ARTICLE IN PRESS

Journal of Quantitative Spectroscopy &

0022-4073/$ - se

doi:10.1016/j.jq

�CorrespondE-mail addr

Radiative Transfer 106 (2007) 378–388

www.elsevier.com/locate/jqsrt

Backreflectance from the blood plexus in the skinunder the low-power laser heating

E.K. Naumenkoa, A.N. Korolevicha,b,�, N.S. Dubinac,S.I. Vecherinskyc, M. Belsleyb

aInstitute of Physics National Academy of Sciences of Belarus, Nezavisimosti Avenue, 68, 220072 Minsk, BelarusbPhysics Department, Minho University, Campus Gualtar, 4709 Braga, PortugalcGP ‘‘MTZ Medservice’’, Stahanovskaia Street, 10A, 220009 Minsk, Belarus

Abstract

The intensity of light backscattered when low-power laser radiation is incident on the skin is investigated in vivo. The

exposure of blood to low-power laser light in the absorption range of haemoglobin leads to an increased intensity of the

backscattered light. The theoretical calculation using the existing optical model of erythrocyte aggregation suggests that

the fragmentation of erythrocyte aggregates is the most probable mechanism leading to the enhanced backscattering.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Blood; Laser irradiation; Backreflectance; Erythrocyte aggregation

1. Introduction

Blood is a two-phase suspension of formed elements (i.e. erythrocytes or red blood cells (RBC), white bloodcells and platelets) suspended in an aqueous solution of organic molecules and salts called plasma. In normalstate, the human RBCs are highly deformable bodies that have a biconcave-disk shape in the absence ofexternal forces. It has been observed that RBCs behave like fluid drop under most flow conditions.

Another important rheological property (i.e. deals with the flow and deformation behaviour of materials orfluids) is their tendency to aggregate into linear arrays, turned rouleaux, in which they are arranged like stacksof coins. In normal blood, coins columns are easily decomposed to their individual cell constituents as bloodflow increases. But in pathological cases (cardiovascular diseases and diabetes), the capillary circulation isseriously affected because nonseparable rouleaux are formed. The cause of this effect is an imbalance betweenaggregation and desegregation forces: electrostatic repulsion between cells and/or elastic (surface binding)energy of the cell membrane [1].

The non-invasive analyses of aggregation–disaggregation process in blood are of great importance becauseit can be used for the diagnosis of many dangerous diseases. Optical scattering of aggregated RBCs differ from

e front matter r 2007 Elsevier Ltd. All rights reserved.

srt.2007.01.045

ing author. Physics Department, Minho University, Campus Gualtar, 4709 Braga, Portugal. Fax: +351 253 678 981.

esses: akaralevich@fisica.uminho.pt, akorolevich@yahoo.com (A.N. Korolevich).

ARTICLE IN PRESSE.K. Naumenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 106 (2007) 378–388 379

that of the nonaggregated ones, therefore, the scattered properties may be used to detect and diagnose tissuepathologies.

The scattering of He–Ne laser radiation by rouleaux is undoubtedly a much more difficult problem thanthat of single erythrocytes. Nevertheless, there are many studies dedicated to investigations of thecharacteristics of scattered radiation both by the separate erythrocytes [2–6], and their aggregates.

However, it is necessary to note that laser irradiation of biological tissues can lead to a change in theirphysical parameters. Heating results in both physiological and optical effects. Therefore, the application ofheat has been widely used for the treatment of many diseases. For example, the stimulation of photobiologicalprocesses in the organism under the conditions in vivo during the low-intensity laser irradiation is used in thelaser therapy [7]. Changes in the characteristics of tissue caused by temperature gradients [8–10] are veryimportant since they have numerous applications, for example with the thermo-therapy (by heating tissue to42–43 1C ) of cancer.

Published results (for example, [11]) have shown that laser irradiation of biological tissues leadsto a change in the rheological characteristics of erythrocytes, mainly deformation. Laser radiation caninduce long-term conformational transition of red blood cell membrane related to changes of botherythrocyte membrane proteins and lipid bilayer (for example, [12]), which led to changes in theirdeformability and their osmotic fragility [13]. Deformation properties, in turn, affect the formation of theaggregates of erythrocytes [14], and therefore it has an essential effect on the normal motion of the blood alongthe capillaries [15].

