Magnetic resonance imaging (MRI) is unique in its sensitivity to a wide array of contrast mechanisms in soft tissue. Unfortunately, the physiological and/or microanatomical characteristics that give rise to this contrast can show significant heterogeneity on the scale of a typical voxel, resulting in an observed nuclear magnetic resonance (NMR) signal that is a summation of these spatially varying characteristics. Multicomponent analysis, which allows one to separate the observed NMR signal into components that represent underlying sub-voxel tissue compartments, can be used to deal with this limitation. For example, this approach has been applied to myelinated tissue (e.g., white matter, peripheral nerve) to measure the so-called myelin water fraction, which has been shown to correlate with myelin content. Despite this promise, several fundamental issues regarding the compartmental models used to describe myelinated tissue exist. These include: 1) quantifying the effect of intercompartmental exchange and 2) the inability to resolve axonal water (water within myelinated axons) from interaxonal water (water outside myelinated axons) in the central nervous system. This dissertation presents a series of studies aimed to address these fundamental issues. To address the effect of exchange, a novel method for quantifying exchange rates, which allows for a significant reduction in scan time relative to existing methods, was developed. This method was tested and compared to existing methods via simulations studies, validated via phantom studies, and applied in excised myelinated tissue samples. To investigate methods for resolving axonal and interaxonal water, compartmental relaxation measurements were performed in myelinated tissue samples before and after administration of contrast reagents as a means to characterize the compartmental enhancement pattern associated with each reagent. The results of these studies suggest that administration of potassium dichromate in white matter and optic nerve ex vivo might allow one to resolve axonal and interaxonal water in these tissues. Though not directly related, an additional study was included, which showed that novel information about tumor microenvironment may be available in vivo using the multicomponent analysis techniques presented throughout this work.

MRI provides an excellent way of visualizing muscle inflammation; however, there are few techniques that serve to quantitatively assess edematous muscle. In this thesis, integrated relaxation measurements were made in vivo on edematous rat muscle with varying degrees of swelling. To investigate the effect of exchange on the observed relaxation parameters a two pool model was created and the Bloch-McConnell equations were solved for the varying amounts of swelling. Results from the simulation of the exchange model were compared to the observed data to extract fitted parameters for the compartmental relaxation times. These simulations also provided a comparison for the observed changes in the long-lived apparent T1. Edematous muscle was found to display both multiexponential T1 and multiexponential T2. Normal muscle, however, was found to exhibit only a single T1-T2 component. It was shown that the apparent T1 of the long-lived signal component in edematous muscle increased monotonically with an increase in the amount of edema. Knowledge of changes in T1 and the exchange kinetics in edematous muscle might help in further characterizing the micro-anatomy of muscle tissue in various stages of injury.

Functional magnetic resonance imaging (fMRI) is a noninvasive imaging technique capable of mapping cognitive networks in the human brain that has become widely used for both research and clinical applications. The activity of focal regions within the brain can be inferred based on their blood oxygen level dependent (BOLD) signal changes through time by recording a series of images while subjects perform carefully designed tasks. Steady state functional connectivity methods generate similar maps of neural networks through analyses of underlying activity. Resting state functional connectivity measurements reduce the demands placed on patient/subject compliance and can potentially extend fMRI to a wider range of applications. However, the factors that affect measurements of steady state functional connectivity remain unclear, and methods of making these measurements may be further improved. This dissertation addresses three different aspects of functional connectivity measured with fMRI. First, we study the effects of cognitive load on measurements of functional connectivity in the working memory and default mode networks, building on previous work in the motor network. We report increases in functional connectivity within both networks, and present evidence of changing connectivity between them. Second, we apply functional connectivity analyses to electroencephalography and fMRI data recorded simultaneously to investigate the regions underlying variance in frontal theta power. Our results show both positive and negative correlations to theta power across working memory loads, and changing correlations in the parahippocampal gyrus, among other regions. Third, we demonstrate two methodological improvements to the measurement of functional connectivity using data acquired at ultra high field (7T) and applying nonlinear measurements of connectivity. We show that detection and significance of functional connectivity can be improved by decreasing voxel volumes to sizes that are not practically achievable without ultra-high magnetic fields, presumably due to decreases in partial volume effects. In addition, we show that mutual information can be used to detect functional connectivity between regions that are commonly left unidentified by measurements based on linear correlation coefficients.

Living cells and tissues experience mechanical forces in their physiological environments that are known to affect many cellular processes. Also of importance are the mechanical properties of cells, as well as the microforces generated by cellular processes in their microenvironments. The difficulty associated with studying these phenomena in vivo has led to alternatives such as using in vitro models. The need for experimental techniques for investigating cellular biomechanics and mechanobiology in vitro has fueled an evolution in the technology used in these studies. Particularly noteworthy are some of the new biomicroelectromechanical systems (BioMEMs) devices and techniques. <p> This study describes cellular micromechanical techniques and methods that have been developed for extit{in vitro} studies. Improvements made to a passive array of vertical microcantilevers, for detecting cellular microforces and studying in vitro cell mechanics are presented. A new technique that uses poly(vinyl alcohol) (PVA) as a lift-off agent to attach structures to the microcantilevers, thereby providing a means to actively move the microcantilevers is introduced. The use of the improved microcantilever array platform as a potential assay for cardiac myofibrillogenesis will also be described. <p> Finally the use of cryogenic etching techniques for making master molds for the microcantilever arrays is described, and subsequent arrays are used to investigate the biological responses of mesenchymal stem cells to forces generated by post deflections.

