The operation of LSCI is having a wide-field illumination of a rough surface through a coherent light source.
Then using photodetectors such as CCD camera or CMOS sensors imaging the resulting laser speckle pattern caused by the interference of coherent light.
[1][2] In biomedical use, the coherent light is typically in the red or near-infrared region to ensure higher penetration depth.
[3] When scattering particles moving during the time, the interference caused by the coherent light will have fluctuations which will lead to the intensity variations detected via the photodetector, and this change of the intensity contain the information of scattering particles' motion.
[4] Through image the speckle patterns with finite exposure time, areas with scattering particles will appear blurred.
The first practical application of utilizing speckle pattern reduction to mapping retinal blood flow was reported by Fercher and Briers in 1982.
This technology was called single-exposure speckle photography at that time.
Due to the lacking of sufficient digital techniques in the 1980s, single-exposure speckle photography has a two-step process which made it not convenient and efficient enough for biomedical research especially in clinical use.
With the development of digital techniques, including the CCD cameras, CMOS sensors, and computers, in the 1990s, Briers and Webster successfully improved single-exposure speckle photography.
The improved technology is called laser speckle contrast imaging (LSCI) which can directly measure the contrast of speckle pattern.
[5] Due to the simple structure of the instrumental setup, LSCI can be integrated into other systems easily.
[2] For a fully developed speckle pattern which formed when the complete coherent and polarized light illuminate a static medium, the contrast (K) range from 0 to 1 is defined by the ratio between the standard deviation and mean intensity:[6][7]
The intensity distribution of the speckle pattern will be used to compute the contrast value.
Autocorrelation functions of electric field are used to measure the relationship between contrast and the motion of scatterers because the intensity fluctuations are produced by electric field changes of scatterers.
Bandyopadhyay et al. showed that the reduced intensity variances of speckle pattern are related to
takes into account the loss of correlation due to the detector pixel size, and depolarization of the light through the medium.
However, the ordered motion in dynamic scatterers follows Gaussian distribution.
When considering the motion distribution, the contrast equation related to the autocorrelation can be updated.
is account for the instability and maximum contrast of each LSCI system.
By adding statics scatterers, the contrast equation can be updated again.
P1 and P2 are two constants that range from 0 to 1, they are determined by fitting this equation to the actual experimental data.
[7] The relationship between the velocity of scatterers and decorrelation time is as follows, velocity of scatterers such as the blood flow is proportional to the decorrelation time,
The method to compute the contrast of speckle patterns can be classified into three categories: s-K (spatial), t-K (temporal), and st-K (Spatio-temporal).
To improve the resolution limitation, scientists also compute the temporal contrast of the speckle pattern.
It can uses simple and cost-effective instrument to return excellent spatial and temporal resolution imaging.
And due to these strengths, laser speckle contrast imaging has been involved in mapping blood flow for decades.
The utilize of LSCI has been extended to many subjects in the biomedical field which include but are not limited to rheumatology, burns, dermatology, neurology, gastrointestinal tract surgery, dentistry, cardiovascular research.
[11] LSCI can be adopted into another system easily for clinical full-field monitoring, measuring, and investigating living processes in almost real-time scale.
[13] Due to the complex vascular anatomy structure, the maximum detection depth of LSCI is limited by 900 micrometers now.
[7][15] The scattering and absorption effect of red blood cell can influence the contrast value.