Therefore, one of the basic reasons for an increase in the perfusion of the blood during the laser irradiationof biological tissues in vivo conditions [16,17] can be caused by the decrease of average size of the aggregates ofthe erythrocytes [18].

Recently in [19], it was shown that laser irradiation of the blood in the spectral ranges of absorptionhaemoglobin leads to increasing the intensity of the backscattered radiation. A theoretical calculation showsthat a possible explanation for this result is a decrease in the mean erythrocyte aggregation size resulting fromlaser-induced fragmentation. At the same time, it is also possible that the change of the refraction index and/orviscosity of plasma due to laser heating can also change the intensity of scattered radiation. The goal of thepresent study is the further research of other mechanisms that can lead to change of intensity of scatteredradiation from the skin under the low-intensity laser irradiation.

2. Theoretical estimate

Under irradiation (small local heating) of biological tissues by a laser, several mechanisms for changingscattered light intensity are possible, namely: (1) changes in refractive index of erythrocytes, (2) changes inrefractive index of plasma, and changes in erythrocyte aggregation degree.

At present, there is no acceptable theory of scattering by particles of arbitrary shape that would be suitablefor accurately describing the processes of interaction of radiation with complex structures similar to blooderythrocytes and their aggregates. Only in the case when individual isolated erythrocytes are investigated isthere sense in using the cumbersome exact and even approximate solutions of the problems of interaction ofelectromagnetic waves with the particles of irregular shape, which requires complex and lengthy calculations.It is practically impossible to use such solutions for blood, where particles of various shapes and sizes arepresent. For this reason, until the present time, the theory of scattering by homogeneous spherical particles hasbeen widely used for interpreting the data of experimental investigations of blood by optical methods. In allprobability, the Mie theory is barely suitable for quantitative analysis. Nevertheless, it can be used as a firstapproximation for revealing the tendencies for any optical characteristic of the single erythrocytes and theiraggregates to be changed.

According to the earlier-proposed optical model of erythrocyte aggregation [4,20], the scattering propertiesof erythrocytes and its aggregates were identified by the properties of the equivalent volume spheres. For thepurpose of volume estimation, the shape of the isolated erythrocytes was approximated by circular cylinderwith concave bases. A model of formation of aggregates in the form of rouleaux from N erythrocytes whenerythrocytes contact by periphery edges was considered. A radius of the sphere of equivalent volume for the

ARTICLE IN PRESSE.K. Naumenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 106 (2007) 378–388380

aggregated erythrocytes was calculated from

R ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3

4pV em

3

r¼

1

2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiN 3r2ðhþ h0Þ �

ðh� h0Þ3

4

� �3

s, (1)

where Vem—the volume of the erythrocyte, h0—the minimal thickness in the centre, h—the maximal thicknesson the edge. The values of backscattering intensity and of extinction coefficient were calculated according to

IðbÞ ¼ CAI 0ðbÞ ¼ CA3

4l2

Z r2

r1

iðr; n;k; l;bÞf ðrÞdr=

Z r2

r1

r3f ðrÞdr

� �, (2)

Kext ¼ CK 0ext ¼ C3

4

Z r2

r1

r2�ðr; n;k; lÞf ðrÞdr=

Z r2

r1

r3f ðrÞdr

� �, (3)

C—the volume concentration of erythrocytes (hematocrit value), A—coefficient, containing parameters ofexperiment, i(r,n,k,l,b) and e(r,n,k,l)—Mie scattering characteristics of the particle (isolated erythrocyte oraggregate of erythrocytes with the radius of equivalent volume sphere—r), n and k—relative values of the realand imaginary parts of the complex refraction index, l—value of wavelength in surrounding media, f(r)—theparticle size distribution function.

The calculations were carried out for polydisperse spherical particles with gamma distribution of particlesizes by integrating within the bounds (1�0.2) R0prp(1+0.2)R0, the parameter of the half-width of theparticle size distribution function was the same for all the R0 (R0—the most probable (modal) radius of sphereof the equivalent volume). The parameters of erythrocytes varied within the limits; minimal thickness in theerythrocyte centre 0.9ph0p1.2 mm, maximal thickness on the erythrocyte edge 1.7php2.4 mm, radius of theerythrocyte base R ¼ 3.5–4.4 mm, the real part of refractive index 1.03pnp1.05, the imaginary part ofrefractive index k ¼ 10�4.

