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A Pressure controlled low-level laser probe to enhance photon density

in soft tissue

Changmin Yeoa, Taeyoon Sona, Heesung Kanga, Junghwan Parka, Young-Heum Leea, b, Kyoung Joung

Leea, b, Byungjo Jung*a, b

aDepartment of Biomedical Engineering, Yonsei University, Wonju, Korea;

bInstitute of Medical Engineering, Yonsei University, Wonju, Korea

ABSTRACT

Noninvasive low-level laser devices have been introduced for therapeutic purpose in medicine. However, low-level laser

cannot deliver enough photon density to expect positive therapeutic results in deep tissue layer due to light scattering

property in tissue. In order to overcome the limitation, this study was aimed to develop a negative pressure applied low-

level laser probe to enhance laser transmission and, therefore, photon density in soft tissue. In order to evaluate clinical

feasibility of developed laser probe, ex-vivo experiments were performed with porcine skin samples and laser

transmissions were quantitatively measured as a function of tissue compression. The laser probe has an air suction hole

to apply negative pressure to skin, a transparent plastic body to observe tissue deformation, and a small metallic optical

fiber guide to support the optical fiber when negative pressure was applied. By applying negative pressure to the laser

probe, the porcine skin under the fiber guide is compressed down and, low-level laser is emitted into the skin. Diffusion

images of laser in the skin samples were acquired with a CCD camera and analyzed. Compared to the intensity without

compression, the peak intensity of laser beam profiles increased about 2~2.5 times and FWHM (Full Width at Half

Maximum) decreased about 1.67~2.85 times. In addition, the peak intensity was linearly increased as a function of

compression. In conclusion, we verified the enhancement of laser transmission and therefore, photon density in tissue by

applying negative pressure to the developed low-level laser probe and its potential for clinical usefulness.

Keywords: Low-level Laser, Pressure probe, Photon density, Biological tissue

Photonic Therapeutics and Diagnostics V, edited by Nikiforos Kollias, Bernard Choi, Haishan Zeng, Reza S. Malek, BrianJet-Fei Wong, Justus F. R. Ilgner, Kenton W. Gregory, Guillermo J. Tearney, Laura Marcu, Henry Hirschberg, Steen J.

Madsen, Proc. of SPIE Vol. 7161, 71610R · © 2009 SPIE · CCC code: 1605-7422/09/$18 · doi: 10.1117/12.809924

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1. INTRODUCTION

Since laser was developed by Maiman in 1960s, it has been used in various fields such as industry, medicine,

information, military, and daily life. [1] In medical applications, laser has been used for therapy: In ophthalmology,

LASIK (Laser-Assisted in SItu Keratomileusis) and LASEK (Laser-Assisted Sub-Epithelial Keratectomy) which use

optical breakdown and vaporization. In dermatology, it has been used to remove tattoo, spot, wrinkle, hair and port-wine

etc., and in orthopedics, it is used for prostate treatment, bladder treatment, removing kidney stone and bloodless surgery

by its own, coagulation. Laser acupuncture which is used as an assistant tool of conventional acupuncture is recently

being performed. [2] The reason why laser has been used in various medical fields is that selective treatment is possible

by using specific lasers corresponding to the absorption wavelength of chromophore in biological tissue.

However, complex interactions between light and tissue decrease the efficiency of laser therapy. When tissue is

exposed to light, four basic interactions occur: transmittance, reflection, scattering, and absorption. Transmittance means

that light passes through tissue, reflections means returning of incident light by surfaces boundary. In case of skin, 4~7%

of light is reflected at the skin surface. [4] Scattering means redirection of light pathway due to heterogeneous tissue

structure and refractive index mismatch. Absorption means that photons are captured by chromophores of tissue and

transformed into heat energy, which is a main mechanism of laser therapy in medicine. By Grothus-Draper law, light

should be absorbed in tissue to result in therapeutic and diagnostic effect, [5] and light transmittance should be enhanced

for better light absorption in deep tissue layer. However, in low-level laser therapy, the laser emitted for specific

chromophore becomes scattered before it reaches to the target. The light scattering property of biological tissue limits

penetration depth of laser and consequently, photon density. Various methods have been studied to overcome the

limitation. We considered that optical clearing agent (OCA), glycerol, and compression are the most clinic friendly

methods to enhance photon density in soft tissue.

