Evaluation of calibrated and uncalibrated optical imaging approaches for relative cerebral oxygen metabolism measurements in awake mice

Objective. The continuous delivery of oxygen is critical to sustain brain function, and therefore, measuring brain oxygen consumption can provide vital physiological insight. In this work, we examine the impact of calibration and cerebral blood flow (CBF) measurements on the computation of the relative changes in the cerebral metabolic rate of oxygen consumption (rCMRO2) from hemoglobin-sensitive intrinsic optical imaging data. Using these data, we calculate rCMRO2, and calibrate the model using an isometabolic stimulus.Approach. We used awake head-fixed rodents to obtain hemoglobin-sensitive optical imaging data to test different calibrated and uncalibrated rCMRO2models. Hypercapnia was used for calibration and whisker stimulation was used to test the impact of calibration.Main results. We found that typical uncalibrated models can provide reasonable estimates of rCMRO2with differences as small as 7%-9% compared to their calibrated models. However, calibrated models showed lower variability and less dependence on baseline hemoglobin concentrations. Lastly, we found that supplying the model with measurements of CBF significantly reduced error and variability in rCMRO2change calculations.Significance. The effect of calibration on rCMRO2calculations remains understudied, and we systematically evaluated different rCMRO2calculation scenarios that consider including different measurement combinations. This study provides a quantitative comparison of these scenarios to evaluate trade-offs that can be vital to the design of blood oxygenation sensitive imaging experiments for rCMRO2calculation.

Long-range inhibitory neurons mediate cortical neurovascular coupling

To meet the high energy demands of brain function, cerebral blood flow (CBF) parallels changes in neuronal activity by a mechanism known as neurovascular coupling (NVC). However, which neurons play a role in mediating NVC is not well understood. Here, we identify in mice and humans a specific population of cortical GABAergic neurons that co-express neuronal nitric oxide synthase and tachykinin receptor 1 (Tacr1). Through whole-tissue clearing, we demonstrate that Tacr1 neurons extend local and long-range projections across functionally connected cortical areas. We show that whisker stimulation elicited Tacr1 neuron activity in the barrel cortex through feedforward excitatory pathways. Additionally, through optogenetic experiments, we demonstrate that Tacr1 neurons are instrumental in mediating CBF through the relaxation of mural cells in a similar fashion to whisker stimulation. Finally, by electron microscopy, we observe that Tacr1 processes contact astrocytic endfeet. These findings suggest that Tacr1 neurons integrate cortical activity to mediate NVC.

Revealing in vivo cellular mechanisms of cerebral microbleeds on neurons and microglia across cortical layers

Cerebral microbleeds (CMBs) are associated with higher risk for various neurological diseases including stroke, dementia, and Alzheimer's disease. However, the understanding of cellular pathology of CMBs, particularly in deep brain regions, remains limited. Utilizing two-photon microscopy and microprism implantation, we longitudinally imaged the impact of CMBs on neuronal and microglial activities across cortical depths in awake mice. A temporary decline in spontaneous neuronal activity occurred throughout cortical layers, followed by recovery within a week. However, significant changes of neuron-neuron activity correlations persisted for weeks. Moreover, microglial contact with neuron soma significantly increased post-microbleeds, indicating an important modulatory role of microglia. Notably, microglial contact, negatively correlated with neuronal firing rate in normal conditions, became uncorrelated after microbleeds, suggesting a decreased neuron-microglia inhibition. These findings reveal chronic alterations in cortical neuronal networks and microglial-neuronal interactions across cortical depths, shedding light on the pathology of CMBs.

Activation and depression of neural and hemodynamic responses induced by the intracortical microstimulation and visual stimulation in the mouse visual cortex

Objective. Intracortical microstimulation (ICMS) can be an effective method for restoring sensory perception in contemporary brain-machine interfaces. However, the mechanisms underlying better control of neuronal responses remain poorly understood, as well as the relationship between neuronal activity and other concomitant phenomena occurring around the stimulation site.Approach. Different microstimulation frequencies were investigatedin vivoon Thy1-GCaMP6s mice using widefield and two-photon imaging to evaluate the evoked excitatory neural responses across multiple spatial scales as well as the induced hemodynamic responses. Specifically, we quantified stimulation-induced neuronal activation and depression in the mouse visual cortex and measured hemodynamic oxyhemoglobin and deoxyhemoglobin signals using mesoscopic-scale widefield imaging.Main results. Our calcium imaging findings revealed a preference for lower-frequency stimulation in driving stronger neuronal activation. A depressive response following the neural activation preferred a slightly higher frequency stimulation compared to the activation. Hemodynamic signals exhibited a comparable spatial spread to neural calcium signals. Oxyhemoglobin concentration around the stimulation site remained elevated during the post-activation (depression) period. Somatic and neuropil calcium responses measured by two-photon microscopy showed similar dependence on stimulation parameters, although the magnitudes measured in soma was greater than in neuropil. Furthermore, higher-frequency stimulation induced a more pronounced activation in soma compared to neuropil, while depression was predominantly induced in soma irrespective of stimulation frequencies.Significance. These results suggest that the mechanism underlying depression differs from activation, requiring ample oxygen supply, and affecting neurons. Our findings provide a novel understanding of evoked excitatory neuronal activity induced by ICMS and offer insights into neuro-devices that utilize both activation and depression phenomena to achieve desired neural responses.

Integrated Microprism and Microelectrode Array for Simultaneous Electrophysiology and Two-Photon Imaging across All Cortical Layers

Cerebral neural electronics play a crucial role in neuroscience research with increasing translational applications such as brain-computer interfaces for sensory input and motor output restoration. While widely utilized for decades, the understanding of the cellular mechanisms underlying this technology remains limited. Although two-photon microscopy (TPM) has shown great promise in imaging superficial neural electrodes, its application to deep-penetrating electrodes is technically difficult. Here, a novel device integrating transparent microelectrode arrays with glass microprisms, enabling electrophysiology recording and stimulation alongside TPM imaging across all cortical layers in a vertical plane, is introduced. Tested in Thy1-GCaMP6 mice for over 4 months, the integrated device demonstrates the capability for multisite electrophysiological recording/stimulation and simultaneous TPM calcium imaging. As a proof of concept, the impact of microstimulation amplitude, frequency, and depth on neural activation patterns is investigated using the setup. With future improvements in material stability and single unit yield, this multimodal tool greatly expands integrated electrophysiology and optical imaging from the superficial brain to the entire cortical column, opening new avenues for neuroscience research and neurotechnology development.

