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Other Laser Surgery Laser Tonsillectomy • Use CO2 with mirror bouncing system • Operation takes 15 minutes, no pain • Cauterizes blood vessels & Lymphatic vessels no blood in throat • Patient eat & drink just after operation unlike regular surgery Gastrointesinal Surgery • Uses fiber to bring in Argon Beam Same system my combine CO2 gas to remove blood • Cauterizes blood vessels • Beam used to stop bleeding internally • Esophagus, Stomach & Intestines Endoscopic (Fiber Optic) Laser Fallopian Tube Surgery • Blockages in Fallopian tubes common cause of infertility in Women • Use Endoscopic Laser fiber optic direction of CO2 beam • Burns away blockages, in 1-2 pulses Throat through Endoscope Laser Dermatology (Skin Operations) • Use the ability of the Laser to penetrate the skin • Most common Argon laser removing skin discolourations • eg Portwine marks: blood coloured birth defects • Argon laser bleaches out blood spots • Green light strongly absorbed by the red blood colour defects • Much less absorbed by skin Almost removes such spots as they are near the surface Laser Tatoo Removal • Laser tattoo removals: done with Nd:Yag • Nd:Yag Near IR light penetrates skin to tattoo depth • Near IR strongly absorbed by dye: bleaches tattoo dye, • but is weakly absorbed by skin so no damage to skin Photoradiation Therapy Herpes • Uses laser light to cause direct or indirect treatment • eg Herpes virus creates lesions in skin and moist tissue • Little in conventional treatment • CO2 used to destroy diseased cells & virus • Beam directed on lesions using a microscope destroys tissue without bleeding Cancer PhotoDynamic Therapy (PDT) • Patient injected with dye (eg HpD) • Dye absorbed preferentially by Cancer tissue normal tissue excretes dye • Exposed to 630 nm HpD has photochemical reaction produces a poison directly only in cancer tissue • 630 nm obtained from Argon pumped Dye laser Laser Acupuncture • He-Ne laser, penetrates 3-10 mm • Laser irradiated for 60 sec at 2 mW controversial process Ophthalmology (eye operations) & Dentistry • Most common attaching detached retinas • Uses Argon laser beam (Sometimes Ruby laser) • Beam strongly absorbed by blood • Creates a burn scar which reattaches retina • Laser cornea shaping with Eximers (already discussed) • Eximer also for Dentistry removal of diseased soft tissue Optical Scattering in Tissue • Within a medium light can be absorbed or scattered • Ideally scattering does not absorb light but only changes direction • But may remove energy from light (change wavelength) • Generally occurs with non-homogeneous mediums • Highly material specific • Dominant effect in air. fog and turbid media e.g. tissue • As object moves into fog it becomes blurred • Reason – scatted light contains little information about object • Scattered light hides the object with distance • E.g. objects in fog disappear when scattering gets high enough Effects • Non-deterministic wave propagation • Focusing of light not really possible • Bolus or large ball of light created Diffusion of Photons in Scattering Media • Light entering Tissue breaks into different types • 1 Photons may be absorbed • 2 Photons may be highly scattered (many paths) until nearly uniform • Scattered photons lose almost all information of internal structure • 3 Photons may travel without scattering: called Ballistic photon • If photon scattered: but nearly ballistic path called quasi-ballistic • 4 Photons may be reflected back from the medium Scattering With Depth • When light in absorbing medium follows Beer Lambert Law • With μa = absorption coefficient (cm-1) I ( z ) = I 0 exp( − μ a z ) • Scattering also follows Beer’s Law but with scattering portion • Now add scattering coefficient μs (cm-1) • Combined effect of absorption+ scattering is I ( z ) = I 0 exp(− [μ a + μ s ]z ) • Here we measure not how much light leaves material, • But rather how much light continuous along original path • Called Ballistic Photons • In tissue, μa and μs are very different • Both are wavelength dependent • Both exhibit molecular specificity • Typical values in breast tissue @λ~635nm: μa =0.2cm-1, μs =400cm-1 @λ~1000nm: μa =0.2cm-1, μs =50cm-1 • Also use Mean Free Path (MFP) = 1/μ Anisotropy and Reduced Scatter Factor • Light in tissue does not scatter evenly • Anisotropy is to alter scattering • In tissue light tends to be more forward scattered • This is measured by an anisotropy factor g • g is average scattered photons into cos(θ) over all directions g = 0 means isotropic scattering g = 1 is total forward scattering g = -1 total reverse scattering (ie reflection) • In most materials 0 < g <1 • Get an effective scattering coefficient μeff (or μ’) where μeff = μ s (1 − g ) • μeff adds up the effect of a random walk of scatters • With g=0.9, then μeff = μs/10 • Note if g = 1 (full forward scattering) μeff = 0 • Reason: fully forward scattering photon continues in original path • Scattering has nill effect Optical Tomography in Tissue • Aim image through tissue like an X-ray • Reason is that X-rays damage tissue • Also different tissue affects different wavelengths • X-ray is only affected by tissue density, not type • Hence can tell state of the tissue by looking at different λ • Example blood changes colour & absorption with oxyget • Oxygenated red, deoxygenated blue • By look at 650 nm see large difference • While at 800 nm both are same • Simple diode sensor used for this • Optical Tomography • Three OT methods: • Time of flight (Time Domain) • Phase Coherence Domain • Angular Domain Imaging Time Domain • Based on path length • Shortest path photons arrive first • Launch very short pulse • Few Femtosec • Ballistic arrive first • Quasi ballistic next • Scattered last • Use high speed shutter to select • Problem: high speed laser expensive • Also FDA worried about impact of short pulses Optical Coherence Tomography • Longer path means that phase shift in coherent light • Consider starting with a coherent source (laser) • 2 paths: one to tissue, other to reference • Michelson interferometer methods • By adjusting reference delay scan return in phase • Hence can separate scattered from unscattered • Can see with high depth & size defination (few microns) • But only with limited depth (few mm) in scattering • Best place eyes (only limited scattering) Angular Domain Imaging OT • Laser source aligned to small acceptance angle angular filters • Ballistic/quasi-ballistic light deviates only small angles • Most scattered light outside acceptance angle ADI Imaging • Use a micromachined collimator for the angular filters • Use high 51 µm diameter x 1 cm length • Tunnels spaced on 102 µm centers • Aspect ratio ~200:1 • Acceptance angle ~0.29º • aspect ratio micromachined tunnels • Use test phantoms in 5 cm scattering fluid • Use lines/spaces (200, 150, 100, 50 µm) • Scan the image • Can detect at 9x109 scattering ratio • Trade off object size (50-100 μm) versus depth (cm’s) Scattered only light ADI imaging of 200-50 um lines