Mike Murphy's Blog

This is a little tricky because it’s not intuitive.

We know, from both clinical tests and computer modelling, that larger spot sizes result in more energy/fluence reaching deeper into the dermis. But how does that happen?

Well, it all comes down to scattering. The dermis is mostly composed of collagen fibres. These fibres are all scattering centres for the wavelengths we typically use in many laser-skin treatments (400 to 1200nm). As a consequence, when light enters the skin, the photons are usually scattered many, many times before being absorbed by something, or emitted back out of the skin altogether (known as ‘back-scattering’).

When a photon is scattered, it is essentially absorbed by an atom, and then re-emitted as a new photon, usually in a new direction. That direction is determined by something called the anisotropy. Longer, red/infrared, wavelengths tend to be ‘forward scattered’ – roughly in the same direction as the original photon direction. Shorter wavelength, blue/green, photons may be directed into a much wider angle.

It is clear that the anisotropy has an effect on the penetration depth of the photons – blue/green photons generally don’t go as deeply as red/infrared photons.

So that’s how wavelength affects penetration depth.  But what has that to do with the spot size?

Well, this is where it gets more interesting.

Imagine a ‘column’ of photons fired into the skin, in a small diameter beam. As soon as this beam hits the skin, the photons begin to scatter.  Many of the photons will move outside of the original beam. This means that the beam diameter will increase in size as it penetrates further into the dermis. The fluence is the total energy divided by the beam diameter, so, the fluence will decrease as the diameter increases. In other words, the fluence decreases in value as the beam penetrates further into the dermis.

We should look at fluence before going any further. Fluence is simply the concentration of energy – the number of photons passing through each square centimetre. So, a fluence of, say, 10 J/cm², requires a certain number of, photons in each square centimetre. So, if we choose to use a bigger spot size, we must increase the number of photons to maintain the same concentration. That means more energy (since energy is merely the total number of photons!)

Now imagine the same scenario as before, but with a bigger spot diameter. To be the same fluence as before, this beam will need more energy (photons). As before, virtually every photon will be scattered on entering the skin. And, as before, the fluence will decrease the deeper it penetrates into the skin.

But, a significant proportion of the scattered photons will be directed back into the beam. If these two beams have the same wavelength, then the ‘spread’ of the beams will be approximately the same.

This means that many of the photons inside the beam will be scattered to another point inside the beam. If we consider the ‘proportionality’, smaller spot sizes have fewer photons inside them, compared with larger spot sizes (of the same fluence). Consequently, larger beam diameters will have many more of the photons scattered ‘internally’ into the beam, whereas smaller beam diameters will essentially ‘lose’ more photons to the surrounding tissues (proportionally).

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Both of these beams have the same fluence (concentration of energy). The beam on the left has a larger diameter than the beam on the right. Due to scattering in the skin, the larger beam diameter delivers more energy deeper into the skin.

You can imagine it as both beams losing photons, mainly from the edge of the beams. Smaller diameter spots will ‘lose’ as many as larger diameters. But larger diameters will ‘retain’ more photons inside them – meaning more fluence there.

Think of a small diameter spot as a three-lane motorway with exit roads on both sides. The cars on the outer edges of the lanes can easily drive off the motorway via the exit roads. If cars are leaving at every exit, then the number of cars left on the motorway after a few exits will be considerably less than the start of the motorway.

Now imagine a ten-lane motorway. Only those cars near the edges can easily leave – the central cars are much less likely to leave the motorway. They may move between lanes (analogous to scattering), but leaving the motorway completely is much more difficult for most of the cars.

The upshot of this is that larger spot diameter beams, with the same fluence as a smaller diameter beam, will deliver more photons deeper into the skin, than the small spots. Or, in other words, the fluence penetrates further!

Summary

The way light travels in the skin is mostly determined by the scattering. The wavelength and anisotropy determine the way light scatters. Larger spot diameters have more photons in them than smaller diameter beams, with the same fluence (concentration).

In large diameter beams, much of this scattering will merely put the photons into other locations within the beam. Proportionally, smaller beams will lose more photons from the original beam, compared with larger diameter beams. So larger diameter beams will inevitably deliver more photons deeper into the skin than smaller beams.

Useful Penetration Depth

Delivering fluence to some particular depth is all fine and well. But, it is only useful if it performs the task we’re trying to achieve. For example, if we need to cook some germ cells in a follicle, we’ll need to be sure to deliver enough fluence to the depth of the germ cells bulge, to heat them properly.

This means we need to define the “Useful Penetration Depth” (click here) – the depth at which the fluence is sufficiently high to do the job in hand. This not only depends on the wavelength and anisotropy; it also depends on the fluence. A relatively low fluence, in a large spot diameter, may deliver some fluence to the depth of the bulge.

But it may not be enough to cook the germ cells properly. A higher fluence will be required to achieve this. This relates to my recent post on ‘kill-stun-null’ zones (click here).

Additional…

There are two additions I should make here:

1. There is a limit to this. Once the spot diameter reaches about 11mm, the depth of penetration does not continue to increase. So, a 15mm spot diameter will not penetrate deeper than an 11mm diameter spot.
2. If the fluence is absorbed by something en route, then, obviously, it diminishes. Less fluence will reach deeper parts of the skin, simply because it has been ‘used up’ by more superficial absorbers. This is particularly important when considering darker skin tones. Those tones will absorb more light energy, especially red light, and thereby, leave less for the intended targets.

I hope this helps,

Mike.

PS Our next MasterClass will be in Liverpool in June. Contact us on DermaLaseMasterClass@gmail.com for more info.