Cocaine-Induced Abnormal Cerebral Hemodynamic Responses to Forepaw Stimulation Assessed by Integrated Multi-wavelength Spectroimaging and Laser Speckle Contrast Imaging

Impairment of cerebral vascular reactivity and resting blood flow in early-staged transgenic AD mice: in vivo optical imaging studies

Background

Alzheimer's disease (AD) is a neurodegenerative disorder with progressive cognitive decline in aging individuals that poses a significant challenge to patients due to an incomplete understanding of its etiology and lack of effective interventions. While "the Amyloid Cascade Hypothesis," the abnormal accumulation of amyloid-β in the brain, has been the most prevalent theory for AD, mounting evidence from clinical and epidemiological studies suggest that defects in cerebral vessels and hypoperfusion appear prior to other pathological manifestations and might contribute to AD, leading to "the Vascular Hypothesis." However, assessment of structural and functional integrity of the cerebral vasculature in vivo in the brain from AD rodent models has been challenging owing to the limited spatiotemporal resolution of conventional imaging technologies.

Methods

We employed two in vivo imaging technologies, i.e., Dual-Wavelength Imaging (DWI) and Optical Coherence Tomography (OCT), to evaluate cerebrovascular reactivity (CVR; responsiveness of blood vessels to vasoconstriction as triggered by cocaine) in a relatively large field of view of the cortex in vivo, and 3D quantitative cerebrovascular blood flow (CBF) imaging in living transgenic AD mice at single vessel resolution.

Results

Our results showed significantly impaired CVR and reduced CBF in basal state in transgenic AD mice compared to non-transgenic littermates in an early stage of AD progression. Changes in total hemoglobin (Δ[HbT]) in response to vasoconstriction were significantly attenuated in AD mice, especially in arteries and tissue, and the recovery time of Δ[HbT] after vasoconstriction was shorter for AD than WT in all types of vessels and cortical tissue, thereby indicating hypoperfusion and reduced vascular flexibility. Additionally, our 3D OCT images revealed that CBF velocities in arteries were slower and that the microvascular network was severely disrupted in the brain of AD mice.

Conclusions

These results suggest significant vascular impairment in basal CBF and dynamic CVR in the neurovascular network in a rodent model of AD at an early stage of the disease. These cutting-edge in vivo optical imaging tools offer an innovative venue for detecting early neurovascular dysfunction in relation to AD pathology and pave the way for clinical translation of early diagnosis and elucidation of AD pathogenesis in the future.

Cocaine's cerebrovascular vasoconstriction is associated with astrocytic Ca2+ increase in mice

Human and animal studies have reported widespread reductions in cerebral blood flow associated with chronic cocaine exposures. However, the molecular and cellular mechanisms underlying cerebral blood flow reductions are not well understood. Here, by combining a multimodal imaging platform with a genetically encoded calcium indicator, we simultaneously measured the effects of acute cocaine on neuronal and astrocytic activity, tissue oxygenation, hemodynamics and vascular diameter changes in the mouse cerebral cortex. Our results showed that cocaine constricted blood vessels (measured by vessel diameter Φ changes), decreasing cerebral total blood volume (HbT) and temporally reducing tissue oxygenation. Cellular imaging showed that the mean astrocytic Ca2+ dependent fluorescence [Formula: see text] increase in response to cocaine was weaker but longer lasting than the mean neuronal Ca2+ dependent fluorescence [Formula: see text] changes. Interestingly, while cocaine-induced [Formula: see text] increase was temporally correlated with tissue oxygenation change, the [Formula: see text] elevation after cocaine was in temporal correspondence with the long-lasting decrease in arterial blood volumes. To determine whether the temporal association between astrocytic activation and cocaine induced vasoconstriction reflected a causal association we inhibited astrocytic Ca2+ using GFAP-DREADD(Gi). Inhibition of astrocytes attenuated the vasoconstriction resulting from cocaine, providing evidence that astrocytes play a critical role in cocaine's vasoconstrictive effects in the brain. These results indicate that neurons and astrocytes play different roles in mediating neurovascular coupling in response to cocaine. Our findings implicate neuronal activation as the main driver of the short-lasting reduction in tissue oxygenation and astrocyte long-lasting activation as the driver of the persistent vasoconstriction with cocaine. Understanding the cellular and vascular interaction induced by cocaine will be helpful for future putative treatments to reduce cerebrovascular pathology from cocaine use.

