Originally Posted by Willis99
Thank you - Just want to make sure I understand correctly - there’s no safety issue with not using US Gel between leg and dorsal side - even if there are a few gaps - it’s mainly for heat conduction and to avoid hotspots?
Which Dragon Skin do you recommend? Their site offers several variants.
Are there other materials that would be good phantoms?
How do you measure temp if you have heat on both sides during treatment phase?
The three things we are concerned with in US are transmission, reflection, and refraction. If the tissues/materials subject to the US waves are of similar refractive index, then the waves will generally transmit undistorted through the mass. The tissue impedence (how much the tissue resists the transmission of sound waves) then determines the attenuation (how much energy is absorbed). It is the absorption that we want, but we want it to happen at a certain depth. Very high frequency is absorbed at the surface and would burn the skin. Very low frequency simply travels through without much absorption. It seems that for our purposes 2mHz might actually be optimal, but it isn’t readily available; we have 1mHz and 3mHz. The former exhibiting much lower attenuation and the latter much higher. So the 3mHz waves can heat much more efficiently, but depending on the wave profile they might not heat evenly throughout or deeply enough, resulting in hot and cold layers. The 1mHz, OTOH, has lower attenuation, so the waves easily travel completely through the mass. Indeed, 1mHz waves can go several inches into the body and still generate heat. This is good and bad for us. On the good side, you get a much more even absorption at various tissue depths. The bad part is that a large amount of energy passes through and out the other side requiring longer warmup time and stronger devices.
This is where your questions come in. What happens to the waves when they exit the other side of the shaft? If the shaft is firmly pressed against other similar tissues (thigh skin and muscle) and there is either sweat or US gel, then the tissues are “index matched”, meaning that the waves with just continue traveling on their intended path with similar attenuation until the energy is mostly absorbed. With the thigh, this generally happens within the adipose layers acting as a heat bank because the adipose layers retain heat very well (they have higher impedence than other tissues and don’t have much blood flow). This is sort of an ideal situation because you get the heat bank effect as well as index matching.
If the materials are not index matched, for example the opposing side is just exposed to air, then the waves will refract. Meaning they will change direction, speed, absorption, etc. They can even reflect directly back at the source. If the waves refract/reflect chaotically then some areas will see a wave canceling while others might see a wave enhancing, in the worst cases creating a standing wave that has double the power. If you feel a pinching of the skin, this is what’s happening. The air-skin interface isn’t index matched and you’re feeling hot and cold spots as a result. A phantom eliminates this by index matching the skin to another material and allowing the waves to continue. If the phantom has enough impedence and thermal mass, it will heat and become a heat bank like the thigh. But at a minimum, it prevents refraction at the surface.
Whether you need this depends on the US wave profile. IME, those devices with high quality profiles are less susceptible to errant refraction resulting in hot spots. If that is the case, then by all means, make your life easier by eliminating the phantom on the opposing side and place the heat pad in direct contact with the skin. If you feel any hot spots, you’ll need a phantom but it doesn’t need to be very thick and if it’s too thick it’ll negate the heat from the pad.
The dragon skin has shown to be reasonably good material. Many here don’t have access to the vacuum chamber that I have, so I would recommend the NV version which stands for “no vacuum” and results in lesser bubbles. I haven’t done a test on the relative insulative and absorptive properties of the various dragon skin.
I no longer measure temperature. After a but of experience and using quality instruments you get a feel for what 42-43C feels like. Your body is dialed in to let you know exactly when that threshold is reached. A protein known as TRPV1 is responsible for indicating heat pain in the body and is triggered at 42C. You will start to feel pain at 42-43C but it is tolerable with sensitization. 44C feels uncomfortable even after sensitizing the tissue, but is tolerable for extended periods for many if they ease into it. 45C feels painful even with tissue sensitization and requires mental focus and determination to resist removing the pain source. 46C just hurts and will cause 2nd-degree burns within several minutes. That’s why I always say, just slightly uncomfortable but not painful is the range we are going for.
