Biomedical Engineering Reference
In-Depth Information
those same arguments to optimize our parameters to enhance
1064-nm LHR. 810-nm light is absorbed roughly 2.5× that of
1064 nm. It follows that scattering and penetration issues aside,
2.5× the fl uence should be required for identical bulb heating
for the two wavelengths. However, more 1064-nm light will
penetrate 3-mm deep into the skin, such that there are about
25% more photons at this depth versus 810 nm (120). It follows
that to achieve identical bulb heating, assuming all other
parameters are held constant, simple algebra suggests that
about 2× more fl uence must be delivered at the surface for sim-
ilar bulb heating. Most YAG lasers, however, do not routinely
allow for fl uences above 80 J/cm
2
with big spots. Also, with
larger spots and high fl uences, although the epidermis is pre-
served, the YAG laser poses a risk for generalized dermal heating
with multiple pulses. It follows that YAG lasers are ideal when
the skin is very dark or the hair is very thick and black (Fig. 1.32).
In these scenarios, the bulb-to-epidermal heating ratio is greater
than 1, and even with lower fl uences,
m
a
of the follicle is so large
(~75 cm
−1
for a thick dark hair vs. 10 cm
−1
for a fi ne brown hair
at 1064 nm) that suffi cient heat is generated (the hair is “satu-
rated” with heat). With lighter fi ner hairs, one can use very high
fl uences and shorter pulses and still achieve some degree of per-
manent LHR with 1064 nm. We were able to see this in sandy
brown haired patient, where a biopsy showed that a combina-
tion of 150 J/cm
2
and a 5-mm spot was necessary to see the
earliest clearly identifi able heat changes in the bulb. Still, this
heating was far less than that seen with a very thick black hair at
50 J/cm
2
. For the author's own fi ne black arm hair, 120 J/cm
2
with 1064 nm achieved the same degree of PFE as 50 J/cm
2
with
810 nm. Hair reduction is a good example of why studies must
be carefully examined before proclaiming victory for one laser
over another (or at least one wavelength over another!). Often a
study will compare lasers in their
commonly deployed
parame-
ters (and not the optimal parameters) and then score the results.
In the investigators' defense, often the optimal parameters are
not available with a particular device. Unfortunately, some YAG
lasers compromise too low fl uences, and shorter wavelength
lasers usually prevail in head-to-head studies unless the studied
hairs are very thick and/or very black. Although the alexandrite
and diode lasers can be used in most skin types (so long as
effective surface cooling is applied), the one exception is very
dark skin (i.e., darkest African-American) (115,121). In this
case, we have found that no matter how long the pulse and
how exquisite the cooling, 1064 nm offers the
greatest
ratio of
effi cacy to safety. This fi nding is consistent with our pigment
readings with refl ectance spectrophotometers, which show that
a dark-skinned African-American is about 1.8× a dark as a
light-skinned African American, but that the light-skinned
African-American is only about 1.2× as dark as a tanned
medium complexion Caucasian. PDT is another potential
means for LHR. The major advantage is its nondependence
on pigment. At this time, no system is readily available for
ALA-assisted hair reduction, and several obstacles (pain, incu-
bation time of ALA) remain before such a system can be
deployed over large areas. A device that combines RF and light
has been proposed to work in whitish hairs (vide supra).
Sebum and Fat
Laser lipolysis has been examined; however, no device has truly
exploited the fat/sebum absorption maxima (relative to water)
at 915, 1200, 1700, and 2200 nm (122). An RF device selectively
heats fat in some applications at some threshold power.
Tattoo Removal by SPT
Tattoos consist of mainly intracellular, submicrometer size
insoluble ink particles that have been ingested by phagocytic
skin cells after intradermal injection. The stability and longev-
ity of most tattoos show that many phagocytic skin cells do not
traffi c or migrate widely, although tattoos do become less dis-
tinct over decades (19). A great variety of inks are used in pro-
fessional tattoos, which consist mainly of insoluble metal salts,
oxides, or organic complexes (123). Amateur tattoos are almost
always carbon in some form—India ink (amorphous carbon),
graphite, or ash. Goldman fi rst noted that tattoos were respon-
sive to pulsed laser treatment, using normal mode ruby laser
pulses (8). A dose-response and histologic study of Q-switched
ruby laser pulses was subsequently performed, which led to
widespread interest in Q-switched ruby lasers in the United
States, followed by further ultrastructural studies of response
(124). It is now apparent that the Q-switched ruby laser is gen-
erally effective and relatively well tolerated in the treatment of
black, blue-black, and green tattoos. Multiple treatments are
required at fl uences ranging from 4 to 10 J/cm
2
. Typically, four
to six treatments given 1-month interval are needed for ama-
teur tattoos, and six to eight for professional tattoos, although
individual response is extremely variable. The risk of scarring
is about 5-10% for the series of treatments, although over
one-fourth of patients have transient textural changes. The
Q-switched ruby laser causes blistering and hypopigmentation
in most patients and permanent depigmentation in about
1-3%. The mechanisms involved in tattoo removal are largely
unknown (123). It is clear that much of the ink, although
apparently removed from the skin, is not removed from the
body. All persons with tattoos have tattoo ink pigmentation of
the lymph nodes draining the tattooed skin, and it is likely that
this is the fate of most of the ink after laser treatment. Lighten-
ing of the tattoo occurs gradually about 1 week after each
treatment and may go on for months. Occasionally, it is clear
that ink is present in the scale crust that forms following epi-
dermal injury and sheds 1-2 weeks after treatment, but it is
equally apparent that tattoos are removed in cases where no
scale crust is formed. Before treatment, ink particles are
Figure 1.32
Selective heating of thicker black hairs with 1064 nm. The thinner
hairs were unchanged on the surface after treatment with 1064-nm laser at
80 J/cm
2
and 40 ms with a 5-mm spot.