Memantine Attenuates Cocaine and neuroHIV Neurotoxicity in the Medial Prefrontal Cortex

Individuals with substance use disorder are at a higher risk of contracting HIV and progress more rapidly to AIDS as drugs of abuse, such as cocaine, potentiate the neurotoxic effects of HIV-associated proteins including, but not limited to, HIV-1 trans-activator of transcription (Tat) and the envelope protein Gp120. Neurotoxicity and neurodegeneration are hallmarks of HIV-1-associated neurocognitive disorders (HANDs), which are hypothesized to occur secondary to excitotoxicity from NMDA-induced neuronal calcium dysregulation, which could be targeted with NMDA antagonist drugs. Multiple studies have examined how Gp120 affects calcium influx and how cocaine potentiates this influx; however, they mostly focused on single cells and did not analyze effects in neuronal and vascular brain networks. Here, we utilize a custom multi-wavelength imaging platform to simultaneously study the neuronal activity (detected using genetically encoded Ca²⁺ indicator, GcaMP6f, expressed in neurons) and hemodynamic changes (measured by total hemoglobin and oxygenated hemoglobin within the tissue) in the prefrontal cortex (PFC) of HIV-1 Tg rats in response to cocaine and evaluate the effects of the selective NMDA antagonist drug memantine on cocaine and HIV neurotoxicity compared to those of non-HIV-1 Tg animals (controls). Our results show that memantine improved cocaine-induced deficit in cerebral blood volume while also attenuating an abnormal increase of the neuronal calcium influx and influx duration in both control rats and HIV-1 Tg rats. Cocaine-induced neuronal and hemodynamic dysregulations were significantly greater in HIV-1 Tg rats than in control rats. With memantine pretreatment, HIV-1 Tg rats showed attenuated cocaine’s effects on neuronal and hemodynamic responses, with responses similar to those observed in control rats. These imaging results document an enhancement of neuronal Ca²⁺ influx, hypoxemia, and ischemia with cocaine in the PFC of HIV-1 Tg rats that were attenuated by memantine pretreatment. Thus, the potential utility of memantine in the treatment of HAND and of cocaine-induced neurotoxicity deserves further investigation.

Ca2+ channel blockade reduces cocaine's vasoconstriction and neurotoxicity in the prefrontal cortex

Cocaine profoundly affects both cerebral blood vessels and neuronal activity in the brain. The vasoconstrictive effects of cocaine, concurrently with its effects on neuronal [Ca²⁺]_i accumulation are likely to jeopardize neuronal tissue that in the prefrontal cortex (PFC) could contribute to impaired self-regulation and compulsive cocaine consumption. Here we used optical imaging to study the cerebrovascular and neuronal effects of acute cocaine (1 mg/kg i.v.) and to examine whether selective blockade of L-type Ca²⁺ channels by Nifedipine (NIF) (0.5 mg/kg i.v.) would alleviate cocaine's effects on hemodynamics (measured with cerebral blood volume, HbT), oxygenation (measured with oxygenated hemoglobin, HbO₂) and neuronal [Ca²⁺]_i, which were concomitantly measured in the PFC of naive rats. Our results show that in the PFC acute cocaine significantly reduced flow delivery (HbT), increased neuronal [Ca²⁺]_i accumulation and profoundly reduced tissue oxygenation (HbO₂) and these effects were significantly attenuated by NIF pretreatment. They also show that cocaine-induced vasoconstriction is distinct from its increase of neuronal [Ca²⁺]_i accumulation though both of them contribute to hypoxemia and both effects were attenuated by NIF. These results provide evidence that blockade of voltage-gated L-type Ca²⁺ channels might be beneficial in preventing vasoconstriction and neurotoxic effects of cocaine and give support for further clinical investigations to determine their value in reducing cocaine's neurotoxicity in cocaine use disorders.

