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A theoretical investigation of human
skin thermal response to near-
infrared laser irradiation
Tianhong Dai, Brian M. Pikkula, Lihong V. Wang, Bahman
Anvari
Tianhong Dai, Brian M. Pikkula, Lihong V. Wang, Bahman Anvari, "A
theoretical investigation of human skin thermal response to near-infrared laser
irradiation," Proc. SPIE 5312, Lasers in Surgery: Advanced Characterization,
Therapeutics, and Systems XIV, (13 July 2004); doi: 10.1117/12.529146
Event: Biomedical Optics 2004, 2004, San Jose, CA, United States
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A theoretical investigation of human skin thermal response to near-
infrared laser irradiation
Tianhong Dai
a
, Brian M. Pikkula
a
, Lihong V. Wang
b
and Bahman Anvari
*a
a
Department of Bioengineering, Rice University, Houston, TX, USA 77251;
b
Department of Biomedical Engineering, Texas A&M University, College Station, TX, USA 77843
ABSTRACT
Near-infrared wavelengths are absorbed less by epidermal melanin mainly located at the basal layer of epidermis
(dermo-epidermal junction), and penetrate deeper into human skin dermis and blood than visible wavelengths.
Therefore, laser irradiation using near-infrared wavelength may improve the therapeutic outcome of cutaneous hyper-
vascular malformations in moderately to heavily pigmented skin patients and those with large-sized blood vessels or
blood vessels extending deeply into the skin. A mathematical model composed of a Monte Carlo algorithm to estimate
the distribution of absorbed light followed by numerical solution of a bio-heat diffusion equation was utilized to
investigate the thermal response of human skin to near-infrared laser irradiation, and compared it with that to visible
laser irradiation. Additionally, the effect of skin surface cooling on epidermal protection was theoretically investigated.
Simulation results indicated that 940 nm wavelength is superior to 810 and 1064 nm in terms of the ratio of light
absorption by targeted blood vessel to the absorption by the basal layer of epidermis, and is more efficient than 595 nm
wavelength for the treatment of patients with large-sized blood vessels and moderately to heavily pigmented skin.
Dermal blood content has a considerable effect on the laser-induced peak temperature at the basal layer of epidermis,
while the effect of blood vessel size is minimum.
Keywords
: Cutaneous hyper-vascular malformations, laser therapy, blood vessel coagulation, epidermal protection
1. INTRDUCTION
Pulsed dye lasers at the wavelengths of 585 and 595 nm have been the common choices for the treatment of cutaneous
hyper-vascular malformations such as telangiectasia
1
, port wine stain
2-5
and hemangiomas
6
. However, clinical studies
have shown that complete blanching of the lesions is not commonly achieved, and multiple treatments are usually
required to obtain optimal blanching
2-6
. Moreover, in some cases, patients are unresponsive to pulsed dye laser
irradiation
7
. The possible reasons for these limited therapeutic outcomes are the limited light penetration depth in large-
sized blood vessels as well as blood vessels extending deeply into the skin dermis, and subsequently non-uniform
heating in various blood vessel layers. Additionally, as epidermal melanin, mainly located at the basal layer of epidermis
(dermo-epidermal junction), competes with subsurface targeted blood vessels in the absorption of laser light, a large
number of the patients, namely those with high melanin concentration skin types, are still excluded from the laser
treatment due to significant light absorption by the epidermal melanin, which can lead to persistent hyper-pigmentation,
textural changes to the skin
8
.
Near-infrared wavelengths are absorbed less by epidermal melanin, and penetrate deeper into skin dermis and blood than
visible wavelengths. Therefore, laser irradiation using near-infrared wavelength may improve the therapeutic outcome of
cutaneous hyper-vascular malformations in moderately to heavily pigmented skin patients and those with large-sized
blood vessels or blood vessels extending deeply into the skin. Using a mathematical model composed of a Monte Carlo
algorithm to estimate the distribution of absorbed light followed by numerical solution of a bio-heat diffusion equation,
we investigated the thermal response of human skin to near-infrared wavelength laser irradiation, and evaluated the
feasibility of near-infrared laser for the treatment of cutaneous hyper-vascular lesions.
