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Angularly Resolved Deep Brain Fluorescence Imaging Using a Single Optical Fiber 时间:2018-08-28 22:15   来源:未知   作者:admin   点击: 次 Abstract:In this article, we report the development of a minimally invasive fiber optic based fluorescence probe which can reach deep brain objects and measure the intensity and spatial distribution of fluorescence signals in the tissue. In this design, the brain is scanned by a single penetrating side-firing optical fiber which delivers excitation light pulses to the tissue at different depths and orientations and simultaneously collects samples of fluorescence emission signals. Signal-to-noise ratio of the measurements is improved by adapting the pulse compression technique and the theory of optimal filters. Effects of each design parameter on the overall performance of the scanner, including the spatial resolution and speed of scanning, are analyzed and experimentally measured. In vivo experiments show that the new device, despite the simplicity of the design, provides valuable information particularly useful in optogenetic stimulation experiments where the exact position of the fiber tip and the radiation orientation can change the outcome of a test.
1. Introduction
  Recent advances in the discovery or synthesis of new fluorescent indicators have been followed by the development of sophisticated technologies for in vitro and in vivo fluorescence imaging offering spatial resolution close to the diffraction limit of adapted optical wavelengths [1]. Nonetheless, in vivo fluorescence imaging has remained limited only to the very immediate superficial areas of the brain tissue. A good example of such imaging modalities is the two-photon excitation microscopy, which allows imaging of living tissue up to one millimeter in depth and the system can image a large portion of the cortex in small rodents [1]. Since most high-level information processing operations take place in the cortex, invention of two-photon microscopes opened new horizons in the study of the cortical dynamics. Fluorescence tomography scanners are also made that employ mathematical (e.g., fluorescence diffuse optical tomography (FDOT) [2]) or statistical algorithms (e.g., fluorescence laminar optical tomography (FLOT) [3, 4]) that model light tissue interaction and use this information to translate the measured data to three-dimensional images that reveal the distribution of fluorescent molecules. Usually such tomography scanners can image slightly deeper areas; however, resolution is significantly sacrificed and spurious objects appear in generated images caused by the instability of algorithms that solve ill-posed inverse problems. As a result, none of these technologies can reliably image deep brain objects, such as the thalamus or hippocampus, which play crucial roles in vital brain functions including the early processing of sensory inputs or memory consolidation. For such applications, the main option is to conduct the experiments in brain slices which is an invasive process and certainly nonreversible. Therefore, there is high demand for the invention of new instrumentation that can reach deep brain objects to perform fluorescence imaging.
   A reasonable approach to achieve optical access to deep brain regions is the use of thin optical fibers. An optical fiber can function as a reliable tool to guide the power of a coherent or incoherent light source to any target area with minimum loss. Such waveguides are used in many challenging biomedical applications, for example, to perform fluorescent detection (e.g., see [5, 6]) spectroscopy (e.g., see [7–9]) or the assessment of tissue optical property (e.g., see [8, 10, 11]) for optical biopsy applications. The performance of such mechanisms is also theoretically and empirically analyzed in recent years (e.g., see [12, 13]).