Selective laser-tissue coupling techniques using oblique incidence radiation

This thesis presents new selective laser-tissue coupling techniques. Lasers are successfully employed in medical procedures because of their ability to selectively interact with tissues. Laser energy can be selectively coupled to specific tissues and not to adjacent tissues by choosing wavelengths that are preferentially absorbed by those specific tissues. Laser energy can also be selectively coupled to a desired penetration depth by choosing the appropriate wavelength. This wavelength dependency imposes practical problems and fundamental limitations. For example, fatty tissues often cannot be selectively targeted because their preferentially absorbed wavelength bands are not always compatible with commercially available lasers and fiber optics delivery systems. In addition, control over the penetration depth of optical energy in tissues requires wavelength adjustment that can be accomplished with multiple lasers or tunable lasers but not with a conventional single medical laser.; In laser medicine, optical energy is conventionally delivered as a free-beam or through optical fibers at normal incidence to tissues. In this thesis, optical energy is delivered to tissues via a selected contacting optical element. Selective laser-tissue coupling is accomplished by selecting the appropriate oblique incident angle. In this thesis, it is shown theoretically and experimentally that fatty tissues having higher refractive index than adjacent water-rich soft tissues can be selectively targeted by selecting incident angles to be above the "critical angle" for lower refractive index tissues, but below the "critical angle" for fatty tissues. It is also shown that optical penetration depth in tissues can be controlled over a wide range by adjusting the incident angle and not the wavelength. These techniques are already being tested for various dermatologic surgical procedures.