Diagnostic photomedicine: probing biological tissues with polarized light
The polarization parameters of light scattered from biological tissue contains rich morphological and functional information of significant biomedical importance. Effectively probing optically-thick turbid media, such as tissues, is challenging due to multiple scattering and the simultaneous occurrence of many polarization effects. These include difficulties in obtaining accurate measurements as well as the extraction and correct interpretation of the constituent polarization parameters. A method that can account for the effects of multiple scatterings and decouple the contributions of polarization effects is needed for the practical application of these probing techniques in a biomedical setting.
Mueller calculus is a mathematical matrix method used to model the transformations of the polarization state of light in its interactions with an optical system. Therefore, a Mueller matrix description contains complete information about all the polarization properties of a given medium. However, when many optical polarization effects occur simultaneously (as is the case of tissues where the most common polarimetry effects are depolarization, linear birefringence, and optical activity), the resulting matrix elements reflect several ‘lumped’ effects, thus hindering their unique interpretation. Each of these, if separately extracted, holds promise as a useful biological metric. For example, chirality-induced optical rotation can be linked to the glucose concentration of the medium, and changes in tissue mechanical anisotropy (resulting from disease progression or treatment response) can be extracted by birefringence measurements. We have developed and validated an expanded Mueller matrix decomposition approach to delineate the individual intrinsic polarimetry characteristics in complex tissue-like turbid media.
Our method treats a ‘lumped’ system Mueller matrix
where a depolarizer matrix
In addition, we have also developed a polarization-sensitive forward Monte Carlo (MC) model capable of simulating all of the simultaneous optical scattering and polarization effects.3 This model is supplemented by a high-sensitivity polarization modulation/synchronous detection experimental system that is capable of measuring the complete Mueller matrix from tissues and tissue-like turbid media.2,3 These three methodologies form a comprehensive turbid polarimetry platform (Figure 1).
We experimentally validated our approach using optical phantoms that had controlled sample-polarizing properties.2 To develop the phantoms, we used polyacrylamide as a base medium with polystyrene microspheres to create turbidity, sucrose to induce optical activity, and mechanical stretching to cause linear birefringence. Additional validation tests were performed using MC-generated Mueller matrices. The derived polarization parameters were in excellent agreement with the controlled inputs, showing self-consistency in inverse decomposition analysis and successful decoupling of the individual polarization effects.2
The initial biomedical application of this approach was monitoring myocardial tissue regeneration following stem-cell therapy. The anisotropic organized nature of myocardial tissues stemming from their fibrous structure leads to linear birefringence. Upon suffering an infarction, a portion of the myocardium is deprived of oxygenated blood and subsequently cardiomyocytes die, replaced by disorganized isotropic scar tissue. Recently, stem-cell-based regenerative treatments for myocardial infarction have been shown to reverse these trends by increasing the muscular constituents and decreasing the scar tissue components.4 These remodeling processes likely affect tissue structural anisotropy, and the newly developed Mueller matrix decomposition method may offer a sensitive probe into the small birefringence alterations that are signatures of regenerative treatments.5 Using the experimental setup, polarized light measurements through 1mm thick ex vivo myocardial samples from Lewis rats after myocardial infarction, both with and without stem-cell treatments, yielded the measured Mueller matrices. These were analyzed using polar decomposition to obtain linear retardance (δ) values as shown in Figure 2. As expected, a large decrease in δ was seen in the infarcted region of the untreated myocardium (Figure 2a). An increase in δ towards native levels was seen in the infarcted region after stem-cell treatment (Figure 2b), indicating regrowth and reorganization of the myocardium. Statistically significant (p < 0.05) differences in derived retardance values were observed between normal and infracted regions and between infracted regions with and without stem-cell treatments (Figure 2c). These results show significant promise for polarized light monitoring of stem-cell-based treatments following myocardial infarction.
To conclude, a novel method for polarimetry analysis of turbid media based on polar Mueller matrix decomposition has been developed and validated. The ability to isolate individual polarization properties in complex turbid media (such as tissues) provides a valuable noninvasive tool for their characterization. In the biological domain, quantification of tissue structural anisotropy and concentration determination of optically active molecules, such as glucose, are two potential applications that have been initially explored, with early indications showing promise and warranting further studies. We are currently expanding our investigations in these biomedical directions, including extending this novel method towards clinical application.
Research support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and collaborations with Richard D.Weisel, Ren-Ke Li, Shu-hong Li, and Brian C.Wilson are gratefully acknowledged.
Alex Vitkin is an engineering physicist/biomedical engineer specializing in medical physics and the application of lasers in medicine. He is currently a professor of medical biophysics and radiation oncology at the University of Toronto, a senior scientist in the division of biophysics and bioimaging at the Ontario Cancer Institute, and a clinical medical physicist at Princess Margaret Hospital (all in Toronto, Ontario, Canada). His research is in the field of biophotonics, with particular emphasis on Doppler optical coherence tomography, tissue polarimetry, and optical-fiber sensors.
Nirmalya Ghosh is currently pursuing his postdoctoral research at the Ontario Cancer Institute, department of medical biophysics, University of Toronto. He also holds the position of scientist at the Raja Ramanna Centre for Advanced Technology, Indore, India. His research is in the field of biomedical optics with particular emphasis on tissue polarimetry.
Michael F. G. Wood is completing his PhD in the department of medical biophysics at the University of Toronto. His thesis research is focused on the use of polarized light for tissue characterization. He completed his undergraduate degree in computer engineering at the University of Western Ontario in 2004.
