Page 100 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Where does the concept of TRT in photothermal treatments come from? Anderson and Parrish wanted to selectively heat up the target vessels in port wine stains but without damaging the tissue surrounding those vessels. So, they decided to construct a set of rules to minimise the potential thermal conduction to adjacent structures. They chose the TRT of the target vessels to set the appropriate laser pulse duration.
This was a perfectly valid choice – it ties the duration of the laser pulse to the size of the target. In doing so, it reduces the time allowed for conduction of the heat generated by the laser energy, to flow into the surrounding tissues.
But it was a random choice! They could just as easily have chosen two times the TRT, or five times or ten times. It was arbitrary!
And it was wrong!!!
Why? Well, the premise of using the TRT as a guide for the pulse duration was flawed. The original idea was to prevent unwanted damage to other tissues by limiting the heating time. But this idea did not consider the amount of time required to damage the target cells.
If you put an egg into a pan of boiling water for 10 seconds, it will not cook through properly. That’s because the egg needs a certain amount of time to cook – typically between 3 and 5 minutes, depending on your preference.
As with all cooking, we know that we must cook our food at some temperature for a certain length of time. The same goes for cells – they must be maintained at the right temperature for the appropriate time, to ensure full denaturation (cell death).
When Anderson and Parrish chose the TRT as the laser pulse duration, they were ONLY considering the heat conduction to the adjacent tissues. They did NOT consider the time needed to kill the target cells sufficiently - the ‘cooking’ time.
So, their choice was based, purely, on saving the tissues around the target, and not on actually destroying the target cells.
We can calculate the cell denaturation time (the cooking time) using an equation developed by the Swedish Nobel-prize laureate physicist, Svante Arrhenius. We have done a lot of work using this equation to try and understand the physical processes which occur in photothermal treatments.
Table 22 - The relaxation time is not the same as the denaturation time
________________________________________________________________________ 100 Chapter 3 Laser/IPL Hair Removal
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Thermal relaxation time
Denaturation time
The cooling time of the hot targets
The cooking time required to destroy the target cells