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LOW-COHERENCE OPTICAL PROBE FOR NON-CONTACT DETECTION OF PHOTOTHERMAL AND PHOTOACOUSTIC PHENOMENA IN BIOMATERIALS Sergey A. Telenkov, Digant P. Dave and Thomas E. Milner Department of Biomedical Engineering, University of Texas at Austin, Austin, TX 78712 ABSTRACT. Laser-induced mechanical deformations in materials can be detected without contact using various optical methods. We have developed a low-coherence optical probe to monitor thermal elastic deformations and acoustic transients in materials exposed to laser excitation. Our approach utilizes principles from low-coherence interferometry with phase-sensitive detection of the coherence function of backscattered light from two spatially separated sites in a test material. High spatial resolution and sensitivity of the optical probe may be used to identify subsurface lightabsorbing structures in turbid media and determine optical properties non-invasively. The lowcoherence optical sensor may prove useful for non-contact studies of tissue-like materials in biomedical engineering. INTRODUCTION Optical low-coherence tomography (OCT) is frequently utilized for non-contact imaging of subsurface structures in biological materials [1]. This technique has proven valuable as a non-invasive imaging modality for studies of microstructures in biological tissue where strong scattering of incident light poses significant difficulties for traditional optical methods. A typical OCT setup utilizes a broadband light source and a Michelson interferometer with variable path length in the reference arm as well as an apparatus for lateral scanning to produce cross-sectional images of test specimens. The short coherence length of the broadband light source (typically less than 20 jam) provides high depth resolution in recorded images. Specific implementation of the OCT setup allows one to detect variations of refractive index [2], birefringence [3] or Doppler shift of scattered light due to flowing blood [4]. Objective of the present study is to demonstrate that low-coherence interferometry can be applied not only to visualize subsurface microstructure but also observe photothermal and photoacoustic phenomena in tissue exposed to laser radiation. Inasmuch as laser sources are often utilized in clinical applications, non-invasive monitoring of laser-tissue interaction and resulting effects of laser exposure has important practical implications. Localized temperature increase is the most evident effect of laser absorption and often used in clinical applications to alter tissue properties non-invasively [5]. Thermoelastic deformation is a response of biological tissue to localized temperature increase, which results in thermal expansion of the heated area. Since laser heating can occur very fast, such localized photothermal deformations produce acoustic waves propagating throughout the specimen. Detection of acoustic transients induced by laser CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti © 2003 American Institute of Physics 0-7354-0117-9/03/S20.00 852 ,SLD Source LiNbO3 EOM Reference Path T I* -<-> Sample Path L n i • L< Polarization Channels^ ^ Sample FIGURE 1. Experimental apparatus for non-contact detection of photothermal and photoacoustic phenomena in tissue using a low-coherence dual-channel optical probe. absorption may also be useful for analysis of laser-tissue interaction. Specific optical properties of biological tissue including strong scattering and rough surfaces pose significant difficulties for application of conventional optical methods for detection of photothermal and photoacoustic phenomena. We believe that low-coherence interferometry may provide an attractive alternative to traditional optical and contact detection techniques providing truly non-invasive visualization of photothermal and photoelastic phenomena in tissue. EXPERIMENTAL APPARATUS Our low-coherence optical probe utilizes a polarization-maintaining (PM) fiberbased dual-channel Michelson interferometer capable of differential phase measurement from two spatially separated probe beams backscattered from the sample (Figure 1). A broadband light source (SLD) with the central wavelength X0 — 1.3 Jim and coherence length of 14 jim is coupled into a PM fiber-optic interferometer and output of the sample arm is focused on the test material. A Wollaston (W) prism in the sample arm allows one to separate two polarization channels in space and a galvanometer mirror (G) provides lateral scanning of both beams parallel to the sample surface. This dual-channel differential configuration significantly improves interferometer stability due to effective cancellation of common-mode low frequency environmental vibrations. Additionally, differential phase of the interference fringes in two coherence functions can be used to determine relative displacement in a light-scattering media caused by thermoelastic deformation following laser excitation. In our experiments, two different Wollaston prisms were used in the sample arm to provide beam separations of 4.5 and 0.5 mm. The relatively large separation distance (4.5 mm) allows identification of a propagating acoustic wavefront by detecting relative displacement between two channels. However, increased beam separation results in increased noise level due to non-synchronous variations of optical pathlength between the two channels. 