Activation and depression of neural and hemodynamic responses induced by the intracortical microstimulation and visual stimulation in the mouse visual cortex

Objective . Intracortical microstimulation can be an effective method for restoring sensory perception in contemporary brain-machine interfaces. However, the mechanisms underlying better control of neuronal responses remain poorly understood, as well as the relationship between neuronal activity and other concomitant phenomena occurring around the stimulation site. Approach . Different microstimulation frequencies were investigated in vivo on Thy1-GCaMP6s mice using widefield and two-photon imaging to evaluate the evoked excitatory neural responses across multiple spatial scales as well as the induced hemodynamic responses. Specifically, we quantified stimulation-induced neuronal activation and depression in the mouse visual cortex and measured hemodynamic oxyhemoglobin and deoxyhemoglobin signals using mesoscopic-scale widefield imaging. Main results . Our calcium imaging findings revealed a preference for lower-frequency stimulation in driving stronger neuronal activation. A depressive response following the neural activation preferred a slightly higher frequency stimulation compared to the activation. Hemodynamic signals exhibited a comparable spatial spread to neural calcium signals. Oxyhemoglobin concentration around the stimulation site remained elevated during the post-activation (depression) period. Somatic and neuropil calcium responses measured by two-photon microscopy showed similar dependence on stimulation parameters, although the magnitudes measured in soma was greater than in neuropil. Furthermore, higher-frequency stimulation induced a more pronounced activation in soma compared to neuropil, while depression was predominantly induced in soma irrespective of stimulation frequencies. Significance . These results suggest that the mechanism underlying depression differs from activation, requiring ample oxygen supply, and affecting neurons. Our findings provide a novel understanding of evoked excitatory neuronal activity induced by intracortical microstimulation and offer insights into neuro-devices that utilize both activation and depression phenomena to achieve desired neural responses.

Electrically Controlled Vasodilator Delivery from PEDOT/Silica Nanoparticle Modulates Vessel Diameter in Mouse Brain

Vascular damage and reduced tissue perfusion are expected to majorly contribute to the loss of neurons or neural signals around implanted electrodes. However, there are limited methods of controlling the vascular dynamics in tissues surrounding these implants. This work utilizes conducting polymer poly(ethylenedioxythiophene) and sulfonated silica nanoparticle composite (PEDOT/SNP) to load and release a vasodilator, sodium nitroprusside, to controllably dilate the vasculature around carbon fiber electrodes (CFEs) implanted in the mouse cortex. The vasodilator release is triggered via electrical stimulation and the amount of release increases with increasing electrical pulses. The vascular dynamics are monitored in real-time using two-photon microscopy, with changes in vessel diameters quantified before, during, and after the release of the vasodilator into the tissues. This work observes significant increases in vessel diameters when the vasodilator is electrically triggered to release, and differential effects of the drug release on vessels of different sizes. In conclusion, the use of nanoparticle reservoirs in conducting polymer-based drug delivery platforms enables the controlled delivery of vasodilator into the implant environment, effectively altering the local vascular dynamics on demand. With further optimization, this technology could be a powerful tool to improve the neural electrode-tissue interface and study neurovascular coupling.

Resuscitation with epinephrine worsens cerebral capillary no-reflow after experimental pediatric cardiac arrest: An in vivo multiphoton microscopy evaluation

Epinephrine is the principal resuscitation therapy for pediatric cardiac arrest (CA). Clinical data suggest that although epinephrine increases the rate of resuscitation, it fails to improve neurological outcome, possibly secondary to reductions in microvascular flow. We characterized the effect of epinephrine vs. placebo administered at resuscitation from pediatric asphyxial CA on microvascular and macrovascular cortical perfusion assessed using in vivo multiphoton microscopy and laser speckle flowmetry, respectively, and on brain tissue oxygenation (PbO₂), behavioral outcomes, and neuropathology in 16-18-day-old rats. Epinephrine-treated rats had a more rapid return of spontaneous circulation and brisk immediate cortical reperfusion during 1-3 min post-CA vs. placebo. However, at the microvascular level, epinephrine-treated rats had penetrating arteriole constriction and increases in both capillary stalling (no-reflow) and cortical capillary transit time 30-60 min post-CA vs. placebo. Placebo-treated rats had increased capillary diameters post-CA. The cortex was hypoxic post-CA in both groups. Epinephrine treatment worsened reference memory performance vs. shams. Hippocampal neuron counts did not differ between groups. Resuscitation with epinephrine enhanced immediate reperfusion but produced microvascular alterations during the first hour post-resuscitation, characterized by vasoconstriction, capillary stasis, prolonged cortical transit time, and absence of compensatory cortical vasodilation. Targeted therapies mitigating the deleterious microvascular effects of epinephrine are needed.

Imaging the Efficiency of Poly(3,4-ethylenedioxythiophene) Doped with Acid-Functionalized Carbon Nanotube and Iridium Oxide Electrode Coatings for Microstimulation

Electrical microstimulation has shown promise in restoring neural deficits in humans. Electrodes coated with materials like the conducting polymer poly(3,4-ethylenedioxythiophene) doped with acid-functionalized carbon nanotubes (PEDOT/CNTs, or PC) exhibit superior charge injection than traditional metals like platinum. However, the stimulation performance of PC remains to be fully characterized. Advanced imaging techniques and transgenic tools allow for real-time observations of neural activity in vivo. Herein, microelectrodes coated with PC and iridium oxide (IrOx) (a commonly used high-charge-injection material) are implanted in GCaMP6s mice and electrical stimulation is applied while imaging neuronal calcium responses. Results show that PC-coated electrodes stimulate more intense and broader GCaMP responses than IrOx. Two-photon microscopy reveals that PC-coated electrodes activate significantly more neuronal soma and neuropil than IrOx-coated electrodes in constant-voltage stimulation and significantly more neuronal soma in constant-current stimulation. Furthermore, with the same injected charge, both materials activate more spatially confined neural elements with shorter pulses than longer pulses, providing a means to tune stimulation selectivity. Finite element analyses reveal that the PC coating creates a denser and nonuniform electric field, increasing the likelihood of activating nearby neural elements. PC coating can significantly improve energy efficiency for electrical stimulation applications.

Contribution of Excitatory and Inhibitory Neuronal Activity to BOLD fMRI

The BOLD fMRI response in the cortex is often assumed to reflect changes in excitatory neural activity. However, the contribution of inhibitory neurons to BOLD fMRI is unclear. Here, the role of inhibitory and excitatory activity was examined using multimodal approaches: electrophysiological recording, 15.2 T fMRI, optical intrinsic signal imaging, and modeling. Inhibitory and excitatory neuronal activity in the somatosensory cortex were selectively modulated by 20-s optogenetic stimulation of VGAT-ChR2 and CaMKII-ChR2 mice, respectively. Somatosensory stimulation and optogenetic stimulation of excitatory neurons induced positive BOLD responses in the somatosensory network, whereas stimulation of inhibitory neurons produced biphasic responses at the stimulation site, initial positive and later negative BOLD signals, and negative BOLD responses at downstream sites. When the stimulation duration was reduced to 5 s, the hemodynamic response of VGAT-ChR2 mice to optogenetic stimulation was only positive. Lastly, modeling performed from neuronal and hemodynamic data shows that the hemodynamic response function (HRF) of excitatory neurons is similar across different conditions, whereas the HRF of inhibitory neurons is highly sensitive to stimulation frequency and peaks earlier than that of excitatory neurons. Our study provides insights into the neurovascular coupling of excitatory and inhibitory neurons and the interpretation of BOLD fMRI signals.

