Page 61 - Veterinary Laser Therapy in Small Animal Practice

Light’s path to all these places in the body 47

Air Medium
Average distance between absorption events = 1/µa
Absorption
Intensity (depth) = initial intensity x e – µa x depth
exponential decay at a rate of µa

Mean free path
(i.e. average distance between ‘Micro’ scatter events)
= 1/µs
Scatter
Reduced mean free path

(i.e. average distance between ‘Macro’ scatter events at average anisotropy)
= 1/µs
Skin/
fur Soft tissue
Fat
Absorption Complex combination of
and scatter spread and decay Connective
tissue

Especially when we Bone
add all the dierent tissues
that make up anatomy into the picture

Figure 6.4 Pictorial representation of absorption, scatter, and the “real-life” combination of both. Illustrator: Elaine Leggett.

an ice pack, only the tissue underneath (or very close energy of the beam incident on a given spot PER UNIT
to) the ice pack is getting cold. But with laser, you will OF TIME and is measured in watts, with 1 watt = 1
be administering therapy well outside of where you see joule per second.
the light. I emphasize the word “incident” because we do not

This is a good thing, but also leads to some confusion. talk (in this industry) about the absorbed energy. In
There are two critical values: energy and power. fields that deal with X-rays, we do talk about absorbed

Energy is the total energy of the beam INCIDENT at a dose (in units of energy per volume or energy per mass
given spot and is measured in joules. Power is the total of tissue) because, for X-rays, absorption is the primary

Figure 6.5 (a, b) The effect of the angle of the incident beam on light dispersion.

REDONDO PRINT (4-COL BLEED).indd 47 08/08/2019 09:47