Below, the analysis of the backscattering intensity and extinction coefficient normalized on a volumeerythrocytes concentration I0(b) and K0ext is examined.

2.1. Influence of the erythrocyte aggregate sizes

The dependences of the backscattered intensity from the degree of aggregation for the different refractiveindex of erythrocytes are presented in Fig. 1.

100 10 1

the number of aggregated erythrocyties

0.0

2.0

4.0

6.0

back-s

cattere

d inte

nsity, a.u

.

1

2

3

4

1

4

Fig. 1. Backscattered intensity from the erythrocyte aggregation degree for the different refractive index: 1—1.048, 2—1.046, 3—1.044,

4—1.041.

ARTICLE IN PRESSE.K. Naumenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 106 (2007) 378–388 381

From data in Fig. 1, it can be seen that fragmentation of the big aggregates (20–100 isolated erythrocytescombined) leads to drastic decreasing of backscattered intensity. It is important to note that a change ofintensity depends also on their refractive index. The greater refractive index stimulates a greater decrease thebackscattered intensity under aggregate fragmentation.

In [21], the scattering of He–Ne laser radiation by a rouleau consisting of n (2pnp8) average-sizederythrocytes was investigated by the much more powerful boundary-element method (BEM). However, as inour case, fragmentation of the small (o10) erythrocyte aggregates resulted in an increase in the intensity ofbackscattered radiation.

In contrast to large aggregates, the aggregates of smaller sizes (Np10) dissociating results in increasingbackscattered intensity, and the values of refractive index of blood plasma do not significantly influence thebackscatter dependance on the number of aggregated erythrocytes. The probability of forming largeaggregates in capillary systems within the measuring zone is very small. Most likely, there are the aggregateswith middle sizes which can break up into isolated erythrocytes under action of laser irradiation.

2.2. Influence of the base medium refractive index

Changes in refractive index of a base medium n1, where particles with radius r are located, lead tosimultaneous changes in several optical parameters determining scattering properties of a layer. The importantparameters are: (1) the real part of the relative complex refractive index of intracellular substance n, (2) it’simaginary part k (given by Eqs. (4) and (5) respectively), (3) the wavelength l and (4) the size parameter of aparticle (expressed in Eq. (6))

n ¼ ne=n1, (4)

k ¼ ke=n1, (5)

r ¼ 2prn1=l. (6)

It is evident from (4) to (6), that the real and imaginary parts of the relative complex refractive indexdecrease with an increasing plasma refractive index, but the erythrocyte diffraction (size) parameters areincreasing.

Let us examine the scattered characteristic of erythrocytes as the plasma refractive index is varied. Below arepresented the results of calculations simulating changes in scattering properties of erythrocytes as the plasmarefractive index is varied. The input parameters were: wavelength l ¼ 0.78 mm, erythrocyte refractive indexne ¼ 1.41. The calculations are made for the non-aggregated erythrocytes with the different geometrical sizes(diameter of the erythrocyte base—R ¼ 3.5 mm, minimum thickness in the erythrocyte centre—h0, andmaximum thickness on the erythrocyte edge—h) (Fig. 2).

A decrease of the backscattered intensity is observed with increasing plasma refractive index for differentsizes of erythrocytes. This would suggest that to explain the experimentally observed increase of backscatteredintensity by this mechanism one must suppose that the refractive index of blood plasma decreases uponheating.

2.3. Influence of the base medium refractive index

Reflection depends not only on the backscattering cross-section bp, but also on the value of attenuationcoefficient e. Approximately, it is possible to consider that the value of backscattering is described by theknown location equation

P ¼ AbpðzÞ expð�2�lÞ, (7)

where P—experimental measurable value of reflected intensity, A—the constant depending from parametersof experimental setup, l—the layer thickness. The parameters of erythrocytes were varied within the limits;minimum thickness in the erythrocyte centre 0.9ph0p1.2 mm, maximum thickness on the erythrocyte edge1.7php2.4 mm, radius of the erythrocyte base r ¼ 3.5 and 4.4 mm, l ¼ 633 nm.

ARTICLE IN PRESS

1.34 1.345 1.35 1.355 1.36

refractive index of plasma

0.32

0.36

0.4

0.44

0.48

extinction, m

km

-1

1

4'

4'

1

Fig. 3. The attenuation coefficient of the erythrocytes on the refractive index of the blood plasma. Curves 1–4—for radius of the

erythrocyte base 3.5mm, 10–40—for 4.5 mm.