*
anvari@rice.edu
; phone 1 713 348-5870; fax 1 713 348-5877; ruf.rice.edu/~banvari
Lasers in Surgery XIV, Bartels, Bass, de Riese, Gregory, Hirschberg, Katzir, Kollias, Madsen, Malek,
McNally-Heintzelman, Paulsen, Robinson, Tate, Trowers, Wong, Eds., Proc. of SPIE Vol. 5312
(SPIE, Bellingham, WA, 2004) · 1605-7422/04/$15 · doi: 10.1117/12.529146
7
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2. METHODOLOGY
2.1 Human skin geometry
The geometry to simulate human skin with cutaneous hyper-vascular malformations consisted of a 60
μ
m thick
epidermis with a 15
μ
m thick basal melanin layer, and a 1940
μ
m thick dermis embedded with discrete ecstatic blood
vessels (Fig. 1). The volume fractions of melanosomes in the basal layer of epidermis were assumed to be 15, 50, and
95% (corresponding to the average volume fractions of 3.8, 12.5, and 23.8% based on the whole epidermis) for lightly,
moderately, and heavily pigmented skin, respectively, based on the data reported in the literature
9
. The blood vessels
were assumed to be straight cylinders running parallel to
y
direction and with infinite lengths. The number of blood
vessels and their sizes could be varied. The haematocrit (
hct
) of the whole blood was 45%. The beam spot was assumed
to be a square flat-top profile with a size of 5mm
×
5mm.
2.2 Optical properties of human skin
In the present study, three near-infrared wavelengths 810, 940 and 1064 nm, and two visible wavelengths 585 and 595
nm were investigated. The determination of skin optical properties at various wavelengths is detailed as follows.
2.2.1 Optical properties of blood
The absorption coefficients of blood
μ
a,blood
at 810, 940, and 1064 nm were converted from the extinction coefficients
for hemoglobin
10
. The original data in units of mM
-1
cm
-1
were multiplied by a factor of 5.4 to obtain the absorption
coefficient of whole blood with
hct
=45% in units of cm
-1
9
. The absorption coefficients of blood at 585 and 595 nm for
hct
=45% were scaled from the data reported by Kienle
et al
, which was for
hct
=40% originally
11
.
The reduced scattering coefficient of blood
μ
'
s,blood
at 810nm was reported by Roggan
et al
12
as 6.6 cm
-1
and 3.9 cm
-1
for
40%
hct
. Thus, the mean value of
μ
'
s,blood
|
hct
=40%
=5.25 cm
-1
was applied to 810
μ
m wavelength for 40%
hct
. We
assumed the corresponding anisotropy factor
g
to be 0.99, and then the scattering coefficient of blood at 810 nm
wavelength for
hct
=40% was:
μ
s,blood
|
hct
=40%
=
μ
'
s,blood
|
hct
=40%
/(1-
g
)=525.00 cm
-1
. (1)
A previous study
13
indicated that, scattering and absorption increase linearly with
hct
if
hct
<50%. As a result, for
hct
=45%, scattering coefficient of blood at 810 nm wavelength was:
μ
s,blood
|
hct
=45%
=
μ
s,blood
|
hct
=40%
×
(45/40)=590.62 cm
-1
. (2)
The scattering coefficients at 940 and 1064 nm wavelengths were derived from the relationship
13
that the scattering
coefficient decreases for wavelength above 800 nm with approximately
λ
-1.7
, where
λ
is the wavelength, and the
scattering coefficient at 810 nm served as the reference value. The anisotropy factors
g
were set to be 0.99 as well
for the
other two near-infrared wavelengths 940 and 1064 nm
14
, and 0.995 for the visible wavelengths 585 and 595 nm
11
.