Postsynaptic activity of inhibitory neurons evokes hemodynamic fMRI responses

Functional MRI responses are localized to the synaptic sites of evoked inhibitory neurons, but it is unknown whether, or by what mechanisms, these neurons initiate functional hyperemia. Here, the neuronal origins of these hemodynamic responses were investigated by fMRI or local field potential and blood flow measurements during topical application of pharmacological agents when GABAergic granule cells in the rat olfactory bulb were synaptically targeted. First, to examine if postsynaptic activation of these inhibitory neurons was required for neurovascular coupling, we applied an NMDA receptor antagonist during cerebral blood volume-weighted fMRI acquisition and found that responses below the drug application site (up to ~1.5 mm) significantly decreased within ~30 min. Similarly, large decreases in granule cell postsynaptic activities and blood flow responses were observed when AMPA or NMDA receptor antagonists were applied. Second, inhibition of nitric oxide synthase preferentially decreased the initial, fast component of the blood flow response, while inhibitors of astrocyte-specific glutamate transporters and vasoactive intestinal peptide receptors did not decrease blood flow responses. Third, inhibition of GABA release with a presynaptic GABAB receptor agonist caused less reduction of neuronal and blood flow responses compared to the postsynaptic glutamate receptor antagonists. In conclusion, local hyperemia by synaptically-evoked inhibitory neurons was primarily driven by their postsynaptic activities, possibly through NMDA receptor-dependent calcium signaling that was not wholly dependent on nitric oxide.

Optogenetic assessment of VIP, PV, SOM and NOS inhibitory neuron activity and cerebral blood flow regulation in mouse somato-sensory cortex

The impact of different neuronal populations on local cerebral blood flow (CBF) regulation is not well known and insight into these relationships could enhance the interpretation of brain function and dysfunction from brain imaging data. We investigated the role of sub-types of inhibitory neuron activity on the regulation of CBF using optogenetics, laser Doppler flowmetry and different transgenic mouse models (parvalbumin (PV), vasoactive intestinal peptide (VIP), somatostatin (SOM) and nitric oxide synthase (NOS)). Whisker stimulation was used to verify that typical CBF responses were obtained in all mice. Photo-stimulation of SOM-cre and NOS-cre mice produced significant increases in CBF that were similar to whisker responses. In NOS-cre mice, CBF responses scaled with the photo-stimulus pulse duration and frequency. In SOM-cre mice, CBF increases were followed by decreases. In VIP-cre mice, photo-stimulation did not consistently produce significant changes in CBF, while slower increases in CBF that peaked 14-18 s after stimulation onset were observed in PV-cre mice. Control experiments performed in non-expressing regions showed no changes in CBF. These findings suggest that dysfunction in NOS or SOM neurons can have a significant impact on vascular responses that are detected by brain imaging methods like functional magnetic resonance imaging (fMRI).

Zwitterionic Polymer Coating Suppresses Microglial Encapsulation to Neural Implants In Vitro and In Vivo

For brain computer interfaces (BCI), the immune response to implanted electrodes is a major biological cause of device failure. Bioactive coatings such as neural adhesion molecule L1 have been shown to improve the biocompatibility, but are difficult to handle or produce in batches. Here, a synthetic zwitterionic polymer coating, poly(sulfobetaine methacrylate) (PSBMA) is developed for neural implants with the goal of reducing the inflammatory host response. In tests in vitro, the zwitterionic coating inhibits protein adsorption and the attachment of fibroblasts and microglia, and remains stable for at least 4 weeks. In vivo two-photon microscopy on CX3CR1-GFP mice shows that the zwitterionic coating significantly suppresses the microglial encapsulation of neural microelectrodes over a 6 h observation period. Furthermore, the lower microglial encapsulation on zwitterionic polymer-coated microelectrodes is revealed to originate from a reduction in the size but not the number of microglial end feet. This work provides a facile method for coating neural implants with zwitterionic polymers and illustrates the initial interaction between microglia and coated surface at high temporal and spatial resolution.

Development of a PET radioligand selective for cerebral amyloid angiopathy

INTRODUCTION

Positron emission tomography (PET) using radiolabeled amyloid-binding compounds has advanced the field of Alzheimer's disease (AD) by enabling detection and longitudinal tracking of fibrillar amyloid-β (Aβ) deposits in living people. However, this technique cannot distinguish between Aβ deposits in brain parenchyma (amyloid plaques) from those in blood vessels (cerebral amyloid angiopathy, CAA). Development of a PET radioligand capable of selectively detecting CAA would help clarify its contribution to global brain amyloidosis and clinical symptoms in AD and would help to characterize side-effects of anti-Aβ immunotherapies in AD patients, such as CAA.

METHODS

A candidate CAA-selective compound (1) from a panel of analogues of the amyloid-binding dye Congo red was synthesized. The binding affinity to Aβ fibrils and lipophilicity of compound 1 were determined and selectivity for CAA versus parenchymal plaque deposits was assessed ex-vivo and in-vivo in transgenic APP/PS1 mice and in postmortem human brain affected with AD pathology.

RESULTS

Compound 1 displays characteristics of Aβ binding dyes, such as thioflavin-S, in that it labels both parenchymal Aβ plaques and CAA when applied to histological sections from both a transgenic APP/PS1 mouse model of Aβ amyloidosis and AD brain. Thus, compound 1 lacks molecular selectivity to distinguish Aβ deposits in CAA from those in plaques. However, when administered to living APP/PS1 mice intravenously, compound 1 preferentially labels CAA when assessed using in-vivo two-photon microscopy and ex-vivo histology and autoradiography.

CONCLUSION

We hypothesize that selectivity of compound 1 for CAA is attributable to its limited penetration of the blood-brain barrier due to the highly polar nature of the carboxylate moiety, thereby limiting access to parenchymal plaques and promoting selective in-vivo labeling of Aβ deposits in the vascular wall (i.e., "delivery selectivity").

Inhibitory Neuron Activity Contributions to Hemodynamic Responses and Metabolic Load Examined Using an Inhibitory Optogenetic Mouse Model

Hemodynamic signals are routinely used to noninvasively assess brain function in humans and animals. This work examined the contribution of inhibitory neuron activity on hemodynamic responses captured by changes in blood flow, volume and oxygenation in the cortex of lightly anesthetized mice. Because cortical activity is not commonly initiated by inhibitory neurons, experiments were conducted to examine the neuronal activity properties elicited by photo-stimulation. We observed comparable increases in neuronal activity evoked by forelimb and photo-stimulation; however, significantly larger increases in blood flow and volume were produced by photo-stimulation of inhibitory neurons compared with forelimb stimulation. Following blockade of glutamate and GABA-A receptors to reduce postsynaptic activity contributions, neuronal activity was reliably modulated and hemodynamic changes persisted, though slightly reduced. More importantly, photo-stimulation-evoked changes in blood flow and volume were suppressed by 75–80% with the administration of a nitric oxide synthase inhibitor, suggesting that inhibitory neurons regulate blood flow mostly via nitric oxide. Lastly, forelimb and photo-stimulation of excitatory neurons produced local decreases in blood oxygenation, while large increases were generated by photo-stimulation of inhibitory neurons. Estimates of oxygen metabolism suggest that inhibitory neuron activity has a small impact on tissue metabolic load, indicating a mismatch between the metabolic demand and blood flow regulation properties of inhibitory and excitatory neurons.

Intracortical Neural Stimulation With Untethered, Ultrasmall Carbon Fiber Electrodes Mediated by the Photoelectric Effect

OBJECTIVE

Neural stimulation with tethered, electrically activated probes is damaging to neural tissue and lacks good spatial selectivity and stable chronic performance. The photoelectric effect, which converts incident light into electric potential and heat, provides an opportunity for a tetherless stimulation method. We propose a novel stimulation paradigm that relies on the photoelectric effect to stimulate neurons around a free-floating, ultrasmall (7-8 μm diameter) carbon fiber probe.

