SignificanceSpeckle contrast analysis is the basis of laser speckle imaging (LSI), a simple, inexpensive, noninvasive technique used in various fields of medicine and engineering. A common application of LSI is the measurement of tissue blood flow. Accurate measurement of speckle contrast is essential to correctly measure blood flow. Variables, such as speckle grain size and camera pixel size, affect the speckle pattern and thus the speckle contrast.AimWe studied the effects of spatial correlation among adjacent camera pixels on the resulting speckle contrast values.ApproachWe derived a model that accounts for the potential correlation of intensity values in the common experimental situation where the speckle grain size is larger than the camera pixel size. In vitro phantom experiments were performed to test the model.ResultsOur spatial correlation model predicts that speckle contrast first increases, then decreases as the speckle grain size increases relative to the pixel size. This decreasing trend opposes what is observed with a standard speckle contrast model that does not consider spatial correlation. Experimental data are in good agreement with the predictions of our spatial correlation model.ConclusionsWe present a spatial correlation model that provides a more accurate measurement of speckle contrast, which should lead to improved accuracy in tissue blood flow measurements. The associated correlation factors only need to be calculated once, and open-source software is provided to assist with the calculation.
Speckle contrast analysis had been used for multiples purposes, for instance, laser speckle contrast imaging (LSCI) has been used to estimate the relative blood flow speed in a non-invasive way. The speckle contrast can be calculated using a spatial or temporal algorithm or a combination of both. Our work focuses into the contrast temporal algorithm. A contrast image calculated with the temporal contrast algorithm requires a sequence of L equal-sized frames. The contrast images are performed pixel by pixel, however, the experimental contrast calculation does not match with the current temporal theoretical model especially when the exposure time T is smaller than the correlation time τc. In this work, we propose to correlate neighboring pixels along the temporal axis to improve the contrast calculation. The contrast measurements using our proposal provide a better agreement than the current models.
A trustworthy speckle contrast calculation is fundamental in many applications, such as “laser speckle contrast Imaging” (LSCI), which is a non-invasive technique commonly employed to estimate relative blood speed. In LSCI, the local contrast of a speckle image is calculated using spatial, temporal analysis or a combination of both. In this work, we focus on the spatial analysis. To calculate the local spatial contrast, typically, a sliding window of 5x5 pixels is used to calculate the standard deviation (σs) and the mean intensity (<I>s) of those 5x5 pixels and the calculated contrast KS=σs/(<I>s) is assigned to the central pixel of the sliding window. In this work, we show that this experimental procedure to calculate the local speckle contrast does not match the corresponding spatial theoretical model and we propose an alternative method that considers correlations of the central pixel with the other ones. We have found a better agreement of the contrast measurement with our numerical calculation.
Optical imaging through highly scattering media such as biological tissue is limited by light scattering. Recently, it has been shown that wavefront shaping is a powerful tool to overcome this problem. In this work, wavefront shaping using spatial light modulators is used to compensate static scattering media (piece of translucent tape) to allow focusing of different intensity distributions. Light propagation is engineered into a specific region of interest. For this purpose, a sequential phase shape algorithm was implemented experimentally. This algorithm is used to encode a phase distribution on an incident beam to pre-compensate phase distortions acquired by the beam after propagating through the tape. The sequential algorithm combined with a spatial light modulator is used to synthesize a phase distribution required for redirecting light using wavefront shaping. The scattered light was re-directed at the detector plane, in order to be: i) focused at a single pixel, ii) at squared regions of 3×3 and 5×5 pixeles and iii) a line pattern of 41 pixels of the camera. Furthermore, the region of interest was placed outside the central area of the camera opening the possibility of image formation.
Previously published data demonstrate that the temporal processing algorithm for laser speckle contrast imaging (LSCI) can improve the visibility of deep blood vessels and is less susceptible to static speckle artifacts when compared with the spatial algorithm. To the best of our knowledge, the extent to which the temporal algorithm can accurately predict the speckle contrast associated with flow in deep blood vessels has not been quantified. Here, we employed two phantom systems and imaging setups (epi-illumination and transillumination) to study the contrast predicted by the spatial and temporal algorithms in subsurface capillary tubes as a function of the camera exposure time and the actual flow speed. Our data with both imaging setups suggest that the contrast predicted by the temporal algorithm, and therefore the relative flow speed, is nearly independent of the degree of static optical scattering that contributes to the overall measured speckle pattern. Collectively, these results strongly suggest the potential of temporal LSCI at a single-exposure time to assess accurately the changes in blood flow even in the presence of substantial static optical scattering.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.