Even though understanding of the biological interaction mechanisms of OCA yet remains incomplete, the OCA might

seem to reduce light scattering property by managing scattering particle density and distribution, scattering particle shape

and size, and relative refractive index of scattering particles. It would be possible to use other optical clearing agents

simultaneously for clinical application, but ultimate optical clearing agent which can be adopted to every tissue is not

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found yet. [6] Other methodology to enhance the light transmittance, tissue compression, has been studied, but the way

to use it practically in clinic was not devised. [7] In this study, we developed a pressure controlled low-level laser probe

to enhance laser transmission and therefore, photon density in tissue. It enhanced the laser photon density in tissue by

applying pressure on tissue. We performed ex-vivo experiments with porcine skin samples and verified its clinical

potential.

2. MATERIALS AND METHODS

2.1 Experiments

(a) (b) (c)

Fig. 1. Pressure controlled low-level laser probe (a) and (b) has an air suction hole, an optical fiber supporter (brass), an

optical fiber, and space to place it. Sample fixation plate (c) is composed of a hole and screws. The plate has upper and

lower aluminum plate.

Fig. 2. Experimental setup to measure laser photon density in porcine skin sample. Setup is composed of three

parts: pressure pumping system, image acquisition, and diode laser parts.

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Current low-level laser therapy has been performed by just emitting it on skin surface. However, in this study, we

applied negative pressure to the laser probe to enhance laser photon density in biological tissue and to independently

sustain the laser probe on skin. The pressure controlled low-level laser probe shown in Fig. 1(a) and (b) was designed to

control both compression and laser factors and Fig. 1(b) shows how it is performed on porcine skin samples. The laser

probe has a hole to apply negative pressure, an optical fiber supporter equipped with an optical fiber, and an acrylic

probe guide. The pressures ranging from 0Kpa through 30Kpa at 5Kpa interval was applied on the probe and

simultaneously laser was emitted to the porcine skin. The frame for the ex-vivo experiments (Fig. 1(c)) was used to fix

the porcine skin samples while pressure was applied. Figure 2 shows experimental setup to obtain the diffusion images

of laser beam profile transmitted the skin sample. It is composed of three parts: pressure pumping system, image

acquisition, and diode laser parts. The pressure pumping system, which is composed of ATMEGA128, DC pump motors,

keypad, and LCD display. The diode part has a diode laser (808nm, 200mW), an optical fiber port (PAF-SMA-11-B), a

diode laser mount (LDM21) equipped with aspheric lenses (C570TM-B, A230TM-B), and a multi-mode optical fiber

patch cord (0.39NA, SMA type, FT1.5EMT). A CCD camera was used to acquire the diffusion images of laser beam

profile transmitted the skin samples. The identical experimental setup was also used to verify the effect of different laser

probe sizes of 20mm, 30mm, and 40mm in diameter. During the experiment, the compression was applied from low to

high level in order to minimize the variation of optical property of skin sample due to the compression. For

reproducibility of experimental results, five repeated trials were performed by using different five skin samples. The 251

×251 pixel window including laser beam profile was extracted from each image and analyzed with a laboratory built

MatLab program.

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3. RESULTS

3.1 Experiment results

Fig. 4. Peak intensity variations of laser beam profiles as a function of compression and the diameter of laser

probes.

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Fig. 5. 3D (left) and 2D (right) analysis images of laser beam profile depending on laser probe size of (a) 20mm,

(b) 30mm, (c) 40mm. At each image, upper one was measured at 0Kpa and lower one 30Kpa.

Figure 4 shows relative laser peak intensity variation in porcine skin samples due to the compression and the different

probe size. The x-axis means applied pressure and y-axis the increasing rate of laser peak intensity. Square, triangle, and

circular symbols represent the laser probe sizes of 20, 30, and 40mm, respectively. At five repeated trials, we found the

linear increasing rate of laser peak intensity as a function of compression and laser probe size. Figure 5 presents 2D and

3D images of laser beam profiles before (0Kpa) and after (30Kpa) pressure application to show laser photon density

variation due to compression. In the Fig. 5, (a), (b), and (c) present diffusion images of laser beam profiles obtained

using different laser probe size of 20, 30, and 40mm, respectively. At each image, upper images were obtained at 0Kpa

and lower images at 30Kpa.