METHODS

A two-photon microscope induced photo-stimulation with a near-infrared laser. Chronoamperometry and chronopotentiometry were used to characterize the electrochemical properties of photo-stimulation, while the fluorescence of Rhodamine-B was used to quantify temperature changes.

RESULTS

Photo-stimulation caused a local cathodic potential pulse with minimal leakage current. Stimulation induced voltage deflections of 0.05-0.4 V in vitro, varying linearly with the power of the laser source (5-40 mW). Temperature increases in the immediate vicinity of the electrode were limited to 2.5 °C, suggesting that this stimulation modality can be used without inducing heat damage. Successful stimulation was supported in vivo by increased calcium fluorescence in local neurons at stimulation onset in a transgenic GCaMP-3 mouse model. Furthermore, cells activated by photo-stimulation were closer to the electrode than in electrical stimulation under similar conditions, indicating increased spatial precision.

CONCLUSION

Our results support the hypothesis that the proposed photoelectric method for neural stimulation is effective.

SIGNIFICANCE

Photoelectric stimulation is precise and avoids the need for a potentially destructive tether, making it a promising alternative to electrical stimulation.

Viral-Mediated Optogenetic Stimulation of Peripheral Motor Nerves in Non-human Primates

Objective: Reanimation of muscles paralyzed by disease states such as spinal cord injury remains a highly sought therapeutic goal of neuroprosthetic research. Optogenetic stimulation of peripheral motor nerves expressing light-sensitive opsins is a promising approach to muscle reanimation that may overcome several drawbacks of traditional methods such as functional electrical stimulation (FES). However, the utility of these methods has only been demonstrated in rodents to date, while translation to clinical practice will likely first require demonstration and refinement of these gene therapy techniques in non-human primates. Approach: Three rhesus macaques were injected intramuscularly with either one or both of two optogenetic constructs (AAV6-hSyn-ChR2-eYFP and/or AAV6-hSyn-Chronos-eYFP) to transduce opsin expression in the corresponding nerves. Neuromuscular junctions were targeted for virus delivery using an electrical stimulating injection technique. Functional opsin expression was periodically evaluated up to 13 weeks post-injection by optically stimulating targeted nerves with a 472 nm fiber-coupled laser while recording electromyographic (EMG) responses. Main Results: One monkey demonstrated functional expression of ChR2 at 8 weeks post-injection in each of two injected muscles, while the second monkey briefly exhibited contractions coupled to optical stimulation in a muscle injected with the Chronos construct at 10 weeks. A third monkey injected only in one muscle with the ChR2 construct showed strong optically coupled contractions at 5 ½ weeks which then disappeared by 9 weeks. EMG responses to optical stimulation of ChR2-transduced nerves demonstrated graded recruitment relative to both stimulus pulse-width and light intensity, and followed stimulus trains up to 16 Hz. In addition, the EMG response to prolonged stimulation showed delayed fatigue over several minutes. Significance: These results demonstrate the feasibility of viral transduction of peripheral motor nerves for functional optical stimulation of motor activity in non-human primates, a variable timeline of opsin expression in a animal model closer to humans, and fundamental EMG response characteristics to optical nerve stimulation. Together, they represent an important step in translating these optogenetic techniques as a clinically viable gene therapy.

Cerebral microcirculatory alterations and the no-reflow phenomenon in vivo after experimental pediatric cardiac arrest

Decreased cerebral blood flow (CBF) after cardiac arrest (CA) contributes to secondary ischemic injury in infants and children. We previously reported cortical hypoperfusion with tissue hypoxia early in a pediatric rat model of asphyxial CA. In order to identify specific alterations as potential therapeutic targets to improve cortical hypoperfusion post-CA, we characterize the CBF alterations at the cortical microvascular level in vivo using multiphoton microscopy. We hypothesize that microvascular constriction and disturbances of capillary red blood cell (RBC) flow contribute to cortical hypoperfusion post-CA. After resuscitation from 9 min asphyxial CA, transient dilation of capillaries and venules at 5 min was followed by pial arteriolar constriction at 30 and 60 min (19.6 ± 1.3, 19.3 ± 1.2 µm at 30, 60 min vs. 22.0 ± 1.2 µm at baseline, p < 0.05). At the capillary level, microcirculatory disturbances were highly heterogeneous, with RBC stasis observed in 25.4% of capillaries at 30 min post-CA. Overall, the capillary plasma mean transit time was increased post-CA by 139.7 ± 51.5%, p < 0.05. In conclusion, pial arteriolar constriction, the no-reflow phenomenon and increased plasma transit time were observed post-CA. Our results detail the microvascular disturbances in a pediatric asphyxial CA model and provide a powerful platform for assessing specific vascular-targeted therapies.

Meningeal inflammatory response and fibrous tissue remodeling around intracortical implants: An in vivo two-photon imaging study

Meningeal inflammation and encapsulation of neural electrode arrays is a leading cause of device failure, yet little is known about how it develops over time or what triggers it. This work characterizes the dynamic changes of meningeal inflammatory cells and collagen-I in order to understand the meningeal tissue response to neural electrode implantation. We use in vivo two-photon microscopy of CX3CR1-GFP mice over the first month after electrode implantation to quantify changes in inflammatory cell behavior as well as meningeal collagen-I remodeling. We define a migratory window during the first day after electrode implantation hallmarked by robust inflammatory cell migration along electrodes in the meninges as well as cell trafficking through meningeal venules. This migratory window attenuates by 2 days post-implant, but over the next month, the meningeal collagen-I remodels to conform to the surface of the electrode and thickens. This work shows that there are distinct time courses for initial meningeal inflammatory cell infiltration and meningeal collagen-I remodeling. This may indicate a therapeutic window early after implantation for modulation and mitigation of meningeal inflammation.

Calcium activation of cortical neurons by continuous electrical stimulation: Frequency dependence, temporal fidelity, and activation density

Electrical stimulation of the brain has become a mainstay of fundamental neuroscience research and an increasingly prevalent clinical therapy. Despite decades of use in basic neuroscience research and the growing prevalence of neuromodulation therapies, gaps in knowledge regarding activation or inactivation of neural elements over time have limited its ability to adequately interpret evoked downstream responses or fine-tune stimulation parameters to focus on desired responses. In this work, in vivo two-photon microscopy was used to image neuronal calcium activity in layer 2/3 neurons of somatosensory cortex (S1) in male C57BL/6J-Tg(Thy1-GCaMP6s)GP4.3Dkim/J mice during 30 s of continuous electrical stimulation at varying frequencies. We show frequency-dependent differences in spatial and temporal somatic responses during continuous stimulation. Our results elucidate conflicting results from prior studies reporting either dense spherical activation of somas biased toward those near the electrode, or sparse activation of somas at a distance via axons near the electrode. These findings indicate that the neural element specific temporal response local to the stimulating electrode changes as a function of applied charge density and frequency. These temporal responses need to be considered to properly interpret downstream circuit responses or determining mechanisms of action in basic science experiments or clinical therapeutic applications.

