Page 248 - Blog Posts v1

E21 - More on Thermal Relaxation Times
I've had a number of enquiries about my article on the relevance of thermal relaxation times in photothermal treatments. They were asking for clarification. So here goes....
Thermal relaxation time (TRT) describes how quickly an object loses heat energy. Typically, it is the time taken for the temperature of an object to drop to either 50% or 36.78% (the 1/e value) of its maximum value, depending on which definition you use.
It is, essentially, a cooling time. The TRT describes how quickly or slowly an object cools!
The TRT was introduced into laser treatments by Anderson and Parrish back in 1981 when they wanted to put a constraint on the laser energy pulse duration to minimise thermal damage to adjacent tissues. Hence, most of the thermal damage would be confined to the absorbing target. So they, arbitrarily, chose the time equal to one TRT as the maximum for any pulsewidth when treating tissues.
That's fine and is a rational choice. However, it was a purely arbitrary choice!
TRT is a COOLING time........ ....... NOT a HEATING time!!
Denaturing tissue
The main aim of photothermal treatments is to use heat energy to destroy ONLY the unwanted tissue(s), without significantly damaging surrounding tissue (the basis of Selective Photothermolysis).
Anderson & Parrish figured that this could be achieved if sufficient energy was deposited in the target tissue but with the TRT constraint to prevent collateral damage. However, their assumption is flawed!!
Mike’s Blog Posts 248