60
μ
m
1940
μ
m
Epidermis
Dermis
Ectatic blood vessels
Basa
l layer of
epidermis
(15
μ
m)
x
y
z
Laser beam
...
5mm
...
Figure 1: Geometry model of human skin with cutaneous hyper-vascular malformation.
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2.2.2 Optical properties of skin epidermis
Based on the model geometry used in the present study, skin epidermis is composed of two layers: a melaninless
epidermis layer and basal melanin-filled layer. The absorption coefficient of melaninless epidermis
μ
a,epi
(cm
-1
) was
given by
9
:
]
2
.
66
/
)
154
(
exp[
3
.
85
244
.
0
base
a,
epi
a,
−
−
+
=
=
λ
μ
μ
(3)
where,
μ
a,base
is the baseline absorption coefficient of skin (i.e., absorption coefficient of melaninless epidermis or
bloodless dermis)
9
.
The total optical absorption coefficient of the basal layer of epidermis
μ
a, ebas
depends on a minor baseline skin
absorption and a dominant melanin absorption due to the melanosomes in the basal layer:
base
a,
mel
mel
a,
mel
ebas
a,
)
1
(
μ
μ
μ
f
f
−
+
⋅
=
(4)
where
f
mel
is the volume fraction of melanosomes in the basal layer of epidermis;
μ
a,mel
(cm
-1
) is the absorption
coefficient of a single melanosome, and was calculated by
9
:
33
.
3
11
mel
a,
10
6
.
6
−
⋅
×
=
λ
μ
(5)
Scattering coefficients of melaninless epidermis and basal layer of epidermis were approximated to be the same, and
were calculated by
9
:
)
1
/(
)
10
2
10
2
(
12
5
s
g
−
⋅
×
+
⋅
×
=
λ
λ
μ
(6)
where
μ
s
is the scattering coefficient (cm
-1
). Values of
g
were assumed to be 0.91 for the near-infrared wavelengths 810,
940, and 1064 nm
14
, and 0.8 for visible wavelengths 585 and 595 nm
11
.
2.2.3 Optical properties of skin dermis
Absorption coefficient of dermis
μ
a,der
is expressed as
base
a,
blood
blood
a,
blood
der
a,
)
1
(
μ
μ
μ
f
f
−
+
=
(7)
where
f
blood
is the volume fraction of blood in the dermis (the blood content of ecstatic blood vessels is not taken into
account here). A typical value of
f
blood
is 0.2% where a homogeneous distribution of blood in the dermis is assumed
9
.
Scattering coefficient and anisotropy factor of the dermis were considered to be the same as those of epidermis.
In summary, the optical properties used in the present study are depicted in Table 1.
Table 1: Human skin optical properties used in the present study
Wavelength
(nm)
Optical
properties
Epidermis
Basal layer
(L)*
Basal layer
(M)*
Basal layer
(H)*
Dermis Blood
μ
a
(cm
-1
)
0.2482 20.65 68.24 129.44 0.2576 4.935
μ
s
(cm
-1
)
148.02 148.02 148.02 148.02 148.02 590.62
810
g
0.91 0.91 0.91 0.91 0.91 0.99
μ
a
(cm
-1
)
0.2446 12.66 41.62 78.85 0.2577 6.791
μ
s
(cm
-1
)
105.57 105.57 105.57 105.57 105.57 458.58
940
g
0.91 0.91 0.91 0.91 0.91 0.99
μ
a
(cm
-1
)
0.2441 8.45 27.59 52.20 0.2501 3.23
μ
s
(cm
-1
)
81.37 81.37 81.37 81.37 81.37 371.48
1064
g
0.91 0.91 0.91 0.91 0.91 0.99
μ
a
(cm
-1
)
0.3531 57.38 190.45 361.53 0.4492 48.4
μ
s
(cm
-1
)
148.69 148.69 148.69 148.69 148.69 523.12
595
g
0.8 0.8 0.8 0.8 0.8 0.995
μ
a
(cm
-1
)
0.3709 60.71 201.50 382.52 0.8 214.9
μ
s
(cm
-1
)
156.06 156.06 156.06 156.06 156.06 525.38
585
g
0.8 0.8 0.8 0.8 0.8 0.995
* L: light pigmentation, M: moderate pigmentation, H: heavy pigmentation
2.3 Mathematical model
Mathematical model consisted of a Monte Carlo algorithm
15
to calculate the distribution of absorbed light in the skin;
and a bio-heat conduction model to calculate the transient temperature distribution. Each simulation was carried out for
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1,000,000 photons. The initial skin temperature was assumed to be 33
o
C. The effect of cryogen (R134A) spray cooling
was also incorporated in the model
(Table 2)
16
.