In vivo imaging of neuronal calcium during electrode implantation: Spatial and temporal mapping of damage and recovery

Implantable electrode devices enable long-term electrophysiological recordings for brain-machine interfaces and basic neuroscience research. Implantation of these devices, however, leads to neuronal damage and progressive neural degeneration that can lead to device failure. The present study uses in vivo two-photon microscopy to study the calcium activity and morphology of neurons before, during, and one month after electrode implantation to determine how implantation trauma injures neurons. We show that implantation leads to prolonged, elevated calcium levels in neurons within 150 μm of the electrode interface. These neurons show signs of mechanical distortion and mechanoporation after implantation, suggesting that calcium influx is related to mechanical trauma. Further, calcium-laden neurites develop signs of axonal injury at 1-3 h post-insert. Over the first month after implantation, physiological neuronal calcium activity increases, suggesting that neurons may be recovering. By defining the mechanisms of neuron damage after electrode implantation, our results suggest new directions for therapies to improve electrode longevity.

Multi-scale, multi-modal analysis uncovers complex relationship at the brain tissue-implant neural interface: new emphasis on the biological interface

OBJECTIVE

Implantable neural electrode devices are important tools for neuroscience research and have an increasing range of clinical applications. However, the intricacies of the biological response after implantation, and their ultimate impact on recording performance, remain challenging to elucidate. Establishing a relationship between the neurobiology and chronic recording performance is confounded by technical challenges related to traditional electrophysiological, material, and histological limitations. This can greatly impact the interpretations of results pertaining to device performance and tissue health surrounding the implant.

APPROACH

In this work, electrophysiological activity and immunohistological analysis are compared after controlling for motion artifacts, quiescent neuronal activity, and material failure of devices in order to better understand the relationship between histology and electrophysiological outcomes.

MAIN RESULTS

Even after carefully accounting for these factors, the presence of viable neurons and lack of glial scarring does not convey single unit recording performance.

SIGNIFICANCE

To better understand the biological factors influencing neural activity, detailed cellular and molecular tissue responses were examined. Decreases in neural activity and blood oxygenation in the tissue surrounding the implant, shift in expression levels of vesicular transporter proteins and ion channels, axon and myelin injury, and interrupted blood flow in nearby capillaries can impact neural activity around implanted neural interfaces. Combined, these tissue changes highlight the need for more comprehensive, basic science research to elucidate the relationship between biology and chronic electrophysiology performance in order to advance neural technologies.

A Materials Roadmap to Functional Neural Interface Design

Advancement in neurotechnologies for electrophysiology, neurochemical sensing, neuromodulation, and optogenetics are revolutionizing scientific understanding of the brain while enabling treatments, cures, and preventative measures for a variety of neurological disorders. The grand challenge in neural interface engineering is to seamlessly integrate the interface between neurobiology and engineered technology, to record from and modulate neurons over chronic timescales. However, the biological inflammatory response to implants, neural degeneration, and long-term material stability diminish the quality of interface overtime. Recent advances in functional materials have been aimed at engineering solutions for chronic neural interfaces. Yet, the development and deployment of neural interfaces designed from novel materials have introduced new challenges that have largely avoided being addressed. Many engineering efforts that solely focus on optimizing individual probe design parameters, such as softness or flexibility, downplay critical multi-dimensional interactions between different physical properties of the device that contribute to overall performance and biocompatibility. Moreover, the use of these new materials present substantial new difficulties that must be addressed before regulatory approval for use in human patients will be achievable. In this review, the interdependence of different electrode components are highlighted to demonstrate the current materials-based challenges facing the field of neural interface engineering.

Enduring disturbances in regional cerebral blood flow and brain oxygenation at 24 h after asphyxial cardiac arrest in developing rats

BACKGROUND

Disturbances in cerebral blood flow (CBF) and brain oxygenation (PbO2) are present early after pediatric cardiac arrest (CA). CBF-targeted therapies improved neurological outcome in our CA model. To assess the therapeutic window for CBF- and PbO2-targeted therapies, we propose to determine if CBF and PbO2 disturbances persist at 24 h after experimental pediatric CA.

METHODS

Regional CBF and PbO2 were measured at 24 h after asphyxial CA in immature rats (n = 26, 6-8/group) using arterial spin label MRI and tissue electrodes, respectively.

RESULTS

In all regions but the thalamus, CBF recovered to sham values by 24 h; thalamic CBF was >32% higher after CA vs. sham. PbO2 values at 24 h after CA in the cortex and thalamus were similar to shams in rats who received supplemental oxygen, however, on room air, cortical PbO2 was lower after CA vs. shams.

CONCLUSION

CBF remains increased in the thalamus at 24 h after CA and PbO2 is decreased to hypoxic levels in cortex at 24 h after CA in rats who do not receive supplemental oxygen. Given the enduring disturbances in this model and the lack of routine CBF or PbO2 monitoring in patients, our data suggest the need for clinical correlation.

Neuroadhesive L1 coating attenuates acute microglial attachment to neural electrodes as revealed by live two-photon microscopy

Implantable neural electrode technologies for chronic neural recordings can restore functional control to paralysis and limb loss victims through brain-machine interfaces. These probes, however, have high failure rates partly due to the biological responses to the probe which generate an inflammatory scar and subsequent neuronal cell death. L1 is a neuronal specific cell adhesion molecule and has been shown to minimize glial scar formation and promote electrode-neuron integration when covalently attached to the surface of neural probes. In this work, the acute microglial response to L1-coated neural probes was evaluated in vivo by implanting coated devices into the cortex of mice with fluorescently labeled microglia, and tracking microglial dynamics with multi-photon microscopy for the ensuing 6 h in order to understand L1's cellular mechanisms of action. Microglia became activated immediately after implantation, extending processes towards both L1-coated and uncoated control probes at similar velocities. After the processes made contact with the probes, microglial processes expanded to cover 47.7% of the control probes' surfaces. For L1-coated probes, however, there was a statistically significant 83% reduction in microglial surface coverage. This effect was sustained through the experiment. At 6 h post-implant, the radius of microglia activation was reduced for the L1 probes by 20%, shifting from 130.0 to 103.5 μm with the coating. Microglia as far as 270 μm from the implant site displayed significantly lower morphological characteristics of activation for the L1 group. These results suggest that the L1 surface treatment works in an acute setting by microglial mediated mechanisms.

Functional Connectivity of Resting Hemodynamic Signals in Submillimeter Orientation Columns of the Visual Cortex

Resting-state functional magnetic resonance imaging has been increasingly used for examining connectivity across brain regions. The spatial scale by which hemodynamic imaging can resolve functional connections at rest remains unknown. To examine this issue, deoxyhemoglobin-weighted intrinsic optical imaging data were acquired from the visual cortex of lightly anesthetized ferrets. The neural activity of orientation domains, which span a distance of 0.7-0.8 mm, has been shown to be correlated during evoked activity and at rest. We performed separate analyses to assess the degree to which the spatial and temporal characteristics of spontaneous hemodynamic signals depend on the known functional organization of orientation columns. As a control, artificial orientation column maps were generated. Spatially, resting hemodynamic patterns showed a higher spatial resemblance to iso-orientation maps than artificially generated maps. Temporally, a correlation analysis was used to establish whether iso-orientation domains are more correlated than orthogonal orientation domains. After accounting for a significant decrease in correlation as a function of distance, a small but significant temporal correlation between iso-orientation domains was found, which decreased with increasing difference in orientation preference. This dependence was abolished when using artificially synthetized orientation maps. Finally, the temporal correlation coefficient as a function of orientation difference at rest showed a correspondence with that calculated during visual stimulation suggesting that the strength of resting connectivity is related to the strength of the visual stimulation response. Our results suggest that temporal coherence of hemodynamic signals measured by optical imaging of intrinsic signals exists at a submillimeter columnar scale in resting state.

