Numerical Simulations of
Laser-Tissue Interactions
Shannon M. Mandel
Sophomore
Intense Laser Physics Theory Unit
Illinois State University
Supervisor : Dr. H. Wanare
Examples of diffusive random media
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Biological Tissue
• Diagnostics of cancerous tissue
• Radiation therapy
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Water and Air
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Atmospheric studies and oceanography
Communications
Remote sensing
Pollution studies
Earth
• Geological studies
• Propagation of pressure waves
• Electromagnetic & acoustic probing
Our Interest
How does light interact with a diffusive
random medium like a tissue?
 Tumors are hidden inside the tissue
tumor
Properties of Random media
Index of refraction n(r) characterizes any medium
Homogeneous media
Continuous n(r)
Inhomogeneous media
Discontinuous n(r)
High Scattering versus
High Absorption
Both phenomena lead to attenuation in tissues
Why not simple X-Ray?
• It can damage the cells
• It only creates a shadowgram
X-ray screen
X-ray source
• CAT scan, PET are again invasive
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
Infrared radiation
• Advantages
 Noninvasive laser-tissue interaction
 High resolution
 Propagates very far in tissue
 Rugged and cheap sources available
 Reliable detectors
• But problems in theoretical modeling ...
Disadvantages of the
Diffusion Approximation
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No coherent effects like interference
No polarization
Inaccurate at low penetration depth
Near-field effects are neglected
need a more complete theory
Exact numerical simulation of
Maxwell’s Equations
Initial pulse satisfies :
E=0
and
B=0
Time evolution given by :
E ⁄t = 1/n2   B
and B ⁄t = –  E
First tests : Snell’s law and Fresnel coefficients
Snell’s law for beams
Reflected
n1
Incident
n2
a1
a2
n1 sin a1 = n2 sin a2
Light bouncing off air-glass
interface
QuickTime™ and a
GIF decompressor
are needed to see this picture.
Time-resolved treatment
Light bouncing off a random
scatterers
QuickTime™ and a
GIF decompressor
are needed to see this picture.
Time-resolved treatment
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