853 FIGURE 2. Interference fringe intensity signal in one channel of the interferometer at modulation frequency of 50 kHz. a) Signal from air-sample interface without optical excitation, b) Interference fringe intensity signal in presence of surface acoustic wave. The reference arm of the Michelson interferometer includes a rapid delay line for depth scanning and a LiNbOs electro-optic phase modulator to establish an interference fringe earner frequency. In experiments reported here, the scanning delay line is used to identify a sample-air interface or a subsurface scattering layer while the LiNbO3 electrooptic phase modulator introduces ± n phase modulation at a 50 kHz interference fringe carrier frequency. The interference fringe intensity signal in one channel recorded from the air-sample interface without laser excitation is shown in Figure 2a. The nearly harmonic waveform becomes significantly distorted (Figure 2b) when an acoustic wave travels across the probing point. Two effects are evident: 1) the Doppler frequency shift of the interference fringe signal is proportional to the surface displacement velocity; and 2) the amplitude modulation due to optical pathlength change of the coherence envelope. Although our technique is similar to heterodyne interferometry described in the literature [6], our system utilizes a low-coherence optical source to identify specific scattering objects and heterogeneous samples such as biological tissue may be investigated. Figure 2b shows that phase of the interference fringe intensity contains information about surface acoustic waves and appropriate phase demodulation processing is required to extract the acoustic waveform. We use a lock-in type synchronous processing algorithm to compute phase as a function of time in each channel and subtract channels from another to determine differential phase A9(t). Relative displacement in two channels is determined using the computed differential phase (A0): Ah = (V4rc)-A0 (1) where XQ = 1.3 jim is central wavelength of the broadband source. We demonstrate two applications of the low-coherence optical probe: 1) detection of photoacoustic transients excited by absorption of pulsed laser radiation; and 2) detection of thermal deformations produced by absorption of periodically modulated laser radiation. Homogeneous and multilayered agarose-based phantoms are used in the experiments to simulate optical properties of tissue. Additionally, to simulate subsurface chromophores in tissue we use a dye-stained gel (Brilliant Blue R dye, Sigma-Aldrich 854 Corp.) with peak absorption at X=585 nm. The multilayered gel samples are excited by a flash-lamp pumped pulsed dye laser (FLPDL) (Candela Corp.) with pulse duration T=500 |is while for homogeneous clear gels a Ho:YAG laser with pulse duration T=300 (is is used. To study periodic laser heating and resulting photothermal deformations we excite a colored polymer sample of polymethyl methacrylate (PMMA) by a modulated CW radiation at A,=532 nm. The laser modulation frequency of 10 Hz is used to induce material heating while probing beams being scanned over the heating area with frequency of 1 Hz. RESULTS AND DISCUSSION Surface acoustic waves (SAW) in gel phantoms can be generated by absorption of pulsed laser radiation in the test sample at a wavelength within the material absorption band. High water content of the gel samples allows use of a Ho:YAG laser at wavelength ^k-2.1 (im (water absorption coefficient a = 5.9 mm"^?]). Photoexcitation relaxation results in a localized temperature increase and the resulting thermoelastic expansion creates an inhomogeneous deformation that propagates away from the excitation region. The surface component of the acoustic field propagates along the air-sample interface and can be detected as a surface displacement with characteristic time consistent with duration of laser excitation. In order to detect the surface acoustic wave, backscattered light from the air-gel interface is coupled into the interferometer and the LiNbOa electrooptic phase modulator generates interference fringes in both polarization channels. Two probe beams were positioned 4.5 mm apart using a 20° Wollaston prism and 5 mm away from the laser excitation spot (Figure 3a). In this setup, lateral scanning is disabled to probe the sample surface at a fixed position. Following pulsed laser excitation, the interference fringe intensity signal is recorded and phase of the signal in both channels is computed. Relative surface displacement is determined from the computed differential phase (A(|)) and equation (1). A typical phase-sensitive measurement is shown in Figure 3b. Arrows in Figure 3b indicate arrival of the SAW at the probing beams. One can see , = 2.1 jim T = 300 us WL L Gel FIGURE 3. Detection of surface acoustic waves in a gel specimen generated by a pulsed Ho:YAG laser, a) Excitation schematics; b) Surface displacement computed from differential phase measurements (arrows indicate arrival of acoustic front at the probing beams). 