Greatest Active Depth

The depth of penetration of laser light depends on the light`s wavelength, on whether the laser is superpulsed, and on the power output , but also on the technical design of the apparatus and the treatment technique used. A laser designed for the treatment of humans is rarely suitable for treating animals with fur / there are, in fact, lasers specially made for this purpose. The special design feature here is that the laser diode (s) obtrude from the treatment probe rather like the teeth on a comb. By delving between the animal`s hair, the laser diode`s glass surface comes in contact with the skin and all the light from the laser is "forced" into tissue.

A factor of some importance here is the compressive removal of blood in the target tissue. When you press lightly with a laser probe against skin, the blood flows to the sides, so that the tissue right in front of the probe (and some distance into tissue) is fairly empty of blood. As the haemoglobin in the blood is responsible for most of the absorption, this mechanical removal of blood greatly increases the depth of penetration of the laser light.

It is of no importance whether the light from a laser probe held in contact with skin is a parallel beam or not in contact treatment.

There is no exact limit with respect to the penetration of the light. The light gets weaker the further from the surface it penetrates. There is, however, a limit at which the light intensity is so low that no biological effect of the light can be registered. This limit, where the effect ceases, we call the greatest active depth. In addition to the factors mentioned above, this depth is also contingent on tissue type, pigmentation, and dirt on the skin. It is worth noting that laser light can even penetrate bone (as well as it can penetrate muscle tissue). Fat tissue is more transparent than muscle tissue.

N. B. We are here talking about direct effect of laser light on cells and not biological effects due to systemic effects from laser therapy.

For example: a He-Ne laser with a power output of 3.5 mW has a greatest active depth of 6 - 8 mm depending on the type of tissue involved. A He-Ne laser with an output of 7 mW has a greatest active depth of 8 - 10 mm. A GaAs laser has a greatest active depth of between 20 - 30 mm (sometimes down to 40 - 50 mm), depending on its peak pulse output (around a thousand times greater than its average power output). If you are working in direct contact with the skin, and press the probe against the skin, then the greatest active depth will be achieved.

Practical experience of treatment shows that it can be more advantageous to treat a smaller area more intensively, and then treat nearby areas later, than to treat a larger area over a longer period of time, on one and the same occasion. This may well be related to the fact that certain substances are released which can be traced in blood and urine, and which can reach disturbing high levels when large areas are treated.

We usually differentiate between local treatment and system treatment. Local treatment refers to direct treatment at the location of the problem - this is the most common form of treatment. System treatment involves the treatment of places some distance from where the problem itself is located. An example of this is a laser acupuncture, where one or more acupuncture points are irradiated with laser light, instead of needles. Other forms of system treatment are the treatment of trigger points and the treatment of back, neck or lumbar vertebrae through which nerves from the problem area pass. The irradiation of lymph glands to activate the immune system is also an example of system treatment.

How Deep does Light Penetrate into Tissue?

It is easy to establish that the light penetrates deeper than 1 - 2 mm in tissue. If you hold your hand in front of a pocket torch, you can see that the light penetrates your fingers, and they are generally thicker than 1 - 2 mm. You will also note that the light which penetrates is red, so is the red light in particular which penetrates, while the blue, green and yellow are absorbed. Infra-red light is not visible, but it is easy to demonstrate that it penetrates deeper than visible light. Therapeutic lasers always emit red or infrared light.

The depth of penetration of the red light has been studied in conjunction with a technique called PDT (photodynamic therapy) with which HPD (haematoporphyrin derivate), for example, is injected and then irradiated in the tumor area with laser light with a wavelength of 630 nm. It has been established, that the depth of penetration is enough to get a biological response (necrosis) as far down as 10 mm in the tissue. 

The depth of penetration is much greater at longer wavelengths - two to three times greater for infrared lasers. The penetration is also very much dependent on the type of tissue involved. Fatty tissue is much more transparent than muscle, mostly due to the latter`s high hemoglobin content. However, by pressing lightly with a laser probe amongst the skin:

1. All light forced into the tissue (optical index matching).
2. The tissue in front of the probe is compressed, forcing the blood away from it.
3. The tissue itself may also be forced aside slightly as a result.

These three mechanisms combine to give better light penetration. Finally, the laser output (average output or pulse-peak output) and power density have a considerable importance with respect to the depth of biological response. The penetration depth depends both on the power density at the skin surface and of the total power. If the power density distribution on the skin surface is kept constant and the total incident power is increased tenfold, the greatest active depth increases by about 50 per cent.

As the GaAs laser is pulsed like a flash gun, with pulse-peak outputs of many watts, we can reach much deeper than with a continuous of the same average output. (It is also possible, via various openings in the body, to make the light reach even deeper. In Russia, experiments have been conducted in which optical fibres have been positioned in cannulae in an attempt to carry the light deeper into the desired area.)

With the above definition (1/e of the intensity on the surface), the depth of penetration is totally independent of the laser`s output, which means that the same depth value is achieved no matter how weak (or strong) the laser. The definition in question specifies, in actual fact, the relative penetration, while the decisive factor for the biological effect is the beam`s momentary absolute value and, as has been mentioned, the absolute penetration. This is decisive for field intensity in a cell membrane deep in the tissue.

Our understanding and experience, based on more than 10 years experiments and practical work, is that LLLT achieves biological effects at a depth of 1 - 4 cm in tissue, depending on the type of laser used, the laser`s output, and other parameters. This does not necessarily depend on the direct effects of light at that depth, but could be the result of other forms of communication in the tissue, such as the transport of transmitters or signal substances.

Stockholm, 31 August, 2000