Biomedical Engineering Reference
In-Depth Information
contained in phagolysosomes in fi broblasts, macrophages, and
mast cells. After treatment by Q-switched ruby laser, electron
microscopy suggests that some ink particles are fractured into
10-100 smaller fragments, which are extracellular, presumably
released by rupture of the phagocytic cells (125). A study com-
paring nanosecond and picosecond 1064-nm pulses showed
that carbon black particles became more electron-lucent after
irradiation both in vivo and in vitro (126). The threshold fl u-
ences for these changes were less for the shorter pulses. Also, a
cuvette of a carbon black suspension was lightened by simply
irradiating its contents. The gradual reduction in absorbance
has been shown to be due to a well-known steam carbon reac-
tion; this reaction occurs when particles are heated suffi ciently
to cause a chemical reaction between carbon and surrounding
water. The products are H
2
and carbon monoxide gases (69).
This suggests that changes in optical properties of the particles
may play a role in the tattoo-lightening process. Observations
strongly suggest but do not prove that the primary effects
of Q-switched lasers are (
i
) fragmentation of ink particles,
(
ii
) laser-induced optical property changes, (
iii
) release into
the extracellular dermal space, (
iv
) partial elimination of ink
in a scale crust if present, (
v
) probably greater elimination into
lymphatics, and (
vi
) rephagocytosis of laser-altered residual
tattoo ink particles. Irreversible changes can occur in tattoo
inks, especially those used in cosmetic skin-colored tattoos.
Photochemical changes may conceivably affect tattoo ink
removal for some kinds of ink. This appears to be especially
true for tattoos containing red (ferric oxide) and titanium
dioxide (vide infra) (127,128). All of the pulsed lasers used for
tattoo removal occasionally cause irreversible immediate
darkening of these tattoos, which may be temporarily obscured
in part by the immediate whitening reaction, discussed above.
It is therefore prudent to perform a small test exposure of cos-
metic red, fl esh-colored, or white tattoos to see whether imme-
diate darkening occurs. In some cases, the darkened ink cannot
be removed by subsequent laser treatments and adds to the
disfi gurement. The mechanism of red tattoo ink darkening is
not known but probably involves reduction of ferric oxide
(rust color) to ferrous oxide (black). Pure ferric oxide is easily
converted in this manner by Q-switched laser pulses in vitro
and is present in many cosmetic tattoos (127). During tattoo
treatment, there is surface whitening (as in epidermal pig-
mented lesions, vide supra). This is accentuated where there is
the highest pigment concentration. The proposed causes
have already been discussed. In treating black tattoos with
1064 nm (where there is little epidermal effect), one can
also see a subsurface transient bright light. This probably rep-
resents laser-induced incandescence with maximum tempera-
tures around 2000 K. At 4000 K carbon particles vaporize
(123). There are a few controlled studies to understand and
optimize tattoo removal. The study cited above addressed the
role of pulse duration (nanosecond vs. picosecond domain
pulses) in tattoo removal (126). One of the study goals was to
examine the role of inertial confi nement in laser-tattoo reac-
tions. Somewhat analogous to the concept of thermal confi ne-
ment, inertial confi nement can be achieved when the laser
pulse is delivered within the time for which pressure can be
relieved from the ink particles. The inertial confi nement time
is simply the particle diameter divided by the speed of sound,
which for tattoo ink particles equals about 1 ns. Because of the
high pressures, shock waves (bulk disruption of the material)
can occur, which should increase the probability of mechani-
cal fracture. It may therefore be that all of the laser pulses being
used now are too long. Ross et al. (126) showed that the pico-
second domain pulses were more effective than nanosecond
pulses in black tattoo removal for the same radiant exposures,
suggesting that tattoo removal is very sensitive to power den-
sity. A theoretical model predicts that the nanosecond pulses
would only generate peak temperatures about 3% that of the
nanosecond pulses, so that photothermal effects alone could
explain the more effi cient tattoo resolution with shorter pulses.
It is unclear if shock waves were generated with the picosecond
pulse. Signifi cant details of wavelength-dependent effects are
unknown. One study has examined the spectra for most colors
and showed absorption peaks for all known colors; however,
many tattoos are resistant to multiple treatments, especially
green tattoos (129). In a study of tattoos at our laboratory,
TiO
2
was detected by X-ray diffraction in 4/5 resistant green
tattoos, and the Q-switched alexandrite laser produced an
immediate blue-black discoloration of TiO
2
ink; the ink is
often used as a brightener for green and other colors. Para-
doxically, with additional treatments, some laser-darkened tat-
toos can be lightened (128). It may be that once all the
photochemistry has taken place that the newly formed prod-
ucts are also capable of absorbing VIS or IR radiation. The
chemical identity of most tattoo inks and how they are affected
by high-intensity laser pulses are unknown and, most impor-
tantly, have never been linked to clinical response. In the previ-
ous edition of this text, several unresolved issues were listed in
tattoo removal, only a few of which have been answered: (
i
) Is
a laser-induced plasma involved? Possibly, if the generation of
surface plasma facilitates ink removal through generation of
shock waves. (
ii
) Must the ink particles be fractured? Ink par-
ticles need not be fractured for clinical resolution. This has
been shown for carbon black, where particles may actually
increase in size yet lighten through a change in chemistry.
It appears that more crystalline particles are more likely to
fracture than relatively amorphous organic substances (130).
(
iii
) Is it the ink or the cells containing it are the targets?
It appears that the ink is the target but that cell death is associ-
ated with clearing, since this allows ink release from the cell.
(
iv
) How is the ink removed, and where does it go? The ink is
removed by the mechanisms cited above; in addition, optical
property changes in the ink itself may contribute to clearing
for some inks. (
v
) What role might phagocytosis by different
cell types play in the retention of ink following treatment,
and how can this be optimized? Attempts have been made to
modulate the immune response in tattoo removal. Recently,
imiquimod cream has been used in the interval between laser
treatment sessions to augment ink removal. Also, at least one
study showed that repeated laser treatment in the same offi ce
visit at 20-minute intervals could accelerate tattoo clearing.
Presumably, the delay allows the whitening response to resolve
and permits enhanced transmission of the subsequent set of
laser impacts (131).
Cooling
Before the availability of surface cooling, fl uence thresholds for
effi cacy and epidermal damage were often close. VIS light tech-
nologies (especially GY light sources such as IPL, KTP laser,