Table 2: Boundary cond
itions of bio-heat co
nduction model
16
Time period
Heat transfer coefficient
(W/m
2
o
C)
Temperature of the medium * right
above the skin surface (
o
C)
Spurt application
4,000
-50
Cryogen pool residence
3,000
-26
Rewarming 10
25
* The medium refers to cryogen film during the spurt duration and cryogen pool residence, and refers to air
during the rewarming period.
The thermo-physical properties of skin used in the study were density
ρ
=1,200 kg/m
3
, specific heat capacity
C
=3,600
J/(kg
o
C), thermal conductivities
k
=0.26, 0.53, 0.53 W/(m
o
C) for epidermis, dermis, and blood, respectively
17
.
Additionally, a damage integral based on Arrhenius relationship
18
was used to estimate the thermal damage to the skin.
The coefficients used in the damage integral for bulk skin were
19
: molecular collision frequency factor
A
=1.8
×
10
51
s
-1
and damage process activation energy
E
=327,000 J/mol. When the damage integral reaches 1, 63% of the tissue is
assumed to be damaged. This limit was taken to be the criteria for irreversible damage. The outputs of the model were
the temperature profiles and coagulated area maps within the skin.
3. RESULTS
3.1 Verification of human skin optical properties
In order to verify the validity of the human skin optical properties used in the present theoretical study (calculated by the
formulas in Section 2.2, depicted in Table 1), we compared the simulation results of threshold incident dosages for
epidermal damage in response to 595 nm laser irradiation (Table 3a) with the experimental results of an
in-vivo
study of
normal human skin irradiated at the same wavelength (Table 3b). The
in-vivo
study was conducted by our group. The
laser pulse duration was 1.5 ms, and cryogen spurt duration 100 ms. For normal human skin, the blood content of the
dermis was assumed to be 0.2% in the simulation
9
.
Table 3a: Simulation results of threshold incident dosages for epidermal damage in response to 595 nm laser irradiation in conj
unction
with a 100 ms cryogen spurt. Laser pulse duration: 1.5 ms.
Skin pigmentation
f
mel
(%)
f '
mel
(%)
D
th
(J/cm
2
)
Light 15 3.8 33
Moderate 50 12.5 16.8
Heavy 95 23.8 12.7
f
mel
: Volume fraction of melanosomes based on the basal layer of epidermis;
f '
mel
: Volume fraction of melanosomes based on the whole epidermis;
D
th
: Threshold incident dosage for epidermal damage.
Table 3b
♦
:
In-vivo
experimental results of threshold incident dosages for epidermal damage in response to 595 nm laser irradiation in
conjunction with a 100 ms cryogen spurt. Laser pulse duration: 1.5 ms.
Sample #
Skin type
+
D
th
(J/cm
2
)
D
th, av
(J/cm
2
)
1 II >30*
2 II >30*
3 II 30
>30
4 III 12
5 III 22
6 III 28
20.7
7 V 8
8 V 14
9 V 5
9
♦
Unpublished data of an
in-vivo
study conducted in our group;
+ Fitzpatric skin classification
20
;
D
th, av
: Average threshold incident dosage for epidermal damage;
* No epidermal damage was observed at 30 J/cm
2
, which was the highest incident dosage used in the experiment.
