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Lasers (The word laser is an acronym for light amplification by stimulated emission of radiation.) lasers were first introduced in 1960, The first successfully optical laser constructed by Maiman (1960), was a ruby crystal surrounded by a helicoilal flash tube enclosed within a polished aluminum cylindrical cavity cooled by forced air. The ruby cylinder forms a Fabry-Perot cavity by optically polishing the ends to be parallel to within a third of a wavelength of light. Each end was coated with evaporated silver; one end was made less reflective to allow some radiation to escape as a beam was produced. Initially the laser was named the invention looking for a job. Photo-pumped by a fast discharge flash-lamp, the first ruby lasers operated in pulsed mode for reasons of heat dissipation and the need for high pumping powers. Nelson and Boyle (1962) constructed a continuous lasing ruby by replacing the flash lamp with an arc lamp.(1) Today lasers are much like those of the early ones and they are widely used in many fields, their uses are wide spread, From fusion physics to the DVD player these are common places where lasers are used. Medicine and surgery are no exceptions from skin resurfacing to eye surgery to correct vision. With the development of lasers Physicians have been able to provide treatment for a large number of medical disorders. Medical lasers have made it possible to treat conditions, which were previously untreatable or difficult to treat. To make the most of the laser technology physicians must maintain a up to date understanding of laser systems and conditions for which each can be applied .To achieve these goals the basic terminology and fundamentals of laser-tissue interaction is needed.
Before we can learn about lasers we need to know a little about light, because that is essentially what a laser is made of. First the speed of light is 3.00*10to the 8th meter per second .The American physicist Albert Michelson found a ratio 1.33 for the speed of light c in a vacuum to the lights speed v in water. This value is equal to the index of refraction n thus n=c/v gives us a way to predict the speed of light in any material once we know its index refraction.(2) Lasers produce a intense beam of bright light that travels in one direction. And a laser has the unique ability to produce one specific color or wave length of light which can be varied in its intensity and pulse.
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The CO2 laser emits a beam of inferred light with a wavelength of 10,600 nm. All the biological tissue containing water also absorbs this light. Approximately 90% of the laser energy are absorbed by the 30um of the tissue. This results in intercellular boiling to 100deg C and vaporization. In earlier uses the CO2 laser, when operated in the continuous mode caused the tissue to become progressively desiccated, and heat accumulated to 600 deg C this heat would then conduct away from the treated site leading to thermal narcosis of up to 200um to 1mm thick. Newer lasers systems can reduce the collateral thermal injury by shortening the duration that the laser beam spends on the target sight.(4) According to bears law “laser energy heats a critical volume of tissue until the temperature exceeds the vaporization threshold.” This threshold for human skin is the thermal relaxation time, and is defined, as the time required for the heated tissue to loss 50% of its heat through surrounding diffusion. And this is approximately 695 to 950 u /sec thus if the lasers energy is delivered in less than the thermal relaxation time the heat dissipates rather than accumulates around the treatment area.(5)
The Erbium laser emits ion radiation at different wavelengths between 2,69 and
2,94 um depending on the host materials, the development of pulsed solid-state
lasers emitting in the 3 um wavelength range has been stimulated by a variety of
applications in medicine and it has a high absorption by tissue water (10to the 4th
cm to -1st ) this makes the erbium laser a very precise instrument in surgery(3).
Lasers have been tested and improved over the last three decades and are continuing to be improved. There is a wide spectrum of laser systems with a large number of beam properties like beam width and frequencies and beam deliver systems. As the Laser beam emerges from the output coupling and its manipulated thereafter using standard optical elements, mirrors lenses fibers etc. To create a system sufficiently flexible for clinical use the beam must be delivered to a highly mobile hand piece or down an electroscope, microscope, or other devices depending on the application. An articulated arm is a series of coupled hollow tubes along which the beam.
