Biomedical Engineering Reference
In-Depth Information
1000
Oxyhemoglobin
Hemoglobin
m
a
dermis
m
a
epidermis light skin (4%)
Reduced scattering coefficient
100
10
1
0.1
500
550
600
650
700
750
800
850
900
950
1000
1050 1100
Wavelength (nm)
Figure 1.16
Hb absorption versus bloodless dermis. Note that Hb absorption always exceeds bloodless dermis.
Abbreviations
:
m
a
, absorption coeffi cient;
Hb, hemoglobin.
Coefficients of absorption of water and human fatty tissue
affect the surface. If the pulse were short enough, a 694-nm
laser, for example, could optically act as a CO
2
laser (28)!
100
Collagen
Dry collagen shows absorption peaks near 6 and 7 µm. With a
free electron laser, these peaks can be exploited for selective
molecular targeting. In this manner, collagen is directly heated
rather than relying on heat conduction due to its close bonds
to tissue water (where Er:YAG and CO
2
lasers work). Ellis et al.
found that this approach provided more effi cient resurfacing
and might allow for less tissue irradiation and less thermal
damage than CO
2
laser (29).
10
1
0.1
selective photothermolysis
With the exception of water, heating in the skin is based on
discrete heating by chromophores of relatively low concentra-
tion (i.e., melanin, hemoglobin). Anderson described the con-
cept of selective photothermolysis (SPT) more than 30 years
ago (24). Although Goldman argued for color as a means to
selectively damage dermal targets as early as 1963, SPT offered
an elegant and mathematically rigorous rationale for develop-
ing different tissue-selective lasers. As described by Anderson,
extreme
localized heating
achieved with SPT relies on the fol-
lowing: (
i
) a wavelength that reaches and is preferentially
absorbed by the desired target structures, (
ii
) an exposure
duration less than or equal to the time necessary for cooling of
the target structures, and (
iii
) suffi cient energy to damage the
target. The heterogeneity of the skin allows for very selective
injury in thousands of microscopic targets. Unlike tracing out
blood vessels with an electric needle (or using a CO
2
laser on a
wart), the physician can apply a large light beam without the
need to aim at the target. The effect is like the legendary “magic
bullet,” which seeks only the desired target. In contrast to gross
thermal injury, SPT (with discrete targets, i.e., melanin and
Hb) allows for focal heating with large volumes of undamaged
skin between chromophores. The focal nature of the heating
decreases the likelihood of catastrophic pancutaneous thermal
Water
Human fatty tissue
0.01
850
1000
1150 1300 1450 1600 1750 1900 2050
Wavelength (nm)
2200 2350
Figure 1.17
Water and fat absorption curves.
Fat
Fat shows strong absorption at 1200 and 1700 nm (Fig. 1.17).
Although the ratios of fat-to-water absorption are small, the
small differences could be exploited with the proper choice of
parameters (or possibly through interstitial therapy).
Carbon
Carbon is not per se a chromophore but rather a product of
prolonged skin heating. Once carbon is formed at the skin sur-
face, the skin becomes “opaque” to most laser wavelengths
(i.e., most energy will be absorbed very superfi cially). It fol-
lows that the dynamics of surface heating changes immedi-
ately once carbon is formed. This can be used creatively as an
advantage. For example, one could use a layer of carbon paper
to convert a deeply penetrating laser to one that would only