Optical imaging of stimulation-evoked cortical activity using GCaMP6f and jRGECO1a

BACKGROUND

Genetically encoded calcium indicators (GECIs), especially the GCaMP-based green fluorescence GECIs have been widely used for in vivo detection of neuronal activity in rodents by measuring intracellular neuronal Ca²⁺ changes. More recently, jRGECO1a, a red shifted GECI, has been reported to detect neuronal Ca²⁺ activation. This opens the possibility of using dual-color GECIs for simultaneous interrogation of different cell populations. However, there has been no report to compare the functional difference between these two GECIs for in vivo imaging. Here, a comparative study is reported on neuronal responses to sensory stimulation using GCaMP6f and jRGECO1a that were virally delivered into the neurons in the somatosensory cortex of two different groups of animals, respectively.

METHODS

GCaMP6f and jRGECO1a GECI were virally delivered to sensory cortex. After 3-4 weeks, the animals were imaged to capture the spatiotemporal changes of neuronal Ca²⁺ and the hemodynamic responses to forepaw electrical stimulation (0.3 mA, 0.3 ms/pulse, 0.03 Hz). The stimulation-evoked neuronal Ca²⁺ transients expressed with GCaMP6f or jRGECO1a were recorded during the baseline period and after an acute cocaine administration (1 mg/kg, i.v.).

RESULTS

Histology confirmed that the efficiency of jRGECO1a and GCaMP6f expression into the cortical neurons was similar, i.e., 34%±3% and 32.7%±1.6%, respectively. Our imaging in vivo showed that the hemodynamic responses to the stimulation were the same between jRGECO1a and GCaMP6f expressed groups. Although the stimulation-evoked fluorescence change (∆F/F) and the time-to-peak of the neuronal Ca²⁺ transients were not significantly different between these two indicators, the full-width-half-maximum (FWHM) duration of the ∆F/F rise in the jRGECO1a-expressed group (0.16±0.02 s) was ~50 ms or 46% longer than that of the GCaMP6f group (0.11±0.003 s), indicating a longer recovery time in jRGECO1a than in GCaMP6f transients (P<0.01). This is likely due to the longer off rate of jRGECO1a than that of GCaMP6f. After cocaine, the time-to-peak of Ca²⁺ transients was delayed and their FWHM duration was prolonged for both expression groups, indicating that these are cocaine's effects on neuronal Ca²⁺ signaling and not artifacts due to the property differences of the GCEIs.

CONCLUSIONS

This study shows that both jRGECO1a and GCaMP6f have sufficient sensitivity for tracking single-stimulation-evoked Ca²⁺ transients to detect neuronal activities from the brain. Since these GECIs are emitted at the different wavelengths, it will be possible to use them together to characterize the activity of different cell types (e.g., neurons and astrocytes) to study brain activation and brain functional changes in normal or diseased brains.

Chronic Brain Imaging Across a Transparent Nanocrystalline Yttria-Stabilized-Zirconia Cranial Implant

