Near-infrared fluorescence (NIRF) imaging has great potential for studying physiological and path... more 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.
ABSTRACT We report on a time-resolved instrument for diffuse NIR-reflectance topography of the ad... more ABSTRACT We report on a time-resolved instrument for diffuse NIR-reflectance topography of the adult head. First results will be presented of time-resolved monitoring of the motor cortex area during finger tapping.
ABSTRACT We developed a time-domain brain imager that is based on picosecond diode lasers, a mult... more ABSTRACT We developed a time-domain brain imager that is based on picosecond diode lasers, a multimode fiber switch and multi-channel time-correlated single photon counting. It allows to record time-resolved diffuse reflectance for 16 source-detector pairs within typically 1 s. Data analysis was based on the evaluation of moments of measured distributions of times of flight of photons. To show the relevance of these moments for achieving depth selectivity, three-dimensional sensitivities of integral, mean time of flight and variance to absorption changes were calculated using a perturbation approach based on the diffusion equation for photon density for a homogeneous semi-infinite medium. It turned out that variance is almost exclusively sensitive to deep layers, whereas the integral reflects changes in deep as well as in superficial layers. The lateral resolution of the imager was demonstrated by a phantom experiment. Results of a motor stimulation experiment on a healthy volunteer strongly suggest that variance reveals mainly the cerebral activation whereas the integral may additionally contain significant systemic contributions.
Advances in Experimental Medicine and Biology, Oct 24, 2009
Near-infrared spectroscopy (NIRS) of the human brain is aiming at the non-invasive determination ... more Near-infrared spectroscopy (NIRS) of the human brain is aiming at the non-invasive determination of concentration changes of oxy- and deoxyhemoglobin in the cortex. However, it usually relies on the assumption of spatially homogeneous absorption changes. To overcome this limitation we performed instrumental and methodological developments of time-resolved NIRS with the aim to achieve depth resolution. We present our recently developed time-domain near-infrared brain imager based on picosecond diode lasers and time-correlated single photon counting (TCSPC) which can be used at the bedside. To achieve depth localization of absorption changes we analysed statistical moments (integral, mean time of flight and variance) of measured time-of-flight distributions of diffusely reflected photons. In particular, variance has a selective sensitivity to deep absorptions changes and provides a suitable representation of cerebral signals. The separation of cerebral and extracerebral changes of hemoglobin concentrations is demonstrated for a motor stimulation experiment.
Journal of Cerebral Blood Flow and Metabolism, Aug 1, 2005
Neuronal activation leads to an increase in regional cerebral blood flow (rCBF) and blood oxygena... more Neuronal activation leads to an increase in regional cerebral blood flow (rCBF) and blood oxygenation (rCBO). The underlying mechanisms of neurovascular coupling are poorly understood. Recently a new model of activity induced vascular regulation was proposed by Stamler et al. (1997) based on the fact that nitric oxide (NO), a potent vasodilator, binds to hemoglobin in the oxygenated state (oxy-Hb) but is released upon deoxygenation of hemoglobin (deoxy-Hb). It was suggested that during cellular activity oxygen consumption locally increases which causes deoxygenation of hemoglobin and release of NO. Indeed it has recently been shown that red blood cell-bound NO mediates hypoxic vasodilation in vitro, and transpulmonary gradients of hemoglobin-bound NO are evident in patients with congestive heart failure (Datta et al., 2004). According to this elegant model of combined oxygen and NO release, blood flow will be directly matched to tissue oxygen demands. To test this model as a physiological mechanism of neurovascular coupling in the brain, we measured rCBF and rCBO responses to functional activation under hyperbaric oxygenation (3 ATA, FiO2 1.0) in the anesthetized rat. During hyperbaric oxygenation oxygen supply to tissue is entirely provided through physically dissolved oxygen. During activation no deoxygenation of hemoglobin and thus no allosteric release of NO occurs. Laser Doppler flowmetry combined with microfiber Hb-spectroscopy was performed through the thinned skull in rats to measure relative changes in rCBF and rCBO and cortical hemoglobin saturation. Averaged rCBF and rCBO responses to electrical forepaw stimulation (3 Hz, 10 s stimulation period) were recorded under normobaric normoxia and compared with responses during hyperbaric hyperoxygenation. Hyperbaric hyperoxygenation increased hemoglobin saturation within the microcirculation from 44 2 to 103 3%. The deoxy-Hb decrease normally occurring during functional activation disappeared. The oxy-Hb increase was unchanged, and the rCBF response to functional activation was increased but not decreased under hyperbaric hyperoxygenation compared with control responses. In most animals the observed increase in the rCBF response was paralleled by an increase in the amplitude of somatosensory evoked potentials. Our results suggest that in contrast to the in-vitro situation and to pathophysiological conditions of severe hypoxia rCBF regulation during physiological neuronal activation involves other mechanisms than oxygen-dependent delivery and release of NO from hemoglobin.
