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Polarized Light for Tissue Characterization

June 23, 2011

Michael Wood

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E E

E E

Linear Circular

Elliptical Unpolarized

Polarized Light

• Electromagnetic wave

• Fields have orientation

• Orientation of electric field is polarization

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Polarized Light in Tissue

Scatteringcells, organelles, nucleus Linear Birefringence

structural proteins (collagen, elastin)

ne

no

δ - Retardance

Optical activity(Circular Birefringence)

proteins, glucose

Δ - Depolarization

α - Optical Rotation

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Myocardial Regeneration

Control Stem Cell

Retardance Imaging• Retardance images, decreased retardance in infarct

• Histology: muscle in red, collagen in blue (Masson’s trichrome)

• Healthy muscle, increased collagen in scar

• Higher retardance in stem cell treated infarct (increasedorganization)

Wood et al, J. Biomedical Optics (2010).

rad

rad

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Polarized Light Mathematics

êx

êy

Ey

Ex

Beam intensity

0° and 90°

±45°

Circular (right/left)

Polarization expressed by Stokes vector

Polarization effects applied by Mueller matrix

incident polarization

output polarization

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Modeling Polarized Light in Tissue

Statistical Method• Sample known probability distributions for interactions

Scattering

Reflections

Absorption

IncidentLight

• Track many photon paths (>108)

• Alter polarization (apply Mueller matrix)

• Record statistics

• Calculate average properties

• Gives macroscopic properties

Calculate macroscopic propertiesfrom many individual interactions

Combining polarization effects in model(Mueller matrices non-commutative AB ≠ BA)

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Mueller Matrix Decomposition

But Mueller matrix contains all effects

M

MΔ MR MD

Measured Matrix

Decomposed Matrices

Decomposed MatricesMΔ: Depolarization Matrix (scattering)

MR: Retardance Matrix (optical activity and birefringence)

MD: Diattenuation Matrix (dichroic absorption)

Ghosh, Wood, and Vitkin, J. Biomedical Optics (2008).

Polar Decomposition

We want to measure polarization effects of tissue(depolarization, birefringence,optical activity, etc)

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Acknowledgments

• Supervisor: Dr. Alex Vitkin

• Polarimetry Group: Dr. Xinxin Guo, Dr. Nirmalya Ghosh, Marika Wallenburg,Shannon Dang, Nasit Vurgun, Manzoor Ahmad, and Sanaz Alali

• Collaborators: Dr. Ren-Ki Li, Shu-Hong Li, and Dr. Dick Weisel (McEwen Centre forRegenerative Medicine)

• Vitkin Lab and Wilson Group