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Lasers (Light Amplification by Stimulated Emission of Radiation)

Figure 6.6 Chromophore absorption spectrum (Reprint from Teichman O, Herrmann T, Bach T. Technical aspects of lasers in urology. World Journal of Urology. June 2007: 25(3); 221–225)
Figure 6.6 Chromophore absorption spectrum
(Reprint from Teichman O, Herrmann T, Bach T.
Technical aspects of lasers in urology. World Journal
of Urology. June 2007: 25(3); 221–225)
(Tables 6-23, 6-24, 6-25, 6-26)
  • Laser light: monochromatic (single, discrete wavelength), spatially coherent (light in phase), collimated (light in parallel fashion)
  • Laser treatment based on principle of selective thermolysis: targeted lesion may be destroyed by chromophore absorption of laser light without significant thermal damage to surrounding normal tissue; pulse duration (exposure time) must be equal to or shorter than the target’s thermal relaxation time (TRT, cooling time or time for target to lose 50% heat) to confine thermal damage
  • Thermal damage time (TDT): time required to irreversibly damage target with sparing of surrounding tissue; pulse duration ≤ TDT allows for efficacy with ↓ epidermal damage
  • Depth of penetration directly proportional to wavelength (i.e., Nd:Yag 1064 nm penetrates deeper than PDL 585 nm); ↑ scattering with decreasing λ (scattering mostly due to collagen)
  • Chromophores: components in skin which absorb laser light
    • Endogenous: hemoglobin, melanin, water; exogenous: tattoo ink
  • Laser characteristics: wavelength, pulse duration, spot size, fluence (J/cm2), power (J/s)
  • Gain medium determines wavelength of light: liquid (dye lasers), gas (argon, CO2, helium-neon), solid (Nd:Yag, ruby)
  • Pulse duration (exposure time of laser):
    • Determines confinement of heat and extent of thermal injury in tissue
    • Best if pulse duration ≤ TRT
  • Spot size : Larger spot size allows for deeper energy penetration (less scattering)
  • Cooling : different types of epidermal cooling to minimize epidermal damage: passive (aqueous gel), active contact cooling (water encased in sapphire or glass housing), dynamic active cooling (cryogen spray), forced air cooling
  • Q-switched or “quality-switched”: allows accumulation of excessive energy in laser cavity prior to emission; extremely short pulses of very high power (nanosecond range); used mainly for removal of tattoo pigment and superficial pigmented lesions
  • Hair removal:
    •    
       TRT: proportional to square of target’s diameter (so shortest TRT in chromophore with smallest size) 
         
      Target thought to be bulge of hair follicle as well as dermal papilla
    • Best when pulse duration ≤ TRT of hair follicle (40–100 ms for terminal hairs) and ≥ TRT of epidermis (3–10 ms) to minimize epidermal damage; thus, optimum pulse duration 10–50 ms; use shorter wavelength laser (ruby) for blond, white, red, and gray hairs since better absorption of melanin
  • Pigmented lesions (epidermal pigment):
    • Use QS lasers (↓↓ pulse duration): QS KTP, QS ruby, QS Nd:Yag (532 nm)
    • Target endpoint: uniform but faint whitening, no epidermal disruption (higher fluences will have solid whitening w/ epidermal disruption and pinpoint bleeding)
  • Tattoos:
    • QS lasers used to remove tattoo pigment; of note, white/peach/pink/flesh-toned tattoo color may turn dark gray immediately after treatment with QS laser (reduction of ferric oxide to ferrous oxide)
    • Amateur tattoo: usually clears after 3–5 treatments with QS laser
    • Professional tattoo: may require ten or more treatments (dense pigment)
   
 
Table 6-23 Chromophores
 ChromophoreAbsorption PeaksLaser
 
Hemoglobin
 
418 nm, 542 nm, 577 nm
 
Argon, copper vapor, KTP, pulsed dye
 
Melanosome
300–1000 nm (peak 335 nm)
PDL, KTP, ruby, alexandrite, diode, Nd:Yag
 
Water
 
1450 nm, 1950 nm, 3000 nm
 
CO2, erbium, diode (1450 nm), Nd:Yag (1320 nm)
      
      
     
