Tomographic imaging of flow and chromophore concentrations in biological tissue

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Understanding the functional makeup of the brain is a holy grail of neuroscience, and imaging tools play a significant role in achieve this aim. Functional imaging is a more difficult goal than anatomical imaging because it depends on establishing a contrast mechanism that relates to physiological function and also can be measured with good accuracy and resolution. Optical techniques are very attractive because of the rich information encoded in the absorption spectra of many different molecules, but they are difficult to use at large scale because of the high degree of scattering that occurs in passing through different tissues of the body. By using time-resolved measurements of the propagation of light from multiple illumination patterns, diffuse optical tomography (DOT) can produced low-resolution images of absorption and scattering properties, and decorrelate these to produce maps of oxygenation in the brain and other organs. At the same time, diffuse correlation spectroscopy (DCS) examines the way in which coherent light is decorrelated from itself when compared over time. This decorrelation naturally occurs due to the Brownian motion of endogenous scattering particles, and blood flow. Coherent optical techniques thus allow the non-invasive monitoring of blood flow and provide an indication of pathological cerebral auto-regulation during, e.g., stroke. Until recently, limitations in coherent detection technology have prevented significant developments towards diffuse correlation tomography (DCT), wherein volumetric images of blood flow can be produced.In this project we aim to develop a system for DCT and time-resolved DOT in one device. This will bring the two techniques together to provide images of cerebral blood flow and cerebral metabolic rate of oxygen extraction in the brain for the first time. We propose the development of theoretical and experimental methods which will enable the development of a new generation of optical instruments for portable, low-cost, continuous simultaneous monitoring of blood flow and chromophore concentrations. The rich images produced by our system have the potential to vastly improve our understanding of underlying neurological processes and pathology, and to allow the efficient use of scarce resources in targeted treatments.