Biomedical Engineering Reference
In-Depth Information
I
i
=
I
o
(1 + 6
R
)
R
I
o
I
i
I
i
/
I
o
l
(nm)
585
694
1064
R
0.3
0.6
0.7
2.8
4.6
5.2
Figure 1.22
How backscattered light can result in subsurface fl uence exceed-
ing incident fl uence.
I
o
is the surface fl uence.
R
is the total light refl ected from
the skin for the respective wavelength. Note that ratio increases with wave-
length over this range.
attenuation. However, for turbid tissue such as the dermis,
where backscattering can be considerable, the “real” penetra-
tion depth can be >
d
. This value may be 2-3×
d
for example,
with 1064 nm (43-45). One will see different charts showing
different penetration depths in different bodies of literature.
For example, in an article by Reinisch (10), the optical pene-
tration depths are somewhat larger than those in another arti-
cle by Anderson (5). The trends are the same in both articles,
and neither chart is “wrong.” The
d
can change based on how
it is defi ned. Therefore one will see a wide range of different
values for similar wavelengths.
One should consider the choice of laser within the context
of the application and the respective absorption and scattering
coeffi cients. If the absorption coeffi cient is greater than
200 cm
−1
, one typically is looking at a “what you see is what
you get laser” (examples are Er:YAG and CO
2
). Between 1 and
200 cm
−1,
one sees a wavelength range where lasers that can
sometimes be useful (i.e., PDL, KTP, alexandrite). Finally,
when one considers
µ
a
less than 1 cm
−1
, we are typically dealing
with deeply penetrating light sources where one can injure the
skin without obvious surface changes [a “what you don't see
can hurt you laser”; an example being the neodynium:yttrium-
aluminum-garnet
(
Nd:YAG)] (20).
Beyond 600 nm, the increase in penetration and a brisk cut-
off in Hb absorption make for a therapeutic window between
600 and 1200 nm. In this range, radiation penetrates biological
tissues at a lower loss, thus enabling treatment of deeper tissue
structures (19). At times, the various skin pigments can play
optical “tricks” on the cutaneous surgeon. For example,
poikiloderma appears to be a mix of hyperpigmentation and
hypervascularity. In fact, although there is some melanin
infl uence—the red-brown appearance—the dyschromia is by
far more a disorder of matted telangiectasia. This is confi rmed
by the response of the condition to the PDL, even with surface
cooling. Additionally, with diascopy, often skin appears no
browner than the surrounding apparently normal skin. The
explanation is that deoxyHb contributes to a “pigmented skin
appearance.” This fi nding follows from the absorption spec-
trum of deoxyHb in the 630-700 nm range, which is very simi-
lar to the absorption spectrum of epidermal melanin. The size
of the vessels in the superfi cial venous plexus is such that the
transmitted light through these vessels is approximately 50%
lower than the incident intensity. These vessels therefore
appear dark (Fig. 1.23) (46).
,
Figure 1.23
Note dark red neck skin of poikiloderma.
Heat Generation
All laser-tissue interactions are guided by the same energy bal-
ance rules that guide all of physics. Heat generation can occur
in one of the two ways. In one scenario, it occurs at discrete
chromophore sites. In this case, there is very precise localized
heating consistent with the theory of SPT. These hot spots
under the skin allow for only the bad guys to be damaged. The
good guys (normal skin), so long as the ratio of absorption
coeffi cients (i.e., blood vs. bloodless dermis) is high enough
(optimally >10), remain unharmed. The primary areas where
SPT is helpful in dermatology are in the treatment of vascular
lesions, tattoos, and pigmented lesions. However, even in
applications where water is the chromophore, the principles of
SPT are useful (24). When treating by heating tissue water, the
injury is typically from “top to bottom,” the exception being
deeper penetrating NIR and MIR wavelengths coupled with
surface cooling. In this scenario, one can coordinate heating
and cooling to damage specifi c slices of subsurface skin.
Once the local subsurface energy density has been deter-
mined using models, heat generation can be predicted by
energy balance (conservation of energy) and the wavelength-
specifi c absorption for that target. Depending on the amount
of thermal diffusion during the pulse, the local peak tempera-
ture can be determined. The temperature increase of a desired
target can be roughly calculated by knowing the absorption
and scattering coeffi cients, surface light dose, size of the target,
and the length of the pulse, as follows:
g
/2
⎛
⎞
F
m
t
rtt
za
r
Δ=
T
(3)
⎜
⎟
c
+
⎝
⎠
r
p
where
F
z
is the local subsurface fl uence,
r
is the density,
c
is the specifi c heat, “
g
” is a geometric factor (“1” for planes,
“2” for cylinders, and “3” for spheres),
t
p
is the laser pulse
duration, and
t
r
is the thermal relaxation time of the
