoacceptors and the subsequent photochemical reactions that take place.

For example, Professor Tiina Karu deals with primary photon interactions with light-receptive molecules (photoacceptors or chromophores) in cells in vitro, and hers' is the most comprehensive work there is on this aspect of phototherapy. However, there is more to laser therapy than the photon-absorption effects, as studied and described by Karu, that characterise other non-laser phototherapies.

Due to their unique property of coherence (all photons/waves are emitted in the same phase), and the effects of the random interference of coherent photons as they are scattered throughout the irradiated tissue mass, laser beams create ‘speckles’.

Speckles are formed because the random interference affects the formation of random non-homogeneities of intensity in space, creating a 3-dimensional field comprising regions of high power density (where the photons have positively interfered), regions of average power density (where no interference takes place between photons in the beam), and regions of lower power density (where negative interference has taken place).

The high power density inside each speckle, relative to the surrounding beam field, creates a higher temperature within the tissue irradiated by each speckle, again relative to that of the surrounding beam field. The higher relative temperature within the speckle creates a pressure gradient at the cellular level, affecting cell membrane permeability and producing mechanical forces which may act upon cells and small particles.

Further, the photons within each speckle are highly polarized (all oscillations are in the same plane), and this polarization is independent of the initial polarization of the source beam. Polarization of the photons within each speckle acts upon certain molecules (those with absorption dipoles, such as porphyrins, cytochrome-c-oxidase, and others) causing rotational force (torque) and stimulating the production of singlet oxygen.

These are laser-specific effects which are not wavelength-dependent.

Although speckle fields are present in any tissue mass irradiated by a laser beam, the coherence length of the laser, the irradiated tissue type, the wavelength and associated tissue optics, and the power density of the incident beam, all combine to affect the depth to which the speckle field will be propagated and the intensity of the effects it generates in the tissue.

For example, due to the very high peak powers emitted by super-pulsed lasers, the speckles that are created have a much greater relative intensity and so, in probability, create stronger effects than do continuous wave lasers. The collagen bleaching effect of the super-pulsed laser also affords deeper effective penetration of these effects in the tissue mass.

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