Improved spatial accuracy of functional maps in the rat olfactory bulb using supervised machine learning approach

Functional MRI (fMRI) is a popular and important tool for noninvasive mapping of neural activity. As fMRI measures the hemodynamic response, the resulting activation maps do not perfectly reflect the underlying neural activity. The purpose of this work was to design a data-driven model to improve the spatial accuracy of fMRI maps in the rat olfactory bulb. This system is an ideal choice for this investigation since the bulb circuit is well characterized, allowing for an accurate definition of activity patterns in order to train the model. We generated models for both cerebral blood volume weighted (CBVw) and blood oxygen level dependent (BOLD) fMRI data. The results indicate that the spatial accuracy of the activation maps is either significantly improved or at worst not significantly different when using the learned models compared to a conventional general linear model approach, particularly for BOLD images and activity patterns involving deep layers of the bulb. Furthermore, the activation maps computed by CBVw and BOLD data show increased agreement when using the learned models, lending more confidence to their accuracy. The models presented here could have an immediate impact on studies of the olfactory bulb, but perhaps more importantly, demonstrate the potential for similar flexible, data-driven models to improve the quality of activation maps calculated using fMRI data.

Neuronal and physiological correlation to hemodynamic resting-state fluctuations in health and disease

Low-frequency, spatially coherent fluctuations present in functional magnetic resonance imaging time series have had a tremendous impact on brain connectomics. This work aims to explore the degree with which hemodynamic connectivity is associated with neuronal, metabolic, and vascular connectivity measures. For this purpose, GCaMP and nontransgenic mice were used to image neuronal activity and oxidative metabolism activity, respectively, along with blood-oxygenation- and cerebral blood volume (CBV)-sensitive hemodynamic changes from the same animals. Although network clusters calculated using either GCaMP (neuronal activity) or optical imaging of intrinsic signal (OIS)-BOLD (blood oxygenation) data did not exhibit strong spatial similarity, the strengths of node-to-node connectivity measured with these modalities were strongly correlated with one another. This finding suggests that hemodynamic connectivity as measured by blood oxygenation measurements, such as functional connectivity magnetic resonance imaging, is a valuable surrogate for the underlying neuronal connectivity. In nontransgenic animals, greater connectivity correlation was observed between tissue oxidative metabolism (flavoprotein autofluorescence imaging [FAI]) and blood oxygenation measurements, suggesting that metabolic contributions to hemodynamic signals are likely responsible for its significant correlation with neuronal connectivity. Lastly, a mouse model of Alzheimer's disease was used to explore the source of decreases in connectivity reported in these mice, a finding that is thought to be associated with amyloid load-driven metabolic decline. The intercluster connectivity measured by metabolic-sensitive measurements (FAI and OIS-BOLD) was maintained while vascular-only signals (OIS-CBV) provided negligible correlation. Therefore, metabolism-sensitive measurements as used in this work are better positioned to capture changes in neuronal connectivity, such that decreases in hemodynamic connectivity likely reflect decreases in oxidative metabolic function.

Neural and hemodynamic responses to optogenetic and sensory stimulation in the rat somatosensory cortex

Introducing optogenetics into neurovascular research can provide novel insights into the cell-specific control of the hemodynamic response. To generalize findings from molecular approaches, it is crucial to determine whether light-activated circuits have the same effect on the vasculature as sensory-activated ones. For that purpose, rats expressing channelrhodopsin (ChR2) specific to excitatory glutamatergic neurons were used to measure neural activity, blood flow, hemoglobin-based optical intrinsic signal, and blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) during optogenetic and sensory stimulation. The magnitude of the evoked hemodynamic responses was monotonically correlated with optogenetic stimulus strength. The BOLD hemodynamic response function was consistent for optogenetic and sensory stimuli. The relationship between electrical activities and hemodynamic responses was comparable for optogenetic and sensory stimuli, and better explained by the local field potential (LFP) than the firing rate. The LFP was well correlated with cerebral blood flow, moderately with cerebral blood volume, and less with deoxyhemoglobin (dHb) level. The presynaptic firing rate had little impact on evoking vascular response. Contribution of the postsynaptic LFP to the blood flow response induced by optogenetic stimulus was further confirmed by the application of glutamate receptor antagonists. Overall, neurovascular coupling during optogenetic control of glutamatergic neurons largely conforms to that of a sensory stimulus.

Brain tissue responses to neural implants impact signal sensitivity and intervention strategies

Implantable biosensors are valuable scientific tools for basic neuroscience research and clinical applications. Neurotechnologies provide direct readouts of neurological signal and neurochemical processes. These tools are generally most valuable when performance capacities extend over months and years to facilitate the study of memory, plasticity, and behavior or to monitor patients’ conditions. These needs have generated a variety of device designs from microelectrodes for fast scan cyclic voltammetry (FSCV) and electrophysiology to microdialysis probes for sampling and detecting various neurochemicals. Regardless of the technology used, the breaching of the blood–brain barrier (BBB) to insert devices triggers a cascade of biochemical pathways resulting in complex molecular and cellular responses to implanted devices. Molecular and cellular changes in the microenvironment surrounding an implant include the introduction of mechanical strain, activation of glial cells, loss of perfusion, secondary metabolic injury, and neuronal degeneration. Changes to the tissue microenvironment surrounding the device can dramatically impact electrochemical and electrophysiological signal sensitivity and stability over time. This review summarizes the magnitude, variability, and time course of the dynamic molecular and cellular level neural tissue responses induced by state-of-the-art implantable devices. Studies show that insertion injuries and foreign body response can impact signal quality across all implanted central nervous system (CNS) sensors to varying degrees over both acute (seconds to minutes) and chronic periods (weeks to months). Understanding the underlying biological processes behind the brain tissue response to the devices at the cellular and molecular level leads to a variety of intervention strategies for improving signal sensitivity and longevity.

Photoelectric artefact from optogenetics and imaging on microelectrodes and bioelectronics: New Challenges and Opportunities

Bioelectronics, electronic technologies that interface with biological systems, are experiencing rapid growth in terms of technology development and applications, especially in neuroscience and neuroprosthetic research. The parallel growth with optogenetics and in vivo multi-photon microscopy has also begun to generate great enthusiasm for simultaneous applications with bioelectronic technologies. However, emerging research showing artefact contaminated data highlight the need for understanding the fundamental physical principles that critically impact experimental results and complicate their interpretation. This review covers four major topics: 1) material dependent properties of the photoelectric effect (conductor, semiconductor, organic, photoelectric work function (band gap)); 2) optic dependent properties of the photoelectric effect (single photon, multiphoton, entangled biphoton, intensity, wavelength, coherence); 3) strategies and limitations for avoiding/minimizing photoelectric effects; and 4) advantages of and applications for light-based bioelectronics (photo-bioelectronics).

