Dynamic study of irradiated artificial skin using OCT

We report a novel application of optical coherence tomography (OCT), to monitor post-laser irradiation collagen injury in skin model. An artificial skin model (RAFT) which closely approximates human skin, was irradiated with a Perovskite laser (λ = 1341 nm). We investigated dynamic changes in a RAFT after laser irradiation through OCT and compared the results to those of histology. OCT images clearly delineated areas of post-irradiation collagen injury and allowed non-invasive monitoring of the wound healing process. Histology was correlated well with OCT images. OCT has advantages because it is non-invasive and allows serial monitoring at the same site over time. Our study showed that OCT may be a useful tool for determination of optimal parameters for non-ablative laser skin rejuvenation (NALSR) using different devices.


I. INTRODUCTION
Optical coherence tomography (OCT) is a non-invasive imaging technique capable of performing highresolution, two-dimensional cross-sectional imaging [1]- [3]. A number of features make OCT attractive for a broad range of applications. Most importantly, OCT is a powerful tool because it enables real-time, in situ visualization of tissue microstructure without the need to excise and process the specimen as in conventional biopsy and histopathology.
Consequently, OCT can be a useful method of dynamic measurement.
Direct visualization of tissue structure offers a unique opportunity for evaluation of tissue effects after laser irradiation. During the last decade, non-ablative laser skin rejuvenation has become a popular procedure for treatment of fine facial wrinkles and acne scars [4]- [7]. A variety of lasers and light source have been evaluated for their potential as NALSR devices [4], [7]. Variable treatment success has been achieved, but to date, an optimal method for NALSR has not been developed [4] effective devices for this clinical indication, it is imperative to visualize non-invasively and dynamically evaluate the tissue wound healing response after laser irradiation. Currently, biopsies and histologic evaluation are used to study the effects of laser irradiation. However, biopsy has difficulties including potential structural changes to the tissue during the excision and are unable to serially image the same site as a function of time [8], [9].
In the present study, we have used OCT to monitor wound healing response in an organotypic RAFT model of the skin. The RAFT tissue culture model of human skin [10], [1 1] was irradiated with a Perovskite laser (X -1341 nm) and the wound healing response followed over a 7-day period using OCT and conventional histopathology.

OCT Instrumentation
The schematic of the OCT system is presented in Fig. 2. The time delay of light backscattered from skin model was measured by a fiber based Michelson interferometer. Light was coupled into the interferometer and split into two paths. One beam was directed toward the model skin and the other to a reference mirror. The OCT system used in this study employed a broadband light source that delivered an output power of 10 mW at a central wavelength of 1310 nm with a bandwidth of 70 nm. A visible aiming beam (633 nm) was used to find and locate the exact imaging position on the sample. In the reference arm, a rapid-scanning optical delay line was used that employs a grating to control the phase and group delays separately so that no phase modulation is generated when the group delay was scanned. The phase modulation was generated through an electro-optic phase modulator that produces a carrier frequency. The axial line scanning rate was 400 Hz, and the modulation frequency of the phase modulator was 500 kHz. Reflected beams from the two arms are recombined in the interferometer and detected on a photodetector. The detected optical interference fringe intensity signals were bandpass filtered at the carrier frequency. Resultant signals were then digitized with an analog-digital converter and transferred to a computer where the structural image was generated. The lateral and axial resolutions of the reconstructed image were 10 and 15 tim, respectively.

Broadband
Source RSOD

Dynamic Measurement
For evaluation of the wound healing process, 8 RAFT specimens were used. Each model was irradiated under identical conditions. One model was imaged daily by OCT over a 7-day period. The imaged area was marked with 100 tm beads so that the exact same position could be relocated every day. Over the 7-day study period, one OCT image of acutely irradiated sample was showed in Fig. 3. On laser irradiated sample, a layer of intact epidermis is noted at the top of the specimens. An area of a dark region demarks the laser-irradiated area is clearly observed. Since OCT measures backscattering light intensity, the dark region, which corresponds to a reduced OCT signals, indicate that backscattering coefficient is reduced in the laser-irradiated collagen. Collagen in the native state forms super helic bundle and has a large spatial variation of density in the microscopic level. The large spatial variation of density results in a large inhomogeneity of the optical refractive index, which results in a relative large scattering coefficient. Our results suggest that thermal energy deposited by laser irradiation alters collagen structure. The photocoagulated and denatured collagen has a more uniform density distribution, which results in a reduced scattering coefficient.

CONCLUSIONS
We have used OCT to monitor wound healing process in an organotypic model of the skin. Our results indicate that OCT can noninvasively monitor changes in collagen structure and thermal damage. This suggests that OCT has potential to be a powerful method for characterization of collagen injury post-laser irradiation and may be a useful tool for evaluation and comparison ofNALSR devices.