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Normally, Fitzpatric skin types I-II, III-IV, and V-VI are considered respectively as lightly, moderately, and heavily
pigmented skin types
21
. For lightly pigmented skin, only type II was obtained in the
in-vivo
experiments (Table 3b), and
the average value of the threshold incident dosage
D
th, av
was higher than 30 J/cm
2
; the corresponding predicted threshold
incident dosage for lightly pigmented skin was 33 J/cm
2
. For moderately pigmented skin, the experimental and
simulation results were 20.7 and 16.8 J/cm
2
, respectively. Considering the experimental result was only from type III
skin and no type IV skin (for which the value is expected to be lower), the prediction can be considered to be reasonable.
For heavily pigmented skin, the predicted value (12.7 J/cm
2
) is 41% higher than the experimental result (9 J/cm
2
). One of
the possible reasons for this discrepancy is the large range of the melanin concentration in heavily pigmented skin: 18-
43% based on the whole epidermis (Table 4).
Table 4: Reported data of volume fraction of melanosomes based on the whole epidermis in different skin types
9
Skin pigmentation
f '
mel
(%)
f '
mel, av
(%)
Light 1.3-6.3 3.8
Moderate 11-16 13.5
Heavy 18-43 30.5
f '
mel, av
: average value of melanosome volume fraction based on the whole epidermis
3.2 Ratios of light absorptions by dermal blood vessel to the basal layer of epidermis
Fig. 2 shows the one-dimensional simulation results of the ratios of light absorptions (absorbed light energy) by dermal
blood vessel versus the basal layer of epidermis at three different commercially available near-infrared wavelengths:
810, 940, and 1064 nm. The values of the light absorptions were the integrals over the whole blood vessel depth and
epidermal basal layer depth, respectively. The blood vessel thickness was 800
μ
m (representing a large-sized blood
vessel) and was located 200
μ
m deep from the skin surface. Moderate pigmentation was assumed here. It can be seen
from Fig. 2 that 940 nm wavelength is superior to 810 and 1064 nm in terms of the ratio of light absorption by dermal
blood vessel versus the basal layer of epidermis. Consistent results were also obtained when varying blood vessel
thickness and location depth, as well as skin pigmentation. Based on the result above, the present study mainly focused
on 940 nm wavelength.
3.3 Light absorption distributions
Fig. 3 illustrates the distributions of absorbed light as a function of skin depth at various wavelengths 585, 595, and 940
nm. All data in Fig.3 were normalized to the peak value at the basal layer of epidermis in response to 585 nm irradiation.
The blood vessel thickness was 800
μ
m and located 200
μ
m deep from the skin surface. First, it can be seen that the
peak values of light absorption by the basal layer of epidermis at 585 and 595 nm wavelengths are almost 5 times higher
than that at 940 nm. Second, unlike the pronounced gradient of absorbed light distribution in blood vessel at 585 and 595
nm wavelengths, the light distribution at 940 nm is approximately uniform. Third, optical selectivity can still be
achieved at 940 nm wavelength, as the light absorption by blood at 940 nm is higher than that by the surrounding dermis.
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
123
Wavelength (nm)
Ratio of absorbed light:
Blood
vessel vs basal layer
810 940 1064
Figure 2: Ratios of absorbed light by blood vessel to the basal layer of epidermis at 810, 940, and 1064 nm wavelengths.
