Biomedical Engineering Reference
In-Depth Information
The probability that absorption will occur depends on specifi c
transitions between allowed electron orbitals and molecular
vibration modes. Thus chromophore molecules exhibit char-
acteristic bands of absorption around certain wavelengths (19).
The molecular basis of LTIs is based on electronic transi-
tions for the UV and VIS wavelengths. However, NIR wave-
lengths and beyond are absorbed via rotational and vibrational
excitations in biomolecules (all of which are hydrocarbons
with the exception of pigments). The reactions can be consid-
ered a two-step process. In the fi rst, the molecule is excited to
an excited state. Then, through a process known as nonradia-
tive decay, there are inelastic collisions with nearby molecules,
giving rise to an increase in kinetic energy and therefore tem-
perature. The temperature rise results from the transfer of
photon energy to kinetic energy. In most biological systems,
tissue constituents show broad absorption bands with only a
few distinct absorption peaks. Goldman and Rockwell (26)
noted years ago that the biggest difference between tissue
necrosis with laser and nonlaser devices, such as high-
frequency electrical current, was the specifi city of the reaction
for color. For example, unlike the electric needle, only the
darkly pigmented areas strongly absorb laser radiation in the
VIS and NIR range. Thus the heterogeneity of the skin (in this
case, melanin in small concentrations in tissue water) allows for
selective heating with chromophore-specifi c wavelengths (27).
From 200 to 290 nm (UVC), light is absorbed by all cellular
constituents (2) and all biological objects (cells and tissue)
absorb energy very strongly. From 290 to 320 (UVB) nm, only
a limited number of biomolecules show absorption (aromatic
amino acids and nucleic acids). For UVA (near-UV) 320-400,
light is weakly absorbed by colorless skin parts. From 400 to
1000 nm, only very few biomolecules absorb (mainly pigments
such as bilirubin, blood, melanin). But it is over this wavelength
range that the heterogeneity of the skin allows for discrete heat-
ing and therefore most of the magical properties of laser. For
>1100 nm, all biomolecules have specifi c strong vibrational
absorption bands. The principal absorber is tissue water, and
all processes are guided by absorption of tissue water.
The absorption spectra of major skin chromophores domi-
nate laser-tissue interactions in dermatology. The absorption
coeffi cient ( µ a ) is the probability per unit path length that a
photon at a particular wavelength will be absorbed. It is there-
fore measured in units of 1/distance and is typically designated
µ a , given as cm −1 . The absorption coeffi cient depends on the
concentration of chromophores present. Skin contains pig-
ments and distinct microscopic structures that have different
absorption spectra (Fig. 1.15) (4).
If tissues were clear, only absorption would be required to
characterize light propagation in skin. However, the dermis is
white because of light scatter. Scattering is responsible for
much of the light's behavior in the skin (beam dispersion, spot
size effects, etc.). The main scattering wavelengths are between
400 and 1200 nm, where the average distance a photon travels
between two scattering events is between 0.05 and 0.2 mm.
Although absorption occurs where the frequencies of the
wavelength equal the natural frequency of the free vibrations
of the particles (absorption is associated with resonance) (4),
scattering takes place at frequencies not corresponding to
those natural frequencies of particles (4). The resulting oscilla-
tion is determined by forced vibration. Scattering is decreased
Diode
Alexandrite
532 Nd:YAG
PDL
Nd:YAG
Melanin
Oxyhemoglobin
Water
300
500
700
Wavelength (nm)
1000
2000
Figure 1.15 Absorption spectra of three major skin chromophores. Abbreviations :
Nd:YAG; neodynium:yttrium-aluminum-garnet; PDL, pulsed dye laser.
as wavelength increases. In most biological tissues, it has been
found that photons are preferably scattered in the forward
direction.
There are three chromophores of interest (water, blood, and
melanin). Water makes up about 65% of the dermis and lower
epidermis. There is some water absorption in the UV. Between
400 and 800 nm, water absorption is quite small (which is con-
sistent with our real-world experience that light propagates
quite readily through a glass of water). Beyond 800 nm, there
is a small peak at 980 nm, followed by larger peaks at 1480 and
10,600 nm. The water maximum is 2940 nm (Er:YAG).
Hemoglobin
There is a large oxyhemoglobin (HbO 2 ) peak at 415 nm, fol-
lowed by smaller peaks at 540 and 577 nm. An even smaller
peak is found at 940 nm. For deoxyhemoglobin (deoxyHb),
the peaks are at 430 and 555 nm. Because of the discrete peaks
of hemoglobin absorption, the laser physician can optimize
heating of the vessel with excellent protection of the surround-
ing structures. If one examines Figure 1.15, there are multiple
opportunities for selective heating blood vessels. Conjugated
double bonds in their structure are responsible for the absorp-
tion of deoxyHb and HbO 2 by VIS light (3).
Melanin
Most pigmented lesions result from “too” much melanin in the
epidermis. By choosing almost any wavelength (
800 nm), one
can preferentially heat epidermal melanin (Fig. 1.16). Shorter
wavelengths will tend to create very high superfi cial epidermal
temperatures, whereas longer wavelengths tend to bypass
epidermal melanin (i.e., 1064 nm).
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