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4) The development of near-infrared fluorescent contrast agents and imaging techniques depends on the deep penetration of excitation light through several centimeters of tissue and the sensitive collection of the re-emitted fluorescence. In this contribution, the sensitivity and depth penetration of various fluorescence-enhanced imaging studies is surveyed and compared with current studies using continuous wave (CW) and frequency-domain photon migration (FDPM) measurements with planar wave illumination of modulated excitation light at 100 MHz and area collection of reemitted fluorescent light using a previously developed modulated intensified charge-- coupled device camera system. Fluorescence was generated from nanomolar to micromolar solutions of indocyanine green (ICG) in a 100 IiL volume submerged at 1-4 cm depths in a 1% Liposyn(R) solution to mimic tissue scattering properties. Enhanced depth penetration and sensitivity are achieved with optimal filter rejection of excitation light, and FDPM rejection of background light is not achieved using CW methods. We show the ability to detect as few as 100 fmol of ICG from area illumination of 785 nm light (5.5 mW/cm2) and FDPM area collection of 830 nm fluorescent light generated from 3 cm below the phantom surface. 5) Light in the visible range is partially absorbed by naturally abundant fluorochromes, including hemoglobin. Photons in the infrared region of the electromagnetic spectrum are partially absorbed by water. The near-infrared region of the electromagnetic spectrum provides a window of opportunity with greater tissue penetration. The fluorochromes reported on in this review typically fluoresce in the 700–800-nm range, wavelengths that allow for tissue penetration on the order of 10–15 cm.4 Fig. 7. Absorption of light versus wavelength. Given the decreased absorption of light in the near-infrared (NIR) region compared with visible light ( 400–650 nm) and infrared light (>900 nm), tissue penetration of NIR photons may be up to 10–15 cm. Fluorochromes used in the reviewed imaging studies fluoresce in this window of opportunity. Effects of light Generating Sources: Laser vs Light Emitting Diodes 1) Dr. Kendric C. Smith at the Department of Radiation Oncology, Stanford University School of Medicine, concludes in an article entitled The Photobiological Effect of Low Level Laser Radiation Therapy (Laser Therapy, Vol. 3, No. 1, Jan - Mar 1991) that, "1. Lasers are just convenient machines that produce radiation. 2. It is the radiation that produces the photobiological and/or photophysical effects and therapeutic gains, not the machines. 3. Radiation must be absorbed to produce a chemical or physical change, which results in a biological response." 5 2) In a study entitled Low-Energy Laser Therapy: Controversies and New Research Findings, Jeffrey R. Basford, M.D. of the Mayo Clinic's Department of Physical Medicine and Rehabilitation, suggests that the coherent aspect of laser may not be the source of its therapeutic effect. He states "firstly, the stimulating effects (from therapeutic light) are reported following irradiation with non-laser sources and secondly, tissue scattering, as well as fiber optic delivery systems used in many experiments rapidly degrade coherency. Thus any effects produced by low-energy lasers may be due to the effects of light in general and not to the unique properties of lasers. In this view, laser therapy is really a form of light therapy, and lasers are important in that they are convenient sources of intense light at wavelengths that stimulate specific physiological functions.6 Etiology of Acute Injury: The Body's Healing Process From the moment a bone breaks or a ligament tears, the body goes to work to repair the damage. Here's what happens at each stage of the healing process:PDF Image | Mechanisms of Action for Infrared Light on Tissue Healing
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