Lasers in Medicine PDF

LASER BEAMS AND THEIR APPLICATION IN MEDICINE The word laser is made up from the initial letters of the words “Light Amplification by the Stimulated Emission of Radiation”. 1958 Objectives: • To describe what is LASER • To define the characteristics of LASER which distinguish it from other forms of...

LASER BEAMS AND THEIR APPLICATION IN MEDICINE The word laser is made up from the initial letters of the words “Light Amplification by the Stimulated Emission of Radiation”. 1958 Objectives: • To describe what is LASER • To define the characteristics of LASER which distinguish it from other forms of light sources • To know some medical applications of LASERs • To define safety regulations Properties of L A S E R • Low divergence / Collimated/ directional: forms a beam High power Properties of L A S E R • Monochromatic • All power/energy concentrated at one wavelength • Very short pulses often have large spread of wavelengths • Coherent : across any cross section of the beam, all parts have the same phase (in phase) Properties of L A S E R • Low divergence / Collimated and forms a beam High power • Monochromatic • All power/energy concentrated at one wavelength • Very short pulses often have large spread of wavelengths Coherent • Waves are in step along and across the beam • Coherence, monochromaticity and low divergence (collimate) leads to high Brightness, power and very small spot sizes when focussed LASER Laser pointer Barcode scanner Optical tweezers Confocal microscope Retina tear (Photocoagulation) Characteristics of laser light 1)Single wavelength 2)Great intensity 3)Narrow beam L A S E R MEDIUMS SOLID: Rubby GAS: Xenon, Helium, CO2 SEMICONDUCTORS dermatology eye conditions Surgery, dermatology Ruby:Dermatology, tattoo removal Surgery Surgery Spectrum of a helium neon laser illustrating its very high spectral purity (limited by the measuring apparatus). The 0.002 nm bandwidth of the lasing medium is well over 10,000 times narrower than the spectral width of a lightemitting diode with the bandwidth of a single longitudinal mode being much narrower still. spectral width of a lightemitting diode n+1 n Initial state absorption If the atom is already in the excited state, it may jump down spontaneously (i.e., no stimulus) to the lower state with the emission of a photon. Absorption Spontaneous emission Stimulated emission Stimulated emission: if a photon with this same energy strikes the excited atom, it can stimulate the atom to make the transition sooner to the lower state Population inversion/inverted population Metastable state—a state in which the electrons remain longer than usual so that the transition to the lower state occurs by stimulated emission rather than spontaneously. Population inversion Optical pumping: the atoms are excited by strong flashes of light of wavelength 550 nm Population inversion/inverted population Mirror Population inversion Optical pumping Mirror Mirror Spontaneous emission Mirror Mirror Stimulated emission Mirror Mirror Feed-back by the cavity Mirror Mirror Stimulated emission Mirror Mirror Feed-back by the cavity Mirror After several round Mirror trips… Mirror Laser beam Features of the Laser Light Photons with: - same energy, same wave length : Monochromatic - same direction of propagation (directional) Spatial coherence - all in synchrony: Temporal coherence - High magnitude Power density - Polarized: vibrations occur in a single plane - Collimated Application of Laser Beams in Medicine LASER SURGERY Used as a knife in surgeries: CO2 Lasers LASER light focused onto small area could burn off and vaporize tissue without damaging neighboring areas ■ Less infections ■ Less bleeding and pain Application of Laser Beams in Medicine OPHTHALMOLOGY ■ Repair of retinal detachments and retinal tears The pinpoint precision of lasers makes them particularly suitable for "welding" detached retinas and sealing broken blood vessels in the eye. The procedure is painless because the laser light passes straight through the patient's eyeball. ■ LASIC: reshaping the cornea, correcting myopia, hyperopia, astigmatism ■ Treat diabetic retinopathy: sealing damaged blood vessels Application of Laser Beams in Medicine The laser tools are used in the medicine such as: Kidney stone treatment: Oil Laser: To end bleeding. Helium/Neon Laser: In treatments, for healing. Optical Coherence Tomography (OCT): microscopic details of the retinal structures (near infrared laser beam, 800nm) Depth information: images up to 3 mm inside the tissue Penetration Depth of LASER Damage Mechanisms for Biological Tissue • Thermal Effects Absorbed light converted to heat – laser burn • Photochemical effects Light induced chemical reactions such as Sunburn Ocular and Skin hazards • Eye is critical organ for laser damage Designed to focus light, possible irreversible damage • Skin can also be damaged No focusing, injury only to small area Worry about long term carcinogenic effects While choosing a laser tool which will be used in medicine, the subjects that must be carefully controlled, are: ■ ■ ■ ■ ■ ■ ■ ■ ■ The suitability of it with the electricity of the city (220V, 50Hz) Type of material (GaAS, HeNe, ..) Emission spectrum Wave Length (with “nm”) Frequency (like 1 – 128 Hz) Penetration range in the tissue/tissue absorption (this also depends on vascularity of the target tissue) Power density of the laser Its output’s being hand-controlled and has to have sound-light alarms. It has to have a control detector. Pulse Oximeter A pulse oximeter uses two LEDs to measure the % oxygen saturation in your blood. One LED is red, 660 nm, and the other IR (900–940 nm). The LED beams pass through a finger (Fig. 29–36) or earlobe and are detected by a photodiode. Oxygenated red blood cells absorb less red and more infrared light than deoxygenated cells. A ratio of absorbed light (red IR) of 0.5 corresponds to nearly 100% saturation; a ratio of 1.0 is about 85% and 2.0 corresponds to about 50% (bad). The LED measures during complete pulses, including blood surges, and the device can also count your heartbeat rate. light-emitting diode (LED) Safety Lasers are usually labeled with a safety class number, which identifies how dangerous the laser is: • Class 1 is inherently safe, usually because the light is contained in an enclosure, for example in CD players. • Class 2 is safe during normal use; the blink reflex of the eye will prevent damage. Usually up to 1 mW power, for example laser pointers. • Class 3R (formerly IIIa) lasers are usually up to 5 mW and involve a small risk of eye damage within the time of the blink reflex. Staring into such a beam for several seconds is likely to cause damage to a spot on the retina. • Class 3B can cause immediate eye damage upon exposure. • Class 4 lasers can burn skin, and in some cases, even scattered light can cause eye and/or skin damage. Many industrial and scientific lasers are in this class. Summary ■ ■ ■ The working of lasers is based on absorption, spontaneous emission, stimulated emission and population inversion To achieve laser action the system must be in a population inversion (more atoms in an excited state than in the ground state) The important characteristics of lasers are ■ monochromaticity: same energy, same wavelength , same frequency ■ Coherence: all in synchrony (in phase) and directional ■ Collimated: waves are parallel with Sharp focus, weak divergence ■ High magnitude Power density