Basics for laser treatment for pigmentation

Characteristics of laser - Coherence, Collimation, Monochromaticity Selective photothermolysis ; laser energy absorption by a target chromophore without thermal damage to surrounding tissue (i) wavelength - absorbed more by target tissue than by surrounding tissue (ii) fluence - sufficiently high to impart enough energy to thermally alter the target tissue (iii) pulse duration - must be shorter than the thermal relaxation time of the chromophore to prevent the spread of thermal energy beyond the targeted chromophore Thermal relaxation time - time required for heat generated by the absorbed light energy within the target chromophore to cool to half of the original value immediately after the laser pulse - tattoo: 10 ns, melanosome; 250 ns, melanocyte: 1 μm, epidermis: 1 ms Laser classification according to pulse duration (i) Continuous wave lasers - non-specific thermal damage (ii) Pulsed lasers - long pulse (milliseconds), short pulse (micro/nanoseconds) (iii) Q-switched laser . ruby, alexandrite, Nd:YAG laser (iv) Picosecond laser Q-switched laser for pigmentation ; Photomechanical effects - sudden thermal expansion that produces stress waves - rupture or increase permeability of cell membranes (i) Q-switched ruby laser (694 nm, 20-80 ns) - higher melanin absorption than alexandrite and Nd:Yag laser - less Hemoglobin absorption → less side effect - epidermal & dermal pigment lesion - black, blue, green colored tattoo (ii) Q-switched alexandrite laser (755nm, 50-100ns) - less absorption by epidermal melanin → ideal for dermal pigment - less tissue splatter - blue, black, green colored tattoos (iii) Q-switched Nd:YAG lasers (1064nm, 5-20ns) - increase dermal penetration, lower epidermal melanin absorption - advantage for treating darker-skinned patients - significant tissue splatter - black, blue colored tattoo - 532 nm: passing the 1064 nm beam through potassium titanyl phosphate (KTP) crystal effective in epidermal pigment, red colored tattoo