SignificanceMotion artifacts in the signals recorded during optical fiber-based measurements can lead to misinterpretation of data. In this work, we address this problem during in-vivo rodent experiments and develop a motion artifacts correction (MAC) algorithm for single-fiber system (SFS) hemodynamics measurements from the brains of rodents.Aim(i) To distinguish the effect of motion artifacts in the SFS signals. (ii) Develop a MAC algorithm by combining information from the experiments and simulations and validate it.ApproachMonte-Carlo (MC) simulations were performed across 450 to 790 nm to identify wavelengths where the reflectance is least sensitive to blood absorption-based changes. This wavelength region is then used to develop a quantitative metric to measure motion artifacts, termed the dissimilarity metric (DM). We used MC simulations to mimic artifacts seen during experiments. Further, we developed a mathematical model describing light intensity at various optical interfaces. Finally, an MAC algorithm was formulated and validated using simulation and experimental data.ResultsWe found that the 670 to 680 nm wavelength region is relatively less sensitive to blood absorption. The standard deviation of DM (σDM) can measure the relative magnitude of motion artifacts in the SFS signals. The artifacts cause rapid shifts in the reflectance data that can be modeled as transmission changes in the optical lightpath. The changes observed during the experiment were found to be in agreement to those obtained from MC simulations. The mathematical model developed to model transmission changes to represent motion artifacts was extended to an MAC algorithm. The MAC algorithm was validated using simulations and experimental data.ConclusionsWe distinguished motion artifacts from SFS signals during in vivo hemodynamic monitoring experiments. From simulation and experimental data, we showed that motion artifacts can be modeled as transmission changes. The developed MAC algorithm was shown to minimize artifactual variations in both simulation and experimental data.
Our understanding of hemodynamic signals, blood perfusion and oxygen saturation (sO2), recorded through fiber-based instruments is limited. To address this, a potential method can be to simultaneously acquire hemodynamic signals with widely used fluorescence fiber photometry signals. We report a novel System for the Simultaneous Measurement of Fluorescence and Hemodynamics (SSMFH) from deep brain regions of freely moving rodents. SSMFH has been developed by modifying our previous white light reflectance single fiber system (SFS) that enabled measurement of blood perfusion and oxygen saturation from freely moving rodents. SSMFH has been designed to be easily integrated with a commercial fluorescence fiber photometry system for time-locked measurement of both hemodynamics and fluorescence signals. SSMFH can be additionally synchronized to behavioral monitoring cameras and other behavioral equipment during experiments. In contrast to previous work in the literature, SSMFH enables hemodynamic recordings from deep brain regions using a wide spectral range (∼ 545nm to 700nm) without the need for injection of an activity-independent fluorescent reporter. The details of the design will be presented along with data to illustrate proof-of-concept through an animal experiment. GCaMP-based fiber photometry and its relation to neuronal activity is well understood. Hemodynamic measurement with simultaneous and co-localized GCaMP based neuronal activity recording can help in understanding variations in in-vivo hemodynamic signals. SSMFH can be used to correct fluorescence measurements which are affected by blood absorption changes.
Oxygen saturation (sO2) and blood perfusion in brain tissue have been known to be modulated with cellular activity in the brain. A single fiber system (SFS) has previously been shown to enable sO2 measurements from localized deep brain regions in freely moving animals. Reflectance spectra (RSF) obtained through the SFS can be used to understand changes in blood perfusion and fit to an empirical model to extract sO2. The sO2 extracted is dependent on the shape of RSF and thus relatively resistant to noise as compared to blood perfusion which is dependent on the magnitude of RSF at specific wavelengths. While slow changes in sO2 have been shown to be robust, sources of certain relatively rapid temporal variations observed in the sO2 signal remains unclear. Potential sources could be variations in cellular activity in the brain or noise due to motion artifacts. In this work, we have described the design of new experiments focused to investigate the effects of motion artifacts on RSF and sO2. Computer simulations and mathematical modelling have been used to explain the experimental findings. Results suggest that the motion artifacts mainly arise from the fiber/brain interface and appear to offset RSF. Using the interpretation from a mathematical model, we also propose a motion artifact correction algorithm which can potentially be used for comparison of perfusion signals.
KEYWORDS: Luminescence, In vivo imaging, Photometry, Optical fibers, Tissue optics, Signal to noise ratio, Signal detection, Calcium, Modulation, Neurons
Fiber photometry uses genetically encoded optical reporters to link specific cellular activity in stereotaxically targeted brain structures to specific behaviors. There are still a number of barriers that have hindered the widespread adoption of this approach. This includes cost, but also the high-levels of light required to excite the fluorophore, limiting commercial systems to the investigation of short-term transients in neuronal activity to avoid damage of tissue by light. Here, we present a cost-effective optoelectronic system for in vivo fiber photometry that achieves high-sensitivity to changes in fluorescence intensity, enabling detection of optical transients of a popular calcium reporter with excitation powers as low as 100 nW. By realizing a coherent detection scheme and by using a photomultiplier tube as a detector, the system demonstrates reliable study of in vivo neuronal activity, positioning it for future use in the experiments inquiring into learning and memory processes. The system was applied to study stress-evoked calcium transients in corticotropin-releasing hormone neurons in the mouse hypothalamus.
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