Laurie's Blogs.

I’m sure I’ll be missing some points here… but this was my quick and dirty dive into the topic of laser light attenuation.  Part of my Laser Therapy 101 series! 

Attenuation is a general term that refers to any reduction in the strength of a signal.  It is a measure of how much the incident energy beam (e.g. ultrasound, laser, x-ray) is weakened by the material it is passing through.

The transmitted power of LLLT on skin, fat, and muscle of tissues decreases with the increase of thicknesses, presenting minor attenuation. Different tissues (rat and pig skin, pig fat, and pig muscle) with different thickness (1.17–14.01 mm) and wavelengths (660, 830, and 904 nm) were tested in the same experimental procedure. In all analyses, the 660 nm showed the major attenuation for the different tissue's samples, followed by 830 nm. The 904 nm showed the minor attenuation in all tissue's samples except for pig skin, where the 830 nm has minor attenuation. (Barbosa 2020)

Wikipedia defines attenuation as follows:

In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at variable attenuation rates.

Picture from https://www.facebook.com/LaserTherapyU/photos/a.411922168826627/3919826444702831/?type=3&paipv=0&eav=Afb3rMG5YxKMCWZIz4j2HTpw9XSf8dVlxKIZnBiWdqE1TlXLDXPNMhgyBABMP5KqpqQ&_rdr 

What you should know to help minimize attenuation of your light energy 

1.  Skin and hair colour.  Darker hair and skin tends to absorb more laser light energy and hence reduces absorption to deeper target tissues.  Cooling of the superficial tissues might aid in enhancing depth of laser penetration. (Farkas et al, 2013)

2. The shorter wavelengths (200–600 nm) have more superficial penetration due to their absorption pattern, and longer absorption wavelengths (650–1200 nm) have deeper penetration in the tissue.  (Farkas et al, 2013)

3.  Power and Spot Size = Spot Density.  A smaller spot size creates a superficial concentration of photons that can lead to an increase in superficial absorption and also heating of tissues.  When the spot size is decreased, so is the depth of penetration due to more rapid scatter of the photons under the tissue surface, creating a more superficial treatment effect.  With larger spot sizes, the scatter is reduced, allowing more photons to be delivered to the desired target in a wavelength-dependent matter.  (Farkas et al, 2013)

4.  As well, it is known that hemoglobin in blood tends to absorb visible light, which could result in an attenuation of the laser energy able to get to the target tissue.  To mitigate this factor, one might be better to use laser prior to exercise or to push the laser probe into the tissue to cause a mild blanching of the superficial tissues overlying the target tissue. (Sikurova et al 2011)

Conclusions:

There are a number of factors that can impact target tissue absorption of laser energy via loss of power (attenuation).  In order to mitigate and minimize attenuation, you can factor in tissue type, tissue colour, wavelength of light, spot density,  and blood flow.  With these aspects factored in, you can then strive to minimize their impact by cooling tissues, choosing an appropriate wavelength, increasing spot density, and reducing superficial blood flow in order to get the best results for using laser on deeper target tissues.  

Now, I know that to those of you smarter than me about lasers (and you know who you are!), I hope I’ve not dumbed down this topic too much!  My goal is to just make the information easy to consume and clinically relevant.  

References:

• Barbosa RI, Guirro ECO, Bachmann L, Brandino HE, Guirro RRJ. Analysis of low-level laser transmission at wavelengths 660, 830 and 904 nm in biological tissue samples. J Photochem Photobiol B. 2020 Aug;209:111914. 
• Attenuation. Wikipedia.   https://en.wikipedia.org/wiki/Attenuation  accessed May 8, 2024.
• McKay Lachlan. Attenuation coefficient.  https://radiopaedia.org/articles/attenuation-coefficient#:~:text=The%20attenuation%20coefficient%20is%20a,material%20it%20is%20passing%20through. Accessed May 8, 2024.
• Farkas JP, Hoopman JE, Kenkel JM. Five Parameters You Must Understand to Master Control of Your Laser/Light-Based Devices, Aesthetic Surgery Journal, Volume 33, Issue 7, September 2013, Pages 1059–1064.
• Sikurova L, Balis P, Zvarik M. Penetration of laser light through red blood cell ghosts. J Photochem Photobiol B. 2011 Jun 2;103(3):230-3. 
• Pantawane, Mangesh & Dahotre, Narendra. (2019). Challenges and Advances in Osteotomy. 2. 1-4. 10.6084/m9.figshare.8216621.