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3.4 Blood vessels coagulation
3.4.1 Lightly pigmented skin
Shown in Fig.4 are the simulation results of laser-induced dermal blood vessel photocoagulation maps in lightly
pigmented skin irradiated by 595 and 940 nm wavelengths at the threshold incident dosages
D
'
th
(The dark areas indicate
the coagulated areas in the skin), where
D
'
th
is defined as the highest value of the incident dosage that does not cause
epidermal damage or perivascular tissue damage. Value of
D
'
th
is dependent on laser wavelength, laser pulse duration,
cryogen spurt duration, and skin pigmentation level. Cryogen spurt duration was
100 ms and the blood vessel sizes were
500
μ
m in diameter. When the laser pulse duration was 1.5 ms, no blood vessel coagulation occurred when irradiate at
595 nm wavelength (Fig. 4a), while one blood vessel was completely coagulated and the other considerably coagulated
at 940 nm wavelength (Fig. 4e). When the laser pulse duration was increased to 40 and 100 ms, blood vessels were
considerably coagulated at 595 nm (Fig. 4b, c) and completely coagulated at 940 nm (Fig. 4f, g).
Figure 4: C
omparison of blood vessel coagulation in response to 595 (a, b, c) and 940 nm (e, f, g) irradiations in lightly pigmented
skin at the threshold incident dosages
D'
th
. Cryogen spurt duration:
100 ms. Laser pulse durations: 1.5 (a, e), 40 (b, f), and 100 ms
(c, g). Circled areas: blood vessels; Dark areas: coagulated areas of blood vessels
.
D'
th
=37 J/cm
2
D'
th
=166 J/cm
2
D'
th
=300 J/cm
2
D'
th
=120 J/cm
2
D'
th
=110 J/cm
2
D'
th
=300 J/cm
2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0
500
1000
1500
2000
Depth (m icrom eter)
Normalized absorbed light
distribution
585 nm
595 nm
940 nm
585 nm
595 nm
940 nm
Figure 3: Predicted distribution of absorbed light in skin at the wavelengths of 585, 595, and 940nm.
e
f
g
g
Blood vessel
Dermo-epidermal junction
Dermis
Dermis
a
b
c
g
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3.4.2 Moderately pigmented skin
The corresponding simulation results for moderately pigmented skin are given in Fig. 5. When irradiated at 595 nm
wavelength, no blood vessel coagulation occurred at all laser pulse durations from 1.5-100 ms (Fig. 5a, b, c). While at
940 nm, almost complete blood coagulation was predicted when laser pulse duration reached 100 ms (Fig. 5f).
3.4.3 Heavily pigmented skin
In heavily pigmented skin, blood vessel coagulation did not occur in response to 595 nm irradiations regardless of the
pulse durations from 1.5-100 ms when the corresponding threshold incident dosages
D'
th
were applied (Fig.6a, b, c). At
940 nm, only little blood vessel coagulation was predicted when the pulse duration was 100 ms (Fig. 6g). However,
when the laser pulse duration was further increased to 200 ms, and cryogen spurt duration to 200 ms, blood vessels were
almost fully coagulated when irradiated at 940 nm (Fig. 7b). In the meantime, still no coagulation was predicted at 595
nm under the same conditions (Fig. 7a).
a
b
c
e
f
g
Figure 5: Comparison of blood vessel coagulation in response to 595 (a, b, c) and 940 nm (e, f, g) irradiations
in moderately
pigmented skin at the threshold incident dosages
D'
th
. Cryogen spurt duration: 100 ms. Laser pulse durations: 1.5 (a, e), 40 (b, f),
and 100 ms (c, g). Circled areas: blood vessels; Dark areas: coagulated areas of blood vessels.
Figure 6: Comparison of blood vessel coagulation in response
to 595 (a, b, c) and 940 nm (d, e, f) irradiations in heavily pigmented
skin at the threshold incident
D'
th
. Cryogen spurt duration:
100 ms. Laser pulse durations: 1.5 (a, e), 40 (b, f), and 100 ms (c, g).
Circled areas: blood vessels; Dark areas: coagulated areas of blood vessels.
