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The role of coherence in laser phototherapy

By Tomas Hode, PhD

Introduction

The importance of coherence and laser speckles in LPT has been discussed over the last 30 years. Coherence [1] is often defined as a property of wave-like states that enables it to exhibit interference. In the case of light, this interference gives rise to laser speckles. This speckle field contains areas/volumes of higher and lower intensity (Fig. 1).  A speckle pattern can be subjective or objective: If visible laser light illuminates a wall paper or some other rough surface, the viewer will see laser speckles in the image plane [2]. If either the viewer or the illuminated target moves, the speckles will also change, and the direction of the perceived changes depends on the viewer’s eyes. This is called a subjective speckle pattern, since the details of the pattern depends on the parameters of the viewing system (for example the shape of the eye). An objective speckle pattern is independent of the parameters of the viewing system, and can be imaged by using, for example, a photographic plate or optical sensor without an objective.

Figure 1. A b/w photograph of a speckle intensity pattern. Note that the areas of higher intensity represent constructive interference, and dark areas represent destructive interference. The average speckle size is in the order of a few micrometers. Photo by L. Hode.

It is well known that if a volume is filled with scattered laser light, a three-dimensional speckle pattern is formed in the volume of tissue that is reached by the scattered coherent light. It is also known that the coherence length decreases as a function of depth in the tissue, and that fluid movement within the tissue causes the speckle pattern to change with time. Historically, such speckles have been considered optical noise since they may interfere with various types of experiments. However, speckles can also be useful in various types of experiments. By analysing the movement of speckles on a surface illuminated by laser, it is possible to describe the movement within the tissue non-invasively. This has, for example, been applied when analysing surface deformation in real time using laser speckles [3]. Other practical uses of laser speckles are monitoring of velocity or flow fields, such as retinal blood-flow visualization by means of speckle photography [4], and transmissive laser speckle imaging (Transmissive LSI), which is proposed as a diagnostic for rheumatoid arthritis [5].

Occasionally, however, it is purported that the coherence is lost as soon as the laser light enters the tissue, and that coherence therefore cannot have any therapeutic significance in phototherapy. Furthermore, it has become commonplace for manufacturers of non-laser light therapy devices to paraphrase as “light is light” the conclusions drawn in articles by Smith [6, 7], commented by Hode [8], and Enwemeka [9, 10]. The basic premise of the arguments was that the principal mechanism of phototherapy is photon absorption, and that it makes no difference [in the fundamental photochemical process] whether the light is coherent or not [11]. These arguments have been supported by in vitro studies [12, 13] as well as in vivo studies on wound healing [14] and other superficial indications [15] that seemingly indicate that LED phototherapy is as effective as laser phototherapy. On the other hand, several comparative studies have shown that laser phototherapy appears to be more effective than LED phototherapy in the treatment of deeper indications [16, 17], as well as in in vitro [18] and wound healing studies [19].

The question is important. If lasers are not needed to acquire an optimised therapeutic effect, then light emitting diodes or halogen lamps with band pass filters could just as well be used with a lower expense for the practitioner. Specifically, there are two main issues that need to be addressed:  First, to what extent is coherence of laser light preserved when it penetrates skin and is diffusely scattered in tissue, and second, what is the significance of laser speckles in tissue (if any)?

Question 1: Is coherence lost in tissue?

First of all, it is important to clarify that coherence is not lost in a scattering media (such as tissue), only reduced. The fact that coherence is not lost upon the light entering the tissue can be demonstrated by a simple experiment [20], first demonstrated by L. Hode at The Ninth Congress of the International Society for Laser Surgery and Medicine in Los Angeles in 1991 in response to claims that coherence is entirely lost upon being diffusely spread in tissue [21, 22, 23]. This experiment is described elsewhere in this book, but for readers’ convince, we summarise it here:

Anyone owning a red laser (for example a HeNe laser) can observe that coherence is not lost by conducting this simple experiment:

1) Press fresh ground beef, e.g. raw hamburger meat, between two glass plates so that a 5 - 10 mm thick slab of ground beef is formed.

2) Aim the light from a 5 - 10 mW red laser at the glass plates with the ground beef slab as shown in Figure 2. A red spot on the back of the minced beef will appear where the light has penetrated.

3) Place a small flashlight beside the laser so that the front end is pressed against the surface of the glass. The flashlight emits normal white light. This light also penetrates the ground beef and forms a light spot beside the one caused by the laser. The spot from the flashlight will also be red (red light is less absorbed than the other visible wavelengths).

