ATTENUATION AND PENETRATION OF VISIBLE 632.8nm AND INVISIBLE INFRA-RED 904nm LIGHT IN SOFT TISSUES

We studied the depth of penetration and the magnitude of attenuation of 632.8nm and 904nm light in skin, muscle, tendon, and cartilagenous tissues of live anaesthetized rabbits.

Tissue specimens were dissected, prepared, and their thicknesses measured. Then, each wavelength of light was applied. Simultaneously, a power meter was used to detect and measure the amount of light transmitted through each tissue. All measurements were made in the dark to minimize interference from extraneous light sources. To determine the influence of pulse rate on beam attenuation, the 632.8nm light was used at two predetermined settings of the machine; continuous mode and 100 pulses per second (pps), at an on:off ratio of 1:1. Similarly, the 904nm infra-red light was applied using two predetermined machine settings: 292 pps and 2,336 pps. Multiple regression analysis of the data obtained showed significant positive correlations between tissue thickness and light attenuation (p < .001). Student's t-tests revealed that beam attenuation was significantly affected by wavelength.

Collectively, our findings warrant the conclusions that (1) the calf muscles of the New Zealand white rabbit attenuates light in direct proportion to its thickness. In this tissue, light attenuation is not significantly affected by the overlying skin, a finding which may be applicable to other muscles. (2) The depth of penetration of a 632.8nm and 904nm light is not related to the average power of the light source. The depth of penetration is the same notwithstanding the average power of the light source. (3) Compared to the 904nm wavelength, 632.8nm light is attenuated more by muscle tissue, suggesting that is is absorbed more readily than the 904nm wavelength or conversely that the 904nm wavelength penetrates more. Thus, wavelength plays a critical role in the depth of penetration of light.

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We investigated the characteristics of low-reactive-level laser therapy (LLLT) and analyzed the effects thermographically in patients with cerebral palsy. We determined transmissivity in exposed bone specimens and in the human body using a camera with a wavelength-specific charge-coupled device. The effects of LLLT were dispersed throughout the entire body, and transmitted through bone tissue as well. At an output level of 100 mW, the laser reached a depth of about 2 cm. To study the effects on the autonomic nervous system, we treated 50 patients with cerebral palsy using the gohkoku acupuncture point (between the bases of the first and second metacarpal bones) of the left hand, and measured changes in the skin temperature of both hands. A GaALAs-diode continuous-wave laser beam with a wavelength of 810 nm and power output of 100mW was applied for 60 seconds and the skin temperature was measured with a Thermotracor TH1106 apparatus (NEC San-ei). The results were classified into 3 categories : temperature decrease, no change, and temperature increase.

For evaluation of the autonomic nervous system, the laser was applied over muscles with increased muscle tone all over the body in 12 patients with cerebral palsy. Color Doppler showed an increase in blood flow in the common carotid artery in eight of the 12 patients. High speed Fourier analysis of the R-R interval in the Holter electrocardiogram showed increases in the high-frequency components originating from the parasympathetic nerves after irradiation in the same eight patients, suggesting the involvement of the autonomic nervous system in producing this change.