Beam Diameter and Divergence

Beam diameter and divergence are important features of single optical sources. Beam size affects light intensity values on the skin surface and within the tissue in accordance with formulae (3, 4). Beam divergence affects light distribution and dosimetry for different tissue layers. For non-contact techniques light spot size and irradiated area S on the skin surface depend on the distance to the irradiated surface h as follows:

S=.pi./4.times.(d+2h.times.TAN .alpha.).sup.2 (5)

where d is the beam diameter near the probe tip, 2.alpha. is the diverging angle, 2h.times.TAN .alpha. is the additional beam diameter due to beam divergence.

Different optical sources (lasers, laser diodes, light emitting diodes, etc.) have different beam divergences. Lasers usually have small beam divergency, laser diodes and LED's have bigger divergences. For different applications particular beam divergences are more convenient. For example, for the treatment of wounds and ulcers, almost parallel beams are less desirable because of the large areas to be treated, and optical sources with some particular divergence are more convenient.

The beam diameter and divergence should be selected based on the three dimensional size and shape of the tissue area affected. Preferably, the beam diameter and divergence should be selected such that the area receiving LEPT is just slightly larger in size than the area affected. The appropriate radius of the beam may be calculated by the following formula:

(R+1).sup.2 /R.sup.2

where R (cm) equals the radius of the area affected by the disorder. In the case of lesions, such as ulcers or other open skin wounds, it is particularly important that too large an area not be illuminated as, where the illuminated area is much larger than the lesion, the skin ulcer (wound) healing rate is not optimized. As the ulcer is treated and healed the area requiring treatment and the beam diameter will have to be reduced.

Dose

The dose D is the light energy provided to the unit of surface (1 cm.sup.2) during a single irradiation and measured in J/cm.sup.2 or mJ/cm.sup.2. The light dose received by the skin surface is

D=I.times.t (6)

where I is the intensity on the skin surface, and t is the exposure time (s). The dose received by subcutaneous tissue layer at the depth z for a parallel beam can be calculated by the following formula:

D=I(z).times.t (7)

where I(z) is given by formula (4).

As mentioned above, the dose alone does not ensure particular photoeffect or healing phenomenon. Only proper selection of the whole set of optical parameters including dose will provide the desirable therapeutic effect. The selection of optical parameters depends on the medical condition, location of the affected areas, person's age, etc.


Frequency and Pulse Duration

Low range frequencies of 0-200 Hz may sensitize release of key neurotransmitters and/or neurohormones (e.g. endorphins, cortisol, serotonin). These frequencies correspond to some basic electromagnetic oscillation frequencies in the peripheral and central nervous system (brain). Once released these neurotransmitters and/or neurohormones can modulate inflammation, pain or other body responses. Analogous phenomena can be expected with "photobiomodulation" within the same range of low frequencies. Certainly, the interaction between living cell and pulsed electromagnetic wave depends on wavelength as well as pulse duration. Pulse repetition rates within the range 1,000-10,000 Hz with different pulse durations (milli-, micro- or nanoseconds) can be used to change average power.

Three Dimensional Light Distribution

Depending on the target tissue for LEPT (e.g. skin, muscle, ligament) a proper three-dimensional light distribution should be provided to get the desirable physiologic and therapeutic response. For single optical sources important parameters affecting light distribution are beam size, divergence, light wavelength as well as biotissue optical properties (reflection, absorption, scattering, refraction). Total reflectance is equal to the sum of the regular reflectance from the skin surface and the remittance from within the tissue (see FIG. 4).

For cluster probes, additional contributive parameters are the distance between diodes and the cluster probe's three-dimensional shape. All these parameters should be physiologically justified to provide optimal biotissue response and requirable three-dimensional light distribution. For example, the distance between diodes can affect vasoactive blood vessel response and average energy density delivered to the treated area. For proper vasoactive response a definite distance between diodes has to be provided depending on particular parameters of a singular diode (power, beam, diameter, divergence).