Repeated non-diffuse optical imaging of the brain is difficult. This is due to the fact that the cranial bone is highly scattering and thus a strong optical barrier. Repeated craniotomies increase the risk of complications and may disrupt the biological systems being imaged. We previously introduced a potential solution in the form of a transparent ceramic cranial implant called the Window to the Brain (WttB) implant. This implant is made of nanocrystalline Yttria-Stabilized Zirconia (nc-YSZ), which possesses the requisite mechanical strength to serve as a permanent optical access window in human patients. In this present study, we demonstrate repeated brain imaging of n = 5 mice using both OCT and LSI across the WttB implant over 4 weeks. The main objectives are to determine if the WttB implant allows for chronic OCT imaging, and to shed further light on the question of whether optical access provided by the WttB implant remains stable over this duration in the body. The Window to the Brain implant allowed for stable repeated imaging of the mouse brain with Optical Coherence Tomography over 28 days, without loss of signal intensity. Repeated Laser Speckle Imaging was also possible over this timeframe, but signal to noise ratio and the sharpness of vessels in the images decreased with time. This can be partially explained by elevated blood flow during the first imaging session in response to trauma from the surgery, which was also detected by OCT flow imaging. These results are promising for long-term optical access through the WttB implant, making feasible chronic in vivo studies in multiple neurological models of brain disease.

Interactions between stimuli-evoked cortical activity and spontaneous low frequency oscillations measured with neuronal calcium

Spontaneous brain activity has been widely used to map brain connectivity. The interactions between task-evoked brain responses and the spontaneous cortical oscillations, especially within the low frequency range of ~0.1 ​Hz, are not fully understood. Trial-to-trial variabilities in brain's response to sensory stimuli and the ability for brain to detect under noisy conditions suggest an appreciable impact of the brain state. Using a multimodality imaging platform, we simultaneously imaged neuronal Ca²⁺ and cerebral hemodynamics at baseline and in response to single-pulse forepaw stimuli in rat's somatosensory cortex. The high sensitivity of this system enables detection of responses to very weak and strong stimuli and real time determination of low frequency oscillations without averaging. Results show that the ongoing neuronal oscillations inversely modulate Ca²⁺ transients evoked by sensory stimuli. High intensity stimuli reset the spontaneous neuronal oscillations to an unpreferable excitability following the stimulus. Cerebral hemodynamic responses also inversely interact with the spontaneous hemodynamic oscillations, correlating with the neuronal Ca²⁺ transient changes. The results reveal competing interactions between spontaneous oscillations and stimulation-evoked brain activities in somatosensory cortex and the resultant hemodynamics.

Optical Access to Arteriovenous Cerebral Microcirculation Through a Transparent Cranial Implant

BACKGROUND AND OBJECTIVE

Microcirculation plays a critical role in physiologic processes and several disease states. Laser speckle imaging (LSI) is a full-field, real-time imaging technique capable of mapping microvessel networks and providing relative flow velocity within the vessels. In this study, we demonstrate that LSI combine with multispectral reflectance imaging (MSRI), which allows for distinction between veins and arteries in the vascular flow maps produced by LSI. We apply this combined technique to mouse cerebral vascular network in vivo, comparing imaging through the skull, to the dura mater and brain directly through a craniectomy, and through a transparent cranial "Window to the Brain" (WttB) implant.

STUDY DESIGN/MATERIALS AND METHODS

The WttB implant used in this study is made of a nanocrystalline Yttria-Stabilized-Zirconia ceramic. MSRI was conducted using white-light illumination and filtering the reflected light for 560, 570, 580, 590, 600, and 610 nm. LSI was conducted using an 810 nm continuous wave near-infrared laser with incident power of 100 mW, and the reflected speckle pattern was captured by a complementary metal-oxide-semiconductor (CMOS) camera.

RESULTS

Seven vessel branches were analyzed and comparison was made between imaging through the skull, craniectomy, and WttB implant. Through the skull, MSRI did not detect any vessels, and LSI could not image microvessels. Imaging through the WttB implant, MSRI was able to identify veins versus arteries, and LSI was able to image microvessels with only slightly higher signal-to-noise ratio and lower sharpness than imaging the brain through a craniectomy.

