Faculty of Mathematics and Physics Deptartment of Physics Modelling and measurements of laser-skin thermal interaction ADVISER: prof. dr. Martin Čopič CO-ADVISER: dr. Marko Marinček Ljubljana, 26th February 2008 Martin Gorjan

Contents ◊ Introduction ◊ Laser ◊ Skin ● Structure and optics ◊ Modelling light propagation ● Monte-Carlo simulation ◊ Modelling thermal interaction ● Types of interactions ● Thermal effects ● Heat diffusion ◊ Photothermal measurements ● Measurement results ◊ Conclusion

Introduction ◊ Use of lasers in medicine: ● Clinical use investigated immediately after the laser invention in 1960 ● Today prominent roles in: ophtalmology, dentistry, dermatology ◊ Use of lasers in dermatology: ● Hair removal ● Tatoo removal ● Skin rejuvenation ● Port wine stains treatment ◊ Important considerations of parameters: ● Wavelength ● Pulse duration ● Pulse energy and power ● Spot size

Laser ◊ Unique properties of laserlight: ● monochromacity, coherence, low divergence, spectral radiance -> ◊ Wavelength (<- laser media): ● Nd:YAG (1064 nm), KTP (532 nm), ruby (694 nm), alexandrite (755 nm),... ◊ Beam properties (<- resonator): ● eigenmodes - TEM xx, collimated beam geometry: spot shape and size ◊ Pulse properties (<- operation): ● cw, quasi-cw, Q-switched, mode-locked

Skin structure ◊ Largest body organ: area ~ 2 m 2, mass ~ 5 kg ◊ Composed of two layers ◊ Epidermis (thickness ~ 100  m) ● Four to five sublayers ● Keratin (90%) gives mechanical protection ● Pigment melanin gives UV protection, colour ◊ Dermis (thickness ~ 1 mm) ● Collagen and elastin (75%) give strength and elasticity ● Blood vessels contain pigment hemoglobin ● Contains receptors, glands, hair follicles ◊ Subcutaneous tissue – lies below skin: ● Made of adipose tissue (fat)

Skin optics ◊ Absorption (->  a ): ● Proteins, = 280 nm ● Melanin, ~ UV-NIR ● Hemoglobin, ~ VIS ● Water, ~ MIR ◊ Scattering (->  s ): ● Measured angular profile mostly forward ● Does not agree with Rayleigh or Mie theory ● Defined phase function (Henyey-Greenstein) -> ◊ Turbid medium (absorption and scattering): ● Cannot be simply treated using separate theories for absorption and scattering ● Treated by photon transport theory or Monte-Carlo simulation

Monte-Carlo simulation ◊ Input parameters: ● Laser beam: N „photons“ with r, s, E distributions ● Skin parameters: n,  a,  s, p(  ) for each layer ◊ Propagation is done in five steps: 1.Photon generation -> initial r, s, E 2.Pathway generation -> l = -ln(x)/(  a +  s ) 3.Absorption ->  E = -E  a /(  a +  s ) 4.Detector count ->  E det = -  E 5.Elimination -> when E < E min ◊ Investigated beams of different spot sizes: ● Intraspot variations in surface temperature ● Maximum surface temperatures ● Penetration depths

Monte-Carlo simulation results 1 ◊ Calculated absorption cross-section on the surface: ◊ Calculated absorption cross-section in depth:

Monte-Carlo simulation results 2 ◊ Temperature and penetration depth vs. spotsize: ● Strong dependance on spot size ● Saturation at spot diameter ~ 8 mm (T) and ~ 6 mm (depth) ◊ Calculated distributions can be used to predict effects on skin...

Types of laser-skin interactions ◊ Five laser-tissue interactions (empirically classified): ● Photochemical ● Thermal ● Photoablation ● Plasma-induced ablation ● Photodisruption ◊ Interaction determined by: ● Laser power density [W/cm 2 ] ● Pulse duration [s] ◊ Thermal interaction: ● 1  s rule ● Most common in dermatology ◊ Five laser-tissue interactions (empirically classified): ● Photochemical -> ~ 1 W/cm 2, exposure ~ 10 s, dependent ● Thermal -> ~ W/cm 2, exposure ~  s - s ● Photoablation -> ~ 10 7 – W/cm 2, exposure ~ 10 ns, depentent ● Plasma-induced ablation -> ~ – W/cm 2, exposure ~ 100 fs – 500 ps ● Photodisruption -> ~ – W/cm 2, exposure ~ 100 fs - ns

Thermal tissue effects ◊ Thermal effects are due to elevated temperature: ● Hyperthermia (45 °C) ● Enzyme deactivation (50 °C) ● Denaturation, coagulation (60 °C) ● Vaporization, ablation (100 °C) ● Carbonization (> 150 °C) ● Melting (> 300 °C) ◊ Duration of elevated temperature: ● Determines amount of damage ● Described by Arrhenius-type „damage integral“ -> ● Concentration of native cells as measure ● Parameters obtained optically or histologically

Thermal diffusion ◊ Thermal diffusion of heat: ● Described by heat equation -> ● Absorbed light as source ● Solved by FDM, FEM ◊ Thermal selectivity theory: ● Due to skin non-homogenity ● Addresses hot-spots caused by „particles“ ● Heat equation solutions for spherical source ● Particle's size (radius) determines its relaxation time ->

Photothermal measurement ◊ IR imaging: ● Bodies at 300 K have thermal emission peak at ~ 10  m ● Thermovision camera can measure temperature profile < 1 K, 1 ms ● Laser pulse and camera frame-taking must be synchronized ◊ Measurement setup: ◊ Measuring techniques:

Photothermal measurement results 1 ◊ Temporal evolution of surface temperature: ◊ Angular average reveals cooling direction and scattering „tails“:

Photothermal measurement results 2 ◊ Variations in spotsize: ◊ Variations between people:

Photothermal measurement results 3 ◊ Temperature vs spotsize: ● Strong dependance on spotsize ● Saturation at spot diameter ~ 6 mm ◊ Temperature vs time: ● Surface relaxation time ~ 20 ms, 10 s ● Hot-spots relaxation time < 100  s

Conclusion ◊ Five types of laser-tissue interactions presented ◊ Calculated and measured results presented: ● Show good degree of agreement ● Tools for tuning laser application parameters ◊ Basic modelling of laser-skin thermal interaction covered: ● Laser beam properties ● Skin optical properties ● Heat generation by light absorption in tissue -> Monte-Carlo simulation ● Thermal diffusion of heat -> heat equation ● Thermal effects prediction -> Arrhenius and thermal selectivity theory ◊ Common technique for measuring skin temperature: ● Photothermal camera synchronized with laser pulse