Laser speckle imaging in the spatial frequency domain.

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.