CONCLUSIONS

This study demonstrates the ability to perform MSRI-LSI across a transparent cranial implant, to allow for cerebral vascular networks to be mapped, including microvessels. These images contain additional information such as vein-artery separation and relative blood flow velocities, information which is of value scientifically and medically. The WttB implant provides substantial improvements over imaging through the murine cranial bone, where microvessels are not visible and MSRI cannot be performed. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.

Enhanced neuronal and blunted hemodynamic reactivity to cocaine in the prefrontal cortex following extended cocaine access: optical imaging study in anesthetized rats

Cocaine addiction is associated with dysfunction of the prefrontal cortex (PFC), which facilitates relapse and compulsive drug taking. To assess if cocaine's effects on both neuronal and vascular activity contribute to PFC dysfunction, we used optical coherence tomography and multi-wavelength laser speckle to measure vascularization and hemodynamics and used GCaMP6f to monitor intracellular Ca²⁺ levels ([Ca²⁺ ]_in ) as a marker of neuronal activity. Rats were given short (1 hour; ShA) or long (6 hours; LgA) access cocaine self-administration. As expected, LgA but not ShA rats escalated cocaine intake. In naïve rats, acute cocaine decreased oxygenated hemoglobin, increased deoxygenated hemoglobin and reduced cerebral blood flow in PFC, likely due to cocaine-induced vasoconstriction. ShA rats showed enhanced hemodynamic response and slower recovery after cocaine, versus naïve. LgA rats showed a blunted hemodynamic response, but an enhanced PFC neuronal [Ca²⁺ ]_in increase after cocaine challenge associated with drug intake. Both ShA and LgA groups had higher vessel density, indicative of angiogenesis, presumably to compensate for cocaine's vasoconstricting effects. Cocaine self-administration modified the PFC cerebrovascular responses enhancing it in ShA and attenuating it in LgA animals. In contrast, LgA but not ShA animals showed sensitized neuronal reactivity to acute cocaine in the PFC. The opposite changes in hemodynamics (decreased) and neuronal responses (enhanced) in LgA rats indicate that these constitute distinct effects and suggest that the neuronal and not the vascular effects are associated with escalation of cocaine intake in addiction whereas its vascular effect in PFC might contribute to cognitive deficits that increase vulnerability to relapse.

Cocaine Decreases Spontaneous Neuronal Activity and Increases Low-Frequency Neuronal and Hemodynamic Cortical Oscillations

Low-frequency oscillations (LFOs) in hemodynamics assessed by fMRI reflect synchronized neuronal activities and are the basis for mapping brain function and its disruption by drugs and disease. Here we assess if cocaine disrupts coupling between neuronal and vascular LFOs by simultaneously measuring cortical field potentials (FP) and cerebral blood flow (CBF) regarding their LFOs (0-1 Hz) spectral bandwidths in the somatosensory cortex of naïve and chronic cocaine-exposed rats at baseline and during cocaine intoxication. While across all conditions the dominant oscillation frequencies for FP and CBF LFOs were ~0.1 Hz, the bandwidth of FP LFOs was about 4.8 ± 0.67 times broader than that of CBF LFOs. Acute cocaine depressed high-frequency FP events but increased the relative intensity of neuronal and hemodynamic LFOs, an effect that was markedly accentuated in magnitude and duration in chronic cocaine-exposed animals. Neuronal LFOs were correlated with CBF LFOs in control animals but not in chronically cocaine-exposed animals, which suggests neurovascular uncoupling. The marked increases in neuronal LFOs with chronic cocaine, which we interpret to reflect increases in neuronal synchronization in the LFOs, and the uncoupling of hemodynamics with resting neuronal activities could contribute to brain dysfunction in cocaine abusers and confound the interpretation of fMRI studies.

