Page 121 - Blog Posts v1

a) penetration into the dermis is strongly affected by the incident wavelength. In the visible and near infra-red part of the spectrum, the penetration depth is dependent on the scattering and absorption coefficients of the skin constituents including blood, melanin, keratin and other chromophores. For this reason the longer wavelengths penetrate deeper into the dermis, hence 755 nm (alexandrite) will penetrate further than 694 nm (ruby) but 1064 nm (Nd:YAG fundamental) will penetrate further than both of those.
The frequency-doubled wavelength available with Nd:YAG laser emits at a wavelength of 532 nm. This wavelength cannot penetrate as far into the dermis as the above wavelengths and so has a lower efficiency compared to them.
b) the wavelength also strongly determines the ability of the ink to absorb the laser energy. In general, darker colours absorb more readily than lighter coloured inks. However, some colours may be more strongly absorbed than others. For example certain red wavelengths are well absorbed by green ink and so this can be exploited by some lasers. Likewise, black ink absorbs more efficiently at the Nd:YAG 1064 nm wavelength than by the other wavelengths listed above. Hence it is important to choose the most appropriate laser wavelength when treating certain colours.
c) the skin colour of the patient can have an important effect on treatment efficiency. Tattoo ink is located in the upper dermis in most cases, just below the epidermal melanin basal layer. Hence, the laser energy must traverse the melanin layer before reaching the tattoo ink. Absorption of light by melanin is stronger in the blue part of the spectrum compared with the red end. Hence wavelengths nearer the blue end will be more strongly absorbed by melanin than red light which may lead to u wanted side-effects. 532nm is absorbed significantly by melanin and so should not be used on darker-skinned individuals. Whereas 1064 nm light is very poorly absorbed by melanin and so is much safer to use on all skin types.
Laser spot size - the spot size is particularly important when considering the effective penetration depth of the energy. Clinical observations, laboratory tests and computer simulations all show that larger spot sizes will deliver energy more efficiently into the dermis than smaller spots. Hence, the laser user should always use the largest spot size available to them while ensuring the correct energy density (fluence).
Treatment interval - when we first tested the Q-switched ruby laser in the '80s we found that a minimum of four weeks was required to ensure no lasting tissue damage. This was adopted as the standard, as a result of this observation. However, clinicians across the world have indicated that longer intervals appear to generate 'better' clinical results. Consequently, many now leave between six and eight weeks between repeat treatment session, with one centre claiming a three month interval. The reason appears to be due to the body's 'clearing' efficiency. As the fragmented ink particles are removed by macrophages, it makes sense to give them more time to do their job. If a patient returns after eight weeks, instead of four, then clearly there will be more ink removed from the treatment site. I have observed a small number of patients who returned for treatments after a year's interval. They exhibited a remarkable loss of ink in that time. I'm not
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