4) Study both the red light-spots on the back of the slab from a distance of a few meters. The laser light-spot shows clear laser speckles, whereas the spot from the flashlight has no laser speckles. It is thus evident that coherence is not lost as the laser light penetrates the meat.

Figure 2. Light spots on the back side of a slab of minced meat through which the light from a HeNe laser and a flashlight has penetrated. The upper left spot originates from the HeNe laser, and the lower right spot originates from the flashlight. Both spots are red after their passage through the meat, which shows that red light has the best penetration of the visible light wavelengths. Infrared radiation penetrates even better. The figure shows that coherence of the laser light is not lost as the light penetrates the meat. The laser speckles can be clearly seen, and it is obvious that there is a difference between laser light and the light from a flashlight.

There is a common misconception that coherence is reduced or eliminated by fluid movement, which is based on the fact that speckles apparently cannot be seen with the naked eye when a laser shines through a finger, or on experiments that appear to demonstrate that speckle contrast is reduced or lost with fluid (e.g. blood) movement in the tissue.  In fact, the speckle pattern is fully present in the tissue at any given moment, but it changes with the perturbations in the tissue. Faster movement in the tissue, such as blood in circulation and interstitial fluid flows, causes the speckle pattern to change configuration quicker. An example of such changes in the configuration of the speckle pattern can be found here: http://www.laser.nu/speckl.MOV. This is a film that was captured after shining laser light through a slab of ground meet, where the changes of the speckle pattern is caused by slight movements (fluids, cells, etc.) in the slab of meat. It is evident that the speckle pattern is shifting over time, although this is not as fast as in a medium with, for example, blood perfusion, such as living tissue, where the movement is quite rapid compared to the slab of ground beef.

This phenomenon is well known and is in fact frequently used in various settings. For example, laser speckles can be used to quantify blood perfusion in highly scattering media: the lower the speckle contrast, the higher the blood flow velocity, essentially capturing the time-integrated speckle pattern at a fixed exposure time. In other words, since the speckle pattern changes during the time of exposure, a time integration will occur, and the faster the speckle pattern changes, the lower the speckle contrast will be for the same exposure time [24].

Surprisingly, it has been stated that this reduction in (apparent) speckle contrast indicates that the coherence of the light decreases with, for example, increased blood velocity (such as in living tissue). This is a misinterpretation of what the actual results of such experiments show. If an image is taken with a short enough exposure time, the speckle pattern will be quite clear. This also explains why a speckle pattern is not visually perceived when a laser shines through (for example) a finger: The reason is not because the coherence is lost due to the blood flow, but because the speckle pattern moves so fast that we are not able to register it.

The question, then, is how fast a movement of the speckle field is too fast to induce biological effects? Li and Champion [25] investigated the vibrational excitation and relaxation of laser-excited chromophores, and found, among other things, that 1) the average photoexcitation rate under photostationary conditions (for a 420 nm laser with average power of 15 mW/cm², giving a photon flux of ~10^27 photons/sec and cm², an a photon absorption cross section of 4×10^16 cm² for the heme chromophore) corresponded to a photon absorption event every 8-9 ps, 2) that thermal saturation of both the chromophore and the protein occurred after ~100 ps, and 3) that the relaxation times for a chromophore-protein-solvent system were on the order of 10 ns once the influx of photons stopped. Since the lifetime of an individual speckle is on the order of a few ms when living tissue is illuminated with a laser, the speckle lifetime is clearly longer (nine orders of magnitude) than reaction times in photo-excitation events.

Having demonstrated that speckle patterns indeed occur when living tissue is illuminated with a laser, and that the lifetime of individual speckles is much longer than the reaction times in photo excitation events, the question then is why the presence of laser speckles in tissue would have a different or improved biological (therapeutic) impact than uniformly distributed (non-coherent) light?

Question 2: What is the significance of laser speckles in tissue (if any)?

Intensity Thresholds

Several studies indicate that the effects of phototherapy depend not only on energy density [8, 26 and references therein], exposure time [27], and wavelength [26], but also on intensity [26, 28, 29].

The intensity thresholds have been shown to be in the order of 5-15 mW/cm² for monochromatic light sources and the biological responses either below or above these intensities appear to be limited. The exact reason as to why such intensity thresholds exist in phototherapy is not well investigated.

The thresholds can be viewed as the minimum rate (flux) at which photons need to be absorbed by the target molecules to reach a certain photobiological effect. The photon absorption rate depends on photon density (intensity) and absorption cross section of the target molecule. The photon absorption cross section, in turn, mainly depends on the wavelength, redox state, polarisation, and to some extent the temperature of the chromophore.

