An excellent clinical report by Scheibner (SCHEIBNER, A., KENNY, G., WHITE, W. and WHEELAND, R.G. (1990), A Superior Method of Tattoo Removal Using the Q-Switched Ruby Laser.) found that the minimum fluence required to induce a good reaction from black tattoo ink, using a Q-switched ruby laser, was 2 Joules/cm2.
This threshold fluence was for black ink, which absorbs much more strongly than virtually all coloured inks. So, the question is, what are the minimum fluences required for other colours – they must be higher than for black since they absorb less energy! (I haven’t seen a good report on this – if anyone knows of one, please let me know).
To answer this question, we need to know how strongly each colour absorbs light (of various wavelengths).
Another excellent study by Choi et.al. (Effects of picosecond laser on the multi-colored tattoo removal using Hartley guinea pig: A preliminary study.) looked at the relative responses in coloured tattoo inks to three wavelengths (532, 755 and 1064nm) from both nanosecond and picosecond lasers. We can use the results from this study to give an indication of the relative absorptions of these coloured inks.
(ns – nanosecond, ps – picosecond)
The Choi study revealed some very interesting results. They found that each of the colours tested responded differently to each wavelength, to various degrees (see table above). Black ink absorbed the most energy, obviously, and reacted better with pico lasers compared with nano lasers (except with the picosecond Alexandrite 755nm wavelength).
Red ink reacted best with the 532nm wavelength, which is green, then with the infrared 1064nm wavelength, and the least with the Alex 755nm wavelength. Orange and yellow inks reacted in the same way as red.
However, green and blue inks reacted best with the red 755nm wavelength, followed by the green 5332nm line then the 1064nm wavelength.
Unfortunately, this study did not investigate the minimum fluences required to induce a reaction. But, their results do shine a light (pun fully intended!!) at the relative absorptions of these coloured inks to those wavelengths – I have shown this in the graph below:
The absorption values above are my own guesstimations, based on the Choi data. They will not be exactly right, but they do give us an indication.
So, if we assume that the minimum fluence needed to induce the desired reaction in black in, with a 1064nm wavelength, is 2 J/cm2, then, it is clear that higher fluences must be applied to the other colours, to achieve the same result.
From the above graph, it appears that red ink may require about 40% more fluence, to match the reaction in black ink; while yellow will require more than four times the fluence needed for black ink – greater than 8 J/cm2.
The same argument can be applied to the 755nm and 532nm wavelengths. In other words, we can use any wavelength to treat any ink colour, as long as the required fluence is applied for that ink’s colour and depth.
This may appear counter-intuitive since many people believe that some colours can only be removed by certain wavelengths – this is incorrect!! The physics is very simple – ink colours with lower absorptions merely require higher fluences.
Ah but…. There is a problem…
However, it is not as simple as this might indicate. The problem is that higher fluences will also induce more potential tissue damage in the dermis. This appears to be even more of an issue following my recent research on these interactions. Up to 90% of all the laser energy we throw into tattooed skin may well be reflected back from the tattoo due to very rapid steam bubble formation, which essentially acts like a mirror. This reflected energy will undoubtedly cause damage to the collagen.
So using higher fluences for poorly absorbing inks might not be a good answer. It may create more problems in the surrounding tissues.
Summary
The Scheibner and Choi reports give us a better understanding of the effects of laser energy in tattoo inks. They also indicate how strongly those coloured inks absorb those wavelengths. Extrapolating from their data we can see, approximately, the ratios of absorptions and how they compare. The physics of these interactions tells us that poor absorbers merely need more energy (fluence) to achieve the same reactions as strong absorbers.
But, the clinical application of higher fluences is fraught with danger – if you do choose to apply higher fluences, be very careful and use lots of cooling post-treatment to extract as much thermal energy as possible.
Huh?!?!
Finally – an interesting point – Choi stated that they believe that the reactions are more dependent on the wavelength than the pulse duration. They state that “wavelength is more influential than pulse duration for the removal of each coloured ink. Regardless of the pulse duration, the 532nm laser was the most effective in clearing red, orange and yellow coloured inks and was mildly effective in removing green and black coloured inks.”
It appears that the Q-switched, nano-second 532nm pulses were very useful…
Hope this helps,
Mike.
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