Abstract
Laser Speckle Imaging (LSI) images interference patterns produced by coherent addition of scattered laser light to map subsurface tissue perfusion. However, the effect of longer path length photons is typically unknown and poses a limitation towards absolute quantification. In this work, LSI is integrated with spatial frequency domain imaging (SFDI) to suppress multiple scattering and absorption effects. First, depth sensitive speckle contrast is shown in phantoms by separating a deep source (4 mm) from a shallow source (2 mm) of speckle contrast by using a high spatial frequency of illumination (0.24 mm−1). We develop an SFD adapted correlation diffusion model and show that with high frequency (0.24 mm−1) illumination, doubling of absorption contrast results in only a 1% change in speckle contrast versus 25% change using a planar unmodulated (0 mm−1) illumination. Similar absorption change is mimicked in vivo imaging a finger occlusion and the relative speckle contrast change from baseline is 10% at 0.26 mm−1 versus 60% at 0 mm−1 during a finger occlusion. These results underscore the importance of path length and optical properties in determining speckle contrast. They provide an integrated approach for simultaneous mapping of blood flow (speckle contrast) and oxygenation (optical properties) which can be used to inform tissue metabolism.
Highlights
Laser Speckle Imaging (LSI) is a wide-field imaging method used for mapping blood flow and mechanical properties in tissue [1,2]
We systematically evaluate the impact of path length for LSI by introducing an approach for modulating LSI in the spatial frequency domain
At fx = 0 mm 1, which is identical to the uniform illumination used typically in LSI, contributions to speckle contrast from scattering liquid in both tubes, was apparent (Fig. 2(c))
Summary
Laser Speckle Imaging (LSI) is a wide-field imaging method used for mapping blood flow and mechanical properties in tissue [1,2]. LSI typically uses a CCD camera to image the interference (i.e., speckle) patterns produced by the coherent addition of scattered laser light propagating with varying path lengths. Fercher and Briers introduced the LSI method using spatial statistics to characterize the fluctuations at the surface [6]. This method enables imaging of two-dimensional structure and flow. A single snapshot taken at an exposure time on the order of the speckle correlation time is analyzed using spatial statistics. The local speckle contrast, K, is calculated from a reflectance image as:
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