Neural and hemodynamic responses elicited by forelimb- and photo-stimulation in channelrhodopsin-2 mice: insights into the hemodynamic point spread function

Hemodynamic responses are commonly used to map brain activity; however, their spatial limits have remained unclear because of the lack of a well-defined and malleable spatial stimulus. To examine the properties of neural activity and hemodynamic responses, multiunit activity, local field potential, cerebral blood volume (CBV)-sensitive optical imaging, and laser Doppler flowmetry were measured from the somatosensory cortex of transgenic mice expressing Channelrhodopsin-2 in cortex Layer 5 pyramidal neurons. The magnitude and extent of neural and hemodynamic responses were modulated using different photo-stimulation parameters and compared with those induced by somatosensory stimulation. Photo-stimulation-evoked spiking activity across cortical layers was similar to forelimb stimulation, although their activity originated in different layers. Hemodynamic responses induced by forelimb- and photo-stimulation were similar in magnitude and shape, although the former were slightly larger in amplitude and wider in extent. Altogether, the neurovascular relationship differed between these 2 stimulation pathways, but photo-stimulation-evoked changes in neural and hemodynamic activities were linearly correlated. Hemodynamic point spread functions were estimated from the photo-stimulation data and its full-width at half-maximum ranged between 103 and 175 µm. Therefore, submillimeter functional structures separated by a few hundred micrometers may be resolved using hemodynamic methods, such as optical imaging and functional magnetic resonance imaging.

Effects of the α₂-adrenergic receptor agonist dexmedetomidine on neural, vascular and BOLD fMRI responses in the somatosensory cortex

This article describes the effects of dexmedetomidine (DEX) - the active ingredient of medetomidine, which is the latest popular sedative for functional magnetic resonance imaging (fMRI) in rodents - on multiple unit activity, local field potential (LFP), cerebral blood flow (CBF), pial vessel diameter [indicative of cerebral blood volume (CBV)], and blood oxygenation level-dependent (BOLD) fMRI. These measurements were obtained from the rat somatosensory cortex during 10 s of forepaw stimulation. We found that the continuous intravascular systemic infusion of DEX (50 μg/kg/h, doses typically used in fMRI studies) caused epileptic activities, and that supplemental isoflurane (ISO) administration of ~0.3% helped to suppress the development of epileptic activities and maintained robust neuronal and hemodynamic responses for up to 3 h. Supplemental administration of N(2)O in addition to DEX nearly abolished hemodynamic responses even if neuronal activity remained. Under DEX + ISO anesthesia, spike firing rate and the delta power of LFP increased, whereas beta and gamma power decreased, as compared with ISO-only anesthesia. DEX administration caused pial arteries and veins to constrict nearly equally, resulting in decreases in baseline CBF and CBV. Evoked LFP and CBF responses to forepaw stimulation were largest at a frequency of 8-10 Hz, and a non-linear relationship was observed. Similarly, BOLD fMRI responses measured at 9.4 T were largest at a frequency of 10 Hz. Both pial arteries and veins dilated rapidly (artery, 32.2%; vein, 5.8%), and venous diameter returned to baseline slower than arterial diameter. These results will be useful for designing, conducting and interpreting fMRI experiments under DEX sedation.

In vivo two-photon microscopy reveals immediate microglial reaction to implantation of microelectrode through extension of processes

OBJECTIVE

Penetrating cortical neural probe technologies allow investigators to record electrical signals in the brain. Implantation of probes results in acute tissue damage, and microglia density increases around implanted devices over weeks. However, the mechanisms underlying this encapsulation are not well understood in the acute temporal domain. The objective here was to evaluate dynamic microglial response to implanted probes using two-photon microscopy.

APPROACH

Using two-photon in vivo microscopy, cortical microglia ∼200 µm below the surface of the visual cortex were imaged every minute in mice with green fluorescent protein-expressing microglia.

MAIN RESULTS

Following probe insertion, nearby microglia immediately extended processes toward the probe at (1.6 ± 1.3) µm min(-1) during the first 30-45 min, but showed negligible cell body movement for the first 6 h. Six hours following probe insertion, microglia at distances <130.0 µm (p = 0.5) from the probe surface exhibit morphological characteristics of transitional stage (T-stage) activation, similar to the microglial response observed with laser-induced blood-brain barrier damage. T-stage morphology and microglia directionality indexes were developed to characterize microglial response to implanted probes. Evidence suggesting vascular reorganization after probe insertion and distant vessel damage was also observed hours after probe insertion.

SIGNIFICANCE

A precise temporal understanding of the cellular response to microelectrode implantation will facilitate the search for molecular cues initiating and attenuating the reactive tissue response.

Cerebral oxygen delivery and consumption during evoked neural activity

Increases in neural activity evoke increases in the delivery and consumption of oxygen. Beyond observations of cerebral tissue and blood oxygen, the role and properties of cerebral oxygen delivery and consumption during changes in brain function are not well understood. This work overviews the current knowledge of functional oxygen delivery and consumption and introduces recent and preliminary findings to explore the mechanisms by which oxygen is delivered to tissue as well as the temporal dynamics of oxygen metabolism. Vascular oxygen tension measurements have shown that a relatively large amount of oxygen exits pial arterioles prior to capillaries. Additionally, increases in cerebral blood flow (CBF) induced by evoked neural activation are accompanied by arterial vasodilation and also by increases in arteriolar oxygenation. This increase contributes not only to the down-stream delivery of oxygen to tissue, but also to delivery of additional oxygen to extra-vascular spaces surrounding the arterioles. On the other hand, the changes in tissue oxygen tension due to functional increases in oxygen consumption have been investigated using a method to suppress the evoked CBF response. The functional decreases in tissue oxygen tension induced by increases in oxygen consumption are slow to evoked changes in CBF under control conditions. Preliminary findings obtained using flavoprotein autofluorescence imaging suggest cellular oxidative metabolism changes at a faster rate than the average changes in tissue oxygen. These issues are important in the determination of the dynamic changes in tissue oxygen metabolism from hemoglobin-based imaging techniques such as blood oxygenation-level dependent functional magnetic resonance imaging (fMRI).

Complex-valued analysis of arterial spin labeling-based functional magnetic resonance imaging signals

Cerebral blood flow-dependent phase differences between tagged and control arterial spin labeling images are reported. A biophysical model is presented to explain the vascular origin of this difference. Arterial spin labeling data indicated that the phase difference is largest when the arterial component of the signals is preserved but is greatly reduced as the arterial contribution is suppressed by postinversion delays or flow-crushing gradients. Arterial vasculature imaging by saturation data of activation and hypercapnia conditions showed increases in phase accompanying blood flow increases.An arterial spin labeling functional magnetic resonance imaging study yielded significant activation by magnitude-only, phase-only, and complex analyses when preserving the whole arterial spin labeling signal. After suppression of the arterial signal by postinversion delays, magnitude-only and complex models yielded similar activation levels, but the phase-only model detected nearly no activation. When flow crushers were used for arterial suppression, magnitude-only activation was slightly lower and fluctuations in phase were dramatically higher than when postinversion delays were used.Although the complex analysis performed did not improve detection, a simulation study indicated that the complex-valued activation model exhibits combined magnitude and phase detection power and thus maximizes sensitivity under ideal conditions. This suggests that, as arterial spin labeling imaging and image correction methods develop, the complex-valued detection model may become helpful in signal detection.

