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54 Cards in this Set

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3 Types of Laser-Tissue Interaction effects
1. Thermal
2. Ionizing
3. Photochemical
When you're blow drying your hair, use heat, ionic blow dryer to make your hair photogenic
2 major thermal laser-tissue interaction effects and their general characteristics
1. Photocoagulation
- pigment dependent
- creates burn
- denatures protein
2. Photovapourization
- pigment dependent
- make holes
- localized microexplosion
Ant and post segment ophthalmic laser procedures using photocoagulation and what the procedure is for
- PRP (saves best supply for retina)
- focal retinal photocoagulation (finding leaks in BV in DM)
- laser iridoplasty (ACG)
- laser trabeculoplastic (increases AH flow)
Comment on photocoagulation is largely pigment-dependent
a. exception
b. type of light used
c. how wavelength is selected
a. unless the laser is at a very high setting, it generally only coagulates tissue pigment
b. UV and IR lasers
c. depends on absortption characteristics of pigment in target tissue. Lower frequency (longer wavelength) penetrates deeper than short
Comment on the laser of these ocular pigments:
a. melanin
b. Hb
c. xanthophyll
a. most commonly targeted in the eye
b. selectively avoided or targeted
avoided --> laser through vitreous hemorrhage
targeted --> vascularizzed lesions in veseels
c. avoided because macula
Comment on photocoagulation creates burns
- pigment absorbs laser energy and converts to heat
- heat sink effect causes pain in some laser therapies (PRP)
- avoid increased energy and longer exposure time b/c will thermal effect will spread to surrounding non-targeted tissue
- when laser is on high power and long exposure, tissue response changes to photocarbonization wwhere tissue becomes charred
Comment on photocoagulation denatures proteisn
- protein denaturation seen as tissue whitening
- moderate inflammation is induced and can be beneficial to make desired scarring and adhesion as collegan contracts
- desirable in laser trabeculoplastic and BPI
- undesirable at other times
- tissue around the burn atrophies and decreases oxygen demand (PRP for DR)
Example of photocoagulation of retinal BV
- blood coagulation --> thrombus
- Collagen contraciton --> BV closes
Photovapourization
- pigment dependent
- thermal alser
- melanin targed
- high-pwoered laser light in brief bursts
- tissue vaporized by light (CO2 and H20)
- localized microexplosiong - tissue change before heat dissipation into adj. structure can occur
- make holes in argon LPI
Ionizing laser-tissue interaction effect
Photodisruption AKA optical breakdown
- ionizes target tissue by strippping them of their electrons
- Tissue reduced to plasma
Photodisruption
- thermal/non-thermal
- describe process
- non-pigment dependent
- TONS of energy, highly localized and brief pulse (um, picosec)
- Em light --> ionizing radiation
- high energy photons hit target tissue and converted to thermal and kinetic energy
- instantaneous, highly localized heat raise
- ionization of tissue
- acceleration e' collide and knock out of orbit "e' avalanche"
- free e' and plasma (ions) expands, gnereating fluid and acoustic forces (sound wave), shearing tissue in all direction
- quick with NO outward disspiation of heat
What is photodisruption used for? example
What is photodisruption not associated with?
1. Cleave undesired membranes in the eye
ex. Posterior capsulotomy
2. Blast holes through tissues
Nd: YAG laser peripheral iridotomy
- BV coagulation --> causes intraocular bleeding
Where does plasma formation occur? What are the undesirable types of plasma formation?
a. most likely at laser's focal point, but can occur anywhere along the beam's path
b. Plasma shielding - too high laser energy plasma before focal point.
- dissipates laser energy and causes undesired damage to adjacent tissues and structures (IOP pits/cracks during posterior capsulotomy)
Photochemical Effects
a. when does it occur
b. three major types
a. occur when light triggers the formation of or breaks moecular bons
b. photoablation, photodynamic therapy and photosynthesis/bleaching of rhodopsin
Photoablation
- dependent/nondependent
- laser wavelength used
- mechanism
- non-thermal dependent
- non-pigment dependent
- uses short UV wave
- cleaves -C bons in cornea and removes tissue in LASIK and PRK
Process of photoablation
- high energy, brief pulse, highly localized
- non-thermal
- tissue reduced to molecular fragments
- "plume" or puff of smoke
- tissue removed one molecular layer at a time with submicron precision and no damage to surrounding tissue
use of excimer laser
- produces far UV light for photoablation
- cornea absorbes far UV light well
- no damage to collateral tissue
- photoablation does not incite an inflammatory healing reaction
Photodynamic Therapy - characteristics
- laser treat tissue INdirectly
- laser activates drug to destroy neovas/cancerous tissue
Process of Photodynamic Therapy
1. Photosensitizing drug
Light-activated drug introduced to living tissue

