Page 90 - LASERS & IPLS IN MEDICAL/AESTHETIC APPLICATIONS

Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.1
In any laser/IPL treatment, it is invariably a better option to use larger spot diameters to induce deeper penetration of the energy. This is particularly true in the treatment of tattoos and hair.
Effect of Pulsewidth
Longer pulses of energy increase the ‘temporal distribution’ of the heat energy. This is obvious! If you put a cake mixture into the oven, at some pre-determined (constant) temperature, then you will know to cook it for a certain length of time. If you leave it too long, then it will blacken (carbonise) and taste horrible! If you remove it before the appropriate cooking time, it will not be fully cooked.
Exactly the same occurs when zapping tissue with laser energy. Longer pulses cause more heat energy to conduct throughout the tissue volume, raising its temperature (assuming the fluence is constant). So, as a direct consequence, long pulses allow for more conduction during the pulse, in all directions, and hence deeper penetration of heat energy. This means that longer pulses will potentially cause more damage in deeper regions than shorter pulses, for the same applied fluence.
If controlled correctly, this can be used to target deeper tissues, such as deep hair follicles. However, applying such a technique requires more efficient skin surface cooling, to mitigate any unwanted effects near the surface.
Summary:
‘Penetration Depth’ depends on –
Wavelength Spot diameter Pulse duration
and ‘Clinically Useful Penetration Depth’ also requires –
Fluence
It should be noted that many IPL systems, and some lasers, can only increase their total output fluence by increasing their pulsewidth! This is not particularly useful when treating skin conditions, as it limits the variability of choice by the operator.
Extra care must be taken with such systems as unwanted tissue damage is much more likely with them!
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
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