<p> <b>Part 1: High-Sensitivity Amplifiers for Detecting Fluorescence </b></p><p> Monitoring electrical activity and Ca<i>_i</i>²⁺ transients in biological tissues and individual cells increasingly utilizes optical sensors based on voltage-dependent and Ca<i>_i</i>²⁺-dependent fluorescent dyes. However, achieving satisfactory signal-to-noise ratios (SNR) often requires increased illumination intensities and/or dye concentrations, which results in photo-toxicity, photo-bleaching and other adverse effects limiting the utility of optical recordings. The most challenging are the recordings from individual cardiac myocytes and neurons. Here we demonstrate that by optimizing a conventional transimpedance topology one can achieve a 10-20 fold increase of sensitivity with photodiode-based recording systems (dependent on application). We provide a detailed comparative analysis of the dynamic and noise characteristics of different transimpedance amplifier topologies as well as the example(s) of their practical implementation.</p><p> <b>Part 2: Light-Scattering Models for Interpretation of Fluorescence Data</b></p><p> Current interest in understanding light transport in cardiac tissue has been motivated in part by increased use of voltage-sensitive and Ca<i>_i</i>²⁺-sensitive fluorescent probes to map electrical impulse propagation and Ca<i>_i</i>²⁺-transients in the heart. The fluorescent signals are recorded using such probes represent contributions from different layers of myocardial tissue and are greatly affected by light scattering. The interpretation of these signals thus requires deconvolution which would not be possible without detailed models of light transport in the respective tissue. Which involves the experimental measurements of the absorption, scattering, and anisotropy coefficients, <i>&mu;_a, &mu;_s,</i> and <i>g</i> respectively.</p><p> The aim of the second part of our thesis was to derive a new method for deriving these parameters from high spatial resolution measurements of forward-directed flux (FDF). To this end, we carried out high spatial resolution measurements of forward-directed flux (FDF) in intact and homogenized cardiac tissue, as well as in intralipid-based tissue phantoms. We demonstrated that in the vicinity of the illuminated surface, the FDF consistently manifested a fast decaying exponent with a space constant comparable to the decay rate of ballistic photons. Using a Monte Carlo model we obtained a simple empirical formula linking the rate of the fast exponent to the scattering coefficient, the anisotropy parameter <i> g,</i> and the numerical aperture of the probe. The estimates of scattering coefficient based on this formula were validated in tissue phantoms. The advantages of the new method are its simplicity and low-cost.</p><p>