Flux or speed? Examining speckle contrast imaging of vascular flows

Abstract

Speckle contrast imaging enables rapid mapping of relative blood flow distributions using camera detection of back-scattered laser light. However, speckle derived flow measures deviate from direct measurements of erythrocyte speeds by 47 ± 15% (n = 13 mice) in vessels of various calibers. Alternatively, deviations with estimates of volumetric flux are on average 91 ± 43%. We highlight and attempt to alleviate this discrepancy by accounting for the effects of multiple dynamic scattering with speckle imaging of microfluidic channels of varying sizes and then with red blood cell (RBC) tracking correlated speckle imaging of vascular flows in the cerebral cortex. By revisiting the governing dynamic light scattering models, we test the ability to predict the degree of multiple dynamic scattering across vessels in order to correct for the observed discrepancies between relative RBC speeds and multi-exposure speckle imaging estimates of inverse correlation times. The analysis reveals that traditional speckle contrast imagery of vascular flows is neither a measure of volumetric flux nor particle speed, but rather the product of speed and vessel diameter. The corrected speckle estimates of the relative RBC speeds have an average 10 ± 3% deviation in vivo with those obtained from RBC tracking.

Cerebral capillary velocimetry based on temporal OCT speckle contrast

“…We note a finding in Ref. 21, suggesting the dependence of number of dynamic scattering events on flow channel size, where 3D Monte Carlo simulation of scattering within whole blood microfluidic channels having three feature sizes (65, 115, and 175μm) showed a linear increase of scattering events with the increase of the channel size. For 65μm in size, in particular, the number of scattering events in the flow channel were less than 3 regardless of objective numerical apertures (N.A.)…”

“…(from 0.12 to 1.0N.A.) [21]. Therefore, it is reasonable to infer that for the channel smaller than 65μm, there would be more pronounced reduction in scattering event and that a single scattering would be predominant for very small channel like capillary (<10μm in size).…”

“…The rate of change in the speckle pattern can be quantified by calculating temporal or spatial statistics of the speckle pattern, providing information on the motion of the scatters [19]. The statistical models have been well-established to quantitatively estimate the motion of the scatters in the form of flow or speed from the observed speckles, making it possible to extract the characteristic decorrelation time τ c of the speckles, which is inversely proportional to the speed of moving scatters [19][20][21].…”

“…For this, we utilize the speckle contrast analytical models that have been already well-established in the many LSCI literatures [21,22]. Table 1 shows the modified Siegert relation based speckle visibility expressions for different scattering regimes and different velocity distributions.…”

Estimation of vessel diameter and blood flow dynamics from laser speckle images

“…In addition to diameter measurements it is also important to understand what SV value stands for. Theory says it is proportional to the mean particle velocity [5,6] or linked to it via complicated relations [15] while experimental circulation research often refers to it as a measure of blood flow [6,7]. As we discussed, velocity is measured at all situations while blood flow measurements require additional information about the diameter (whether it is constant or changing).…”