Download Week 13 Tues. Notes (Lesson 23): laser surgery

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