Biomedical Engineering Reference
In-Depth Information
tissue splatter seen, is most likely caused by the lack of thermal
injury to collagen. The dermis and epidermis sustain mechani-
cal injury from the photoacoustic wave, but this trauma is
apparently highly reparable. The authors also noted a bright
flash of white light from the tattoo during laser exposure, as
mentioned with the ruby laser. Because 1064-nm light is not
visible, this flash must result from either laser-stimulated
plasma or incandescence of tattoo ink particles. These both
imply extreme temperatures greater than 500°C.
The Q-switched Nd:YAG laser has a great advantage for
treating darker-skinned patients. Both Jones et al. (217) and
Grevelink et al. (218) demonstrated effective tattoo removal
with minimal hypopigmentation or hyperpigmentation. This
provides a significant benefit over the Q-switched ruby laser
for darker-skinned patients in whom melanin absorption is a
hindrance and for cosmetic eyeliner tattoos, where care must
be taken not to damage the eyelashes or retinal melanin (219).
In addition, the smaller spot size available with most
Q-switched Nd:YAG lasers (1.5-mm diameter) allows for more
accurate distinction of tattoo in sensitive areas.
A frequency-doubling potassium titanyl phosphate crystal is a
feature of the Nd:YAG laser that reduces the standard 1064-nm
wavelength by 50% to produce a 532 nm, green appearing beam
to treat red ink very effectively (Fig. 4.7). Greater than 75%
removal of red ink has been reported with three treatment
sessions, and orange and some purples respond almost as well;
however, yellow ink responds poorly, presumably because of its
dramatic drop in absorbance between 510 and 520 nm, as do
green and blue inks, where absorption is greater at 600 nm or
longer (220).
In summary, the Q-switched Nd:YAG laser has proved
to be effective in removal of black ink (Fig. 4.8), with rare
textural changes and almost no hypopigmentation. These
improvements are attributed to the longer wavelength,
higher fluence, and shorter pulse width. However, the faster
repetition rate (1-10 Hz) shortens the treatment session,
although this is somewhat counterbalanced by the use of a
smaller beam size (2-4 mm vs. 5-6.5 mm for the ruby laser).
Larger spot sizes are now available with new, higher-powered
Nd:YAG laser systems, enabling deeper penetration and
more effective treatment of deeper, denser tattoos (221).
Better beam profiles have minimized epidermal damage,
decreasing bleeding, tissue splatter, and transient textural
changes.
( A ) ( B )
Figure 4.7 Red ink. ( A ) A red ink tattoo; ( B ) complete red ink resolution after treatment with 532 nm via neodymium-doped:yttrium-aluminum-garnet.
( A ) ( B )
Figure 4.8 Black ink. ( A ) A black ink tattoo; ( B ) complete resolution of black ink after treatment with 1064-nm neodymium-doped:yttrium-aluminum-garnet.
 
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