Biomedical Engineering Reference
In-Depth Information
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Laser-tissue interactions*
E. Victor Ross and R. Rox Anderson
introduction
When using a light source, many physicians dial in “suggested”
settings and proceed, using identifi able endpoints to deter-
mine if the parameters are appropriate. For example, when
treating a port-wine stain (PWS) with the pulsed dye laser
(PDL), the physician looks for immediate purpura and is vigi-
lant to ensure that no surface whitening occurs. Although this
is a good way, perhaps the best, to ensure that a favorable out-
come is secured, it is instructive for the operator to know from
fi rst principles how the clinical endpoint was achieved. This
will allow the physician to expand his laser repertoire and
optimize the use of this very expensive equipment. For exam-
ple, armed with an education in laser-tissue interactions
(LTIs), one can cajole the PDL to treat pigmented lesions
without purpura by compressing the lesions with a clear plas-
tic piece (Fig. 1.1). This removes blood as a chromophore and
increases the ratio of melanin to vascular heating (1). There
are many other creative ways to use light-based technologies,
but without a basic understanding of LTIs, one is reduced to
treating skin ailments like one microwaves popcorn (accord-
ing only to the instructions with no license to do it better).
In short, an understanding of light-tissue and electrical-tissue
interactions optimizes clinical outcomes in cutaneous laser
surgery.
skin (4). Depending on the angle between the light beam and
the skin, this value varies considerably. More light is refl ected
as the angle of incidence between the beam and the surface
approaches zero. It follows that, in most laser applications, we
want to maintain a perpendicular angle to minimize refl ective
losses (5,6). This regular refl ectance is about 4-7% for light
incident at right angles to the skin (Fig. 1.2). One can reduce
interface losses by applying an alcohol solution ( n = 1.4)
or even water ( n = 1.33). This allows for optical coupling
(vide infra). On the other hand, because of multiple index of
refraction mismatches (keratin-air-keratin-air, etc.), dry
skin refl ects a great deal of light (hence the white appearance
of a psoriasis plaque). Light not refl ected at the skin surface
penetrates into the epidermis. At this point, further light
propagation in skin is determined by wavelength-dependent
localized absorption and scattering (vide infra) (4). Overall,
because of scattering, much incident light is remitted (remit-
tance refers to the total light returned to the environment due
to multiple scattering in the epidermis and dermis, as well as
the regular refl ection from the surface; Fig. 1.2). The amount
of light “wasted” because of remittance varies from 15% to as
much as 70% depending on wavelength and skin type. For
example, for 1064 nm, 60% of an incident laser beam may be
remitted. One can easily verify this by holding a fi nger just
adjacent to the beam near the skin surface. Considerable
warmth will be felt with higher fl uences, all of which is
due to a remitted portion of the beam. For our purposes, light
can be divided into the ultraviolet (UV; 200-400 nm), visible
(VIS; 400-760 nm), near-infrared (NIR; 760-1400 nm), mid-
infrared (MIR; 1.4-3
macroscopic basis of lti
Light demonstrates both wave and particle properties. Nor-
mally, wave behavior accurately describes light's behavior in
space and at large interfaces (i.e., skin and air) (2). However,
the particle properties (and quantum physics) are more use-
ful in characterizing electromagnetic radiation (EMR)-tissue
interactions on a molecular level (vide infra) (3). On a mac-
roscopic level, light behavior conforms to various laws and
equations that are consistent with our “eyeball” observations
in nature. For example, we are familiar with refraction and
refl ection (4). Normally, the percentage of incident light
refl ected from the skin surface is determined by the index of
refraction mismatch between the skin surface (stratum cor-
neum, n = 1.55) and air ( n = 1) (2). The Fresnel equations can
be used to describe how much light will be refl ected from the
m and
beyond). These are the wavelength ranges that are important
in laser dermatology (2).
μ
m), and far-infrared (FIR; 3
μ
types of lasers and properties of lasers
Lasers as light sources are useful because they can, depending
on parameters, allow for exquisite control of where and how
much one heats. However, most reactions with biological sys-
tems are nonspecifi c to radiation from laser sources and
at least in principle could be induced by thermal sources as
well (3). What are the advantages of laser? For the most part,
properties of laser light (i.e., coherence) are irrelevant insofar
as the way light interacts with skin in therapeutic applications.
This contention is supported by the increasing use of fi ltered
fl ashlamps in dermatology. There are four properties that are
common to all laser types: ( i ) beam directionality, ( ii ) narrow
beam divergence, ( iii ) spatial and temporal coherence of the
beam, and ( iv ) high intensity of the beam (7).
* The reader should note that although the title of this chapter is
“Laser-tissue interactions”, the introduction of many new and diverse
technologies makes the term somewhat obsolete. We will continue to
use the term, but a more appropriate is “electromagnetic radiation
(EMR)-tissue interactions.” Both terms are used interchangeably in
the remainder of the text.
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