Methodological notes on physical parameters of low-level laser irradiation. Part 1. Penetration depth of laser light

Purpose . The general purpose of present methodological article, consisting of two parts, is to provide a unifying theoretical approach to the still debated problem of determining the depth of penetration of laser light into tissues and the dosage of laser therapeutic effects from the standpoint of modern medical physics. The purpose of the first part of the article is to discuss the depth of laser light penetration into tissues and organs during diagnostic and therapeutic procedures and to formulate practical recommendations for its measurement. Material and methods . The review is devoted to the current problem on the depth of laser light penetration, with terms and definitions. Based on different approximations of the radiation transfer equation known in physics, numerical theoretical estimates of the penetration depth of laser light with different wavelengths into the skin are given for different approaches. The Monte Carlo method was used to simulate soft tissue volume in which radiation energy is absorbed up to 95 %. The depth of light penetration was estimated using linear dimensions of the volume. Results . As it turned out, the classical theoretical depth of laser light penetration into tissues and organs highly depends not only on wavelength and tissue optical properties, but also on the chosen approximation and calculation method. The penetration depth, defined by the calculated volume in which up to 95 % of laser radiation is absorbed, is about 3 times greater than the classical theoretical penetration depth, which better complies with known experimental findings. Conclusion . The depth of laser light penetration into tissues can be more reasonably determined for therapeutic and diagnostic procedures via the effective irradiated volume of tissues.