Assessment of the restoration of load-bearing function is the central goal in the study of fracture healing process. During the fracture healing, two critical aspects affect its analysis: (1) material properties of the callus components, and (2) the spatio-temporal architecture of the callus with respect to cartilage and new bone formation. In this study, an inverse problem methodology is used which takes into account both features and yields material property estimates that can analyze the healing changes. Six stabilized fractured mouse tibias are obtained at two time points during the most active phase of the healing process, respectively 10 days (n=3), and 14 days (n=3) after fracture. Under the same displacement conditions, the inverse procedure estimations of the callus material properties are generated and compared to other fracture healing metrics. The FEA estimated property is the only metric shown to be statistically significant (p=0.0194) in detecting the changes in the stiffness that occur during the healing time points. In addition, simulation studies regarding sensitivity to initial guess and noise are presented, as well as the influence of callus architecture on the FEA estimated material property metric. The finite element model inverse analysis developed can be used to determine the effects of genetics or therapeutic manipulations on fracture healing in rodents.

We present a multi-dye ratiometric method for the correction of optical artifacts and unequal illumination encountered by researchers that utilize complex microfluidic systems. Using a novel chemotaxis system that provides cells with passively generated chemoattractant gradients, we demonstrate that the currently utilized method of single-color epi-fluorescence is limited in its ability to characterize gradient formation in the presence of differing channel heights and in the proximity of micron-sized features. As future devices strive to mimic the microtopography of physiological environments, this deficit will become increasingly relevant. The presented multi-color methodology allows for the correction of standard wide-field images and, with the incorporation of laser scanning confocal microscopy, explores three-dimensionally resolved gradient analysis. Using a validated numerical model, we also analyze the potential distortion in gradient formation introduced by device microstructure and the very cells that the system is designed to examine. Our analysis has important implications for the sensitivity of microfabricated systems to cell loading density and channel aspect ratio. In conclusion, the imaging and modeling methodologies introduced in this work will provide complementary information for the rational design and validation of microfluidic devices with cell-sized features.

We demonstrate a flexible, tunable scheme for synthesizing multifunctional, ultrasmall superparamagnetic iron oxide nanoparticles (USPIOs) and its application to the area of magnetic relaxation switches. USPIO cores (10nm) were functionalized with a poly(propylene sulfide)-bl-poly(ethylene glycol) (PPS-PEG) copolymer, yielding ~40nm micelles. PPS-PEG-ssDNA conjugates and fluorophore-conjugated PPS-PEG are incorporated into the micelle synthesis process, yielding ssDNA-coated magnetofluorescent particles. To form magnetic relaxation switches, we generated USPIO populations that display complementary ssDNA sequences. Mixing of complementary USPIOs leads to clustering, resulting in a significant increase in R2 magnetic relaxation. Treatment of the DNA-crosslinked USPIO clusters with restriction enzymes specific for the crosslinking sequence results in an irreversible return of R2 relaxation to baseline levels. The constructs demonstrate their utility as nanoscale sensors of restriction enzyme activity. The presented functionalization scheme can be extended to the generation of biosensors for other sources of proteolytic activity, for diagnostics and imaging applications for cancer and atherosclerosis.

Within the last decade, magnetic resonance imaging (MRI) scanners with ultra-high magnetic field strengths of 7 Tesla and above have become available. While high magnetic fields provide theoretical improvements in the nuclear magnetic resonance signal-to-noise ratio (SNR) and in the blood-oxygenation-level dependent (BOLD) contrast used for functional brain imaging, they also introduce technical issues such as increased bulk magnetic susceptibility effects and increased physiological noise in human data. Acquiring MRI data with multiple receiver coils in parallel can alleviate some of these issues at a cost to SNR and BOLD sensitivity that depends on several experiment-specific factors. This work attempts to address some of the issues surrounding high field fMRI in the context of parallel imaging and suggests methods for realizing the theoretical benefits of functional MRI with high magnetic fields.

Patient safety and emergency department functionality are compromised when inefficient coordination between hospital departments impedes ED patients access to inpatient cardiac care. The objective of this dissertation was to create a discrete event simulation model of hospital patient flow in order to determine how bed demand from competing cardiology admission sources affects ED patients access to inpatient cardiac care. The simulation employed survival analysis regression to model competition for inpatient cardiac beds and predict delays in ED patients access. The novel simulation strategy was used to demonstrate how altering outpatient schedules and creating informed bed management practices can optimize hospital capacity and improve ED patient access and how interventions designed to increase inpatient throughput or add capacity will have the most significant effect on highest priority patients.

Previous studies have reported that fluorescence in human tissue is a temperature dependent phenomenon. The most apparent effect is the inverse relationship between fluorescence intensity and temperature. Here, we present the effects of temperature on fluorescence in human tissue using skin and adipose tissue in vitro and skin in vivo. Fluorescence and diffuse reflectance measurements were made while the temperature of the specimens was increased, and an inverse Monte Carlo algorithm was used to calculate optical properties as a function of temperature. In vitro and in vivo experiments showed a decrease in fluorescence intensity due to temperature increase. The fluorescence intensity showed no relationship to the optical properties in the physiological temperature range, suggesting that changes in optical properties are not the primary mechanism by which fluorescence is affected by temperature. This study confirms that fluorescence decreases with increasing temperature in human tissue in vitro and in vivo. Our results further suggest the presence of a temperature dependent non-radiative decay mechanism.