ECSE-4962 Introduction to Subsurface Sensing and Imaging Systems

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ECSE-4962Introduction to Subsurface Sensing and
Imaging Systems

• Lecture 24 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

• Molecular Imaging has tremendous potential.
• MI is the result from a tight coupling of biology
  subsurface imaging technologies.
• Pursuit of activities in this area will require a
  good grounding in cell biology, biochemistry.
• PET, nuclear will be most likely the first
  modalities esp. in human imaging.
• Optical imaging, MRI are receiving much attention
  in animal studies.
• There is a very exciting potential for a
  fundamental change in diagnostic therapeutic
  medicine.
• Todays Goals
• Cover several loose ends involving scattering in
  both optical acoustic domains.

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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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What are scattered waves?

• Scattered waves originate through the interaction
  of primary (and scattered) waves with
  heterogeneities.
• The primary waves are often referred to as
  incident waves.
• They do not exist in a homogenous medium.
• In pulse-echo imaging, scattering is the means
  for getting info from the target.
• With transmission imaging, scattering is a noise
  source

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Scattering
Note, this is microscopic, or single-particle
cross section
138.5.51.241/index/downloads/ 2004/Fall04/FS04-Cla
ss7_Ultrasound_Theory.ppt
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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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Rayleigh and Mie
Rayleigh Occurs when radiation interacts with
particles or molecules much smaller in diameter
than the wavelength of the radiation Strongly
wavelength dependent Elastic no change in
energy (frequency) of photon (Inelastic
scattering is also known as Raman Scattering)
Source hyperphysics.phy-astr.gsu.edu
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Mie Scattering

• Less wavelength dependent
• The forward lobe gives us the white glare around
  the sun when lots of large particulate matter is
  present in the air. Also gives us the whiteness
  of mist and fog (white indicates wavelength
  independence)

Fun stuff To generate Mie scattering profiles,
try the website http//omlc.ogi.edu/calc/mie_calc
.html
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Raman Scattering

• Rayleigh scattering is elastic, no energy loss or
  gain.
• Raman scattering is inelastic
• scattered photons are shifted in frequency
• With polarizable molecules, an incident photon
  can excite vibrational modes
• These yield scattered photons with less energy
• Application example lased beam directed at an
  industrial smokestack to monitor released gases.

J is the rotational state of the molecule hit by
the photon.
http//hyperphysics.phy-astr.gsu.edu/hbase/atmos/r
aman.htmlc1
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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?
• What are the comparable quantities in
  electromagnetics?

Light scattering from ice crystals in atmosphere.
http//wwwold.first.fhg.de/persons/bwalter/html/do
ktor.html
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Halo Effect

• Often associated ice crystals in cirrus clouds.
• Faceted crystals are great for this.
• Lot of folks are doing computer modeling of this
• Ray tracing
• Diffraction
• Can you make money doing this?

http//strc.herts.ac.uk/ls/ise.html
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Light in a Turbid Medium
Input
Scattering
Absorption
Direct Transmission
Diffuse Transmission
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Scattering in Breast Tissue
2 Photons
1 Diffuse Photon
1 Snake Photon
Transit time 6 tb
106 Photons
10298 Photons
Transit time tb
1 Ballistic Photon
Transit time º tb
Presented by Lihong Wang at Bios98
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Scattering Contrast Mechanisms

• Near infrared (NIR) light is used in breast
  imaging.
• NIR imaging supplies info on
• Oxyhemoglobin
• Deoxyhemoglobin
• Water fraction
• Scattering parameters
• Usual method of detection is spectroscopy.
• But, the resolution is poor.

http//www.dartmouth.edu/biolaser/papers/2004/JBO
20intersubject20variation20paper202004.pdf
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Scattering Contrast Mechanisms

• In x-ray mammography, photon energies are near 20
  KeV. Hence
• Compton scattering
• Photoelectric effect
• In NIR, Mie scattering is the dominant
  interaction
• This is mainly due to intracellular fluctuations
  in refractive index.

http//www.dartmouth.edu/biolaser/papers/2004/JBO
20intersubject20variation20paper202004.pdf
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Acoustic 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 micro-calcifications
• 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 is easier discerned
  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 Lecture 24

• 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?)
• Can Raman scattering occur with acoustic waves?
  How so?

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Acknowledgments

• Thanks to Drs. Chuck DiMarzio (NU), Dana Brooks
  (NU), and Bahaa Saleh (BU) for optics related
  slides and illustrations
• www.es.ucsc.edu/jsr/PPT/scattering.ppt

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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 Laraine Michaelides (michal_at_rpi.edu),
  518-276-8525

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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 Laraine Michaelides (michal_at_rpi.edu),
  518-276-8525

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Monopoles and Dipole Scatterers

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