Page 54 - An Introduction to Laser Tattoo Removal

Chapter 2 – Fundamentals of Laser Tattoo Removal v1.0
This explains why a typical picosecond laser is used at relatively low fluences compared with nanosecond lasers. Looking at the published clinical literature shows that many nanosecond lasers use somewhere around 2 to 8 J/cm2, whereas many picosecond lasers will use fluences in the range 0.2 to 4 J/cm2. (Occasionally you might see picosecond lasers using higher fluences, but these are the exception and not the rule). Our original QS ruby laser which we used to develop this technique back in the 1980s had a pulsewidth of 30 ns and we typically used between 5 and 12 J/cm2 in those days (which we now think was too high!!).
This is purely due to the fact that the picosecond pulses do not dissipate heat energy nearly as much as nanosecond pulses, during the delivery time of the pulse, and so the lower energy from the picosecond laser will result in higher maximum (or peak) temperatures (in some of the ink particles depending on their size).
Chasing the ‘crack’ sound
Some laser operators want to hear a significant ‘crack’ sound when treating tattoos. They have been told that this is required to ensure a successful treatment. This is not correct.
This ‘cracking’ sound is generated by using an excessive fluence (energy density) on the skin surface. Mike demonstrated this in a video when firing QS 1064 and 532nm laser energy at non- tattooed skin (see his blog post). When both wavelengths were set to just under 4 J/cm2 using 5 mm spot diameters, there is a very subtle sound produced when the energy hits the skin surface.
However, when he sets the spot size to only 1mm, there is a very audible crack sound when the laser is fired. At this spot diameter, the delivered fluence is around 60 J/cm2. Even though there is no tattoo ink present the crack is very clear. This occurs whether the skin is pale or tanned, indicating that the skin colour is also irrelevant in this process.
What is happening? Well, it appears that the fluence, or more correctly, the irradiance (power per unit area) is sufficiently high to break down the air molecules. This intense power forces the oxygen molecules to disassociate creating a small amount of ozone. However, this reaction also generates large pressure differences in the air, and it is these pressure changes we hear as the loud cracking sound!
You can test this for yourself by firing QS laser energy at white paper – which will not absorb much of the energy. Set the spot size for a very small diameter, around 1 mm. Or, pull the handpiece tip from the paper slowly, while firing, and you will hear a loud crack at some distance (depending on the focussing lens in your laser handpiece).
In terms of tattoo treatments, we don’t want to be firing this excessive fluence at the skin as it can cause too much trauma in the dermis which may lead to scarring.
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Chapter 2 LEVEL A Laser Tattoo Removal
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