Absorption, scattering, and penetration
Updated: Nov 30, 2021
I'd like to talk to you about the optics of tissues. Now this is going to be diving into super nerd territory again for the second week in a row. So, if that's not your cup of tea, that's fine. If you go back to last week's post, that will talk about how the wavelengths interact in the tissues specifically. But today, kind of flipping it around and talking about the tissues and how light interacts with different tissues. I'm going to be talking about three different keywords. They are penetration, scattering and absorption. What do we mean when we're talking about those three things? Why do they matter when it comes to applying laser or light therapy? Well, you need to know about those different factors in addition to knowing about wavelengths if you're going to utilize light therapy to it's maximum.
This information is pulled primarily from a paper titled "Optical Properties of Biological Tissues." The review was published in the Physics in Medicine and Biology journal back in 2013. It is a very, very, in-depth look at optics in tissues.
When we say penetration, we're talking about how deep the light can get through the tissues; how much of it can get through those tissues. When we talk about scattering that has to do with how much light diffuses into the tissues instead of traveling on in a linear fashion. Absorption refers to how the tissues take up the light. Now, these are all interrelated factors. If you have increased scattering in the tissues then you're going to have decreased penetration, you can't get light deeper through the tissues. If you have really high absorption in a particular type of tissue, then you also lose penetration. All of this matters because to get light to have a certain action in tissue, it has to be absorbed. That means that there's this balancing act between penetration and absorption in particular, that determines how effective light can be in the tissues. That's where you have to start talking about what your therapeutic target is. Are you trying to get light in deep or are you trying to get light to cover a large amount of tissue? And further, what type of tissue you're getting into, because different tissue types have a different rate of absorption for different wave wavelengths. They have different rates of scattering and you've got different penetration based on the wavelength through those different types of tissue.
This can get complicated really, really quick.
That being said, let's start at the surface. The skin is the biggest barrier to getting light into deeper tissues. I talk a lot about getting light into deeper tissues, because most of the targets that we want to address with light therapy outside of wound healing really come down to getting light to those injured, deeper tissues, whether it be joint or muscle or tendon or nerve or bone. So the way that we talk about these things is very important when you start designing protocols or designing laser equipment, or just thinking about selecting the right type of settings for the injury and the patient that you're working with.
The skin has a very high rate of scattering, and depending on the wavelength, skin can have a high rate of absorption as well. What I want you to take away from my writing today is that the higher the wavelength the less problems you have with absorption and scattering in the skin. The less absorption you have in the skin, the more light you can get into those deeper tissues. At 600 nanometers, you have a decent amount of absorption in the melanocytes or the melanosomes, whereas at 400 it is very high. And that is why the color dependent lasers, the greens and blues, really can't get to targets deeper than the skin because they are highly absorbed in the colored part of the skin.
Once you get to about 800nm, the amount of transmission through the skin improves; the absorption rate of the skin is much lower and that continues to go lower all the way to at least 1200 nanometers from what we can tell. That means you're going to get much better penetration of light through the skin in that 800 to 1200 nanometers zone.
After you get through the skin, what's the next layer that you usually are going to have to deal with? That is the adipose tissue, the subcutaneous fat tissue. Looking at the absorption and the scattering and the penetration that light can have through fat, the dominant absorption peak of fat is at 930 nanometers. That means that if you are using a 930 nanometer laser, you're going to get pretty good penetration through the skin, but you're going to get quite a bit of absorption in the fat.
Once you get through skin and then the adipose or fat tissue, your next barrier to penetration is going to be blood and water in blood. In particular, we're talking about the hemoglobin being the chromophore that really absorbs light the most, and hemoglobin has different absorption peaks based on whether it is oxygenated or deoxygenated. For both deoxygenated and oxygenated hemoglobin, absorption peaks at about 806 nanometers. But for oxygenated hemoglobin right around 900nm is a very nice sweet spot in the curve for absorption in hemoglobin. What we theorize happens is when oxygenated hemoglobin absorbs light in that 900nm range, it will change confirmation. It will drop oxygen and become deoxygenated. If you're dropping oxygen off from hemoglobin and you're talking about blood vessels, then the idea is you are delivering more oxygen to the tissues, which of course is critical for healing.
The next one is water and it is a critical, and really the most prolific chromophore that you'll find in the tissues because you've got bound water within the cells and within the mitochondria. Also, there is free water that floats in the interstitial fluid. This study says that bound water has a peak absorption at 970 nanometers.
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