1.342 1.346 1.35 1.354 1.358

refractive index of blood plasma

1.2

1.6

2.0

2.4

2.8

back-s

cattere

d inte

nsity, a.u

.

1

2

3

4

Fig. 2. Influence of the refractive index of plasma on the backscattered intensity: 1, 2—d0 ¼ 7 mm, 3, 4—d0 ¼ 8, 8mm, 1, 3—h0 ¼ 0.9mm,

h ¼ 1.7mm, 2, 4—h0 ¼ 1.2 mm, h ¼ 2, 4mm.

E.K. Naumenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 106 (2007) 378–388382

As follows from Eq. (7) for the estimation of the scattered radiation with a change in the refractive index,the knowledge of extinction coefficient is necessary. The results of the calculations of the attenuationcoefficient from the refractive index of plasma are represented in Fig. 3.

Let us examine the ratio of the signals P0/Pi,

P0=Pi ¼b0bi

exp½�2�0lð1� �i=�0Þ�, (8)

where—P0 the value of the intensity of scattering at the initial moment of the time (t ¼ 0, exp(�2e0l) ¼ 0.1,npl ¼ n0 ¼ 1.35), Pi—intensity of the scattered light changed by variations of the refractive index induced bythe laser heating.

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1.34 1.345 1.35 1.355 1.36

refractive index of plasma

0.8

1

1.2

P0/P

i

1

1

23

4

1'

2', 3'

4'

4'

Fig. 4. Influence of the refractive index of plasma on the relation P0/Pi for the different radius of erythrocyte base: 1–4—3.5mm, 10–40—

4.5 mm.

E.K. Naumenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 106 (2007) 378–388 383

For the relatively small particles (curves 1–4), an increase of backscattering intensity can reach 20–30%,while for the larger one it lies inside 7–12% (Fig. 4).

3. Backreflectance measurements

3.1. Objects for study

The effect of exposure time on skin backreflectance in, and outside of, the irradiated region of a He–Ne laser(wavelength 632.8 nm, power 1.1mW) was studied experimentally in vivo. The measurements were made in thetemple zone where the fatty layer is practically absent and the number of capillaries and vessels is relativelylarge. There are nearly 25–40 capillaries present underneath 1mm2 of skin in the region of the elbow, while60–70 capillaries are present in the region of the temple. The measurements were carried out on 14 patients ofboth sexes with cardio-vascular disease, ages of the patients ranged from 43 to 65 years.

Irradiation of the patient’s temporal zone was carried using a low-power continuous-wave helium–neonlaser with following parameters; wavelength of 632.8 nm, power of 1.1mW, beam diameter of 2.5mm,irradiance of approximately 225W/m2. As the optical characteristics of the skin and blood microcirculationare dissimilar for the different patients, we therefore performed the relative measurements, i.e., measurementin the radiation zone, also, out of the radiation zone. All studies were carried out at room temperature, 25 1C.Results are presented as mean 7 standard deviation (SD).

The penetration depth of laser radiation within biological tissues depends on the optical properties of thesetissues, namely, on the values of scattering coefficient, ms, absorption coefficient, ma, anisotropy factor, g, andthe separation between the source and the observation point. From the separation distance between the opticalfibres, it is possible to estimate the probable trajectory of the scattered light [22]: ZðxÞ ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

2xðr� xÞ=ffiffiffiffiffiffiffiffiffiffiffiffi3mams

pqð1� gÞr; where r is the separation between the source and the observation point and

the x-axis is directed along the skin surface. Besides the parameters described, the scattering properties ofthe skin’s layers varies with the patient’s sex, age and the body location. For example, the thickness of theepidermis varies from 0.1mm in the eyelids to nearly 1mm on the palm and soles. The exact volume of thetissue being measured is difficult to estimate. Roughly speaking for the strong absorption limit, the maximumpenetration depth is Z(x)Er/2. It is typically of order 1mm [23]. Therefore, all elements of the dermis from thesuperficial nutritional capillaries to deeper thermoregulatory vessels are typically probed by this method.

