Austrian Research Centers GmbH - ARC 16. Oktober 2008 Laser Bioeffects and Protection Karl Schulmeister Austrian Research Centers Eye Ultraviolett 100 nm - 400 nm Skin Visible + near IR 400 nm - 1400 nm Far Infrared 1400 nm - 1mm UV: Photokeratitis courtesy of Per Söderberg Cataract courtesy of University of Michigan, Kellogg Eye Center Pulses: Ablation medicineworld.org Far IR Courtesy of Tsutomu Okuno Effects superthreshold: Accidents, Medical at threshold: Exposure limits courtesy of Joe Zuclich Threshold Experiments Uncertainty: ~ 20 % Safety Factor 3 for larger spots retina 3 for UV 1 h macula 1 h ex-macula 24 h macula 24 h ex-macula 100 Energy [mJ] 10 for small spots and retina 10 1 d Mo el E PL ME 0,1 1 10 100 Nominal retinal laser beam diameter [µm] Bioeffect Dependencies Wavelength: 100 nm to 1 mm Pulse duration: 10-14 s to 10 000 s Spot size: 10 µm to 1 m Power: µW to TW 4 orders 18 orders 5 orders 21 orders 1000 Bioeffect for Laser AND broadband radiation Depends on Wavelength Where? » Location of absorption » Type of interaction (thermal, photochem.) How? Pulse duration stu i e L g n Bestrahlungsdauer Power/spot area = power density (irradiance W/m2) How strong? „Temporal concentration“ – pulse duration „Spatial concentration“ – spot size One Photon (light particle) has certain energy Laser This energy has „action“ on tissue (heating, chemical change) Power = Photons emitted per time Power = Energy emitted per time Energy Power = Time Energy = Power x Time 1 Joule 1 Watt = 1 Second Photon concentration 1 Joule = 1 Ws How many photons? 1 Joule Energy 10 ns Pulse duration (= 10-8 s) Power 1 Joule 10-8 Second = 108 Watt = 100 MW time [s] Bioeffect < ~ 10 µs: Microcavitation Melanosomes (pigment particle) in Retinal Pigment Epithelium (RPE) „Vapour Bubble“ „Thermomechanical“ Thermomechanical“ Retina Choroid 5 µm Pigment layer in RPE Temporal Concentration Spatial Concentration Laser: „point point source“ source (usually) Broadband: „extended source“ Absorbing layer: 5 µm „thick“ Very high power densities (W/m3) Retina Choroid 5 µm Pigment layer in RPE Temporal concentration low damage thresholds! Spatial concentration 1.E+07 ED50, visible lasers ED50, NIR lasers MPE, visible lasers MPE, NIR lasers 1.E+06 1.E+05 1.E+04 TIE, μJ 1.E+03 1.E+02 1.E+01 1.E+00 1.E-01 1.E-02 1.E-03 1.E-14 Plot courtesy of DJ Lund 1.E-12 1.E-10 1.E-08 1.E-06 1.E-04 1.E-02 1.E+00 1.E+02 1.E+04 Exposure Duration, s Wavelength Dependence Retina Total Intraocular Energy - μJ 10000000 1000000 100000 Qp - 0.1 s fit 0.1 S data 0.1 s MPE Qp - ns fit ns data OPO data ns MPE 10000 1000 100 10 1 0.1 400 600 800 1000 WAVELENGTH, nm David J. Lund 1200 1400 Spot Size Dependence 532 nm, ex-vivo cow eye -2 Radiant Exposure [J cm ] 1000 100 2s Thermally induced Damage 655 ms 10 31 Thresholds 5000 exposures 100 ms 10 ms 1 1 ms 0.1 ms 0,1 10 100 1000 Diameter [µm] Temporal concentration (high peak powers): -> temperature rise „Thermal damage“ damage“ Low powers (no heating) heating) but long enough to accumulate photons for blue and UV: „Photochemical damage“ damage“ Thermal Increase of Temp critical Temperature Æ damage („Burn“, coagulation, enzyme inactivation) Photochemical „High Energy“ Photons (UV, blue) Chemical Bonds are changed, Molecules activated Æ “Phototoxic“ (e.g. Sunburn) E= h * ν Photo-chemical Effects • Vision (Isomerisation) • Photo-film exposure • Vitamin-D production UV-B • „Sunburn“ of skin and cornea • Photoretinitis („Sunburn“ of Retina, Blue light hazard“) Thermal Photochemical Additivity of microdamages „Dose“ relationship – J/m² mainly by pulses Leistung Prolonged exposures, low power, Energy adds up Bestrahlungsdauer spot size dependence NO spot size dependence strong wavelength dependence „action spectrum“ wavelength dependence: optical absorption 38,46 100 3,85 10 0,38 Laser ICNIRP broadband incoherent 0,04 1 400 420 440 460 480 500 520 Wavelength [nm] 540 560 580 600 Retinal photochemical Class 1 AEL [mW] 1000 -2 Retinal photochemical MPE [W m ] Wavelength dependence – photochem Retina Photochemical and Thermal „compete“ wavelength pulse duration spot size Sun (high in sky) naked eye: photochemical damage (blue light) telescope: thermal damage within ~ 100 ms Overview Bioeffects From: „Laser Safety“ Henderson & Schulmeister
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