Page 57 - Veterinary Laser Therapy in Small Animal Practice

CHAPTER 6

Light’s path to all these places in the body

We live in a society exquisitely dependent on science after reading this entire chapter, you will get familiar
and technology, in which hardly anyone knows anything enough with everything that is going on to then go

about science and technology. back and find which tidbit you find most useful toward

— Carl Sagan whichever clinical effect you aim to target.
The goal of this chapter is to give you just enough 6.1 Attenuation via absorption and scatter

understanding about light transport through tissue
so that you can make good guesses about the clinical The amount of light present in the beam decays (or is

impact you can expect. So we will very broadly cover attenuated) with depth inside your patient. This idea
what light does when it runs into stuff, how its accu- teeters between intuitive and counter-intuitive among

mulation is quantified, the different ways to deliver it, different people. On one hand, you realize that the

and some clinically important implications of all of this. thicker the curtains you use to cover your windows, the
Or if you prefer, we can go right into deriving and less light gets through. On the other hand, especially
solving these equations … with today’s state-of-the-art digital radiography, you
see right through your patients with ease and little-to-
no technique manipulation on your X-ray machine, no
matter the size of the patient.
But it’s time to be publicly honest (more so than
some laser manufacturers/distributors) about where
laser therapy falls in that spectrum: infrared light is
I didn’t think so. You are much more interested in STRONGLY attenuated by the body.
how to better predict the clinical outcomes of your I’ve always avoided (like the plague) the request
therapy based on what flavors of light you are shining to answer the question of, “So how deep does it pen-

at the patient. To do so, you’ll need to know some very etrate?” because that is an ill-posed question. Light is
fundamental pieces of the puzzle, and so my discussion not like a needle that will go to a certain depth and then
will be guided throughout by its utility in helping you not exist beyond that point. Instead, it is a decaying
practice better medicine (and integrating partial dif- intensity that eventually dwindles down to being neg-
ferential equations is not gonna get you there). What ligible at a certain depth … given a certain time. Wow
follows, then, is a very high-level, albeit informal and … how vague, right? True, but that depth and that
qualitative discussion with quantitative analysis only time very much depend on what you start with at the
when truly necessary. Keep in mind, as you will learn surface: how much power of which wavelength of light
very quickly, that this is not a “skim through and find you shine for what amount of time.

the ‘right’ value” kind of section, mostly because no I promise to shed more light than that, but not before
individual subsection tells the whole story. Instead, I explain a little more about where the light goes.

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