Hemodynamic and neuronal responses to cocaine differ in awake versus anesthetized animals: Optical brain imaging study

Cocaine is a highly addictive drug with complex pharmacological effects. Most preclinical imaging studies investigating the effects of cocaine in the brain have been performed under anesthesia, which confounds findings. To tackle this problem, we used optical imaging to compare the effects of cocaine in the awake versus the anesthetized states. For this purpose, we customized an air floating mobile cage to fit the multi-wavelength spectral and laser speckle optical imaging system and implanted a multi-layer cranial window over the mouse somatosensory cortex. Results showed significant differences in neuronal activity and hemodynamics at baseline and in response to cocaine between the awake and the anesthetized states (isoflurane anesthesia). Specifically, 1) at baseline isoflurane dilated cerebral vessels, increased cerebral blood flow and depressed neuronal Ca²⁺ activity compared to the awake state; 2) acute cocaine (1 mg/kg iv) vasoconstricted blood vessels (arteries and veins) and decreased cerebral blood flow and oxygenated hemoglobin in the anesthetized state but not in the awake condition; 3) cocaine increased the accumulation of mean intracellular Ca²⁺ in neurons in the anesthetized state but not in the awake condition; and 4) in the awake state acute cocaine increased neuronal activities (increased the frequency of Ca²⁺ transients) and increased neuronal synchronization. We also corroborated that in the awake state cocaine also disrupted neurovascular coupling. These findings indicate that both vascular and neuronal responses to cocaine are influenced by isoflurane anesthesia, which highlights the importance of imaging awake animals when studying the effects of cocaine or other drugs in the brain.

Evaluation of a transparent cranial implant as a permanent window for cerebral blood flow imaging

Laser speckle imaging (LSI) of mouse cerebral blood flow was compared through a transparent nanocrystalline yttria-stabilized zirconia (nc-YSZ) cranial implant over time (at days 0, 14, and 28, n = 3 mice), and vs. LSI through native skull (at day 60, n = 1 mouse). The average sharpness of imaged vessels was found to remain stable, with relative change in sharpness under 7.69% ± 1.2% over 28 days. Through-implant images of vessels at day 60 appeared sharper and smaller on average, with microvessels clearly visible, compared to through-skull images where vessels appeared blurred and distorted. These results suggest that long-term imaging through this implant is feasible.

Dual-Wavelength Laser Speckle Contrast Imaging (dwLSCI) Improves Chronic Measurement of Superficial Blood Flow in Hands

Laser speckle contrast imaging (LSCI) has been widely used to determine blood flow and perfusion in biological tissues. The physical model of traditional LSCI ignores the effects of scattering property distribution in relation to speckle correlation time τ_c and blood flow v, which further results in biased estimation. In this study, we developed a dual-wavelength laser speckle contrast imaging (dwLSCI) method and a portable device for imaging the blood flow and tissue perfusion in human hands. Experimental data showed that dwLSCI could retrieve the vein vasculatures under the surface skin, and it further provided accurate measurements of vein blood flow signals, tissue perfusion signals, and fingertip perfusion signals, which assist with assessments of rehabilitation therapy for stroke patients. Fingertip perfusion signals demonstrated better performance in early assessments, while vein blood flow signals assisted the Fugl-Meyer Assessment Scale (FMA) and the Wolf Motor Function Test (WMFT) behavior assessments. As a general noninvasive imaging method, dwLSCI can be applied in clinical studies related to hand functions combined with behavior assessments.

Long-term optical imaging of neurovascular coupling in mouse cortex using GCaMP6f and intrinsic hemodynamic signals

