Tissue Interaction And Laser Safety Flashcards
Lower power laser
He-Ne gas laser Continuous beam Helium is the pump Helium causes stimulated emission Lower power lasered mW range Wavelength=632.8nm
CO2 laser
- gas laser
- continues beam
- N, He pump
- CO2 stimulated emission
- kW range
- 1060nm
Can cut metal for 2”
Excimer (ArF)
- gas laser
- pulsed
- no ground state
- ArF stimulated emission
- 50-100 W to MW
- 193nm
RUby Laser (1st laser)
- solid state laser (3 level laser)
- pulsed
- high pumping with optical flash
- 694.3nm
Nd_YAG
- solid state laser (4 level)
- continues or short pulsed
- Nd pumping, YAG SE
- cont-kW
- pulse-tetraW
- 1060nm
Semiconductors, LEDs
- solid state diode
- pulse of the size of microns
- PN junction diode
- mW-MW
- 400nm
Dye laser
- liquid
- pulsed
- pumping with flash lamp
- tunable over large wavelength range
Common lasers in ophthalmology
Diode
Nd:YAG
Excimer
Ar+, Kr+
The near infrared and visible laseawvelvnths are transmitted by the cornea enabling ________ to be delivered to the interior of the eye without surgically cutting the eye open
Large amounts of energy
The energy being focused in the eye
In a small area
The produce the surgical effect, the target tissue ______ the laser light. (If not, there is no energy gain in the target tissue, and therefore no effect)
Must Absorb
The surgical effects of the laser beam depend on _______ and on the amount of _____ delivered )which in turn depends on the laser pulse duration)
It’s wavelgnth (Frequency)
-power
The surgical effects produced by the laser are either _____, ______, or _____
Photochemical
Ionizing
Thermal
Most ophthalmic surgical laser operate in the ____ range
Watt
Properties of laser
Exposure
Irradiance
Wavelgnths
Photochemical effect
1us exposure
Low-moderate irradiance
<320nm wavelength
Thermal effect
1us exposure
Low-moderate
Longer wavelgnth
Photodisruption effect
20ns
Higher irradiance
Photochemical effects
Photoradiation
Photoablation
Photoradiation
- a process in which the absorption of light triggers a chemical reaction that produces the desired effect
- example: a chemical injected into the blood stream
- the particular chemical is taken up and retained by metabolically active tumor tissue
- a specific laser with a wavelgnth that is absorbed by the chemical and then subsequently irradiates the tissue
- the absorbed energy triggers a chemical reaction that ultimately produces a cytotoxic derivative that kills the tumor tissue
Photoablation
- a single photon of UV radiation have enough energy to break chemical bonds
- when a surface is hit with a high irradiance of this UV radiation during a short exposure time, the bond breakage is very precise resulting in “clean” cuts with no collateral damage
- these cuts cause ejection of molecular fragments at supersonic velocities
- since the etch depth per laser pulse is predictable, photoablation can remove material layer by later in very thin laters
- photoablation is not a thermal effect and is considered a “cold” process
Thermal effects
Photocoagulation
Photovaporization
Photocoagulation
- the structure of biological tissue is stabilized by the relatively weak van der Waals force between molecules
- a temperature increase of 10-20C overcomes the van der Waals force causing the coagulation (clotting) of the target tissue
- the extend of the thermal injury is proportional to the product of the irradiance times the exposure time (the total energy delivered)
- a higher irradiance is required for an immediate effect, there may be damage to collateral tissue by either heat spreading or by scattered laser light
Photovaporization
-when laser irradiance higher than those required for photocoagulation are applied to the target tissue, the tissue temperature may reach the boiling point of water, the a rapidly expanding water vapor (an explosion) rips the tissue apart (tissue disruption) before coagulation can cauterize the tissue. This is called photovaporization
In many situations, cauteriation will subsequently occur
Secondary mechanical disruption
Photodisruption/photochemical
- a very high irradiance beam, achieved by Q-switching or mode locked lasers, with a focusing abiltiy of small spot size could ionize the maternal by releasing or freeing the bound electrons
- the free election plasma will expand rapidly producing shock waves that will rip the material apart. This process is called photodisruption
Cataract removal
- fairly common complication of cataract removal and IOL implant is that’s the remaining lens capsule subsequently loses it transparency due to epithelial cell proliferation