In all three cases, laser was more centralized, resulting in enhancement of the laser photon density after pressure

application (lower ones at each image). The photon density was also increased when the laser probe size was increased.

Although the patterns of diffused laser beam profile were anisotropic because of morphological non-homogeneity of

biological tissues, the results show the enhancement of laser photon density. Laser peak intensity increased about 3.45

times in average: 2.89, 3.63, and 3.83 times at the laser probe sizes of 20mm, 30mm, and 40mm, respectively. FWHM

(full width at half maximum) decreases 1.81 ~ 2.61 times in average after maximum pressure (30Kpa) application, and

the larger probe size was, the higher decrease rate was: 1.81, 1.99, 2.61 times at the laser probe sizes of 20mm, 30mm,

and 40mm, respectively. 3D images in Fig. 5 representatively showed the results.

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4. CONCLUSIONS

Laser has been used in various fields, and, now, it became one of indispensable tools in medical therapy. Currently,

low-level laser therapy is being widely used and various therapeutic laser devices have been developed because of its

advantages of non-invasive property, selectivity of CW/Pulse mode, and selective wavelength. However, it is hard to

expect good therapeutic effect in deep tissue layer because of low transmittance and photon density of laser by light

scattering property of tissue. In this study, we developed a pressure controlled low-level laser probe to enhance laser

photon density and therefore, to potentially improve the effect of low-level laser therapy. In tissue morphology,

deformation of cells influences on scattering and thinning of epidermis, blood vessel dilation and depth. A mathematical

modeling show that the thicknesses of epidermis linearly decrease as pressure increased. [9] In this study, we did not

measure the positive pressure between skin and laser probe guide, and the change of sample thickness by negative

pressure. When pressure is applied on tissue, the space between cells decreases, the thickness of epidermis decreases, so

the optical path length also decreases, and finally, much more light can be delivered into tissue. In addition, moisture in

collagen fibrils is dehydrated, so combination of protein and mucopolysacharide increases, and then the cells in tissue

make environment like refractive index match, and finally, the transmittance increases. [7, 8] To control both laser

emission and pressure simultaneously, we confirmed airtight of the laser probe for applying negative pressure on tissue

through the laser probe and its independent sustain. Figure 4 shows quantitative analysis of laser peak intensity at

different probe sizes and at the pressure ranging from 0Kpa through 30Kpa. Rapid increase of laser peak intensity at

5Kpa might be due to the tissue deformation happened more rapidly than other sections. The other sections between

5Kpa and 30Kpa, in all laser probe sizes, the laser peak intensity linearly increased as pressure increased. Figure 4 also

shows that the larger laser probe was, the higher laser peak intensity was in a specific pressure value. It represents tissue

compression influence skin optical property in laser transmission. The both 2D and 3D images in Fig. 5 show that the

laser probe can centralize transmitted laser beam profile and therefore, increase laser photon density.

The result of the experiment through Fig. 4 and Fig. 5 consequently shows that the laser probe improves the laser

photon density in tissue, and can be adopted to existing low-level laser treatment equipments. The photon density in

tissue might be linearly changed in the pressure range of 5Kpa ~ 34Kpa. This study shows the clinical feasibility of

pressure controlled laser probe system. The laser probe can be built in various sizes for therapeutic purposes and applied

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body locations. Laser photon density reaching in deep tissue layer can be further controlled by changing diameter or

shape of optical fiber.

ACKNOWLEDGEMENT

This study was supported by a grant of the Next Generation New Technology Development Program, Ministry of

Knowledge Economy, Republic of Korea (10028424).

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[3] R. R. Anderson, J. A. Parrish, "Selective photothermolysis: precise micro-surgery by selective absorption of pulsed

radiation," Science 220, 524-527(1983).

[4] R. R. Anderson, J. A. Parrish, "The optics of human skin," J. Invest. Dermatol. 77(1), 13-19(1981).

[5] L. Carroll, T. R. Humphreys, "LASER-tissue interactions," Clin. Dermatol. 24, 2-7(2006).

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[9] Michael A. Childers, Walfre Franco, J. Stuart Nelson, and Guillermo Aguilar, “Laser Surgery of Port Wine Stains

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