ARTICLE IN PRESSE.K. Naumenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 106 (2007) 378–388384

From an optical point of view, a whole blood is the high concentrated turbid medium consists of about55 vol% plasma (90% water, 10% proteins) and 45 vol% cells (99% red blood cells ‘‘erythrocytes,’’ 1%leukocytes and thrombocytes) [24]. Therefore, conditions the intensity of scattered radiation is determined,mainly by the optical characteristics of erythrocytes, and the influence of the remaining regular elements of theblood (leukocytes and thrombocytes) can be disregarded.

The measurements were carried out using a commercially available instrument, Periflux 4001 from Perimed(Stockholm, Sweden). This instrument uses single-mode laser radiation a wavelength of 780 nm and power of0.8mW to probe the light used for measuring the blood microcirculation in the skin. The radiation from thelaser is passed through a lens to be coupled into a light guiding fibre through which it is guided to the surfaceof the object being investigated. A flexible fibre-optics laser probe was attached to the patient’s body so thatthe laser radiations incident perpendicularly to the skin’s surface.

The light diffusely backscattering from the tissue is collected by optical fibres the same dimensions as that ofthe source fibre. These two fibres were held parallel to each other by a mechanical support. The light isdelivered to the probes via a flexible optical fibre with a very little loss of light. The flexible probe holdersmould easily to the skin surface and are fixed to the skin with a double-sided adhesive disk.

For the control of our signal, we normalized by the signal coming from the temple region on the oppositeside of the head without the He–Ne laser irradiation.

3.2. Results of measurements

The RBC backreflectance records of volunteers showed typically cyclic variations with time at two basicfrequencies with heart rate, and another slower frequency more variable amplitude cycle, presumed to bevasomotion [25]. Therefore, to estimate the backreflectance value we had to make an average during 1min anduse this value /RhS for the estimation of backreflectance during the laser irradiation.

A typical result is presented at Fig. 5. Laser irradiation leads to increase the intensity of backscatteringradiation. The statistical analysis of the results obtained shows that the character/behaviour Rh is different forthe patients, but the value of backscattering radiation increased during the irradiation time.

In general, the measured values of perfusion vary greatly among the subjects, probably as a result of theindividual variations in the actual state of health and the optical characteristics of the dermis. Therefore, tobetter ascertain the effect of the low-power laser radiation on each patient, control measurements of bloodreflection were simultaneously made without the presence of He–Ne laser light (curves 2, 4, 6, 8).

Further confirmation that low-power laser radiation can effectively alter the local scattering was obtainedby interrupting the exposure to the laser light during the measurement process. One of these results ispresented in Fig. 6. The value of the scattered light increased in the zone of exposure from the beginning ofmeasurements up to the 13th minute when the laser light was blocked. Then at 18min, the laser light was againpermitted to reach the skin. The response in the observed blood scattered is striking. During the intervalbetween 13 and 18min, the intensity oscillates around a mean value but does not increase. After the exposureto laser light is resumed, there is once again a clear increase in the scattered light.

The intensity of Rh begins growing after 1min of irradiation for 50% (7) of patients, and for only one after amaximum latency of 10min. For most of the cases, the value of backscattered radiation reaches saturationafter 3–5min of the irradiation. Obviously that dynamics of change and the saturation value Rh will depend onthe optical characteristics of the skin and blood.

The aggregation state of erythrocytes is very sensitive to the processes in the human organism especially incases of different pathologies. Characteristic times for formation of linear (T1) and 3-D (T2) erythrocyteaggregates at low shear stress is significantly altered in different pathologies (psoriasis, coronary diseases, andso on [6]).

Cardiovascular disease is prevalent among these clinical conditions with well-established haemorheologicalconsequences. Red blood cell aggregation is increased in various conditions associated with stable ischeamicheart disease (for example, [26]), myocardial infarction (for example, [27]) and so on. Ischeamic diseases ofvarious organs are known to be associated with haemorheological impairment [28]. In short, all vascularinsufficiencies and ischeamic diseases may result from the disturbance of local homeostasis [29]. Therefore, we

ARTICLE IN PRESS

0 6 10 126.4

6.5

6.6

6.7

6.8

6.9

7.0

7.1

7.2a

reflecta

nce, a.u

.

irradiation time, min

1

2

-2 0 10 12 145.1

5.2

6.6

6.7

6.8

6.9

7.0b

reflecta

nce,

a.u

.

irradiation time, min

3

4

-2 0 2 4 6 8 10 12 14 166.55

6.60

6.65

6.70

6.75

6.80

6.85

6.90

6.95

c

reflecta

nce, a.u

.

irradiation time, min

5

6

-2 0 2 4 6 8 10 12 14 165.2

5.4

5.6

6.6

6.8

7.0re

flecta

nce, a.u

.

irradiation time, min

d

7

8

2 4 8 2 4 6 8

Fig. 5. Dependence of the backreflectance on the irradiation time for the four patients under (curves 1, 3, 5, 7) and without (curves 2, 4, 6,

8) laser irradiation. Results are presented as mean 7 standard deviation SD.

