Biomedical Engineering Reference
In-Depth Information
from melanin, GY wavelength (532-595 nm) laser pulses
appear to work well for treating larger vessel (0.1- to 1-mm
diameter) PWSs or telangiectasia, and at the longer pulses
appropriate to PWS treatment, pigment cell injury is mini-
mized. By adding a second pass to vascular lesions, the dermal
blood fraction will have increased from the fi rst pass, and the
blood absorption can increase manyfold. This has been shown
to result in more comprehensive vessel heating within a vol-
ume of treated tissue (107). Part of the improvement may also
stem from methemoglobin production (108,109). With GY
lasers and IPLs, longer pulses (or pulse sequences) allow for
some epidermal cooling between vascular events. The ratio-
nale is that the vessel will have a longer
t
than the DE junction.
It follows that longer pulses should allow for a greater ratio of
vascular to epidermal heating (Fig. 1.19). This is supported by
our work, which shows that longer IPL pulses allowed for
greater epidermal sparing (Fig. 1.18). Recently, an enhanced
role for PDT in vascular lesions has been reported. In these
cases, vascular-specifi c photosensitizers, such as hematopor-
phyrin monomethyl ether, have been applied to PWS (110,111).
Hair
In this application, the target is either the bulge or the bulb, and
the innocent bystander is the DE junction. The goal is to maxi-
mize the ratio of bulb to epidermal heating. The melanin den-
sity of the bulb normally exceeds the DE junction (most people
have darker hair than skin). The challenge in laser hair reduc-
tion (LHR) is that the bulb lies deep in the skin (about 1-3 mm),
such that the local fl uence at the DE junction will exceed
the bulb fl uence. However, we can maximize the bulb-to-
epidermal temperature ratio by (
i
) extending the pulse,
(
ii
) using longer wavelengths, (
iii
) applying epidermal cooling,
and (
iv
) compressing the skin (which decreases the bulb-to-
surface distance) (112). In LHR, the heater (shaft) and the tar-
get (bulb or bulge) are not collocated. It follows that pws should
be designed that exceed the
t
of the shaft but not be so long that
the dermis is overheated beyond the bulb (113). Too short
pulses provide only temporary hair reduction and for the same
reasons, very short pulses are ineffective in treating vascular
lesions-heating is too confi ned to the immediate chromophore
and does not extend to the intended target. Nanosecond pulses
result in vaporization, but with or without a carbon suspen-
sion, hair reduction is akin to that of laser waxing. Very short
nanosecond pulses expel the melanosomes outside the shaft
but do not allow for signifi cant heat conduction, and an imme-
diate leukotrichia may be observed (the pulses are so short that
the skeleton of the hair shaft remains) (114). With longer pulses
(1-3 ms) and high power densities, the entire shaft is often
coagulated, and it tends to coil up on the surface. Thus, like the
blood vessel, one observes a continuum in the immediate laser
light-hair response as a function of pulse duration. What about
immediate endpoints and their relationship to the laser-tissue
interaction? We normally think of perifollicular edema (PFE)
as a necessary (and possibly) suffi cient condition for semi- or
permanent hair reduction. However, PFE can be a deceptive
endpoint. Most likely it represents peribulbar damage. How-
ever, no one has characterized an association between the level
of damage and the PFE severity. The failure of PDL, for exam-
ple, to cause PFE despite obvious hair vaporization at the sur-
face, suggests that deeper heating is required for PFE. Shorter
Figure 1.31
Note perifollicular edema increases with decreasing pulse width
for same fl uence with 1064-nm laser.
pulses, at least in a range from 3 to 100 ms) cause more PFE
than longer ones with other parameters held constant, most
likely due to more “intense” short-term damage to the follicle
(Fig. 1.31). It follows that there is no reliable endpoint for per-
manent reduction. The presence of PFE always signals short-
term reduction, but its absence does not preclude delayed LHR.
For example, with a very long pulse (200-400 ms) 810-nm
diode laser (Super Long Pulse, Palomar, Burlington, Massachu-
setts, USA), we have observed permanent LHR without PFE.
What is the optimal wavelength for LHR? Theory suggests that
longer wavelengths (e.g., 1064 vs. 755 nm) will always achieve a
greater ratio of dermal-to-epidermal heating (112). But studies
have not confi rmed that longer wavelengths are necessarily
more effective in LHR (115-119). Overall, the greatest limita-
tion in using longer wavelengths is the absolute decrease in
melanin absorption. For example, let us take the typical case of
a young type II patient with dark coarse hair on the neck. If one
compares the 810-nm diode laser with the 1064-nm laser, for
example, treating one side with a 10-mm spot at 810 nm and
44 J/cm
2
and 21 ms and the other side with identical parame-
ters with the YAG laser, although both lasers will achieve
marked hair reduction, LHR will be better on the 810-nm side.
We can use arguments from the skin optics and heat generation
sections (vide supra) to support this result. We can also use
