Biomedical Optics Theory

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Alvin Higgins
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1 Introduction Biomedical Optics Theory Diffuse reflectance spectroscopy (DRS) and Laser Doppler Flowmetry (LDF) are booth optical techniques that can quantify a number of microcirculatory parameters. Prof Tomas Strömberg will present how these techniques can be used. But what is the principle behind these techniques? Page 2 Presentation outline Optical properties of tissue Beer Lambert Law Absorption Scattering Quasi elastic scattering Photon propagation Monte Carlo Simulation Fundamental Principle of Optical Modalities DRS Diffuse Reflectance Spectroscopy LDF Laser Doppler Flowmetry Beer Lambert Law Why does the amount of light decrease when passing thru tissue? Page 3 Page 4

Page 2 Presentation outline Optical properties of tissue Beer Lambert Law Absorption Scattering Quasi elastic scattering Photon propagation Monte Carlo Simulation

2 Beer Lambert Law Why does the amount of light decrease when passing thru tissue? Beer Lambert Law Why does the amount of light decrease when passing thru tissue? Page 5 Page 6 Beer Lambert Law Beer Lambert law: 0 describes the amount of light that passes through a tissue with: Absorption coefficient μ [1/mm] Thickness [mm] The absorption coefficient μ describes the probability of being absorbed. 1/µ a describes the mean free path length before absorption: Intensity Beer-Lambert law - exp(-µ a z) 1 μ µ a z Page 7 Page 8

coefficient μ [1/mm] Thickness [mm] The absorption coefficient μ describes the probability of being absorbed.

3 Skin chromophores: Other chromophores: Page 9 Page 10 Absorbed photons?: Heat. Fluorescence & Phosphorescence. Chemical reactions. Absorption phenomenon is used in: Laser surgery/therapy. Photodynamic therapy. Diffuse reflectance spectroscopy. Opto acoustic methods. Absorbed photons?: Heat. Fluorescence & Phosphorescence. Chemical reactions. Absorption phenomenon is used in: Laser surgery/therapy. Photodynamic therapy. Diffuse reflectance spectroscopy. Opto acoustic methods. Can be measured with DRS µ a Concentration of chromophore µ a Concentration of chromophore Page 11 Page 12

Diffuse reflectance spectroscopy. Opto acoustic methods. Absorbed photons?: Heat. Fluorescence & Phosphorescence. Chemical reactions.

4 Scattering Scattering What else affects photon propagation in tissue? What else affects photon propagation in tissue? Scattering Page 13 Page 14 Scattering Coefficient µ s Scattering anisotropy g The average number of scattering occassions per mm is desribed by the scattering coefficient μ : The scattering angle is described by the anisotropy factor: cos μ Page 15 Page 16

Scattering Page 13 Page 14 Scattering Coefficient µ s Scattering anisotropy g The average

5 Scattering Scattering is caused by small objects with a refractive index mismatch, for example: Cellular membranes Mitochondria Ribosomes Fat globules Simulation of photon propagation Is it possible to predict where a photon will end up if the absorption and scattering is known? Scattering depends on: Concentration of scattering particles Particle size Refractive index mismatch Page 17 Page 18 Simulation of photon propagation Is it possible to predict where a photon will end up if the absorption and scattering is known? With simulation techniques it is possible to determine the most likely position. Simulation methods: Diffusion Approximation - Deterministic - Fast, but not always accurate Monte Carlo Simulation - Stochastic - Slow, but in general accurate Monte Carlo Simulation Based on random numbers Large number of photons statistically reliable results. Typically at least 1E6 simulated photons. Complex simulations can take hours. Page 19 Page 20

$Scattering depends on: Concentration of scattering particles Particle size Refractive index mismatch Page 17 Page 18 Simulation of photon propagation Is it possible With simulation techniques it is$

6 Monte Carlo simulation example High absorption Joint effect of absorption and scattering Page 22 Page 23 Why simulating photon transport? Predict the amount of destructed tissue in laser surgery/therapy. Why simulating photon transport? Theoretical evaluation of optical techniques. Does it work on all types of tissues? What does the optimal probe configuration look like? How large is the sampling volume? Page 24 Page 25

Why simulating photon transport? Theoretical evaluation of optical techniques.

7 Why simulating photon transport? Estimating the in vivo tissue optical properties by solving the inverse problem. Estimate blood amount and saturation. Cancer tumour detection. Quantifying drug delivery. Diffuse Reflectance Spectroscopy White light spectroscopy measures the amount of backscattered light as a function of wavelength. Probe based or imaging systems. Page 26 Page 27 Diffuse Reflectance Spectroscopy Can estimate the amount of tissue chromophores : Blood amount Blood saturation Inverse of the Beer Lambert Law: Laser Doppler Flowmetry Laser Doppler flowmetry can indirectly estimate the tissue perfusion by analysing intensity fluctuations induced by quasi elastics Doppler effects. quasi elastics Doppler effects? I 0 ln I μ μ conc chromophore But z depends on scattering Page 28 Page 29

Page 26 Page 27 Diffuse Reflectance Spectroscopy Can estimate the amount of tissue chromophores : Blood amount Blood saturation Inverse of the Beer Lambert Law: Laser Doppler Flowmetry Laser Doppler

8 Laser Doppler Flowmetry Laser Doppler flowmetry can indirectly estimate the tissue perfusion by analysing intensity fluctuations induced by quasi elastics Doppler effects. quasi elastics Doppler effects? Laser Doppler Flowmetry Laser Doppler flowmetry can indirectly estimate the tissue perfusion by analysing intensity fluctuations induced by quasi elastics Doppler effects. quasi elastics Doppler effects? Page 30 Page 31 Doppler shift and speckle patterns Detected photo current Page 32 Page 33

quasi elastics Doppler effects? Page 30 Page 31 Doppler shift and speckle patterns Detected photo current Page 32 Page 33

9 Concentration of Moving red Blood Cells CMBC LDF tissue Perfusion perf Page 34 Page 35 Summary Photon propagation depends on tissue scattering and absorption. Photon propagation can be simulated with Monte Carlo technique. Diffuse Reflectance Spectroscopy can measure Amount of blood Blood saturation Other chromophores Laser Doppler Flowmetry can measure: Tissue perfusion Amount of moving red blood cells Page 36

Photon propagation can be simulated with Monte Carlo technique.

Permanent City Research Online URL: http://openaccess.city.ac.uk/13270/

Permanent City Research Online URL: http://openaccess.city.ac.uk/13270/ Chatterjee, S., Phillips, J. P. & Kyriacou, P. A. (2015). Differential pathlength factor estimation for brain-like tissue from a single-layer Monte Carlo model. 37th Annual International Conference of