Page 64 - Veterinary Laser Therapy in Small Animal Practice

50 Veterinary Laser Therapy in Small Animal Practice

Power vs. penetration
After I gave my annoying, imprecise “a few depth. In fact, any percentage increase in power will
centimeters” answer on how far light penetrates, I lead to that same percentage decrease in treatment
made a comment on accumulation of dose at depth time to deliver the same dose to the same depth. So it
over time and promised you more detail. I come back should be obvious that if you use twice the power, then
to it here, as it relates to power. you’ll get the same dose to the same depth in half the
The penetration profile of a beam is determined time. But that doesn’t say anything about penetration.
by the tissue being treated and the wavelength(s) of Or does it?
light used. Light that interacts (via either scatter or The flip side of that same coin is what happens if
Percent
Intensity
you fix the treatment time.
Let’s consider the example of
treating for a fixed amount
of time and calculate the
depth to which each laser
will deliver the same dose.
In Figure 6.7, you have two
identical lasers treating at
different power settings,
with an exponential decay
in intensity as you go deeper
� 1 W for 20 seconds with a 2 watt
50% 4 W � into the patient. Treating
laser will deliver 20 joules
of energy to depth A (50%
intensity of 2 joules per
1 W
�
second times 20 seconds).
12.5% � Now, even though the
.25 W Depth Inside transmission percentage is
four times lower at depth B
A B Patient (where the beam has decayed
Figure 6.7 to 12.5% of its intensity),
absorption) with tissue in a stronger way leads to a because you start off with four times more photons
steeper decay in intensity with depth. But no matter with the higher-powered laser, the same 20 joules of
what the shape of that attenuation, if you start with energy can be delivered to this deeper target (B vs. A)
more light, then you are left with more light at each in the same treatment time.

But for the scope of this book, average power A simple picture may help here (Fig. 6.8). Our vari-
tells the story. With continuous wave (light) where ables are pulse width t , dark period t , duty cycle DC,
off
on
the light is always on, peak and average power are pulse frequency f , peak power P , average power P , and
a
p
p
one and the same. But when you pulse the light, energy per pulse E .
p
they diverge … importantly. Chapter 8 dives into the You need t to tell you if the light absorbed during
on
importance of pulsing and the variety of effects it has the pulse is going to cause an acoustic wave (i.e. if t is
on
on different tissues in the body. But here we will limit in the low microsecond range) or if it can be dissipated
our discussion to what the light is doing and how it away during the pulse. You need t to tell you if the
off
is structured. time between pulses is going to coincide with which-
Pulse frequency simply describes the number of ever thermal or biochemical process you aim to take
pulses per second. If you try to reconcile which realm advantage of. But you can get either from the other if
of thermal or biochemical effects discussed above is you have the frequency or the duty cycle, so you need
relevant to a laser pulsing at 20 Hz, good luck to you. one of those (you could also have t and t or f and
p
on
off

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