Hode et al. [30] performed a Monte Carlo simulation on the speckle intensity distribution in a scattering media (such as tissue), and found that intensities of up to 5 x mean can be expected in tissue (Figure 3). As a consequence, volumes of higher intensities will occur in the tissue at greater depths in the tissue than the average intensity otherwise would allow, thus increasing the effective depth of penetration of a laser as compared to a non-coherent light source with otherwise the same parameters (wavelength, power, etc.).

Figure 3. Simulation of intensity distribution of a speckle field in a highly scattering media (such as tissue). a) Polarised speckle pattern (see also fig. 2 below) with exponential intensity statistics. b) Regions with intensity greater than mean. c) Regions with intensity greater than 2 x mean. d) Regions with intensity greater than 5 x mean.

Polarisation

It is not only the intensity that differs between coherent and non-coherent light penetrating into tissue. The laser speckles so formed also differ from non-coherent light of the same intensity – it is locally polarised, or, at least, partially polarised.  Speckles are a result of interference and interference only occurs if the interfering light is co-polarised and has some degree of coherence. If the E-fields contributing to the intensity at a single point are not aligned, there will be no constructive or destructive interference, i.e., no speckle. The laser speckles are formed also if the penetrating light from the illuminating laser is not polarised.

The simulations by Hode et al. [30] explored the statistics of the superposition of two orthogonally polarised statistically independent speckle patterns, specifically with regard to the degree of polarisation. The simulation showed that the degree of polarisation has a complex spatial distribution that looks somewhat like a speckle pattern, and the first order statistics are uniform. The results of the simulations suggest that even though an incident polarised beam becomes depolarised by the time it reaches a few transport mean-free paths as it penetrates the tissue, there still remains a complex spatial distribution of polarised light deep within the tissue (Fig. 4).

 

Figure 4. Simulation of degree of polarisation in a speckle field. a) Complex spatial distribution of the degree of polarisation somewhat similar to a speckle pattern. b) Distribution of the theoretical vs. actual degree of polarisation.

Interestingly, there is a polarisation dependence for the chromophore photon absorption cross section. Tolkachev [31] compared chromophore excitation in non-polarised vs. polarised light, and concluded that the maximum excitation occurs when the polarisation vector is parallel to the direction of the dipole moment of the chromophore (or close to it). Tolkachev [31] concluded that on an orientationally stable, fixed individual chromophore the excitation effect is two times larger than the effect of non-polarised light. In other words, the probability for photon absorption (cross section) is polarisation-dependent and up to two times as high if the direction of the polarised light is aligned with the dipole moment of the molecule.

Combining the effects of the intensity distribution and polarisation patterns in tissue, it can be concluded that the rate of photon absorption may increase with up to an order of magnitude compared to non-coherent light sources. In tissue, where the light absorption is high, these effects have a significant impact on the effective penetration depth. In a clinical setting, this increased effective depth of penetration can make the difference between having to rely on systemic effects vs. getting direct effects in the target tissue.

Superpulsing

Another factor that is typical for lasers, rather than LEDs and other non-coherent sources, is the ability to produce pulses with very high peak power. By so-called super-pulsing it is possible to simultaneously have high momentary photon density and low average power, thus giving low thermal influence. A continuous source for the same photon density would create a high temperature. A typical super-pulsed laser is the GaAs type laser (904 nm) with pulse lengths around 100-200 ns and peak powers up to 25-100 watts (Fig. 5). As a consequence of the high peak powers, the intensity in the tissue increases with an additional 1-2 orders of magnitude.

Figure 5. Diagram of a super-pulsed GaAs laser pulse profile. Typical pulse widths range between 100-200 ns, with peak powers of up to 100 W. The average output power is significantly lower and generally range between 5 and 100 mW.

Biostimulating effects have also been noticed from skin treatment by means of IPL instruments. IPL instruments are wide band flash light devices, usually used for hair removal. The pulses from said devices are usually of rather high power (pulse energies of sometimes up to 100 joules over a surface of typically 3 cm² i.e. an energy density of  >30 J/cm²) and with typical pulse lengths of 3-5 ms. In the case of broad band light sources higher power densities (150-300 mW/cm²) and energy densities (20-30 J/cm²) are required [28], possibly because the relative intensity of the portion of the spectrum that is absorbed by the target molecules needs to be high enough to reach the 5-15 mW/cm² threshold.

