Page 65 - Veterinary Laser Therapy in Small Animal Practice

Light’s path to all these places in the body 51

P

Pp

Ep

Pa

t
ton toff

1 ton
fp = DC = Pa = Pp x DC Ep = Pp x ton
ton + toff ton + toff

x
x
= ton fp = Pp x ton fp
Complete combinations

OR toff) AND OR DC) OR Pa)
(ton (fp AND (Pp
PLUS
2
Spot size (cm ) AND Wavelength (nm)
Figure 6.8 Relevant parameters describing pulsed light.

DC to get what you need). Then you need one of the as a laser pointer (in the low milliwatt range), which
power values to determine the energy per pulse. would take hours or days to deliver the necessary clin-
Obviously the wavelength and spot size round out ical dose.
the story, wavelength giving you information as to the But how long is too long?
absorption/scatter/penetration/target chromophore of
the light, and spot size giving you the values of energy 6.7 Treatment time and reciprocity
density (dose) and power density.
All that said, the average power P is what matters In general, a serious question is whether the accumula-
a
when it comes to understanding how much time is tion of dose is a sufficient predictor of clinical effect.
required to deliver your clinical effect. You will hear the With respect to pulsing, it is clear that the rule of reci-
term “superpulsed” thrown around a lot in this indus- procity (i.e. that total exposure or dose determines clin-
try. That word in itself can be good or bad. In the good ical effect) does not hold across the board. The whole
form, there are superpulsed lasers that deliver high beneficial nature of pulsing relies on a specific timing of
average power, but with enough time between pulses pulses (either in the time they are on or in the time the
to allow for thermal relaxation of the tissue. In the bad light is off between them) and so it is clear to see that if
form, there are superpulsed lasers that have substan- you modified the pulse structure (e.g. by decreasing the
tially high peak power, but their pulses are so short peak power but increasing the pulse width proportion-
and so few that the total energy delivered per second ally) you could expect a very different intra-tissue effect,
(average power, by definition) is only about as much even though the total energy per pulse and therefore

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