Cerebral hemodynamics are modulated in response to changes in neuronal activity, a process termed neurovascular coupling (NVC), which can be disrupted by neuropsychiatric diseases (e.g., stroke, Alzheimer's disease). Thus, there is growing interest to image long-term NVC dynamics with high spatiotemporal resolutions. Here, by combining the use of a genetically-encoded calcium indicator with optical techniques, we develop a longitudinal multimodal optical imaging platform (MIP) that enabled time-lapse tracking of NVC over a relatively large field of view in the mouse somatosensory cortex at single cell and single vessel resolutions. Specifically, GCaMP6f was used as marker of neuronal activity, which along with MIP allowed us to simultaneously measure the changes in neuronal [Ca²⁺]_i fluorescence, cerebral blood flow velocity (CBFv) and hemodynamics longitudinally for more than eight weeks. We show that [Ca²⁺]_i fluorescence was detectable one week post viral injection and the damage to local microvasculature and perfusion recovered two weeks after injection. By three weeks post viral injection, maximal neuronal and CBFv responses to hindpaw stimulations were observed. Moreover, single neuronal activation in response to hindpaw stimulation was consistently recorded, followed by ∼2 s delayed dilation of contiguous microvessels. Additionally, resting-state spontaneous neuronal and hemodynamic oscillations were detectable throughout the eight weeks of study. Our results demonstrate the capability of MIP for longitudinal investigation of the organization and plasticity of the neurovascular network during resting state and during stimulation-evoked neuronal activation at high spatiotemporal resolutions.

Stepwise method based on Wiener estimation for spectral reconstruction in spectroscopic Raman imaging

Raman spectroscopy has demonstrated great potential in biomedical applications. However, spectroscopic Raman imaging is limited in the investigation of fast changing phenomena because of slow data acquisition. Our previous studies have indicated that spectroscopic Raman imaging can be significantly sped up using the approach of narrow-band imaging followed by spectral reconstruction. A multi-channel system was built to demonstrate the feasibility of fast wide-field spectroscopic Raman imaging using the approach of simultaneous narrow-band image acquisition followed by spectral reconstruction based on Wiener estimation in phantoms. To further improve the accuracy of reconstructed Raman spectra, we propose a stepwise spectral reconstruction method in this study, which can be combined with the earlier developed sequential weighted Wiener estimation to improve spectral reconstruction accuracy. The stepwise spectral reconstruction method first reconstructs the fluorescence background spectrum from narrow-band measurements and then the pure Raman narrow-band measurements can be estimated by subtracting the estimated fluorescence background from the overall narrow-band measurements. Thereafter, the pure Raman spectrum can be reconstructed from the estimated pure Raman narrow-band measurements. The result indicates that the stepwise spectral reconstruction method can improve spectral reconstruction accuracy significantly when combined with sequential weighted Wiener estimation, compared with the traditional Wiener estimation. In addition, qualitatively accurate cell Raman spectra were successfully reconstructed using the stepwise spectral reconstruction method from the narrow-band measurements acquired by a four-channel wide-field Raman spectroscopic imaging system. This method can potentially facilitate the adoption of spectroscopic Raman imaging to the investigation of fast changing phenomena.

High-speed swept source optical coherence Doppler tomography for deep brain microvascular imaging

Noninvasive microvascular imaging using optical coherence Doppler tomography (ODT) has shown great promise in brain studies; however, high-speed microcirculatory imaging in deep brain remains an open quest. A high-speed 1.3 μm swept-source ODT (SS-ODT) system is reported which was based on a 200 kHz vertical-cavity-surface-emitting laser. Phase errors induced by sweep-trigger desynchronization were effectively reduced by spectral phase encoding and instantaneous correlation among the A-scans. Phantom studies have revealed a significant reduction in phase noise, thus an enhancement of minimally detectable flow down to 268.2 μm/s. Further in vivo validation was performed, in which 3D cerebral-blood-flow (CBF) networks in mouse brain over a large field-of-view (FOV: 8.5 × 5 × 3.2 mm³) was scanned through thinned skull. Results showed that fast flows up to 3 cm/s in pial vessels and minute flows down to 0.3 mm/s in arterioles or venules were readily detectable at depths down to 3.2 mm. Moreover, the dynamic changes of the CBF networks elicited by acute cocaine such as heterogeneous responses in various vessel compartments and at different cortical layers as well as transient ischemic events were tracked, suggesting the potential of SS-ODT for brain functional imaging that requires high flow sensitivity and dynamic range, fast frame rate and a large FOV to cover different brain regions.