1. Numerical Simulations of Laser-Tissue Interactions
Shannon M. Mandel
Sophomore
Intense Laser Physics Theory Unit
Illinois State University
Supervisor : Dr. H. Wanare

2. Examples of diffusive random media
Biological Tissue
Diagnostics of cancerous tissue
Radiation therapy
Water and Air
Atmospheric studies and oceanography
Communications
Remote sensing
Pollution studies
Earth
Geological studies
Propagation of pressure waves
Electromagnetic & acoustic probing

3. Our Interest
How does light interact with a diffusive random medium like a tissue?
Tumors are hidden inside the tissue
tumor

4. Properties of Random media
Index of refraction n(r) characterizes any medium
Homogeneous media Inhomogeneous media
Continuous n(r) Discontinuous n(r)

5. Both phenomena lead to attenuation in tissues
High Scattering versus High Absorption
Both phenomena lead to attenuation in tissues

6. Why not simple X-Ray? It can damage the cells
It only creates a shadowgram
CAT scan, PET are again invasive
X-ray screen
X-ray source

7. Existing non-invasive techniques
Magnetic resonance imaging
Bulky and Expensive
Photodynamic therapy
Requires tumor seeking photosensitive dyes
Ultrasound methods
Cannot detect tumors of size < 1 cm
Problem: Resolution
Solution: Infrared light

8. Infrared radiation Advantages But problems in theoretical modeling ...
Noninvasive laser-tissue interaction
High resolution
Propagates very far in tissue
Rugged and cheap sources available
Reliable detectors
But problems in theoretical modeling ...

9. Disadvantages of the Diffusion Approximation
No coherent effects like interference
No polarization
Inaccurate at low penetration depth
Near-field effects are neglected
need a more complete theory

10. Exact numerical simulation of Maxwell’s Equations
Initial pulse satisfies :
  E = and   B = 0
Time evolution given by :
E ⁄t = 1/n2   B and B ⁄t = –  E
First tests : Snell’s law and Fresnel coefficients

11. Snell’s law for beams n1 n2 n1 sin a1 = n2 sin a2 a1 a2 Reflected
Incident a2
Refracted
n1 sin a1 = n2 sin a2

12. Light bouncing off air-glass interface
Time-resolved treatment

13. Light bouncing off a random scatterers
Time-resolved treatment

14. Summary and Outlook Exact solution of the Maxwell’s equations
Model a tissue as a collection of spheroids of random refractive indices
Systematically test the conventional diffusion approximation
Understand near-field effects