Biomedical Engineering Reference
In-Depth Information
Table 1.2
Thermal Relaxation Time of Some Potential
Targets
Target
80
75
Thermal Relaxation Time
Erythrocyte
2 μs
70
200-μm hair follicle
40 ms
0.5-μm melanosome
0.25 μs
65
10-μm nevus call
0.1 ms
0.1-mm diameter vessel
10 ms
60
10
100
0.1
1
0.4-mm diameter vessel
80 ms
0.8-mm diameter vessel
300 ms
Time (s)
Figure 1.24
Denaturation as a function of
t
and
T
.
Abbreviations
:
t
, time;
T
, temperature.
and/or vitiligo, as well as striae. The presumed action
is immunomodulatory. The XeCl excimer laser emits
at 308 nm, near the peak action spectrum for psoria-
sis. Other UV nonlaser sources have also been used for
hypopigmentation, striae, and various infl ammatory
diseases (48,49).
2.
Violet IPL spectra and low-power 410-nm LED and
fl uorescent lamps
:
both are used either alone or with
ALA. Alone, the devices take advantage of endog-
enous porphyrins and kill
P. acnes
(50-53). After
application of ALA, this wavelength range is highly
effective in creating singlet O
2
after absorption by
PpIX. Uses include treatment of AKs, actinic cheilitis,
and basal cell carcinomas (BCCs) (54).
3.
VIS (GY):
these wavelengths are highly absorbed by
Hb and melanin and are especially useful in treating
epidermal pigmented lesions and superfi cial vessels
(11,12,23,55-59). The relatively poor penetration
in skin (and the even poorer penetration in blood,
Table 1.2) makes them poor choices for treatment
of
deeper
pigmented lesions or
deeper larger
vessels.
The effective portions of many IPL spectra include
the GY range. There are absorption peaks for PpIX
in the GY range, making these wavelengths useful
for PDT (i.e., sodium lamp, IPL, or PDL) (52,60).
4.
Near-IR(A) (755-810 nm)
: these two popular wave-
lengths are used primarily in hair reduction but have
also been used to treat blood vessels and hyperpig-
mented lesions. They are positioned in the absorp-
tion spectrum for blood and melanin between the
GY wavelengths and 1064 nm. They will penetrate
deeply enough in blood to coagulate vessels up to
2 mm (61,62); also, they are reasonably tolerant of
epidermal pigment in hair reduction (with surface
cooling) so long as
very
dark skin is not treated (63).
5.
Near-IR(B) 940 and Nd:YAG
: these two wavelengths
have been used extensively for various sized vessels
on the legs and face (64-66). They occupy a unique
place in the absorption spectrum of our “big 3”
chromophores (i.e.,
blood, melanin, and water
).
Because of the depth of penetration (on the order
of millimeters), they are especially useful for hair
reduction and coagulation of deeper blood ves-
sels. On the other hand, they are not well suited for
epidermal-pigmented lesions. Also, although water
absorption is poor, it exceeds that of the VIS and
near-VIS wavelengths. The result is that 940 and
1064 nm can cause large volume mild temperature
target (time for target to cool to 37% of peak temperature),
defi ned by
2
D
g
(4)
t
=
r
k
where
D
is the thickness or diameter of the target. Thus one
can perform some quick algebraic calculations to estimate the
peak temperatures of local targets in the skin.
Spatially selective temperature elevation is possible when
(
i
) the absorption coeffi cient of the target exceeds that of col-
lateral tissue (SPT) or (
ii
) when the “innocent bystander” tis-
sues are cooled so their peak temperatures do not exceed some
damage threshold. Localized heating, for example, in telangiec-
tasia and lentigines, follows from the concentrations of blood
and melanin there, respectively, such that
m
a
is focally increased.
Coagulation
As noted above, denaturation depends on time and tempera-
ture, and at least over times greater than 1 second, appears
to conform to a rate process as described by the Arrhenius equa-
tion (Fig. 1.24). The characteristic behavior of the Arrhenius-
type kinetic damage model is that, below a threshold temperature,
the rate of damage accumulation is negligible; it increases pre-
cipitously when this value is exceeded. This behavior is to be
expected from the exponential nature of the function. Pearce
and Thomsen defi ne a critical temperature,
T
crit
, as the tempera-
ture at which there is roughly 100% damage in 1 second. A range
of values for
T
crit
from 60°C to 85°C have been reported for vari-
ous human tissues (47).
Heat Conduction Away from the Chromophore
Once heat is generated, heat losses are based on heat conduc-
tion, heat convection, or radiation. Radiation can be neglected
in most types of laser applications. A good example of heat
convection is transfer from blood fl ow. Heat conduction is a
considerable heat lost term and is the primary mechanism by
which heat is transferred to unexposed tissue structures.
wavelength ranges useful
for cutaneous surgery
In this section, wavelength ranges useful for cutaneous surgery
are examined briefl y:
1.
UV laser and light sources
: they have been used primar-
ily for the treatment of infl ammatory skin diseases