Changes in cerebral arterial, tissue and venous oxygenation with evoked neural stimulation: implications for hemoglobin-based functional neuroimaging

Little is known regarding the changes in blood oxygen tension (P(O2)) with changes in brain function. This work aimed to measure the blood P(O2) in surface arteries and veins as well as tissue with evoked somato-sensory stimulation in the anesthetized rat. Electrical stimulation of the forepaw induced average increases in blood flow of 44% as well as increases in the tissue P(O2) of 28%. More importantly, increases in P(O2) throughout pial arteries (resting diameters=59 to 129 microm) and pial veins (resting diameters=62 to 361 microm) were observed. The largest increases in vascular P(O2) were observed in the small veins (from 33 to 40 mm Hg) and small arteries (from 78 to 88 mm Hg). The changes in oxygen saturation (S(O2)) were calculated and the largest increases were observed in small veins (Delta=+11%) while its increase in small arteries was small (Delta=+4%). The average diameter of arterial vessels was observed to increase by 4 to 6% while that of veins was not observed to change with evoked stimulation. These findings show that the increases in arterial P(O2) contribute to the hyper-oxygenation of tissue and, mostly likely, also to the signal changes in hemoglobin-based functional imaging methods (e.g. BOLD fMRI).

Apparent wall thickening of cystic renal lesions on MRI

PURPOSE

To show that cystic renal lesions that would otherwise meet criteria for simple cysts can demonstrate perceptible walls or increased wall thickness on MRI, sometimes causing these lesions to be "upgraded." It was hypothesized that thickening of cyst walls on MRI can be artifactual, due to data truncation, applied filtering, and low signal-to-noise ratio (SNR).

MATERIALS AND METHODS

k-Space data for a 4-cm cyst were created in a 40-cm field of view (FOV) (512 x 512 matrix). Additional data sets were created using the central 512 x 256 and 512 x 128 points. Noise was simulated so that the cyst SNR was approximately 7, 14, and 20, respectively. Actual wall thickness was set at 0.25 mm, and cyst:wall signal at 1:4. An inverse two-dimensional (2D) fast Fourier transform (FFT) yielded simulated images. A Fermi filter was applied to reduce ringing. Images/projections were examined for wall thickening. Seven patients with initially thick-walled cysts on fat-saturated spoiled gradient-echo (FS-SPGR) images were scanned with increasing resolution (256 x 128 and 256 x 256; four patients were also scanned with 512 x 512). Average wall thickness at each resolution was compared using a two-tailed paired Student's t-test.

RESULTS

Simulations showed apparent wall thickening at low resolution, improving with higher resolutions. Low SNR and application of the Fermi filter made it difficult to identify ringing as the cause of this thickening. The simulation results were confirmed on seven patients, whose cyst walls proved to be artifactually thickened (P < 0.01).

CONCLUSION

Thickening of cyst walls on MRI can be artifactual. Upon encountering thick-walled cystic renal lesions, high-resolution images can be acquired to exclude apparent thickening.

Dynamics of oxygen delivery and consumption during evoked neural stimulation using a compartment model and CBF and tissue P(O2) measurements

The dynamics of blood oxygen delivery and tissue consumption produced by evoked stimulation of the rat somato-sensory cortex were investigated. Tissue oxygen tension (P(O2)) and laser Doppler flowmetry (LDF) measurements were recorded under two experimental conditions: normal, which represented both oxygen delivery and consumption, and suppressed CBF (achieved using a vasodilator), which only represented tissue oxygen consumption. Forepaw stimulation for 10 s produced increases of 27.7% and 48.8% in tissue P(O2) and LDF signal under normal conditions, respectively. The tissue P(O2) response peaked 9.8 s after stimulation onset and did not show any early transient decreases indicating that measurable oxygen deficits are not required to increase the delivery of oxygen by blood flow. Under suppressed CBF conditions, the LDF signal was mostly suppressed while the tissue P(O2) decreased by 11.7% and reached a minimum 10.8 s after stimulation onset. These data were analyzed using a dynamic model that described the transport of oxygen from blood to tissue. In order to explain the differences between the model prediction of the tissue P(O2) changes and the experimental data, several hypothetical scenarios were considered, such as changes in the vascular volume, permeability-surface area or arterial oxygenation. The increase in tissue P(O2) was found to probably require the recruitment of upstream oxygen from larger arteries as well as increases in the vascular volume at the oxygen exchange sites. The amplitude of the estimated tissue tension of oxygen delivered was about 2.7 x larger than the estimated consumption under normal conditions (45.7% vs. 17.1%, respectively).

Application of selective saturation to image the dynamics of arterial blood flow during brain activation using magnetic resonance imaging

A saturation-based approach is proposed to image the arterial blood flow signal with temporal resolution of 1 to 2 s and in-plane spatial resolution of a few millimeters. Using a saturation approach to suppress the undesired background stationary signal allows the blood water that enters the slice to be imaged at some specified later time. Since the blood protons that are being imaged are not restricted to the intravascular space, this technique is also sensitive to tissue perfusion signal contributions. The signal uptake characteristics of the saturation method proposed were used to study the different signal contributions as a function of the acquisition parameters. A typical perfusion acquisition (FAIR) was also used for comparison. The proposed method was demonstrated in a functional motor activation experiment and the observed signal changes were smaller than those obtained using the FAIR acquisition. The dynamics of the saturation method and FAIR temporal signal changes were investigated and time constants between 2 and 44 s were estimated. The tissue signal contribution to the saturation method's signal was small over the range of acquisition parameters that sensitized it to the arterial compartment.

Vascular dynamics and BOLD fMRI: CBF level effects and analysis considerations

Changes in the cerebral blood flow (CBF) baseline produce significant changes to the hemodynamic response. This work shows that increases in the baseline blood flow level produce blood oxygenation-level dependent (BOLD) and blood flow responses that are slower and lower in amplitude, while decreases in the baseline blood flow level produce faster and higher amplitude hemodynamic responses. This effect was characterized using a vascular model of the hemodynamic response that separated arterial blood flow response from the venous blood volume response and linked the input stimulus to the vascular response. The model predicted the baseline blood flow level effects to be dominated by changes in the arterial vasculature. Specifically, it predicted changes in the arterial blood flow time constant and venous blood volume time constant parameters of +294% and -24%, respectively, for a 27% increase in the baseline blood flow. The vascular model performance was compared to an empirical model of the hemodynamic response. The vascular and empirical hemodynamic models captured most of the baseline blood flow level effects observed and can be used to correct for these effects in fMRI data. While the empirical hemodynamic model is easy to implement, it did not incorporate any explicit physiological information.

Fast, pseudo-continuous arterial spin labeling for functional imaging using a two-coil system

A fast, two-coil, pseudo-continuous labeling scheme is presented. This new scheme permits the collection of a multislice subtraction pair in <3 s, depending on the subject's arterial transit times. The method consists of acquiring both control and tag images immediately after a labeling period that matches the arterial transit time. The theoretical basis of the technique, and simulations of the signal during changes in both transit time and perfusion are presented. Experimental data from functional imaging experiments were collected to demonstrate the technique and its characteristics.

Evaluation of respiratory artifact correction techniques in multishot spiral functional MRI using receiver operator characteristic analyses

Navigator corrections and low-spatial frequency (LSF) oversampling are investigated as methods for reducing respiration-related effects in multishot functional MRI. Both techniques take advantage of the smoothly varying or nearly constant phase variations linked to the respiration cycle. These techniques were tested in functional MRI studies with spiral k-space acquisitions. Receiver operator characteristic (ROC) analyses and the temporal variance averaged across the brain were used to evaluate their effectiveness. Both methods were found to increase the area under the ROC curve and to reduce the standard deviation, with the LSF oversampling method being more effective.