  ↑ Absorption of melanin at lower λ (300–600 nm) but ↑ scattering, ↓ penetration, and competing chromophores (Hgb) occur with lower λ lasers, which is why higher λ used (694+) for hair removal and often for pigmented lesions  
     
 
   

   
 
Table 6-24 Lasers
 LaserWavelengthChromophoreComments
 
Excimer (XeCl)
 
308 nm
 
Protein
 
Psoriasis
 
Argon
488 nm, 514 nm
Melanin, Hgb
Vascular and pigmented lesions, ↑ risk scarring
 
Pulsed dye
(short wavelength)
510 nm
Melanin
Pigmented lesions
 
Copper vapor
511 nm, 578 nm
Melanin, Hgb
Vascular and pigmented lesions
 
KTP
(potassium titanyl phosphate)
532 nm
Melanin, Hgb
Pigmented and superficial vascular lesions
 
QS Nd:Yag
(frequency doubled)
532 nm
Tattoo pigment
Superficial pigmented lesions, red/orange/yellow tattoos
 
Pulsed dye (PDL)
585–595 nm
Hgb
Vascular lesions, hypertrophic scars, verrucae
 
Ruby
694 nm
Melanin
Hair removal, nevus of Ota
 
QS Ruby
694 nm
Melanin, tattoo pigment
Superficial pigmented lesions
(i.e., solar lentigo), blue/black/green tattoos
 
Alexandrite
755 nm
Melanin
Hair removal
 
QS Alexandrite
755 nm
Melanin, tattoo pigment
Pigmented lesions, blue/black/green tattoos
 
Diode
800–810 nm
Melanin, Hgb
Hair removal, leg veins
 
Nd:Yag (long-pulsed)

{Concern for retinal damage with Nd:Yag as laser penetrates deep and emits invisible radiation}
1064 nm
Melanin, Hgb
Hair removal, nonablative dermal remodeling, leg veins
 
QS Nd:Yag
1064 nm
Melanin, tattoo pigment
Pigmented lesions, blue/black tattoos
 
Nd:Yag (long-pulsed)
 
1320 nm
 
Water
 
Nonablative remodeling
 
Diode
1450 nm
Water
Nonablative remodeling
 
Er:Yag
 
2940 nm
 
Water
 
Ablative remodeling
 
CO2
10,600 nm
Water
Ablative resurfacing, actinic cheilitis
        
 
   

   
 
Table 6-25 Lasers for Hair Removal
 LaserWavelengthHair ColorSkin TypeComments
 
Ruby
694 nm
Blond, red white, gray, brown
I, II
Significant dose-related side effects (epidermal crusting, vesiculation, dyschromia) due to ↑ melanin absorption
(vs. Nd:Yag)
 
Alexandrite
755 nm
Red, gray, brown
I, II
Longer λ so ↑ penetration; slightly ↓ risk of epidermal damage than Ruby
 
Diode
800–810 nm
Brown, black
I, II, III, IV, V
↑ Penetration, ↓ epidermal injury
 
Nd:Yag
1064 nm
Brown, black
II, IV, V
Deeply penetrating λ, ↓ melanin absorption requires ↑ influence for melanin injury; not as effective as Ruby with lighter hair; safe for darker skin types
 
IPL
515–1200
Varies based of cutoff filters
Varies based on cutoff filters
Nonlaser, noncoherent, multi-wavelength light; filters placed for more selective treatment (shorter λ for lighter skin, longer λ for darker skin)
          
 
   


   
 
Table 6-26 Lasers for Tattoo Pigment Removal
 Laser TypeWavelengthTattoo-Ink Color Treats
 
Pulsed dye (short wavelength)
510nm (green light)
Yellow, red, orange, purple
 
QS Nd:Yag (frequency doubled)
532nm (green light)
Red, orange, yellow
 
QS Ruby
694nm (red light)
Blue, black, green
 
QS Alexandrite
755nm (red light)
Green, blue, black
 
QS Nd:Yag
1064nm
Blue, black
      
      
     
  Red pigments reflect red light and maximally absorb green light (therefore, ruby/alex is not effective in removing red tattoos); green pigment reflects green light and maximally absorbs red light (so frequency doubled NdYag is not effective)  
     
 
   

 

 

 

 

 

 

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