Is guided by reflection at precisely aligned mirrors or prisms contained within the tube originally the articulated arms were difficult to use and needed constant adjustment, however modern systems are flexible, easy to use and require little maintenance. Fiber optic’s delivery systems are more popular because of their ease of use, despite their minor disadvantages like a slight reduction in power delivered to the output and mild incases in divergence. The fibers are made up of quartz and glass and the beam is guided through their length by total internal reflection. All visible wavelengths and near infrared spectrums can be transmitted through these fibers however many Q-switched lasers (e.g. ruby, Nd:YAG) are also transmitted by means of the articulated arm system to avoid the problems of high-power pulsed transmissions in these fiber optic guides. These signals are delivered to a hand piece, tip or micromanipulator. Most dermatologic lasers in current use, use non-contact hand pieces; where only the laser beam itself is allowed to contact the tissue. A contact delivery system or hot tip contains elements, crystals or sculptured fiber ends coated with absorbing material, the absorb the laser light and are heated up by it then the tips are placed into contact with tissue to be treated and the energy is transmitted by means of conduction. In a non-contact system, the hand piece is typically a simple hand held light weight device that consists of a converging lens system and a pointing system which allows the surgeon to control the distance between the hand piece and tissue.(5) The optical properties of the skin consists of medium that is optically both variable (site to site person to person) and dynamic (changing with time) most simply the skin can be considered to be a two layered structure- the stratum conium/ epidermis and the demise the optics of each layer is relatively noticeable differently The stratum cornea (10um)and epidermis (100um) form optical barriers because of absorption which is high across a wide band , including the wave lengths of most lasers . Major absorbing chromophores include the following, by wave length absorption : Less than 300nm- melanin peptide bonds aromatic amino acids, nucleic acids urocainic acid more than 320nm (to 1500nm ) – Melanim More than 1100 nm – Water Each chromophores has a strong wave length dependant absorption spectrum characterized by an absorption coefficient Below is a table of the Effects of absorption and scattering on the irradiate volume,
Excimer Argon KTP ND:YAG CO2
351 nm 488-514nm 532nm 1064nm
Penetration Depth <0.5mm 0.5-2mm 0.5-2mm 2-6mm 0.5m
The stratum corneum/epidermis is composed of a matrix of chromophores that range in various type and concentration . For a particular type of wave length , its total absorbs ion depends on the absorbs ion coefficient and concentration of the total sum of chromophores present this is characterized by an abortion length which is equal to the distance to light of a certain wavelength travels before it falls by 63% solely because of absorbs ion this is determined by shorter absorption length is stronger absorbs ion scattering id sue to the in homogeneities in the skins tissue structure ,organs cells molecules etc. The main effect of scattering is to modify the region where the radiation is absorbed thus the greater the degree of scatter make s a reduction in the depth of penetration this is due to the photon having to travel to reach a certain depth Scattering can result in a significant amount of energy being deposited outside the incident diameter and the depth of penetration is reduced there for a small but significant amount of backscattering occurs which can contribute to losses and create a potential hazard. In general light incident on the surface undergoes reflection, scattering and absorbs ion or a combination the interaction that occurs determine the light leaving the tissue and the irradiated volume and therapeutic effects .
When a laser beam is incident on the surface of the skin a certain portion of the radiation is reflected . This occurs because there is an abrupt change in the refractive index at the interface where n= 1 for air and n = 1.55 for stratum corneum. The portion reflected depends on the angle of incidence although is relatively independent of the wavelength of 250nm to over 3000nm and skin type this may depend on skin condition such as psoriasis which increases reflection, some reflection is unavoidable it can how ever be minimized by ensuring the angle of incidence is equal to 0 although there is a limited scatter this causes the physicians and all who are in the surgical room are to wear protective eyewear.(5) Lasers have had major break through in the last 40 years and with all of the hazards involved there are many benefits to laser surgery, One can see once again with out special optics, scares can be removed ,old tattoos disappear, these are only a few of the many benefits one can obtain by having surgery with the use of lasers and this writer feels that the benefits out weight the risks.