E.K. Naumenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 106 (2007) 378–388 385

can suppose that changes in the backscattered radiation (as dependent on the average size of the erythrocyteaggregates) caused by laser heating should be related to the diagnosis of disease.

In fact, the detailed analysis of anamnesis of diseases showed that the dynamics of a change in the intensityof scattered radiation during the laser irradiation depends on the diagnosis of the disease. Both the dynamic ofbackscattered radiation and the saturated intensity are correlated with diagnosis of the cardiovascular disease(Fig. 7).

As follows from Fig. 6 an increase in the scattering for the patients with the diagnosis ‘‘atherosclerosis’’(curves 1–3) begins, as a rule, not immediately, but 3–5min after the beginning of irradiation. For the patientswith other diagnoses of cardiovascular diseases (curves 4–7) and healthy volunteers (curve 8), an increase inthe scattering begins practically immediately after the beginning of irradiation. It is interesting that theintensity of scattering with the identical exposure time is substantially more than for patients withatherosclerosis (curves 1–3). One of the possible reasons for this effect can be the aggregative properties of theblood with atherosclerosis that reduce the separation of the erythrocyte aggregates during the small laserheating.

From another side, atherosclerosis begins with a thickening of the intimal, i.e. the inner layer is composedprimarily of endothelial cells, layer of blood vessels [29]. This inner layer is composed of elastin, collagen, and

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-20

24

68

1012

1416 1,0

87

65

43

2

1

patients

backre

lecta

nce, a.u

.

irradiation time, min

Fig. 7. Influence of laser irradiation on the intensity of scattered radiation for different diagnoses of the cardio-vascular diseases: curves 1,

2, 3—atherosclerosis, curves 4–7—cardiosclerosis, tension angina pectoris, and so on, 8—healthy volunteer.

70

65

60

55

50

45

40

35

30

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Inte

nsity, a.u

.

Irradiation time, min

Fig. 6. The intensity as a function of exposure time to low-power laser light. The laser light was incident for the first 13min, and then

blocked between the 13th and 18th minute, when the exposure was once again resumed. Note the break in the steady rise that occurs

during the interval during which the laser light was blocked.

E.K. Naumenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 106 (2007) 378–388386

smooth muscle, largely determines the elastic properties of the vessel. In other words, with atherosclerosis itchanges the geometric and elastic properties of the walls of vessels. However, it is possible to assume that theoptical parameters of blood vessels during the disease were changed also. Therefore, the laser irradiation of theblood leads to the smaller and slower heating of the blood and with respect to a smaller increase in theintensity of scattered radiation.

4. Conclusion

The influence of low-intensity laser irradiation on the scattered intensity from the skin in the area ofvascular plexus is investigated in vivo. By the backreflectance technique we found that low intensity laser

ARTICLE IN PRESSE.K. Naumenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 106 (2007) 378–388 387

irradiation of the blood in the absorption range of haemoglobin leads to increasing intensity of thebackscattered light.

The small enough temperature gradients are sufficient to produce structural and conformational changes inthe cellular proteins of the erythrocytes and/or to produce a change in the permeability [10] and of membranedeformability [11]. These changes lead to a change in the aggregative characteristics of erythrocytes (time ofaggregation, the sizes and so on) [29].

The theoretical calculation using the existing optical model of erythrocyte aggregation suggests that thefragmentation of erythrocyte aggregates is the most probable mechanism leading to the enhancedbackscattering. Therefore, we conclude that besides the photobiological processes occurring with the lasertherapy [1] physical mechanisms can also occur, for example heating biological tissues by laser radiation.

Acknowledgements

This work was supported in part by the Portuguese Foundation for Science and Technology, Grant SERH/BPD/5556/2001, by the Byelorussian Foundations for Basic Research, Project F04-050, and Luso-AmericanoFoundation, project 608-04.

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