Studying neurovascular blood flow function in cerebrovascular activities requires accurate visualization and characterization of blood flow volume as well as the dynamics of blood cells in microcirculation. In this study, we present a novel integration of laser speckle contrast imaging (LSCI) and spectral domain optical coherence tomography (SD-OCT) for rapid volumetric imaging of blood flow in cortical capillaries. LSCI uses the illumination of wide-field near infrared light (NIR) and monitors back scattered light to characterize the relative dynamics of blood flow in microcirculation. Absolute measurement of blood cells and blood volume requires high-resolution volumetric structural information. SD-OCT system uses coherence gating to measure scattered light from a small volume within high structural resolution. The structural imaging system rapidly assesses large number of capillaries for spatio-temporal tracking of red blood cells (RBC).
A very fast-ultra resolution SD-OCT system was developed for imaging high-resolution volumetric samples. The system employed an ultra wideband light source (1310 ± 200 nm in wavelength) corresponding to an axial resolution of 3 micrometers in tissue. The spectrometer of the SD-OCT was customized for a maximum scanning rate of 147,000 line/s. We demonstrated a fast volumetric OCT angiography algorithm to visualize large numbers of vessels in a 2-mm deep sample volume. A LSCI system that has been developed previously in our group was integrated to the imaging system for the characterization of dynamic blood cells. The conjunction data from LSCI and SD-OCT systems imply the feasibility of accurate quantification of absolute cortical blood flow.
Time-resolved measurement of early-arriving photons has been shown by a number of groups to effectively reduce
photon scatter and improve resolution in diffuse optical tomography (DOT) and fluorescence mediated tomography
(FMT). Recently, we experimentally showed that measurement of early-arriving photons resulted in the reduction of
the instrument photon density sensitivity function (PDSF) width by a factor of 2 to 2.5 over a wide range of relevant
small-animal imaging conditions using a picosecond pulsed laser and time-resolved photon counting combination.
However, we also showed that this experimental improvement was less than predicted from time-resolved Monte
Carlo simulations. Specifically, a reduction by a factor of 4 or better was predicted, but this could not be achieved
with our system. To better understand this, in this work we have experimentally tested the effect of a series
instrumentation (hardware) parameters on the experimentally measured time-dependant PDSFs including, i) source
and detector geometry, ii) detector sensitivity, iii) laser illumination intensity, and iv) instrument temporal impulse
response function. Our ongoing research indicates that all of these parameters affected the relative PDSF width by as
much as 10-25%, particularly at early time points. The results of this work are significant because they show in a
number of cases that significant disagreement between experimental PDSFs and theoretical models exist as a result
of minor changes in experimental configuration. We also anticipate that these results will be useful in the design of
future time-resolved DOT and DFT imaging systems.
Time-resolved measurement of early arriving photons through diffusive media has been shown to effectively reduce the high degree of light scatter in biological tissue. However, the experimentally achievable reduction in photon scatter and the impact of time-gated detection on instrument noise performance is not well understood. We measure time-dependent photon density sensitivity functions (PDSFs) between a pulsed laser source and a photomultiplier tube operating in time-correlated single-photon-counting mode. Our data show that with our system, measurement of early arriving photons reduces the full width half maximum of PDSFs on average by about 40 to 60% versus quasicontinuous wave photons over a range of experimental conditions similar to those encountered in small animal tomography, corresponding to a 64 to 84% reduction in PDSF volume. Factoring in noise considerations, the optimal operating point of our instrument is determined to be about the 10% point on the rising edge of the transmitted intensity curve. Time-dependant Monte Carlo simulations and the time-resolved diffusion approximation are used to model photon propagation and are evaluated for agreement with experimental data.