2. Tissue irradiation
Tissue exposed to UV radiation, which activates drug
- neither the drug/laser along can exert effect
Step 1: Photosensitizing drug
- IV drug binds to cell lipoprotein
- drug contains porphyrins (component of Hb der. fr animals)
- absorb light and tansfer this energy to surroudning oxygen
Ideal drug for photodynamic therapy
- selectively accmulate in the target tissue
- activated by a wavelength capable of penetrating deeply through tissue/blood
- cear quicly from body to reduce skin phototoxicity
Step 2 of Photodynamic therapy
Tissue irradiation
- porphyrins uses laser energy to produce toxic oxygen species
- light catalyzes conversion of O2 into signlet oxgen and free radicals
- damaged by radicals, cancer cell dies
Why use laser for phtoodynamic therapy?
- restriction
- hjghly selective (monochromatic)
- precised focused
- intense (short tx time)
- restr: as long as tissue is accesible to laser light
Ex. of a photosensitizing drug
- what does it treat
- which wavelenght activates it?
Visudyne
- treats WET ARM
- activated by 689nm laser light (red diode laser)
What must be considered when choosing the best laser for a surgical procedure?
- tissue type
- media the laser must pass through
- desired action
Laser variables
1. wavelength
2. cumulative energy delievered
3. spot size
4. mode of operation
Tissue variables in the way laser light affects ocular tissue
- transparency of ocular tissues and media
- ocular pigments
- how loss of trasnrency affect light absorption
Parts of Visible and Near-visible EM spectrum
Near visible
UVC (far UV) 100-290
UVB (med UV)290-320
UVA (near UV) 320-400
Visible
400-750
Near Visible
IR A (near IR) 750-1400
IR B (med IR) 1400-3000
IR C (far IR) 3000-10000
Spectrum transmission of transparent vs. non-transparent ocular tissue/media
transparent
- similar to water
- transmit wavelengths <1000nm
- increase absor'n 1000-1400
- total absor'n at >1400nm

Non-transparent ocular structures
- depends on absoprtion char of their pigments
Type of light and tissue affected
UVC+UVB
- cornea
- when epithelium slough of, photokeratitis; seen in welder's flash or snow blindness
- never reach the AQ in AC

UVA
- increasingly transmit by cornea
- through AQ and absorbed by lens
- culprit of cataract

Visible light
- completely trasmitted to lens, vitreous body
- wavelength used to treate pigmented structures in the eye

Near IR
- some IR completed transmitted also and treat pigmented structures
- near IR 1000--> 1400nm, lens increasingly absorbes radiation

Medium and Far IR
- mainly absorbed by lens (causes Glassblower's cataract)
- min absorbed by cornea
>1900nm --> absorbed by cornea and causes corn burns
- cannot treat retina
What type of light must be blocked by cataract IOL?
UVA
Application of Medium IR in laser
Laser Thermal Keratoplasty (LTK) - create a pattern of corneal burns
Laser Presbyopia Reversal - thin sclera
General rule of thumb re: pigment absorption and light penetration
- less tissue absorption, the deeper the light can penetrate
Location of melanin
Darker fundus
Tessellated fundus
- TM
- CB
- Iris stroma and pigment epithelium
- RPE and choroid

Darker fundus - easier to photocoagulate
Tessellated funus - uneven melanin distribution in RPE
Absorption char of melanin
- absorbs visible spectrum
--> orange/red less than G/Y
- absorbs near IR but less effectively than visible light

G/Y/O light --> shallow/intermediate/deep intra-retinal burns
Red and IR light --> subretinal and choroidal burns
Absorption char of Hb
- effective absorbs B/G/Y <575nm
- poorly absorbs >575 (O/R/IR)
therefore, use red/IR to photocoagulate melanin-pigmented targets deep to regions of hemorrhage
Absorption char of xanthophyll
- completely transmits wavelengths > 514 nm (green)
B and B-G laser will therally damage foveal cones (central soctoma)
Y/O/R/IR --> treats subfoveal retinal lesions safely
How Loss of transparency affects light transmission in cornea, AQ/VH, lens
Cornea
- scarring, infiltrates, edema, dystrophies, degenerations
- reduce laser light transmission
- risky to do anterior segment laser procedure (LPI) when the cornea is edematous (acute angle clusre attack) and predisposes px to corneal burns

