We have proposed and developed a multi-modal non-invasive biomedical optical imager. It was combined from the
subsystems of optical microangiography and dual-wavelength laser speckle contrast imaging. The system was designed
to maintain the performances of both subsystems. It was capable of simultaneously imaging the hemodynamic and
metabolic responses in tissue environment in vivo. To achieve such requirements, we utilized unique optical setup, such
as paired dichroic mirrors to compensate dispersion, additional relay lens to increase working distance and translational
sample probe to freely select imaging area and focal plane. The multi-functionality of the system was demonstrated in an
investigation of hemodynamic and metabolic responses on an acute wound healing model in mouse pinna in vivo. The
microvasculature, blood flow and hemoglobin concentration from millimeter down to capillary level were
comprehensively visualized. The captured instantaneous responses to wound onset differed greatly between localized
areas; after that blood flow had a rebalance tendency, and hemoglobin concentration dynamically recovered to baseline
situation.
We use rodent parietal cortex as a model system and utilize a synchronized dual wavelength laser speckle imaging (SDW-LSCI) technique to explore the hemodynamic response of infarct and penumbra to a brain injury (middle cerebral artery occlusion (MCAO) model). The SDW-LSCI system is able to take snapshots rapidly (maximum 500 Hz) over the entire brain surface, providing key information about the hemodynamic response, in terms of which it may be used to elucidate evolution of penumbra region from onsite to 90 min of MCAO. Changes in flow are quantified as to the flow experiencing physical occlusions of the MCA normalized to that of baseline. Furthermore, the system is capable of providing information as to the changes of the concentration of oxygenated, (HbO) deoxygenated (Hb), and total hemoglobin (HbT) in the cortex based on the spectral characteristics of HbO and Hb. We observe that the oxygenation variations in the four regions are detectable and distinct. Combining the useful information, four regions of interest (ROI), infarct, penumbra, reduced flow and contralateral portions in the brain upon ischemic injury may be differentiated. Implications of our results are discussed with respect to current understanding of the mechanisms underlying MCAO. We anticipate that SDW-LSCI holds promise for rapid and large field of view localization of ischemic injury.
Ability to non-invasively monitor and quantify of blood flow, blood vessel morphology, oxygenation and tissue morphology is important for improved diagnosis, treatment and management of various neurovascular disorders, e.g., stroke. Currently, no imaging technique is available that can satisfactorily extract these parameters from in vivo microcirculatory tissue beds, with large field of view and sufficient resolution at defined depth without any harm to the tissue. In order for more effective therapeutics, we need to determine the area of brain that is damaged but not yet dead after focal ischemia. Here we develop an integrated multi-functional imaging system, in which SDW-LSCI (synchronized dual wavelength laser speckle imaging) is used as a guiding tool for OMAG (optical microangiography) to investigate the fine detail of tissue hemodynamics, such as vessel flow, profile, and flow direction. We determine the utility of the integrated system for serial monitoring afore mentioned parameters in experimental stroke, middle cerebral artery occlusion (MCAO) in mice. For 90 min MCAO, onsite and 24 hours following reperfusion, we use SDW-LSCI to determine distinct flow and oxygenation variations for differentiation of the infarction, peri-infarct, reduced flow and contralateral regions. The blood volumes are quantifiable and distinct in afore mentioned regions. We also demonstrate the behaviors of flow and flow direction in the arterials connected to MCA play important role in the time course of MCAO. These achievements may improve our understanding of vascular involvement under pathologic and physiological conditions, and ultimately facilitate clinical diagnosis, monitoring and therapeutic interventions of neurovascular diseases, such as ischemic stroke.
Evaluation of spatiotemporal hemodynamic and metabolic responses during neural activation is crucial in studying brain function. We explore the use of a noninvasive multifunctional optical imaging system to measure these responses in a mouse brain upon electrically stimulated neural activation, with the cranium left intact. The system is developed by integrating an optical microangiography (OMAG) imaging system with a dual-wavelength laser speckle imaging (DW-LSI) system. The DW-LSI, running at an image acquisition speed of ∼100 Hz, is used to extract the large-scale two-dimensional map, revealing the localized response of blood flow, hemoglobin concentration, and metabolic rate of oxygen change. Guided by DW-LSI, the OMAG is, however, used to image the response of individual blood vessels with its unique depth-resolved capability. We show that the integrated system is capable of investigating neural activation, thus is potentially valuable in the preclinical study of the mechanism of neurovascular coupling.
