Mike Murphy's Blog

There’s more…

One of my readers, Dan Jackson, asked me about pulsewidth (in a roundabout way) after my recent posts about ink colours (Part 1, Part 2). I had forgotten that the pulsewidth does have an effect on the threshold (minimum) fluences required to instigate the desired response in tattoo ink colours.

A few years ago I did a wee review of tattoo laser treatments through the decades. I found that the fluences typically used were directly related to the pulsewidths (see figure below).

Shorter pulses need less fluence than longer ones, regardless of wavelength. The shortest pulse I could find was used by Ross et.al. with an experimental picosecond laser of duration 35 ps. This was not a commercial system.

They found that they could stimulate a response using only 0.65 J/cm², whereas a conventional picosecond laser required around 1.0 J/cm² – current “picosecond” lasers utilise pulsewidths between 350 and 800 ps, according to their manufacturers. (Which means that they are better describes as ‘sub-nanosecond’ since these equate to 0.3 and 0.8 ns…)

Fluence threshold vs pulsewidth

So, using the same calculation method as in Part 1, but with a ‘base point’ of now only 1 J/cm², I have recalculated the minimum fluences required to induce a response, for different pulsewidths:

Wavelength	532nm	694 nm	755 nm	1064 nm
Ink colour
Black	1.2	1.0	1.0	1.0
Blue	2	1.2	1.2	1.6
Green	2	1.2	1.2	1.8
Red	1.2	2.8	3.1	2.6
Yellow	1.6	4.1	4.1	4.2

This clearly indicates that shorter pulses need less fluence. My original calculations were based on a study from 1990 when a ruby laser was used to determine the threshold fluence in black ink. But this system had a pulsewidth of around 40 nanoseconds – quite long by today’s standards.

Many of the Q-switched lasers we use today are in the range 5 to 10 ns, with the alexandrite and ruby lasers utilising longer pulses.

The tables show the difference between the pulsewidths – these are approximations since it is not possible to calculate these for all pulsewidths (insufficient data!!)

Wavelength	532nm	694 nm	755 nm	1064 nm
Ink colour
Black	2.2	2.0	2.0	1.9
Blue	3.8	2.2	2.3	3.1
Green	3.8	2.2	2.3	3.4
Red	2.2	5.3	5.9	4.9
Yellow	3.0	7.8	7.8	8.1

Picosecond lasers may use even lower fluences to generate the desired reaction (in superficial ink)!

The advantage of using lower fluences is that there is less chance of collateral damage to the surrounding tissues. Every laser tattoo removal procedure results in some micro-scarring, mostly due to the physical tears caused by high-speed fragments of ink tearing through the dermis and epidermis.

Of course, fluences must be increased when tackling deeper ink. But any micro-scarring at depth will not be readily visible at the surface.

Fluences required for shorter pulses, less than 10 ns

Conclusion

Shorter pulsewidths require lower fluences to induce the reaction on the ink particles, but all wavelengths can be used to tackle all ink colours, if sufficient fluence is applied.

It is likely that short pulsewidths add more energy into the individual ink particles, before the steam production prevents further absorption (see my BMLA presentation on this stuff).

Hope this helps,

Mike.