Page 106 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Middle zone – this zone occupies a range of depths where there is insufficient available energy to ensure the desired response in the target tissues. During photothermal treatments some of the targets may be only “reversibly denatured” leading to a disappointing clinical outcome. Hair follicles can recover and regrow as thinner, finer hairs – which are more difficult to destroy. Deeper follicles, such as bikini or arm pits, consequently, require higher fluences at the skin surface to ensure their destruction. We call this the ‘Stun’ zone.
Deeper zone – at this depth there is insufficient absorption of the incoming light energy to induce any significant or useful reaction in the target tissues. Increasing the incident irradiance at the skin surface will increase the amount of available energy at these depths and push the non-reactive zone deeper. But this strategy may result in excessive thermal damage near the skin surface. Hence, much more cooling would also be required. We call this the ‘Null’ zone.
As a consequence, we must choose the most appropriate parameters (settings) to ensure the desired results at the required depths (where our targets are located), whilst protecting the upper layers of the skin.
If we then take the same 0.6 Joules of light energy and focus it into a 2 mm spot diameter, then it becomes much more ‘concentrated’ than before. In this scenario, the fluence has jumped to 19.1 J/cm2 – more than 6 times the previous value!
Figure 56 – The same 0.6 J concentrated into a 2 mm spot diameter will yield a fluence of 19.1 J/cm2 – more than 6 times the fluence in a 5 mm spot
Since fluence is defined as energy / spot size area, and the area is increasing with depth, then the fluence will drop, as the light penetrates deeper into the skin.
In, figure 58 a fluence of 5 J/cm2 is applied to the skin surface on the left side of the image. At some depth, due to the beam spreading out and increasing the beam diameter, the fluence will drop to 4 J/cm2. At a deeper depth, the beam diameter will be larger and the fluence will drop to 3 J/cm2. Then, it will be 2 J/cm2 at a deeper level.
Now, if the treatment needs a minimum fluence of 4 J/cm2 to ensure sufficient cooking of the stem cells, then this treatment will not be very successful since most of the follicle is below the 4 J/cm2 level!
To get around this problem, we increase the fluence at the surface - on the right side of the image the applied skin surface fluence is now 10 J/cm2. So, the depth at which the fluence fell to 4 J/cm2 in the previous situation, the fluence is now 8 J/cm2. Similarly for the two deeper cases.
________________________________________________________________________ 106 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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