Summary
Filter‐exchange imaging (FEXI) has already been utilized in several biomedical studies for evaluating the permeability of cell membranes. The method relies on suppressing the extracellular signal using strong diffusion weighting (the mobility filter causing a reduction in the overall diffusivity) and monitoring the subsequent diffusivity recovery. Using Monte Carlo simulations, we demonstrate that FEXI is sensitive not uniquely to the transcytolemmal exchange but also to the geometry of involved compartments: complex geometry offers locations where spins remain unaffected by the mobility filter; moving to other locations afterwards, such spins contribute to the diffusivity recovery without actually permeating any membrane. This exchange mechanism is a warning for those who aim to use FEXI in complex media such as brain gray matter and opens wide scope for investigation towards crystallizing the genuine membrane permeation and characterizing the compartment geometry.
In this paper, we propose a variational approach for fusion of two co-registered high-resolution panchromatic (HRP) and low-resolution multi-spectral (LRM) images to reach the high-resolution multi-spectral (HRM) one, i.e. pan-sharpening. In this fusion technique, there is a trade-off between structural information of HRP image and spectral information of LRM one. To reconstruct the HRM image, which benefits from the best characteristics of both images, we consider several fidelity terms. The structural fidelity term is used to transfer structural information of HRP image to HRM one, and spectral fidelity term is utilized to preserve spectral consistency between HRM and LRM images throughout the fusion process. To reduce the spectral distortion occurred due to the discrepancy between intensity values of HRP and LRM images, a novel spatial-spectral fidelity term is designed to keep the intensity ratio between multispectral and panchromatic pixels in the high-resolution space as the same as the low-resolution space. Moreover, the total variation (TV) regularization term is employed as a prior to promote the sparseness of gradient in HRM bands. These fidelity terms were formulated in a convex optimization problem. However, the structural and TV terms made this optimization problem nondifferentiable. Therefore, we developed an efficient majorizationminimization (MM) algorithm for solving the optimization problem. The proposed method applied to three datasets, acquired by WorldView-3, Deimos-2, and QuickBird satellites. To assess the effectiveness of the proposed method, visual analysis, as well as quantitative comparison to various pan-sharpening methods, were carried out. The experimental results suggested that the proposed method outperformed the competitors visually and quantitatively.
Filter-EXchange Imaging (FEXI) evaluates the permeability of cell membranes, which is already used in clinical research. It relies on suppressing the extracellular signal using strong diffusion weighting (mobility filter causing a reduction in the overall diffusivity) and monitoring the subsequent diffusivity recovery. Using simulations, we demonstrate the presence of a spurious exchange: In compartments with complex geometry, there are locations where spins remain unaffected by the mobility filter. Moving to other locations afterward contributes to the diffusivity recovery, thus mimicking exchange. The discovered phenomenon opens large room for investigation towards characterizing the compartment geometry and crystallizing the genuine membrane permeation.
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