Diffuse optical tomography

Diffuse Optical Tomography (DOT)Modeling and Reconstruction

M. Rajendra Kumar

School of Medical Science and Technology,

Indian Institute of Technology Kharagpur.

Motivation

• If you shine a flashlight

onto your hand, you can

clearly see that light can

travel through a few

centimeters of tissue and

still be detected.

• Can this light be used to

“see,” or image, inside the

body?

12/3/2012 DOT Modeling and Reconstruction - Rajendra 2

Source: http://www.comhs.org

How photons interact with biological

tissues?


Outline

• Absorption & Scattering in tissues

• Light diffusion in a biological tissue

• Photon transport model

• Single source DOT system

• Numerical modeling of the forward problem

• Introduction to inverse solutions

• Application: Brain imaging

• Summary


Absorption in Tissues


Scattering is caused by Tissue ultrastructure


Source: (http://omlc.ogi.edu)

µa : Tissue absorption coefficient

µs’ : Tissue scattering coefficient

BF: Blood Flow

Absorbers: Hemoglobin, Water, Lipids (µa)

Scatterers: Organelles, Mitochondria (µs’), Moving Blood Cells (BF)

Tissue Optical Properties:

r

Light diffuses in biological tissue

A presentation on “Near-infrared Diffuse Optical Measurement of Tissue Blood Flow, Oxygenation and Metabolism” by Guoqiang

Yu,Bio-photonics lab, University of Kentucky.

12/3/2012 7DOT Modeling and Reconstruction - Rajendra

Photon transport in multiple scattering

media like human tissue

• Can be described as a diffusive process and can be modeled through diffusion equation (DE).

• The DE in its simplest form is given as:

photon flux through r

photon diffusion coefficient given by:

absorption coefficient

reduced scattering coefficient


' 1( ) {3[ ( ) ( )]}a sk r r r

0( ) ( ) ( ) ( )a r k r r q r

Intensity of the source

Major Assumptions for Diffuse

Approximation

• Regard photons as particles.

• Neglect interference and

polarization properties.

• Highly turbid media

(scattering >> absorption).


Single source DOT system

• illuminate the phantom with a NIR laser source, modulated by 100 MHz sinusoidal signal.

• The intensity and phase measurements are taken on the detectors placed on the opposite side of the phantom facing the probing light source.

• For each of the source positions, detector measurements are carried out at 13 locations, equiangle spaced around the point diagonally opposite the source.


Single source illumination of the tissue with a detector

located on the opposite side of the object. S is the source.

D-n -Dn are the detector positions[2].

Single source DOT system• The source is rotated by an

angle of 15 and the intensity and phase measurements are repeated for all 13 detector positions from −90to 90 positions.

• This is repeated for various source locations totally 24 positions that span the object around the phantom. The modulated source is given by:


0( , ) cos( ).dc acq r t A A t Single source illumination of the tissue with a detector

located on the opposite side of the object. S is the source.

D-n -Dn are the detector positions[2].

Numerical modeling of the DE

Forward Problem

• To study the propagation of light in diffuse

tissue and to be able to solve inverse

parameters.

• Forward problems is predicting fluence at the

detectors given a geometric model of:


1. Optical parameters

2. Background parameters, and

3. Source and detector locations and functionality.

Numerical modeling of the DE

Forward Problem

• Direct approaches

• Standard methods for numerical approximation of Partial Differential Equations.


•Analytical solutions applied only to restricted geometries

•Monte Carlo simulations treats photons as distinct particles

with certain probability of scattering in a discrete geometry.

•Finite Element & Finite Difference method

Introduction to Inverse Solutions

• The usual goal of DOT imaging is to reconstruct a

spatial map of the optical scattering coefficient,

absorption coefficient, or both, from fluence

measurements, using a forward model of the photon

propagation.

• From these maps other biological characteristics,

such as a map of blood volume or oxygen

concentration, can be derived.


Application: Brain Imaging

• http://www.martinos.org/martinos/rese

arch/MultimediaGallery/DOT_materials

/dot.html

Source :

Athinoula A. Martinos Center for Biomedical Imaging

Charlestown, Massachusetts, USA.


http://www.martinos.org/martinos/research/MultimediaGallery/DOT_materials/dot.html



Breast (Detecting tumors)

Brain (Stroke, Trauma, Therapy Monitoring)

DOT Technologies:

• Noninvasive

• Inexpensive

• Portable

• Microvasculature

• Limited penetration

• Low spatial resolution

Summary: Translation to Clinic

A presentation on “Near-infrared Diffuse Optical Measurement of Tissue Blood Flow, Oxygenation and Metabolism” by Guoqiang Yu,

Bio-photonics lab, University of Kentucky.


References[1] D. A. Boas, D. H. Brooks, E. L. Miller, C. A. DiMarzio, M. Kilmer, R. J. Gaudette, and Q. Zhang “Diffuse Optical Tomography”, IEEE Signal Processing Magazine, vol.18, no. 6, pp. 57-75, Nov. 2001.

[2] Samir Kumar Biswas, K Rajan, and R. M. Vasu “Diffuse optical tomographic imager using a single light source”, Journal of Applied Physics, vol. 105, no.2, Jan. 2009.

[3] Simon R Arridge and John C Schotland “Optical Tomography:forward and inverse solutions”, IOPscience, vol.25, no.12, Dec. 2009

[4] V Vijayakumar and P K Dutta “Instrumentation of phased array system for detecting breast cancer using diffuse optical tomography”, M.Tech Theses , Dept. Elect. Eng., IIT Kharagpur, Kharagpur, WB, 2007.


Acknowledgements

• Prof. Pranab K. Dutta

• Rusha Patra

• Debdoot Sheet


Education

Diffuse optical tomography