The three-dimensional light distribution in tissues such as the skin and underlying tissue layers may be calculated based on diffusion approximation and/or the Monte Carlo approach (L. Wang and S. Jacques, Hybrid model of Monte Carlo Simulation and diffusion theory for light reflectance by turbid media, J. Opt. Soc. Am. A/Vol. 10, No. 8, 1993, pp 1746-1752; A. Welch et al., Practical Models for Light Distribution in Laser-Irradiated Tissue, Lasers in Surg. Med. 6: 488-493, 1987). Wavelength


Wavelength

Wavelength.lambda. (nm) is the basic electromagnetic wave feature which is directly linked to the energy of an individual light quantum (photon). The more wavelength the less photon energy. Wavelength is also linked to the monochromatic light color. Visible monochromatic light changes its color with wavelength, increasing from violet and blue (shorter wavelengths) to orange and red (longer wavelengths). Cell culture experiments have indicated that there is a selectivity in photoinduced phenomena related to wavelengths. Experiments on different cell cultures (microbe and mammalian) have revealed the ranges of wavelengths (360-440 nm, 630-680 nm. 740-760 nm) where photoinduced phenomena are observed (Karu, Health Physics, 56:691-704, 1989; Karu, IEEE J. of Quantum Electronics, QE23:1703-1717, 1987). Photoeffect can be induced by monochromatic light, only in cases, where a cell contains photoacceptors, substances which are able to absorb monochromatic light of this particular wavelength. No photoinduced cell phenomena can be observed if there are no wavelength specific photoacceptors in a cell.

The following factors have to be taken into account when considering LEPT dosimetry for monochromatic light of a particular wavelength .lambda.. The dose required for "photobiomodulation" strongly depends on the wavelength. In general, the longer the wavelength the more dose is required to induce photoeffect. For example, in experiments on cell cultures, doses required for DNA synthesis stimulation are 10-100 times less with blue light (.lambda.=404 nm) than with red (.lambda.=680 nm) or near infrared (.lambda.=760 nm) light.

Wavelengths in the range of from 400 to 10,000 nm may be used for LEPT, preferably from 500 to 2,000, more preferably from 600 to 1,100, most preferably from 600 to 700 nm and 800-1,100. There appears to be some optimal wavelength range to induce every particular photoeffect or healing phenomenon. For example, light having a wavelength of from 600 to 700, preferably from 630-680 nm, may be used for wound and ulcer healing. For chronic soft tissue pathology monochromatic light in near infrared wavelength range (800-1,100) is more suitable.

Biotissue optical parameters (reflection, scattering, refraction, absorption and depth penetration) depend on wavelength. Therefore, light wavelength affects three-dimensional light distribution in biotissue. For example in a specific wavelength range, the longer wavelength the more light penetration depth. The darker skin the more light absorption, therefore the dose for a black skin has to be less then for a white skin.

Monochromaticity (Bandwidth)

Light source is described by its spectrum, which shows the range of wavelengths of the emitted light. Strictly monochromatic light source is a source of radiation with exactly the same wavelengths. This is never achieved in practice even with a laser. Every light source can be described by its spectrum bandwidth .DELTA..lambda.(nm). The smaller the bandwidth the more monochromaticity of the light source. The following considerations are important in regards to light source monochromaticity.

Biological objects became adapted to wide-band solar radiation through evolution. Therefore, pronounced photoinduced phenomena in living cells can be observed only under irradiation by a light source with narrow enough bandwidth. The exact restrictions on light bandwidth may differ for various biological objects.

Simultaneous irradiation by wide bandwidth and monochromatic light can lead to decrease or even disappearance of "photobiomodulation" effect. Therefore, it is recommended to provide some LEPT treatments in a darkened room.

Difference in wavelengths emitted by optical source is leading to dispersion in light reflection, scattering, refraction and absorption which can affect three-dimensional light distribution and LEPT dosimetry.

Bandwidth of the optical source can affect optimal intensity and dose values required to induce a particular healing phenomenon. The full bandwidth of monochromatic light to activate healing phenomena should not exceed 30-40 nm.