855 characteristic inversion of the profile when the surface acoustic wave reaches the second probe beam. The time delay required for the SAW to travel 4.5 mm (beam separation distance) gives a straightforward method to estimate SAW phase velocity. In an agarose gel specimen, SAW velocity is 40.9 cm/s. These experiments demonstrate that our lowcoherence optical probe may be used to identify a scattering surface and detect acoustic transients excited by absorption of pulsed laser radiation. Acoustic waves generated due to photoelastic effect in subsurface chromophores can be detected from surface displacement measurements. For these experiments we utilized a layered gel specimen with a subsurface layer stained by Brilliant Blue R dye, positioned 0.8 mm below the air-gel interface and 1 mm thick (Figure 4a). Photoexcitation of the specimen is carried out using a flash-lamp pumped pulsed dye laser at the wavelength of 585 nm which corresponds to an absorption peak of the staining dye. This phantom simulates subsurface blood vessels heated by pulsed laser radiation. The interferometer probe beam is backscattered from the air-gel interface and coupled back into the interferometer to produce an interference fringe signal. The surface displacement due to an acoustic wave excited in the subsurface layer is detected in one polarization channel (Figure 4b). The first wavefront is followed by gradually decreasing oscillations due to multiple reflections of the acoustic wave in the superficial layer. Periodically modulated laser radiation can be used to produce a temperature increase, which is often referred to as a thermal wave [8]. The oscillating temperature field is characterized by an exponentially decreasing amplitude with characteristic thermal diffusion length LD=(D/7ifm)1/2, where D is thermal diffusivity of a specimen and f m is the modulation frequency of the laser radiation. In materials with low thermal diffusivity or at high fm, the thermal wave is confined to the excitation region while at greater distances only a DC component of the temperature field is present. We demonstrate that laser-induced stationary photothermal deformations can be detected by our low-coherence optical probe with lateral scanning. A small angle Wollaston prism (2°) spatially separates orthogonal polarization channels (separation distance: d = 500 |im) and galvanometer mirror (G) provides lateral scanning at the frequency f s = IHz (Figure 5a). In our experiments we use light-absorbing polymethyl methacrylate Ch1 A, = 585 nm = 500 us FIGURE 4. Detection of photoexcited acoustic waves in a subsurface chromophore layer, a) Excitation and detection setup; b) Surface displacement due to acoustic wave detected by one polarization channel. 856 = 532nm f m =10Hz FIGURE 5. Photothermal deformation of the PMMA surface measured by our low-coherence optical probe, a) Excitation and detection setup; b) Surface displacement [h(x)] probed across the excitation beam. (PMMA) samples and CW radiation of the second harmonic of a Nd:YAG laser (A, = 532 nm). Incident radiation is modulated at f m = 10 Hz and focused by a cylindrical lens (CL) on the surface into a line shape 1mm wide and 10 mm long. The laser-induced thermal "bump" on the surface is measured by scanning interferometer probe beams across the line of laser excitation. When beam separation distance d is much less than size of the "bump" then differential phase (Ac))) is proportional to derivative of the surface profile h(x), i.e. A6 = [4rc/A,](dh/dx)-d (2) Measuring differential phase and computing the integral of A6 allows reconstruction of the surface profile [h(x)]. Figure 5b shows the surface profile obtained from differential phase measurements at incident laser power P = 40 mW. Our data indicate that lateral extent of the laser-induced surface deformation in PMMA samples significantly exceeds the area of optical excitation. CONCLUSION We have demonstrated application of a phase-sensitive low-coherence optical probe for non-contact detection of laser-induced photothermal and photoelastic deformations in materials simulating physical properties of biological tissue. This technique can be used to identify light scattering structures in test specimens to measure photothermal deformations and propagating acoustic waves induced by absorption of laser radiation. Additional experiments are required to fully realize the quantitative imaging capabilities of our system. Such experiments can provide depth information to visualize photothermal deformations in subsurface layers of tissue. Measurements of subsurface deformations may be used to determine laser-induced temperature increase in tissue as well as to determine thermal and elastic properties of tissue constituents. 857 ACKNOWLEDGEMENTS This work was supported by a grant from the National Institutes of Health, the National Center for Research Resources (RR 14069). REFERENCES 1.. 2. 3. 4. 5. 6. 7. 8. M. E. Brezinski and J. G. Fujimoto, IEEE J. Selct. Topics in Quant Elect. 5, 1185 (1999). D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. 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