ECSE4963 Introduction to Subsurface Sensing and Imaging Systems

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ECSE-4963Introduction to Subsurface Sensing and
Imaging Systems

• Lecture 17 Scattering
• Kai Thomenius1 Badri Roysam2
• 1Chief Technologist, Imaging technologies,
• General Electric Global Research Center
• 2Professor, Rensselaer Polytechnic Institute

Center for Sub-Surface Imaging Sensing
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Recap

• We have reviewed the use of phase in coherent
  signal processing.
• A key application of this takes advantage of the
  Doppler effect.
• Blood flow detection in medical ultrasound has
  been used as an example.
• Basic equations governing Doppler processing were
  derived.

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Acoustic Imaging SSI
Probes
Detectors
Surface
Medium
object
Medium
Object
Probe
Optical/IR
Electro- magnetic
Fluorescence
Absorption
X-ray
Acoustic
Absorption
Nonlinear Absorption
Dispersion
CW
Pulsed
Modulated
Nonlinear Scattering
Scattering
Scattering
Multi- Spectral
Partially Coherent
Coherent
Diffusion
Diffusive
Phase Object
Clutter
Quantum
Classical
Depolarizing
Inhomogeneous/ Layered
Outside
Inside
Auxiliary
Stationary
Moving
Rough Surface
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Some More on Scattering
When a plane wave strikes a small object
(particle), a portion of the wave is scattered
into all directions. The scattered wave has an
angular distribution that depends on the
scatterers geometry and dimensions in relation
to the wavelength of the incoming wave, as well
as the contrast between the scatterer and the
surrounding medium.
l
Rayleigh Scattering Dimensions of scatterer are
much smaller than l
Incident wave
Back scattering
Forward scattering
l
Incident wave
Mie Scattering Dimensions of scatterer are NOT
much smaller than l
l
Incident wave
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Some more on scattering

• Some terminology
• Scattering target size similar or less than
  wavelength
• Reflection target size much larger than
  wavelength
• In acoustics, scattering or reflection arises
  from variations in either density or
  compressibility
• Why are density or compressibility important?
• Are there comparable quantities in
  electromagnetics?

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More on Scattering

• There is a difference in scattering caused by
  density variation compressibility variation
• Monopole scattering
• Compressibility variation
• Dipole scattering
• Density variation

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Scattering
Assumes l gtgt size of obstacle
Scattered waves
Cross section
s scattered intensity/unit solid angle
incident wave intensity
incident
For a particle of volume V in homogenous medium
k compressibility r mass density
p particle m surrounding
medium
Blood shows f4 dependence, other tissue types
weaker dependence
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More on Scattering

• Why is this monopole/dipole distinction
  important?
• There are many targets with large density or
  compressibility variations.
• Land mines
• Breast microcalcifications
• Hand grenades in luggage
• To identify dipole scattering, we have to measure
  angular backscatter.
• Clinical research into angular scatter on-going.

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Images from scattering targets

• So far, we have talked about backscatter from
  single scatterers.
• Image will be a convolution of the scattering
  cross section and the point spread function.
• What if we have a huge number of random
  non-resolvable scatterers?
• Image will be composed of speckle noise.
• Speckle noise image does not represent the
  underlying structure.

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Scattering and Speckle scattering
element is smaller than dimensions of ultrasound
pulse
Echoes from multiple scattering points generated
simultaneously
Pattern of constructive and destructive
interference from random scatterers in
homogeneous tissue gives rise to speckle in
resulting ultrasound image
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Speckle noise reduction by Spatial Diversity

• Left hand image shows a conventional ultrasound
  acquisition.
• Right hand image shows acquisition from multiple
  angles.
• The images from these angles have independent
  speckle.
• Adding these images after detection will improve
  the SNR.

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Some examples

• The images shown are of a breast cyst.
• Left hand image shows the speckle noise
  structure.
• Right hand image shows the improved detectability
  of tissue structures cyst with spatial
  compounding.

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More examples

• A secondary breast tumor was discovered adjacent
  to the primary tumor using spatial compounding.

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On Things That Stick out in SSI Images

• Probe/target interactions what has been
  helpful?
• X-ray, CT, Optics
• Attenuation variations
• Acoustics
• Variation in backscatter strength
  (compressibility, density)
• Tissue (e.g blood) motion
• Two types of interactions
• Ones needed to form structure of basic image
• Our basic contrast mechanisms
• Humans needed to view images, make decisions
• Smoking guns for a given clinical situation,
  target identification
• Luggage inspection, explosives detection (image
  processing based)
• PET, Nuclear medicine
• High velocity Doppler jet, hence a leaky cardiac
  valve

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Homework 13

• Identify two other probes (other than ultrasound)
  which exhibit speckle noise.
• Discuss means for speckle noise reduction for
  those probes.
• Identify analogs for compressibility and density
  variations for electromagnetic probes. Justify
  your choices. (In other words, what causes
  scattering with electromagnetic radiation?)

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Instructor Contact Information

• Badri Roysam
• Professor of Electrical, Computer, Systems
  Engineering
• Office JEC 7010
• Rensselaer Polytechnic Institute
• 110, 8th Street, Troy, New York 12180
• Phone [phone]
• Fax [phone]
• Email roysam_at_ecse.rpi.edu
• Website http//www.rpi.edu/roysab
• NetMeeting ID (for off-campus students)
  128.113.61.80
• Secretary Betty Lawson, JEC 7012, (518) 276
  8525, lawsob_at_.rpi.edu

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Instructor Contact Information

• Kai E Thomenius
• Chief Technologist, Ultrasound Biomedical
• Office KW-C300A
• GE Global Research
• Imaging Technologies
• Niskayuna, New York 12309
• Phone [phone]
• Fax [phone]
• Email thomeniu_at_crd.ge.com, thomenius_at_ecse.rpi.edu
  
• Secretary Betty Lawson, JEC 7012, (518) 276
  8525, lawsob_at_.rpi.edu