We propose a simple and optimized method for acquiring a wide velocity range of blood flow using Doppler optical microangiography. After characterizing the behavior of the scanner in the fast scan axis, a step-scanning protocol is developed by utilizing repeated A-scans at each step. Multiple velocity range images are obtained by the high-pass filtering and Doppler processing of complex signals between A-scans within each step with different time intervals. A phase variance mask is then employed to segment meaningful Doppler flow signals from noisy phase background. The technique is demonstrated by imaging in vivo mouse brain with skull left intact to provide bidirectional images of cerebral blood flow with high quality and wide velocity range.
Skipping various numbers of A-lines is effective to obtain multi-range velocimetry using Doppler optical coherence
tomography (DOCT). High correlation between A-lines is a fundamental prerequisite for DOCT processing. Therefore,
high oversampling is normally necessary, especially when skipping A-lines. That requires quite a long time of imaging,
which might not be acceptable on some occasions. Step-scanning protocol, which captures repeated A-scans, has been
employed for multi-range DOCT previously. We develop it by waiting for the scanner to stabilize, but not capturing
continuously. In this way the cross-correlation of step scanning maintains almost constant for all the captured A-lines,
and is higher than that of conventional linear scanning with similar imaging time. Due to the limited numbers of A-lines
at each lateral position, we choose previously proposed high-pass filter in ultrahigh sensitive optical microangiography
(OMAG) to enhance flow sensitivity. Doppler processing is implemented after the filter, both of which utilize A-line
skipping to achieve variable velocity ranges. The obtained Doppler signal in blood flow is encircled by much noise in
non-flow area. This is because that the static components are rejected by the filter, leaving random phase noise. The
phase variance, which is a flow indicator, is employed to generate a binary mask to extract the Doppler signal out of
noise. The technique is demonstrated in bi-directional cross-section and maximum projection en face images of middle
cerebral artery (MCA) occluded mouse model. The vasculature responses from artery down to capillary during baseline,
occlusion and reperfusion are illustrated. In some arteries that branch from MCA the flow is reversed but not simply
reduced or vanished.
Large-size diffraction gratings are essential for pulse compressors in chirped-pulse-amplified high-power laser
systems, spectroscopic telescopes, etc. Fabricating large gratings requires large-aperture laser beams with collimated
and aberration-free wavefronts. As an alternative a method of making monolithic gratings by optical mosaic has been
proposed, which makes multiple-exposures in different areas of a substrate to enlarge the grating size. Between
exposures the position and attitude of substrate must be adjusted to minimize the overall wavefront errors, with very tight
accuracy requirements (~ dozens of nanometers and tenths of micro-radians, respectively).
We fully utilize the latent fringes (exposed but undeveloped fringes in photoresist) as the core reference object and
the exposure beams as the adjustment beams to adjust and lock the position and attitude between consecutive exposures.
This approach greatly simplifies the alignment system and eliminates many system errors. However, the diffraction
efficiency of a latent grating is extremely weak (~10-5) and excessive exposure of the latent fringes during the position
and attitude adjustment should be avoided. We overcome these difficulties by carefully blocking stray lights and using a
high-sensitivity CCD to monitor the interference fringes of the 1st- and 0th-order latent-fringe diffracted wavefronts.
Experimentally we have made 2 x 2 mosaics of (60+28) x (53+30) mm2 grating area. Typical peak-valley and rootmean-
square values of the measured 1st-order diffraction wavefront errors are 0.06 λ and 0.01 λ, respectively.
The mosaic conditions, detailed alignment steps, and experimental results showing position and attitude
controllability will be presented. Important issues of extending the present work to large-size (possibly sub-meter)
fabrication will also be discussed.
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