Investigators: Young Kim, Yang Liu, Vladimir Turzhitsky, Dingxin Wang and Hariharan Subramanian

Although the phenomenon of enhanced backscattering (EBS), otherwise known as coherent backscattering (CBS), in non-biological media has generated substantial research interest, observing EBS in biological tissue has been extremely difficult. Thus, this phenomenon has awaited its applications in tissue optics over the last two decades.  CBS is a self-interference effect in multiple light-scattering that gives rise to an enhanced backscattering of light by random media.  Recently we have reported a new phenomenon, low-coherence enhanced backscattering (LEBS), and have introduced LEBS spectroscopy as a new tool for probing the structure of random media.  Specifically, the combination of: 1) low spatial coherence, broadband illumination and 2) low temporal coherence, spectrally-resolved detection overcomes all major impediments that had prevented the widespread application of CBS in tissue optics.  

Low-coherence effect facilitates CBS measurements in biological tissue and other media with long transport mean free pathlengths, which had been previously beyond the reach of conventional EBS investigations.  In particular, low spatial coherence illumination results in an anomalous broadening of a CBS peak thus rendering the peak easily detectable.  Observation of conventional ultra-sharp CBS peaks in tissue has been extremely difficult.  Moreover, the combination of low spatial coherence illumination and low temporal coherence detection dramatically suppresses speckle and, thus, enables observation of CBS without being obscured by speckle.  Furthermore, LEBS allows depth-selective assessment of tissue.  The depth resolution can be achieved by several complementary means.  The penetration depth can be controlled by varying the spatial coherence length of illumination or by sampling CBS signals at different angles within a single CBS peak.  The depth resolution is achieved by exploiting the nature of the LEBS peak that contains information about a wide range of tissue depths.  This presents an easy and inexpensive means to selectively probe living biological tissues and other random media at desired depths. 

LEBS spectroscopy has the potential for identifying the location of the origin of precancerous transformations in the colon at an early, previously undetectable stage. The unprecedented sensitivity of EBS signatures supports the potential of LEBS spectroscopy both for initial-detection techniques  and for chemoprevention. In colon cancer, unfortunately, the most widely used risk-stratification techniques such as fecal occult blood test and flexible sigmoidoscopy are plagued by low sensitivity.  Thus, the LEBS signatures, which are sensitive, non-expansive, and minimally invasive without the need for complete colonoscopy, would be of tremendous clinical benefit.  In addition, the ability to detect the earliest changes associated with the actions of chemopreventive agents is crucial for the development of effective anticancer strategies.  Although a variety of agents have demonstrated chemopreventive efficacy in experimental studies, clinical studies remain difficult, expensive, and time-consuming.  The EBS signatures would quantitatively assess the efficacy of a chemopreventive strategy early in the course of the therapy, which is of great benefit to patients undergoing the therapy, pharmaceutical companies developing or evaluating the agent, and biomedical researchers investigating the mechanisms of carcinogenesis and chemoprevention.  Moreover, the development of an endoscopically compatible probe will facilitate those applications with real-time optical alteration assessments.  Therefore, LEBS spectroscopy may find potential applications in tissue diagnosis among other currently used optical spectroscopic techniques such as diffuse reflectance spectroscopy, light scattering spectroscopy, fluorescence spectroscopy, and spectroscopic optical coherence tomography. 

 

Figure 1.  Normalized angular distributions of backscattering from a rat colon tissue. (a) Experimental data, low spatial coherence illumination (xenon lamp). (b) Experimental data, coherent illumination (He-Ne laser). A CBS  peak is masked by the speckle. (c) Simulation using the conventional diffusion-approximation-based CBS theory .

Figure 2. (a) LCBS intensity obtained from a rat colon tissue under low spatial coherence illumination as a function of wavelength.  LCBS exhibits unique features not present in conventional CBS recorded using coherent light sources.  LCBS intensity is speckle free and dramatically broader than a conventional CBS peak. (b) Radial intensity distribution P(g, l) obtained from the inverse Fourier transform of I_CBS(q, l). 

 

Figure 3. (a) Penetration depth L_p(q) as a function of q.  (b) L_p(r) as a function of r.  L_p(q) decreases with q.  L_p(r) increases with r.

Publications

1. Y.L. Kim, Y. Liu, V. M. Turzhitsky, H.K. Roy, R.K. Wali, and V. Backman. "Coherent backscattering spectroscopy," Optics Letters, 29(16), 1906-1908, 2004.

2. Y.L. Kim, Y. Liu, H.K. Roy, R.K. Wali, and V. Backman. "Low-coherent backscattering spectroscopy for tissue characterization," Applied Optics, in press, 2005.

3.  Y.L. Kim, Y. Liu, V. M. Turzhitsky, R.K. Wali, H.K. Roy, and V. Backman. "Depth-resolved low-coherent backscattering in tissue," Optics Letters, in press, 2005.