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Abstract
Imaging methods are a powerful tool for diagnostic purposes. In this chapter, the most important light-imaging methods, their advantages, and drawbacks are described. The advantages of radiation-free light-based imaging methods relative to traditional radiation methods, such as X-ray, magnetic resonance, or positron emission imaging, are indicated, and the recent advances to improve probing depth, contrast, and resolution in thick tissues are demonstrated. Some historical aspects and recent improvements in light-imaging methods, such as optical coherence tomography, speckle-imaging, second harmonic generation, or light-sheet microscopies, are presented. Due to the recent combination of optical immersion clearing with light-based imaging methods, several studies have been reported, where high-quality images and 3D reconstruction have been obtained for various tissues, providing an alternative to traditional histology or histopathology methods. The purpose of optical clearing is to reduce light scattering, but tissue clearing is obtained through three mechanisms: tissue dehydration, refractive index matching, and protein dissociation. This last mechanism leads to a reduction in the intensity of protein fluorescence, which can be a disadvantage in fluorescence imaging methods. The selection of certain clearing protocols that minimizes or eliminates protein dissociation has been made by some researchers, and a review of such literature is made in the various sections of this chapter.

Abstract
There are several types of measurements that can be performed with biological tissues during optical clearing treatments. When analyzing these methods, two major modes of study must be provided: ex vivo and in vivo. Measurements made from ex vivo samples are more flexible, allowing, for instance, to measure tissue transmittance or sample thickness kinetics. The results obtained from these measurements do not mimic exactly the in vivo situation. In the case of in vivo tissues, results from measurements are more realistic, but a more restrict number is possible, based only on reflectance or imaging methods. In this chapter, we make a brief description and analysis of the various measurement procedures that can be made during treatments of tissues ex vivo and in vivo and present some studies where important information was collected. The valuable results already obtained or possible to obtain in future from measurements described here will be presented and explained in the following sections. A particular case with great interest not only for biophotonics but also for food industry or organ preservation is the estimation of the diffusion properties of water and agents. Such evaluation of parameters is based only on collimated transmittance and thickness measurements made from ex vivo tissues. We will describe these measurements here and exploit their use in the study of diffusion in Chap. 7.

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Abstract
Evaluating diffusion properties of novel optical clearing (OC) agents is critical for advancing medical imaging. Tartrazine (TTZ), a strong absorbing dye, has shown promise in enhancing tissue transparency, yet its diffusion properties remain uncharacterized. In this work, OC treatments with TTZ-water solutions with varying osmolarities were performed, and the diffusion times (tau) that characterize the tissue dehydration and the RI matching mechanisms were estimated. From kinetic T-c measurements during treatment, tau values of water and TTZ were estimated in muscles as 60.0 s and 416.0 s, respectively. Corresponding diffusion coefficients (D) were derived from sample thickness data measured during treatments where the unique fluxes of TTZ and water occur. The respective D values were then calculated as 1.9 x 10(-6) cm(2)/s for water and 3.6 x 10(-7) cm(2)/s for TTZ. These findings provide key insights into TTZ diffusion in skeletal muscle and support its potential as an effective OC agent.