Near-infrared fluorescence (NIRF) imaging has great potential for studying physiological and path... more 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.
ABSTRACT We report on a time-resolved instrument for diffuse NIR-reflectance topography of the ad... more ABSTRACT We report on a time-resolved instrument for diffuse NIR-reflectance topography of the adult head. First results will be presented of time-resolved monitoring of the motor cortex area during finger tapping.
ABSTRACT We developed a time-domain brain imager that is based on picosecond diode lasers, a mult... more ABSTRACT We developed a time-domain brain imager that is based on picosecond diode lasers, a multimode fiber switch and multi-channel time-correlated single photon counting. It allows to record time-resolved diffuse reflectance for 16 source-detector pairs within typically 1 s. Data analysis was based on the evaluation of moments of measured distributions of times of flight of photons. To show the relevance of these moments for achieving depth selectivity, three-dimensional sensitivities of integral, mean time of flight and variance to absorption changes were calculated using a perturbation approach based on the diffusion equation for photon density for a homogeneous semi-infinite medium. It turned out that variance is almost exclusively sensitive to deep layers, whereas the integral reflects changes in deep as well as in superficial layers. The lateral resolution of the imager was demonstrated by a phantom experiment. Results of a motor stimulation experiment on a healthy volunteer strongly suggest that variance reveals mainly the cerebral activation whereas the integral may additionally contain significant systemic contributions.
Advances in Experimental Medicine and Biology, Oct 24, 2009
Near-infrared spectroscopy (NIRS) of the human brain is aiming at the non-invasive determination ... more Near-infrared spectroscopy (NIRS) of the human brain is aiming at the non-invasive determination of concentration changes of oxy- and deoxyhemoglobin in the cortex. However, it usually relies on the assumption of spatially homogeneous absorption changes. To overcome this limitation we performed instrumental and methodological developments of time-resolved NIRS with the aim to achieve depth resolution. We present our recently developed time-domain near-infrared brain imager based on picosecond diode lasers and time-correlated single photon counting (TCSPC) which can be used at the bedside. To achieve depth localization of absorption changes we analysed statistical moments (integral, mean time of flight and variance) of measured time-of-flight distributions of diffusely reflected photons. In particular, variance has a selective sensitivity to deep absorptions changes and provides a suitable representation of cerebral signals. The separation of cerebral and extracerebral changes of hemoglobin concentrations is demonstrated for a motor stimulation experiment.
Journal of Cerebral Blood Flow and Metabolism, Aug 1, 2005
Neuronal activation leads to an increase in regional cerebral blood flow (rCBF) and blood oxygena... more Neuronal activation leads to an increase in regional cerebral blood flow (rCBF) and blood oxygenation (rCBO). The underlying mechanisms of neurovascular coupling are poorly understood. Recently a new model of activity induced vascular regulation was proposed by Stamler et al. (1997) based on the fact that nitric oxide (NO), a potent vasodilator, binds to hemoglobin in the oxygenated state (oxy-Hb) but is released upon deoxygenation of hemoglobin (deoxy-Hb). It was suggested that during cellular activity oxygen consumption locally increases which causes deoxygenation of hemoglobin and release of NO. Indeed it has recently been shown that red blood cell-bound NO mediates hypoxic vasodilation in vitro, and transpulmonary gradients of hemoglobin-bound NO are evident in patients with congestive heart failure (Datta et al., 2004). According to this elegant model of combined oxygen and NO release, blood flow will be directly matched to tissue oxygen demands. To test this model as a physiological mechanism of neurovascular coupling in the brain, we measured rCBF and rCBO responses to functional activation under hyperbaric oxygenation (3 ATA, FiO2 1.0) in the anesthetized rat. During hyperbaric oxygenation oxygen supply to tissue is entirely provided through physically dissolved oxygen. During activation no deoxygenation of hemoglobin and thus no allosteric release of NO occurs. Laser Doppler flowmetry combined with microfiber Hb-spectroscopy was performed through the thinned skull in rats to measure relative changes in rCBF and rCBO and cortical hemoglobin saturation. Averaged rCBF and rCBO responses to electrical forepaw stimulation (3 Hz, 10 s stimulation period) were recorded under normobaric normoxia and compared with responses during hyperbaric hyperoxygenation. Hyperbaric hyperoxygenation increased hemoglobin saturation within the microcirculation from 44 2 to 103 3%. The deoxy-Hb decrease normally occurring during functional activation disappeared. The oxy-Hb increase was unchanged, and the rCBF response to functional activation was increased but not decreased under hyperbaric hyperoxygenation compared with control responses. In most animals the observed increase in the rCBF response was paralleled by an increase in the amplitude of somatosensory evoked potentials. Our results suggest that in contrast to the in-vitro situation and to pathophysiological conditions of severe hypoxia rCBF regulation during physiological neuronal activation involves other mechanisms than oxygen-dependent delivery and release of NO from hemoglobin.
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