Due to modeling and experimental imperfections, multispectral optoacoustic tomography images are often afflicted with negative values, which are further amplified when propagating into the spectrally unmixed images of chromophore concentrations. Since negative values have no physical meaning, accuracy can potentially be improved by imposing non-negativity constraints on the initial reconstructions and the unmixing steps. Herein, we compare several non-negative constrained approaches with reconstruction and spectral unmixing performed separately or combined in a single inverse step. The quantitative performance and sensitivity of the different approaches in detecting small amounts of spectrally-distinct chromophores are studied in tissue-mimicking phantoms and mouse experiments.
KEYWORDS: Image processing, Photography, Image compression, Digital photography, Clouds, Image enhancement, Process control, Digital imaging, Spatial frequencies, Digital cameras
Digital photography systems often render an image from a scene-referred description with very wide dynamic range to an output-referred description of much lesser dynamic range. Global tone maps are often used for this purpose, but can fail when called upon to perform a large amount of range compression. A luminance formulation of the Retinex ratio-reset-product-average algorithm produces a smoothly changing contrast mask of great benefit, but it too can fail where high contrast edges are encountered. A slight but critical modification to the Retinex equation - introducing a ratio modification operator - changes the nature of the generated contrast mask so that it is simultaneously smooth in regions of small contrast ratios, but extremely sharp at high contrast edges. A mask produced in this way compresses large and undesirable contrast ratios while preserving, or optionally enhancing, small ratios critical to the sensation of image contrast. Processed images may appear to have a greater contrast despite having a shorter global contrast range. Adjusting the new operator prior to processing gives control of the degree of compression at high contrast edges. Changing the operator during processing gives control over spatial frequency response.
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