Page 98 - Lasers and IPLs in Medical/Aesthetic Applications v1.2
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Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.2
There are other reactions and processes which may occur during laser/IPL interactions, but the above four are the main ones found in medical/aesthetic treatments.
Thermal Relaxation Time
The concept of ‘thermal relaxation time’ is a cornerstone of the Anderson and Parrish theory of Selective Photothermolysis, which has proven to be relatively successful in many laser/IPL treatments. But, it seems to be poorly understood by many in the field.
A ‘thermal relaxation’ describes when a hot object is cooling. It is ‘relaxing’ in terms of thermal (heat) energy.
So, a thermal relaxation time is how long it takes for a hot object to cool. We usually choose the time when it has cooled to 50% of its added temperature – if an object starts with a temperature of 20C, and is heated to 80C, then it has increased by 60C. Its thermal relaxation time is how long it take to cool by 30C, i.e. when it reaches 50C.
When Anderson and Parrish introduced this concept in their SP theory, it was chosen to minimise the amount of heat energy spreading out from a hot blood vessel into the surrounding tissues, and hence limit any collateral damage.
They decided that if the laser pulsewidth was limited to one thermal relaxation time of the target vessel, them a minimal amount of damage would occur in the surrounding area.
While this is, to a limited extent, true, it does not really consider the amount of damage to the actual target, which we are trying to destroy (or alter in some way). This requires a different approach!!
We published a paper on this topic in the journal ‘Lasers in Medical Science’ back in 2013 – you can see it here.
We will discuss this topic in much more detail in Chapter 3 of this book – it is important. ________________________________________________________________________ 97
Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
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