Biomedical Engineering Reference
In-Depth Information
and PDL) are popular in cutaneous laser surgery. They are also
the wavelength ranges where epidermal damage is most likely.
The epidermis is an innocent bystander in cutaneous laser
applications where the intended targets, such as hair follicles or
blood vessels, are located in the dermis. Specifi cally, absorption
of light by epidermal melanin causes epidermal heating. Mela-
nin is distributed throughout the epidermis but is especially
concentrated in the basal cell layer. Melanin absorption of VIS
light causes heating of melanosomes and through thermal dif-
fusion, subsequent damage to the entire epidermis. This is
especially true for GY light, but the risk of selective DE junction-
derived epidermal injury extends to wavelengths as long as
1064 nm. Overall, shorter wavelengths pose a greater risk to the
skin surface, because the ratio of epidermal-to-dermal heating
is higher. This ratio derives from (
i
) a higher absorption of
melanin by shorter wavelengths and (
ii
) a tendency for pho-
ton scatter to limit penetration of shorter wavelengths. This
leads to an accumulation of energy near the DE junction
(58,132-134).
Beyond VIS light (green, yellow, and red) sources, surface
cooling has also been employed in NIR and MIR lasers. With
NIR lasers, surface cooling is important, but not only because
of DE junction-derived epidermal heating. In addition, deep
beam penetration may cause catastrophic bulk heating. With
MIR lasers (1.32, 1.45, and 1.54
In the above equation,
T
ic
and
T
i
are basal layer temperatures
before laser irradiation with and without cooling, respectively.
T
crit
is the critical temperature at which thermal injury occurs.
The detailed calculations described later indicate that if the ini-
tial skin temperature is 30°C, contact cooling reduces the tem-
perature of the basal layer to about 20°C. If
T
crit
is assumed to
be 60°C (it is actually somewhat higher for the brief laser
exposure times in this analysis), this would give the CPF as
(60-20)/(60-30) or 1.33. Similarly, cryogen cooling reduces
the temperature to about 0°C, thus giving a CPF value as
(60-0)/(60-30) or 2.0. In summary, the CPF values provided
by CSC and contact cooling are predicted as 2.0 and 1.33,
respectively. Finally, there is convective air cooling, where cold
air is commonly used in skin chilling. The Zimmer (Cyro5,
Zimmer MedizinSysteme, Ulm, Germany) directs −10°C air
at the skin at a rapid rate (1000 L/min). This system proves for
good bulk cooling but spatial localization of the cooling is
poor. The CPF, depending on the air temperature and nozzle
velocity, is near that of contact cooling (142).
Besides protection of the DE junction from pigment
“unfriendly” wavelengths, sometimes bulk cooling is required
because the volume heated is large and there is a risk for
large volume overheating and catastrophic scarring (i.e., with
1064 nm). There are some scenarios where cooling is unlikely
to prove benefi cial: (
i
) when the absorption of the wavelength
is very strong by water (i.e., Er:YAG and CO
2
), here the cooling
and heating zone are collocated so that preservation of epider-
mal
viability
is unfeasible; also (
ii
) when using Q-switched
lasers in the range from 532 to 1064 nm, cooling achieves pain
reduction but will only modestly reduce the high peak tem-
peratures generated by these ultrashort pulses in melanin and
exogenous inks (tattoos).
m), the chromophore is
water. It follows that with even very low fl uences, surface cool-
ing is imperative. Without cooling, water's ubiquitous nature
in the skin causes laser-induced top-to-bottom injury. There is
no discrete heating. All of the techniques are susceptible to
operator error and device failure. It follows that as physicians
rely more heavily on cooling devices, any lack of their proper
deployment unveils the
dark
side of cooling.
The fi rst goal of surface cooling is preservation of the epider-
mis. Unintentional heating of the basal cell layer can lead to
vesiculation, crusting, and, at times, scarring. The second and
related goal of surface cooling is to allow for delivery of higher
fl uences to the intended target (i.e., the hair bulb and/or bulge
or a subsurface blood vessel). Often, the highest fl uence that
can be used in targeting hair and/or subsurface vessels is limited
by heating of the epidermis. By cooling the epidermis, higher
fl uences and therefore higher temperature elevations are pos-
sible in the targeted structures in the dermis. Another benefi t of
surface cooling is analgesia, as almost all cooling strategies will
provide some pain relief (135-141).
The timing of the cooling relative to the laser pulse is impor-
tant. Cooling can be before the pulse (pre), during the pulse
(parallel), or after the pulse (post) (134). All three cooling peri-
ods are important. For example, postcooling may prevent retro-
grade heating (i.e., from the vessel back to the epidermis) from
damaging the skin surface. A cooling protection factor (CPF)
has been proposed by Anderson, who likens it to the sun pro-
tection factor concept used in sunscreen assessments. The cool-
ing protection factor is the ratio of fl uence, with and without
surface cooling, that spares the epidermis. It can be evaluated
from the following equation:
μ
some interesting concepts and ideas
in laser
tissue interactions
Focusing the Laser Beam
A trick to increase the dermal-to-epidermal damage ratio is
use of a convergent lens. This tool increases the local photon
density in the dermis (targeting the hair bulb, a blood vessel, or
dermal water). Theoretically, one should be able to use smaller
incident fl uences, therefore achieving some protection of the
epidermis (143). Because of scattering and subsequent beam
broadening, the increased subsurface photon densities that
would be predicted in a transparent medium are not realized.
Still, there should be a relative increase in deeper photon den-
sity versus using a more collimated beam.
-
Vacuuming the Target in the Laser Beam
A company (Aesthera, Livermore, California, USA) has pro-
posed a pneumatic device whereby the skin is vacuumed into
the light path, such that the light penetration into skin is
enhanced (Fig. 1.33). In this way more energetic high-
frequency photons can be delivered, for example, to the hair
follicle, with relative epidermal sparing.
Pixilated Injury (Also Known As Fractional
Photothermolysis)
One can use a “pixilated” injury with water as a chromo-
phore in what is called fractional photothermolysis. Roughly
=
TT
−
−
c
ic
CPF
TT
(5)
c
i
