Laser Speckle Imaging in the Spatial Frequency Domain

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. tomography of sub-surface heterogeneities using spatially modulated structured light," Opt.

Combined effects of scattering and absorption on laser speckle contrast imaging

“…This decrease in contrast with an increase in μ Likewise, in this study, contrast decreased linearly as absorption increased. Others 8,9 have reported an increase in contrast as absorption increases. At initial glance, it may appear that these results are contradictory.…”

“…The confounding effect of changes in absorption was not considered in Khaksari and Kirkpatrick. 5 Changes in observed speckle contrast in LSCI due to changes in hematocrit 8 or directly due to changes in μ s and μ a in phantom materials have been described by others in the literature; 9 however, a systematic study of the combined effects of absorption and scattering on the observed contrast values in LSCI has yet to be reported.…”

“…As will be discussed below, this reduction in contrast is due to the attenuation of the light reaching the static scattering block below the moving fluid and, thus, reducing the influence of these static scatterers on the overall contrast. Others 8,9 have observed an increase in contrast with an increase in absorption; however, in their experiments, there was no underlying layer of static scatterers and their scattering mediums were optically semi-infinite or close to it.…”

Speckle contrast optical tomography: A new method for deep tissue three-dimensional tomography of blood flow

“…In another study [41], the authors have used a transmission geometry through the finger joint, 1.0-1.5 cm, but did not use a proper physical model or achieved any depth resolution. Depth sensitive speckle contrast measurement using a sinusoidally modulated source is used in [42], where a pair of confined flows separated 2 mm apart and at a depth of 2 mm and 4 mm were measured. Although this work presents the relation connecting speckle contrast to the electric field autocorrelation for sinusoidal source excitation, a quantitative recovery of the effective flow velocity from the measured speckle contrast was not presented.…”

Assessing blood coagulation status with laser speckle rheology

“…In addition to the effects of sample mechanics, the evolution of the g 2 (t) curve is also influenced in part by factors such as the optical properties of blood (Eq. (2) and speckle contrast [19,43,44]. We have previously demonstrated that an increase in optical scattering can accelerate the decay rate of the measured g 2 (t) curve independent of sample mechanical properties due to an increase in the number of scattering events per optical path [19].…”