D'
th
=17.7 J/cm
2
D'
th
=54.4 J/cm
2
D
'
th
=86.1 J/cm
2
D'
th
=61.5 J/cm
2
D'
th
=186.9 J/cm
2
D
'
th
=270 J/cm
2
D
'
th
=116 J/cm
2
D
'
th
=37.2 J/cm
2
D
'
th
=168 J/cm
2
a b c
e
f
g
D
'
th
=13.4 J/cm
2
D
'
th
=40.9 J/cm
2
D
'
th
=60.9 J/cm
2
e
f
g
a
b
c
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3.5 Effect of dermal blood content
The simulation results for the effect of dermal blood content on the laser-induced peak temperature at the basal layer of
epidermis are presented in Fig. 8a. The irradiation wavelength
λ
=940 nm, incident dosage
D
0
=100 J/cm
2
, cryogen spurt
duration 100 ms, and the blood vessel size 500
μ
m in diameter. Lower dermal blood content led to higher laser-induced
epidermal peak temperature. At the laser pulse duration of 1.5 ms, the laser-induced epidermal peak temperature
increased from 130 to 156
°
C when the dermal blood content decreased from 12% to 0.2%. When 100 ms pulse duration
was applied, the peak temperatures were 24 and 28.5
°
C, respectively.
Accordingly, the threshold incident dosage for epidermal damage
D
th
increased with increasing dermal blood content
(Fig. 8b). At 1.5 ms pulse duration,
D
th
increased from 54 to 65 J/cm
2
(increased by 20.3%) when the dermal blood
content increased from 0.2% to 12%. When the pulse duration was 100 ms, the threshold incident dosages were 238 and
280 J/cm
2
(increased by 17.6%) for 0.2% and 12% dermal blood content, respectively.
3.6 Effect of dermal blood vessel size
Simulation results showed that when the dermal blood content is constant, the effect of blood vessel size on the
epidermal peak temperature and accordingly the threshold incident dosage for epidermal damage is minimum compared
to that of dermal blood content (Fig. 9). For the example of 1.5 ms pulse duration, the threshold incident dosage
increased from 59.6 to 61.5 J/cm
2
(increased only by 3.2%) when the dermal blood size increased from 50 to 500
μ
m
(Fig. 9b).
Figure 7: Comparison of blood vessel coagulation in response to (a) 595 and (b) 940 nm irradiations in heavily pigmented skin
at
the threshold incident dosages
D'
th
. Cryogen spurt duration:
200 ms. Laser pulse duration: 200 ms. Circled areas: blood vessels;
Dark areas: coagulated areas of blood vessels.
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
1.5
40
100
Laser pulse durtion (ms)
Peak temperature at the basal layer
of epidermis (oC)
(a) (b)
Figure 8: Effect of dermal blood content on (a): the laser-induced temperature at the basal layer of epidermis (incident dosag
e
D
0
=100 J/cm
2
) and (b): the threshold incident dosage for epidermal damage. Dermal blood content: .
D
0
=100 J/cm
2
λ
=940 nm
D
'
th
=260 J/cm
2
D
'
th
=96 J/cm
2
a b
0.0
50.0
100.0
150.0
200.0
250.0
300.0
1.5
40
100
Laser pulse duration (ms)
Threshold Incident dosage for
epidermal damage (J/cm
2
)
λ
=940 nm
14 Proc. of SPIE Vol. 5312
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4. DISCUSSION
Results of this theoretical study indicated that optical selectivity could still be achieved at 940nm wavelength. The
absorption coefficient in blood at 940 nm is about 25 timers higher than that in dermis (Table 1). Unlike the pronounced
gradient of light energy absorption in large-sized blood vessel at 585 and 595 nm wavelengths, the light energy
distribution at 940 nm is approximately uniform within bloo
d vessels (Fig. 3). This will give rise to more uniform
heating of the blood vessel, and subsequently be beneficial to the photocoagulation of the entire blood vessel. The result
also explains why pulsed dye lasers are inefficient in treating large-sized ectatic blood vessels. Most of the energy of
pulsed dye laser irradiation, especially at 585 nm wavelength, is absorbed by very superficial layer of the blood vessel.
