Nitric oxide (NO) is a highly reactive molecule that is synthesized by a variety of biological tissues. It plays a major role in the regulation of blood pressure, in nerve cell communication, in the destruction of pathogens, and it has been implicated in numerous other physiological process in ways yet to be elucidated. The need to understand how or when this molecule participates in a chemical pathway in vivo, has made it necessary to develop methods for its detection in biological matrices and fluids. In this lecture we review some of the optical methods that have gained acceptance in the biological community, the controversies that they have engendered, and some of the technical challenges that lie ahead for this area of research.
Much attention has been devoted to the enzymatic production of nitric oxide (NO) by the endothelial layer lining blood vessel walls, which regulates among other things local vasodilatation and platelet adhesion. Considerably less attention, however, has been paid to the accumulation of NO-related products in the vascular wall itself. Such local storage of NO products could conceivably contribute to the local regulation of blood flow and provide additional anti-adhesive protection, if biochemically activated to regenerate NO. Since little is known about their chemical nature, concentrations, and possible role in vascular biology we sought to characterize those species basally resent in rat aorta. To this end we developed a functional form of optical spectroscopy that allows us not only to identify NO-stores in intact tissues but also to monitor their production and disappearance in real-time. The method is based on the ability of NO stores to reversibly release NO when illuminated with light of particular wavelengths, which can be detected as a robust relaxation of vascular smooth muscle (photorelaxation). Characterization of NO-stores is achieved through a careful assessment of photorelaxation action spectra, taking into account the light scattering properties of the tissue, and of depletion of the NO-stores induced by exposure to controlled levels of light. This functional form of optical spectroscopy is applied to rat aortic tissue where the results suggest that the NO photolytically released from tissue stores originated from a low-molecular-weight RSNO as well as from nitrite. The significance of these findings to vascular physiology and pathophysiology is discussed.
KEYWORDS: Brain, Blood, Absorption, Near infrared spectroscopy, Spectroscopy, Oxygen, Near infrared, Signal attenuation, Animal model studies, Magnetic resonance imaging
Transient global cerebral ischemia accompanying cardiac arrest (CA) often leads to permanent brain damage with poor neurological outcome. The precise chain of events underlying the cerebral damage after CA is still not fully understood. Progress in this area may profit from the development of new non-invasive tools that provide real-time information on the vascular and cellular processes preceding the damage. One way to assess these processes is through near-IR spectroscopy, which has demonstrated the ability to quantify changes in blood volume, hemoglobin oxygenation, cytochrome oxidase redox state, and tissue water content. Here we report on the successful implementation of this form of spectroscopy in a rat model of asphyxial CA and resuscitation, under hypothermic and normothermic conditions. Preliminary results are shown that provide a new temporal insight into the cerebral circulation during CA and post-resuscitation.
Many diseased states of the brain can result in the displacement of brain tissues and restrict cerebral blood flow, disrupting function in a life-threatening manner. Clinical examples where displacements are observed include venous thromboses, hematomas, strokes, tumors, abscesses, and, particularly, brain edema. For the latter, the brain tissue swells, displacing the cerebral spinal fluid (CSF) layer that surrounds it, eventually pressing itself against the skull. Under such conditions, catheters are often inserted into the brain's ventricles or the subarachnoid space to monitor increased pressure. These are invasive procedures that incur increased risk of infection and consequently are used reluctantly by clinicians. Recent studies in the field of biomedical optics have suggested that the presence or absence of the CSF layer can lead to dramatic changes in NIR signals obtained from diffuse reflectance measurements around the head. In this study, we consider how this sensitivity of NIR signals to CSF might be exploited to non-invasively monitor the onset and resolution of brain edema.
A novel approach to the quantitative image reconstruction in diffuse optical tomography is proposed. The special structure of the transport equation is used to formulate the iterative image reconstruction algorithm as a process updating the estimates of the optical properties from the solution of an intermediate tomographic problem The ability of the technique to reconstruct simultaneously maps of both absorption and reduced scattering coefficients in 2D geometry is demonstrated using simulated frequency-domain data. The potential advantages of the new approach include its ability to fully retain the non-linear character of the inverse problem while at the same time avoiding either gradient or Jacobian calculations and eliminating the need in an additional regularization mechanism.
Optical tomography has recently demonstrated the potential for deep imaging of tissue oxygenation and blood volume, non- invasively, using relatively simple and inexpensive instrumentation. Prior demonstrations of this form of tomography have relied on scanning and data collecting methods limited to imaging bandwidths of Hz or slower. Here we report on an approach that significantly accelerates the imaging rate of optical tomographs based on time-domain methods to well beyond the kHz range. Such high bandwidths are critical for extending the capabilities of optical tomographs to include deep imaging of blood flow and neural activity.
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