Laser Speckle Imaging Biomedical Application of Optics: MEE4004

Laser Speckle Imaging

Biomedical Application of Optics: MEE4004

By Byungjo Jung

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

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

What is Interference?

Scanning Laser Doppler Imaging (SLDI)

Low power laser beam scans the tissue recording measurement

spots(4096=64 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

How Does Laser Speckle Imaging (LSI) Works ?

Laser Speckle Imaging System

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.

Basic Theories: Speckle Contrast

1

2

c

2T{ [1 exp( )]}

I 2Ts c

T : exposure time of the CCD

: correlation time

: standard deviation of the speckle pattern intensity

: mean intensity of the speckle pattern 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.

Raw Vs. Speckle Contrast Image

raw speckle image 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 > higher blood flow

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

Single speckle size

Magnification of lens

Wavelength of laser

Camera F-stop

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.

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

(1/ )cV

Comparison Four Blood Perfusion Imager

Comparison between LSI and SLDI

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