Abstract
Near-infrared fluorescence (NIRF) imaging has great potential for studying physiological and pathophysiological processes noninvasively in several locations of the body. In this study, we evaluated the feasibility of NIRF imaging to visualize fluorescent compounds within the brains of live mice commonly used in brain research. To simulate the presence of a molecular NIRF reporter agent at the site of a lesion, we developed a new in vivo phantom model wherein capsules containing different amounts of an NIRF dye (Cy5.5) were stereotactically implanted deep into the left hemispheres of living mice. To precisely locate the implanted capsules, magnetic resonance imaging (MRI) was performed. Fluorescence reflectance imaging (FRI) and transillumination fluorescence imaging (TFI) were conducted to analyze and compare sensitivity and target-to-background ratios of the two methods. The sensitivities of FRI and TFI to background fluorescence from circulating dye was tested by imaging fluorescent capsules in mice intravenously injected with increasing amounts of long-circulating Cy5.5-dextran. The results show that capsules containing dye amounts as low as 10(-12) mol can be detected. TFI yielded significantly higher target-to-background ratios than FRI at 10(-11) mol (p < .05). Comparatively low amounts of fluorescence in the blood vessels can extinguish the signal. We conclude that keeping the signal from circulating NIRF dye low, NIRF imaging offers high sensitivity in detecting fluorochromes noninvasively within brains of mice, especially by using TFI. This encourages the application of NIRF for molecular imaging in the mouse brain using NIRF reporters.
Highlights
In recent years, numerous near-infrared fluorescence (NIRF) reporters have been designed and successfully used for noninvasive NIRF imaging of biological processes in a variety of small-animal disease models including cancer [1], cardiovascular diseases [2], and arthritis [3]
To simulate the presence of an accumulating fluorescent molecular reporter at the site of a lesion, we developed a new in vivo phantom model wherein capsules containing different amounts of an NIR fluorochrome (Cy5.5) were implanted into the left hemispheres of live C57Bl6 mice, a pigmented mouse strain that is commonly used in brain research [9]
NIRF imaging using fluorescent reporters is a promising technology for visualizing biological events noninvasively in small animals and potentially in clinical settings
Summary
Numerous near-infrared fluorescence (NIRF) reporters have been designed and successfully used for noninvasive NIRF imaging of biological processes in a variety of small-animal disease models including cancer [1], cardiovascular diseases [2], and arthritis [3]. A number of NIRF imaging devices with different setups, modes of data collection, and features have become available and have been tested for small-animal imaging purposes. The majority of these systems are based on so-called planar fluorescence imaging. More complex tomographic systems (fluorescence-mediated tomography [FMT]) have recently emerged. In these systems, light is coupled sequentially to the object at different locations for a fixed position of the detectors, delivering 3-D reconstructions of the fluorochrome distribution in an object and data for quantitative analysis [4 –6]
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