AQ/VH
- inflammatory cells, flare, hyphema, and vitreous hemorrahge
- heating denatures and coagulates protein-rich ocular fluids

Lens
- cataract, nuclear sclerosis and brunescences
- cannot perform reitnal laser surgeies and can speed up cataract progression
Laser Variables - UV wavelengths
Wavelengths
- UVC UVB --> absorbed by cornea; excimer lasers that emit UVC at 193nm to ablate the cornea in LASIK/PRK, epi-LASIK procedures

UVA - partially cornea and aq, mostly lens
not used in eye care
Laser variables - visible light char
Visible lgiht
- transmitted by ocular media
- photocoagulation
Laser variables - Blue
Blue
- poor penetration through opacities
- absorbs well by all 3 ocular pigments --> cannot be sued on or near macular
- not a popular wavelength
Laser variables - Green
Green
- poor penetration through media opcacities
- some absorption by xanthophylls
- absorbs well by melanin and hb
- most popular for anterior segment photocoagulation (LPI and laer trabeculoplasty) and retinal photocoagulation procedures (focal and PRP) but not used at the macula
Laser variables - Yellow
Yellow
- slightly better penetration through media opaciites
- poorly absoprbed by xanthophyll --> can be used for macular photocoagulation
absorbed very well by hb -->but cannot treat hemorrhage
absorbs well by melanin --> good choice for a pale fundus
- used fo retinal photco. procedures and will treat at deeper level than green light
Laser variables - Orange
Orange
- fairly good penetration through opacities
- used for macular photocoagulation and photocoagulation at deeper level than yellow light
- poorly absorbed by Hb --> treat deeper retina even with vitreous hemorrhage
- ok melanin absorption but not good choice for a pale fundus
Laser variables - red
- macular photocoagulation, treat retina at deeper level than orange light
- used in photodynamic therapy to activate Visudyne
- good penetration thru opacities
- very poorly absorbed by xanthophyll
Laser variables - Near Infrared
750-1000nm
- excellent transmiattion by clear ocular tissues, media and media opocities
- poorly absorbed by xanthophyll and Hb
- absobes by melanin <red --> not a good choice for a pale fundus
- treats reitna at a deeper level than red light, but not as strong

1000-1400nm
- lens increasing absorbs radiation, not reach retina effectively
- Q switched Nd:YAG laser has a radiation output in this range (1064nm) used for non-pigment dependent (photodisruptive) procedures like posterior capsulotomy and LPI
Laser variables - Med and Far IR
- not used for retinal photocoagulation since these wavelengths do not reach the pigmented structures in the retina
- risks are in the cornea and cataract
- used in LTK to create corneal burns and in LPR to thin the sclera
Laser variables - Cumulative Energy Delivered
Power/Energy Level
Number of Applications
Duration of Applications/Exposure Time
Cumulative energy determines laser-tissue interaction will be thermal, ionizing, or photochemical
Increase impact on target and surrounding tissue with
- increased energy level
- increased # applications
- longer exposure time

- all incrases cummulative energy
Best application of laser energy
- lowest effective energy setting
- fewer number of shots
- shortest duration pulses
Laser variables - spot size
Nd:YAG have fixed spot size
Argon has variable spot size
- cornal CL (Lgodlmann) decreases spot size. Focused spots are more intense and must decrease power to avoid over-treatment
Laser variables - mode of operation
Pulsed
Continous wave
Q-switched
Pulsed laser
AKA long pulse laser
- beam lasts ms
pump is usually a flash lamp
multiple flashes= multi pulses
Laser Variable - Continuous wave laser
- pump must be continous and typically electric current
Laser Variable - Q-switched laser
- emitted as an extremely brief, powerful pulse of light
- during light amplifaction, light is held back insdier the cavity by a shuttter Q-switch until high amounts of energy builds up
- enough energy accum. in resonator --> shutter opens and coherent photons are released
- intense pulse ns
- usually inoizing lasers and produce photodisruptive effect on tissue
Q = quality factor (how well energy is being stored)