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Rojas and Gonzalez-Lima Dovepress penetration and they produce a constant beam width that offers the advantage of energy delivery on circumscribed areas. For example, a Food and Drug Administration (FDA)- cleared laser known as HD LaserTM CG5000 (HD Laser Center, Dallas, TX) has a beam size of 45 mm, a power density range up to 1.6 W/cm2 and a 1064 nm wavelength that maximizes tissue penetration. The beam width of lasers can be modified by coupling them into fiber optic, which allows delivering energy to larger areas. Yet, areas of tissues that can be treated with lasers could be insufficient for some transcranial applications, and repeated single beam exposures are usually necessary. LEDs produce about 95% of light between a narrow range of wavelengths (4–10 nm) and light is not coherent. Beam noncoherence of LEDs accounts for the significant difference in the amount of energy delivered to a single cm2 of target surface compared to lasers. While lasers are capable of heat production that can induce tissue damage, LEDs generate negligible amounts of heat, thus reducing the risk of thermal injury.8 But there is no risk of thermal injury at low irradiances if the laser output power is chosen correctly. LEDs can be mounted on arrays with ergo- nomic features that allow efficient energy delivery, which is Table 1 Major parameters of low-level light therapy (LLLT) relevant when the target organ has a large surface area, such as the brain. LED arrays and diode lasers are compact and portable, which is relevant in a clinical setting, and LEDs have achieved nonsignificant risk status for human trials by the FDA.9 Major dosimetric parameters relevant for LLLT studies are starting to become more standardized. Confusion about the effects of LLLT is evident in the literature, and it derives from lack of standardization of parameters relevant for LLLT. Until more is known about the dosimetry, the con- vention should be to report all relevant parameters involved in a particular LLLT use (Table 1, Figure 2). In contrast to traditional pharmacology, in which dose is a major determi- nant of the effect, LLLT is also dependent on power density, energy density, frequency, fractionation, wavelength, contact modality, source, and physicochemical properties of the target tissue. Hormetic effects of LLLT An accurate description of LLLT dosimetry should take into account the dose-response phenomenon of hormesis. A hormetic dose-response (also known as U-shaped, Parameter wavelength Energy Power irradiance Radiant exposure Exposure time wave type Fraction protocol Aperture Delivery mode Unit nm (nanometers) J (joules) w (watts) w/cm2 J/cm2 Seconds Continuous versus pulsed Number of fractions Area of the light beam Distance of the beam source to the target tissue Explanation wavelength (λ) is the distance between wave peaks. Light is a form of energy with wave behavior. Photoacceptors exhibit different sensitivities to different wavelengths. The most effective LLLT wavelength range is 600–1100 nm. Light visible to the human eye is 400–700 nm. The higher the wavelength the lower the energy. Energy (E) is the frequency (v) of radiation by Planck’s constant (h) of 6.626 × 10-34 J sec (E = hv). Energy of a photon depends on the frequency of radiation (Ephoton = hv). A photon is a particle of electromagnetic radiation with zero mass and a quantum of energy (minimum E gained or lost by atom). Energy (J) = Power (w) × Time (seconds). Amount of energy (J) transferred or flowing per unit of time (W = J/seconds). Power (w) per surface area (cm2). Also called power density or light “intensity”. irradiance = Power (w) / Area (cm2). Energy (J) per surface area (cm2). Equivalent to power density per unit of time (seconds). Also called fluence, energy density, or light “dose.” Thus, “dose” can be easily varied by changes in exposure time. However, at the same energy density (J/cm2) variations in either irradiance (w/cm2) or time may cause different LLLT effects on tissues. Time during which the target tissue is exposed to light. Continuous waves may be advantageous for transcranial applications. Pulse waves may decrease thermal effects. Pulse Average Power = Peak Power (w) × Pulse width (seconds) × Pulse Frequency (Hz). Total dose can be divided in treatment sessions or fractions of specific duration and separated by specific intervals of time (eg, minutes, hours, days). Can be parallel, convergent, or divergent. Aperture may influence efficiency and tissue penetration. Types: shallow (or noncontact), contact, and deep. Shallow is preferable when larger areas need to be exposed, but offers lower tissue penetration for light-emitting diodes. Deep delivery implicates pressure of the beam source on the target tissue. Eye and Brain 2011:3 52 submit your manuscript | www.dovepress.com DovepressPDF Image | Low-level light therapy of the eye and brain
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