- in a capsulotomy, a Nd:YAG laser is used to “zap” (photodisruption) the capsule causing it to split and pull back leaving a clear area of 3-4mm in size
Angle closure glaucoma
- in cases of angle closure glaucoma due to a pupillary block, photodisruptiuon by a Nd:YAG is also used for peripheral iridectomy (opening a hole in the peripheral iris) on lightly colorful (poorly absorbing) it is
- for darkly colored iris (better absorbers), a peripheral iridectomy can be achieved by photovaporization with an argon ion laser
Argon laser for angle closure
- argon lasers with a large spot size are used to create burns (photocoagulation) in the extreme periphery of the iris
- the resulting burns cause tissue shrinkage, which then pulls the iris back and opens the angle
- in glaucoma due to a lack of drainage through the TM, argon lasers are used to place burins athe juncture of the pigmented and non pigmented TM
- the postulated theory is that the burns shrink the collage, thereby tightening the ring of meshwork, and opening up the drainage spaces
Retinal photocoagulation
- the argon ion laser is also commonly used for retina
- this can seal/weld retinal holes and tears, and destroy new (leaky) blood vessles growth in DR, ARMD, or BRVO
- it may also be used in panretinal photocoagualtion, which is desutrciotn of selected peripheral retinal tissue to decrease the demand for oxygen so that the new (leaky) blood vessel growth I inhibited
Photocoagualtion and the macula
- in retinal photocoagulation near the macula, care may be taken not to damage the macual region
- Xanthophyll, the yellow pigment in the macular region, absorbs strongly for incident wavelgnths below 500nm, and only weakly for wavelgnths above 500nm
- therefore, an argon green laser or better a frequency doubled YAG, or even better a krypton red laser would supply better protection against accidental damage to the macula than an argon blue laser
Photocoagualtion through hemorrhages
- oxyhemoglobin strongly absorbers wavelgnths below 630, while hemoglobin absorbers strongly below 600
- in the presence of a hemorrhage, blood strongly absorbs argon blue, argon green, and frequency doubled YAG laser light
- on the other hand, hemoglobin is very weak absorber for krypton red while oxyhemoglobin is a moderate weka absorber of krypton red
- krypton red lasers are used for retinal photocoagualtion though hemorrhages
- krypton red laser light also penetrates deeper into the choroid than either argon blue or green laser light, and is thus used for photocoagualtion of deeper choroid a structures
PRK
- corners is transparent to visible light, but absorbed (is opaque) out in the UV region
- therefore the ArFl axcimer laser can be used to operate on the cornea by photoablation
In PRK
- the epithelial layer of the cornea is scarped away
- the excimer is used to change the corneal curvature by removing material (laser sculpting) in 5-9mm diameter zone
PRL refractive correction
- in myopia, the cornea is flattened by removing more material in the center than a the edges of the zone
- moderate myopia up to 10% of the corneal thickness may be removed
- higher myope, up to 30% of CT may be removed
- the material removed includes bowmans membrane together with some stroma, and bowmans membrane does not grow back
- the wounded cornea must heal, and the steroid regimen that is used during the healing may also influence the final outcome. Regression occurs in some cases
LASIK
- Micokeratome is used to cut a thin slice in the anterior cornea
2the slice is then flapped back to expose the stroma - The ArFl excimer laser is then used to reshape the stroma
- The slice is then repositioned on top the reshaped stroma, where normal suction hold she flap in place
- Bowman membrane is in the flap, so its is not destroyed iwth LASIK
- LASIK is a riskier procedure than PRK, but appears to have a higher satisfaction rate
For myopia: laserrmoves more tissue centrally and flattens central cornea
For hyperopia: laser removes mroe tissue in the peripheral and steepens the central cornea
Wavefront guided LASIK
- wavefront sensory provides an estimate of the lower and higher order aberrations inherent in a patient’s eye
- the wavefront error can be documented and then transferred to the excimer laser electronically
- a corneal ablation profile (that is opposite in shape to the wave aberration) can be formulated to correct the aberration pattern
- this technique is called customized (or wavefront guided) ablation
Laser fo the cornea
CO2 laser
Excimer
Laser for lens
Nd:YAG laser pulsed
Laser for retina
Argon-ion laser
