Page 276 - Mike Murphy's Blog Posts

Plasmas are very unlikely to form under laser tattoo removal conditions. There is simply no where near sufficient energy applied to induce either multi-photon optical breakdown or thermally initiated plasma formation. For example, a 350 picosecond pulsewidth would require a fluence of around 35 J/cm2 applied at the ink surfaces to induce multi-photon optical breakdown. Whereas a 10 nanosecond pulse requires nearer 1000 J/cm2 to do the same!
2nd Presentation – OP20
The second presentation is “Longer pulsewidths are clinically better when treating hair/blood vessels with laser/IPL systems.”
It is generally accepted that shorter pulses are ‘better’ for smaller, thinner targets, while longer pulses are more suited for larger targets. This is incorrect. The aim of photothermal treatments is to denaturate a sufficient volume of target proteins such that the tissue is irreversibly destroyed.
The amount of cell denaturation is a complex function of temperature and time, using the Arrhenius rate equation. The final denaturation state depends exponentially on temperature, while it is a linear dependency on time.
Achieving the desired goal requires that the target cells attain a suitable temperature for a sufficient period of time. Thin objects lose heat rapidly through conduction. Hence such targets need to have a high temperature maintained to ensure sufficient denaturation. If they cool down too quickly then they will not achieve the required level of denaturation.
This can be ensured by applying a longer energy pulse, thereby maintaining the required temperature for the required time.
Similarly, a larger target must also achieve the necessary denaturation level. However, larger targets have greater mass than thin targets. This results in a lower temperature rise for the same incident energy/fluence. In addition, due to the thermodynamics, larger targets lose heat slower than thinner targets. Consequently, larger targets are cooler than thin targets, but they maintain their elevated temperature for a longer period.
However, to ensure irreversible denaturation, larger targets will need longer energy pulses so that the lower temperature will have sufficient time to generate the required amount of cell denaturation.
In summary, to achieve the desired clinical result in hair or blood vessels, the cells/proteins need to be heated for a required length of time corresponding to the local temperature. For small or thin targets, which cool quickly, this can be achieved by extending the heating time.
Mike’s Blog Posts 276