Page 58 - Veterinary Laser Therapy in Small Animal Practice

44 Veterinary Laser Therapy in Small Animal Practice

we would lose count for a single photon very quickly,
Reflection
not to mention the billion billion photons delivered
Transmission every second. Instead, we talk about probabilities and
the average distances traveled, with terms like attenua-
tion coefficients and mean free paths.

6.3 How much and in which direction?
Absorption I know I promised to spill the beans and give you an
answer, but not quite yet. There are two very distinct
and equally important concepts here: amplitude and
direction. So you are not allowed to jump to any con-
clusions before I explain both.
For amplitude, I have to talk a VERY little about math
Scatter and the idea of exponential decay. I won’t bore you with
the differential equations that govern light transport,
Figure 6.1 The four basic interactions of light.
but the amount of light that remains unattenuated (i.e.
not absorbed or scattered) from a beam over distance is
We talked about absorption earlier. This is the way an exponentially decaying proportion that looks some-
light “disappears” from the beam. Its energy gets trans- thing like I(x) = I0 e - x, where I is intensity at distance
μ
ferred to the tissues and therefore, there is no more x, and I0 is the starting intensity (at distance 0), e is the
light. And this is the fate of virtually ALL of the light natural logarithm, and μ is the attenuation coefficient.
that enters the patient. That said, absorption is BY OK, that’s all the math you need. In English, this says
FAR the weaker of the interactions of laser light in that intensity will decrease by 36.8% for every section
your patients. The much more dominant interaction is of distance equal to 1/μ. This distance (1/μ) is called the
scatter (Fig. 6.1). mean free path, which is basically the average distance
between attenuating interactions.
6.2 Scatter: the primary interaction NOTE: Notice I’ve used only percentages here in
my description. This is exactly how these values are
Scatter is when light bounces off small particles within defined, as the ratio of the quantity you are left with
the tissue, changing the course of individual photons over the quantity you started with. Flexibility like this is
along its long and windy path to ultimate absorption. very useful to understand how we SHOULD be talking
In the wavelength ranges relevant to therapy and given about penetration.
the body as the incident material, the type of scatter Now, as we’ve covered, attenuation is a combination
that occurs is termed “elastic,” which means the light of absorption and scattering, so the attenuation coef-
retains all of its energy as it bounces around. This is ficient μ is made up of two components, the absorp-
an important concept that merits being repeated in tion coefficient μ and the scattering coefficient μ . The
s
a
slightly different terms: light does not lose pieces of inverse of these numbers is then the mean free path (or
its energy along its path, so there is no slowing down the average distance) between absorption and scatter-
during the pinball game. It remains in full effect until ing events, respectively.
it gets absorbed, and when it does, it is completely Remember how I said that scatter dominates over
absorbed – all the energy gone. So the only thing that absorption when infrared light enters the body? Well,
changes is the direction of the photon as it gets pinged Table 6.1 shows exactly that. You can see that the
around the body. average 800 nm photon scatters once about every mil-
To give some scope to the relative difference between limeter in soft tissue, but is only absorbed about once
these interactions, a single photon is likely to bounce every 33 centimeters.
around almost a THOUSAND times before it gets This is awesome. Laser therapy penetrates SO far
absorbed. Now granted, we don’t talk about scatter (or into tissue. WRONG. Those equations hold for inter-
absorption) in terms of the paths of individual photons; actions ALONG THE PATH of a photon. And so yes,

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