Thus, the light penetration depth in blood is very limited, and subsequently, non-uniform heating of the blood vessel is
induced.
595 nm wavelength is now widely used in clinical settings instead of 585 nm with the intention to increase the light
penetration depth in large-sized blood vessels or blood vessels extending deeply into dermis. As the light absorption by
blood at 595 nm decreases by a factor of 5 comparing with 585 nm
22
, higher incident dosage is required to generate
sufficient heat within blood vessels for coagulation. However, the light absorption by epidermal melanin at 595 nm is
almost identical to that at 585 nm, limiting the usage of high incident dosage and resulting in a poor treatment efficacy in
moderately to heavily pigmented skin patients. A comprehensive comparison of the treatment efficacies for large-sized
blood vessels between 595 and 940 nm wavelengths was carried out in the present study.
For lightly pigmented skin, in which epidermal melanin plays a less critical role, 940 nm wavelength shows the
advantage over 595 nm in the photocoagulation depth in larger-sized blood vessels (Fig. 4). For moderately to heavily
pigmented skin, 595 nm wavelength is constrained by the epidermal light absorption for using sufficient incident dosage
to coagulate the blood vessel, even in conjunction with long cryogen spurt. In contrast, when long laser pulse duration
(e.g. 200 ms) and long cryogen spurt duration (e.g. 200 ms) are applied, 940 nm even shows good efficacy in treating
large-sized blood vessels in heavily pigmented skin (Fig. 7).
Dermal blood content has a considerable effect on the laser-induced peak temperature at the basal layer of epidermis.
Lower dermal blood content leads to less light absorption by dermal blood and subsequently more back scattering from
the dermis to the epidermis, resulting in higher laser-induced peak temperature in the basal layer, and accordingly, lower
threshold incident dosage for epidermal damage. The laser-induced epidermal peak temperature is predicted to be
minimally dependent on the size of dermal blood vessels.
(a)
(b)
Figure 9: Effect of dermal blood vessel size on (a): the laser-induced temperature at the basal layer of epidermis (incident d
osage
D
0
=100 J/cm
2
) and (b): the threshold incident dosage for epidermal damage. Blood vessel size (
μ
m): .
0.0
50.0
100.0
150.0
200.0
250.0
300.0
1.5
40
100
Laser pulse duration (ms)
Threshold incident dosage for
epidermal damage (J/cm
2
)
D
0
=100 J/cm
2
λ
=940 nm
λ
=940 nm
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
1.5
40
100
Laser pulse duration (ms)
Proc. of SPIE Vol. 5312 15
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In summary, this theoretical investigation predicted that near-infrared wavelength 940 nm is promising in treating
cutaneous hyper-vascular malformation patients with large-sized ectatic blood vessels and moderately to heavily
pigmented skin types. Future experimental studies are needed to verify the results of the present study.
5. CONCULSIONS
This theoretical investigation indicated that laser irradiation using 940 nm wavelength is superior to 595 nm for the
treatment of cutaneous hyper-vascular malformation patients with large-sized blood vessels and moderately to heavily
pigmented skin types. Using long laser pulse duration and long cryogen spray duration, 940 nm wavelength is predicted
to be efficient in treating dark skin patients. Dermal blood content has a considerable effect on the laser-induced peak
temperature at the basal layer of epidermis, while the effect of blood vessel size is minimum when the dermal blood
content is constant.
ACKNOWLEDGEMENTS
This study was supported in part by grants from the Institute of Arthritis and Musculoskeletal and Skin Disease (IR01-
AR47996) at the National Institutes of Health and Texas Higher Education Coordinating Board. We thank Dr. James W.
Tunnell from G.R. Harrison Spectroscopy Laboratory at Massachusetts Institute of Technology for his fruitful
discussions.
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