Laser Speckle Imaging

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Laser Speckle Imaging
Biomedical Application of Optics MEE4004 By
Byungjo Jung
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What is Laser Speckle ?

• The speckle was discovered as an unexpected
  phenomenon when the first lasers
• were in operation. It was about the year
  1960
• optical interference effect that can be
  observed when objects are illuminated with
• laser light. This effect is grainy in
  appearance, with light and dark "speckles"
• caused by constructive and destructive
  interference, respectively, of scattered laser
• light.
• In other words, a interference of elementary
  coherent (partially coherent) beams
• of radiation from many secondary light point
  sources located on the rough surface
• of the object.
• Laser speckle offers the possibility of
  developing a full-field technique for velocity
• map imaging which produces an instantaneous
  map of velocities in real time
• blood flow measurement in assessing
  condition such as inflamation, healing
• process, burn assessment, intra-operative
  measurement, dermatology (psoriasis,
• skin flap failure, skin irritation),
  physiology

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What is Coherent Light ?

• Coherent Light
• Light in which the phases of all electromagnetic
  waves at each point on a line normal to the
  direction of the the beam are identical. Coherent
  light is usually monochromatic, and the most
  common source of such light for practical uses is
  from a laser.
• Coherence is one of the unique properties of
  laser light. It arises from the stimulated
  emission process which provides the
  amplification. Since a common stimulus triggers
  the emission events which provide the amplified
  light, the emitted photons are "in step" and have
  a definite phase relation to each other. This
  coherence is described in terms of temporal
  coherence and spatial coherence, both of which
  are important in producing the interference

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What is Interference?
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Scanning Laser Doppler Imaging (SLDI)

• Low power laser beam scans the tissue recording
  measurement
• spots(409664 by 64). In the tissue, the
  laser light is scattered
• and changes wavelength when it hits moving
  blood cell (Doppler
• shift).
• A fraction of the backscattered light is
  detected by a photo detector
• and the data is recorded and processed by
  software.
• Perfusion image concentration of moving blood
  cell mean
• velocity of these blood cells
• The concentration is related to the magnitude of
  the Doppler signal
• Velocity is related to the frequency shift

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How Does Laser Speckle Imaging (LSI) Works ?

• It is important to select a laser wavelength
  suitable to the tissue under observation, as it
• is necessary to achieve some tissue surface
  penetration with the laser light for blood flow
• mapping.
• Each acquired image will display a slightly
  different speckle pattern, caused by the change
  of
• position of moving scatterers in the area of
  interest. If the time lapse between images is
• known, it is possible to examine the
  intensity variation of individual speckles at the
  same
• position in each image and calculate the
  velocity of the scatterers responsible for the
  variation.

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Basic Theories Speckle Contrast

• The size of the region over which the speckle
  contrast is computed must be large
• enough to contain a sufficient number of
  pixels to ensure accurate determination of
• standard deviation mean intensity, yet not
  so large that significant spatial
• resolution is lost
• In practice a 5x5 or 7x7 region of pixels is
  typically used to compute the speckle
• contrast.

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Raw Vs. Speckle Contrast Image

• 785 nm laser diode was expanded to illuminate a
  6x4mm area of
• rat cortex (skull removed, dura intact) and
  the area was imaged
• onto an 8-bit CCD camera
• Speckle contrast range 0 1
• 1 no blurring of the speckle pattern
• 0 the scatters are moving fast enough to
  average out all of the
• speckles
• darker color gt higher blood flow

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Basic Theories Speckle Size
To ensure proper sampling of the speckle pattern,
the size of a single speckle should be
approximately equal to the size of a single pixel
in the image
F-stop the size of the aperture in a lens.
Larger the F-stops give smaller lens openings.
i.e. f2.8 gives a larger aperture (and more
exposure) than f11.
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Basic Theories Velocity

• The velocity (in arbitrary unit) is computed
  from the inverse of the correlation
• time
• Theoretically, it is possible to relate the
  correlation time to the absolute velocities
• of the red blood cell. However, it is
  difficult to do in practice because the
• number of moving particles that the light
  interact with and their orientation are
• unknown.
• However, relative spatial and temporal
  measurement of velocity can be easily
• obtained from the ratios of the correlation
  times.
• The relative blood flow is computed by taking the
  ratio of the correlation time
• image at any point and a baseline image

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Comparison Four Blood Perfusion Imager
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Comparison between LSI and SLDI
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