Dynamic environment

As previously discussed, a speckle field in tissue will change continuously due to e.g. blood flow. In an arbitrary point, the optical field will vary in intensity, temperature, degree of polarisation and direction of the Stokes vector in a random way, which may have clinical significance. For example, Horvath and Donko [32] measured the intensity differences in a speckle field, and concluded that although the actual temperature differences are low (in the order of micro-degrees), the thermal micro-gradients are very steep, which has the effect of increasing the rate of diffusion in accordance with Fick’s equations. Furthermore, Rubinov [33] showed in a series of experiments that illumination of biological tissue by coherent laser light “unavoidably leads to strong intensity gradients of the radiation in the tissue due to speckle formation”. This causes the appearance of inter- and intracellular gradient forces whose action may significantly influence the paths and speeds of biological processes. In contrast to the photochemical action of light, which is accompanied by absorption of quanta and has a specific character (i.e. is characterised by a specific spectrum of action), the action of the gradient field is of non-resonant type. It is not accompanied by photon absorption and has a universal character - it depends weakly on the radiation wavelength, but requires a high degree of coherence”. These effects will not take place with illumination by incoherent monochromatic light of the same intensity.

Length of coherence

 A HeNe laser (a gas laser) can have a coherence length of several metres, whereas a diode laser of the same wavelength has a considerably shorter length of coherence. When treating in tissue contact, this would appear to have little importance. However, a study by Qadri et al. [34] showed that the results were in favour of the HeNe laser when compared to a diode laser of the same power and dose. HeNe lasers are no longer very common, but it is reasonable to believe that the energies quoted in HeNe studies should at least be increased by 100% if a diode laser of the same wavelength interval is to be used.

Summary

Coherence is a unique property of laser light, and it gives rise to a speckle field when an object is illuminated. It can easily be demonstrated that coherence is not lost in scattering media, such as tissue, which means that a three-dimensional speckle field will occur inside the tissue when illuminated by a laser. Furthermore, studies indicate intensities of up to five times the mean intensity occur in a speckle field. Together with the fact that polarisation patterns occur in speckle fields (independent of whether the light was originally polarised), and that the photon absorption cross section is polarisation dependent and up to two times more effective if the polarisation vector is aligned with the dipole moment of the target photoreceptor molecule, the rate of photon absorption can locally be up to an order of magnitude higher if a coherent light-source is used instead of a non-coherent light-source. Because of these intensity and polarisation distributions, the speckle pattern has a significant impact on the effective penetration depth of in vivo laser phototherapy (Fig. 6), since it becomes easier to reach the required intensity thresholds.

Figure 6. Illustration of the difference in effective penetration between coherent and non-coherent light-sources. For any given power density threshold (for example 5 mW/cm²) there will be a depth at which the average intensity will be lower than the required power density for phototherapeutic effects to take place. However, in the case of coherent light (where a speckle field is present), there will be individual speckles with intensities of up to 5× the average intensity, which means that tissue situated deeper than the average power density threshold cut-off will be exposed to power densities above the phototherapeutic threshold. In other words, the effective penetration depth of coherent light-sources (i.e. lasers) is greater than for non-coherent light-sources (e.g. LED’s).

Also, if a super-pulsed laser is used (e.g. GaAs laser, with peak powers between 5 and 100 W), intensities of additional 1-2 orders of magnitude will occur in the tissue. These observations are important for the clinical application of phototherapy. In deep tissue the intensity is lost to such an extent (as the light passes the tissue above) that it becomes hard to reach the necessary intensity thresholds of 5-15 mW/cm² [26, 28, 29]. With coherent light sources, and super-pulsed lasers in particular, the photon absorption rate is increased by up to 2-3 orders of magnitude, which translated into a clinical setting means an increased effective penetration depth of an additional 2-3 cm compared to non-coherent light sources.

These observations could also explain why LED phototherapy in many cases appears to be comparable to laser phototherapy for superficial indications such as wound healing [14], but not as effective as laser phototherapy in cases of deeper-seated indications [16, 17]. In LED phototherapy of superficial tissue (e.g. wounds, in vitro, etc.) the intensity-loss is not so much of a factor, and as a consequence it becomes easier to reach the required intensity thresholds than it is to reach the given thresholds in deep tissue. But a chronic wound is not only what meets the eye; large areas around the visible wound are affected by inflammatory processes and poor microcirculation. For these areas the laser seems to have advantages.

 

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Editorial note: This article was originally published in “Laser phototherapy – clinical practice and scientific background”, Hode-Tunér, Prima Books, 2014

 

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