Nd:YAH laser (cw)
Laser safety
-a well collimated laser beam entering the pupil will focus the collimated laser on the smaller area on the retina, sometimes causing serious retinal bran
Fovieal burn
Results in severe and irreversible loss in acuity
Exposure to special reflections of the laser beams
Avoid it
-use interference filters to protect the eye by reflecting the laser wavelgnth while transmitting other waventlths
Class I lasers
Very low power lasers (<0.5 uW)
Present no eye or skin hazard from full day exposure, and are expect from labeling requirements
Class II lasers
Low power (<1mW)
- visible radiation that cannot injure by a short accidental exposure, but retinal damage can occur if the beam is directly viewing for a long period of time
- present not eye hazard from intrabeam espsoirue within the aversion reflex time
Class IIIa lasers
Produce visible beams of power <5mW capable of producing damage on accidental exposure through an optical aid (more powerful laser pointers fall into this class)
Class IIIb laser
Present eye hazards from intrabeam exposure within the aversion reflex time. Diffuse reflections from a class IIIb laser may present eye and skin hazards. The beam should be viewed only via diffuse reflection, from a distance over 50mm from the diffuse surface, for less than 10s, and with the diffuse image diameter being greater than 5.5mm
Class IV lasers
Present eye and skin hazard from intrabeam viewing and from diffuse reflection viewing and are a fire hazard if combustible materials were exposed
Laser protective eyewear chosen based on
Determine the laser source
- Laser operating mode
- Wavelgnth range
- Optical density (OD)
The most popular method of excimer laser refractive surgery. It creates a corneal flap using a microkeratome or a femtosecond laser. The corneal flap is retracted, the underlying cornea is reshaped with the excimer laser, and then the flap is repositioned over the ablated cornea
LASIK
The superficial layer of the cornea, the corneal epithelium, is removed and the laser treatment is applied. A therapeutic or “bandage” SCL is applied to the cornea postop, and drops are give to minimize discomfort
PRK
The corneal epithelium is peeled back often after the application of diluted alcohol to loosen the corneal epithelium, the laser treatment is applied, the epithelial layer is floated back over the treated cornea, and a bandage CL is applied over the cornea for comfort
LASEK
LASIK vs PRK
- the recovery time for patients after LASIK is typically much shorter than in other forms of excimer laser ablation, as most patients recover in 24-48 hrs after treatment
- lasik is used to the typical patient while PRK or LASEK are used more commonly in patients that have thin corneas that are not deep enough for LASIK treatments or in pateitns with large refractive errors requiring too deep an ablation
Major complications with laser surgery
- major complications including loss of an eye, loss of detail vision, reduction in BCVA, etc
- glare, halos, starbursts, reduction in ability to see low contrast objects, and objects that are seen may not appear as crisp as before
- higher pre surgery refractive errors, and wider ablation zones seem to result in more complications
- eyes that have undergone laser surgery currently have more optical third and higher order aberrations than presurgery eyes
Maximum permissible exposure
- exposure levels below those known to cause damage to eye and skin
- levels are determined for each laser as a function of wavelgnth total beam power, and delivery configuration of the beam. The addition of a condensing lens in the beam imparts significant focal characteristics to the beam that, in turn, causes the beam to diverge much more rapidly than would normally be the case. Such divergence creates a lower energy density, thus shortening significantly the distance over which an MPR can be exceeded
MPE for argon ion
0.0018
MPE for He-Ne
0.0018
MPE for diode
0.0062
MPE for Nd:YAG
0.0090
MPE for CO2 laser
0.56
Lasers operating in the ____ spectrum are absorbed in the anterior segment of the eye, primarily by the cornea as well as by the lens
UV
Below 400nm
Laser radiation in the _____ spectrum is absorbed primarily within the retina. An ideal eye can focus a collimated beam by as much as 100,000 times
Visible
Laser radiation in the _____ spectrum is absorbed primarily within the retina. The eye will focus the energy, but _______
Near infrared
It is not visible and this creates a very dangerous situation
Laser radiation in the _______ spectrum primarily affects the cornea
Far infrared
