74 research outputs found
Multiple quantum coherences experiment, simulations and applications
No full textMULTIPLE QUANTUM COHERENCES EXPERIMENT, SIMULATIONS AND APPLICATIONS
ARMAN SAINOVITCH KUSSAINOV
Dissertation under the direction of Professor Daniel F. Gochberg
This dissertation discuses the intermolecular Multilpe Quantum Coherence (iMQC) phenomenon in nuclear magnetic resonance (NMR). This effect originates from the coherent long range dipole-dipole interaction between spins. A short introduction to nuclear magnetic resonance and the CRAZED experiment (the primary method to observe and study iMQC) is given. We perform numerical simulations of the CRAZED experiment, including relaxation processes, instantaneous radio frequency pulses and gradients, as well as phase cycling, phantom positioning, and sample rotation. The signal dependence on the sample
orientation, applied gradient strength and direction, and the underlying sample structure length scale has been determined experimentally and by numerical simulations for two biological tissues: rat sciatic nerve and mice fibrotic liver. The results indicate that the CRAZED signal may provide a unique means for elucidating sample structures at the hundreds of micrometers distance scale
Multi-exponential T2 Myelin Water Imaging in Ex-Vivo Rodent Brain at 7T and 15.2T
No full textTo apply MWI techniques to excised, fixed rodent brains, multi-exponential T2 (MET2) data were acquired at high (7T) and ultra-high (15.2T) fields with and without tissue doping with Gadolinium to increase SNR efficiency. From relaxivity measurements, optimal scan parameters are found based on the minimal Cramér-Rao lower bounds (CRLB) of variance of myelin water fraction (MWF). Subsequently acquired MET2 data were analyzed to obtain myelin water fraction (MWF) maps with and without contrast agent at each field strength with various techniques, displaying the potential for MWI in rodent brain in the study of myelin content across whole brain
Development and Characterization of NMR and MRI Methods for Assessment of Tumor Oxygen Consumption
No full textA common feature of invasive cancers is that they have an upregulation of glucose metabolism and reduced oxygen utilization. The overall goal of this research was to develop a 17Oxygen MRI method to access this aberrant tumor metabolism using a T1ρ weighted pulse sequence on a Bruker NMR Spectrometer. An assessment of the reliability of this pulse sequence for quantifying the ratio of concentrations of H217O (necessary for measuring the metabolic rate in vivo) water in 4 pairs of phantoms was preformed. Specifically, we varied the pH and concentration of Gadopentetic acid (Gd-DTPA) (which modulated T1 and T2 profiles) in 4 different combinations to mimic conditions in the tumor microeviorment. We then used the signal decay data collected at a low and high spin lock power and inserted this data into an eqaution. This equation was then used to compute the ratio of concentration between an enriched and unenriched phantom in each of the 4 pairs.
The two -frequency method was able to quantify [H217O] successfully in the phantoms without Gd-DTPA using the fitted signal data but failed to yield the correct result for the raw data. Thus, to further evaluate these approaches we need employ studies at clinical field strengths. Nevertheless, our results suggest that the quantification of H217O is insensitive to pH but sensitive to changes in T1 and T2
Computational and Experimental Investigation of DSC-MRI Signal Behavior in Magnetically Inhomogeneous Media
No full textStudies of the design, use, and characteristics of methacrylic acid-based polymer gel dosimeters
No full textStudies of the design, use, and characteristics of methacrylic acid-based polymer gel dosimeters
No full textPolymer gel dosimeters are three-dimensional radiation-sensitive materials comprised of monomers and other chemicals distributed in an aqueous gelatin matrix. Upon irradiation by high energy X or gamma rays, free radicals formed within the water initiate polymerization of the monomers, resulting in distributions of polymer that reflect the distribution of radiation dose. The polymers in turn affect the local nuclear magnetic resonance (NMR) properties of water, so that complex, integrated radiation dose distributions can be measured with high spatial resolution using magnetic resonance imaging. Previous studies have demonstrated the use of polymer gels for applications in dosimetry for clinical radiation therapy. However, there are several aspects of polymer gel dosimetry that remain unresolved. In this thesis some of these problems are addressed. In particular, the design and composition of gels for optimal dose response, the characterization of their dose responses for different NMR properties, the development of improved imaging methods, and the underlying mechanisms of dose response, are each considered. Methacrylic acid-based dosimeters have been optimized for measurements of dose based on transverse relaxation rates. In addition, measurements of other NMR parameters, such as the rates that govern magnetization transfer, are made, are considered and a new magnetization transfer parameter, the magnetization transfer proportion, is introduced as a simplified measure of dose response that is less susceptible to imaging errors than more traditional measures. A simple model is introduced to explain the dose response in terms of an increase in the number of efficiently relaxing protons through a chemical exchange relaxation mechanism, and the parameters of this model are derived from a series of appropriate NMR experiments
Studies Of Proton Nuclear Magnetic Resonance Relaxation In Human Cortical Bone: From Ex Vivo Spectroscopy To Clinical Imaging Methods
No full textCurrent clinical bone diagnostic measures rely predominantly on X-ray-based contrast and are primarily sensitive to bone mineral content. Since bone also contains collagen and water components, which are heavily implicated in fracture resistance, these X-ray measures are micro-anatomically incomplete and do not identify individuals who will fracture. This dissertation aims to improve clinical bone fracture risk assessment through the use of novel magnetic resonance imaging (MRI) methods, which provide quantitative measures of the non-mineral bone components. The overall goal is to advance our understanding of 1H nuclear magnetic resonance (NMR) relaxation in human cortical bone to the point that diagnostically-relevant parameters may be extracted from in vivo bone MRI measurements. To accomplish this, custom NMR hardware was first developed for a rigorous, NMR relaxation-based characterization of ex vivo cortical bone. Such characterization was used to identify the micro-anatomical origins of cortical bone NMR signal components, which included collagen, bound water, and pore water protons. These signal components correlated well with various bone mechanical properties, indicating diagnostic relevance. Using the well-characterized cortical bone relaxation characteristics, novel and clinically practical methods for quantitative, diagnostic bone MRI were developed and validated. Collectively, this work represents a biophysical basis for cortical bone MRI, which stands ready for translation to clinical and research studies
Computational and Experimental Investigation of DSC-MRI Signal Behavior in Magnetically Inhomogeneous Media
No full textThe systematic investigation of susceptibility-induced contrast in MRI is important to improve our understanding of the influence of tissue microstructure on dynamic susceptibility contrast (DSC)-MRI derived perfusion data. The Finite Perturber Method (FPM) has previously been used to investigate susceptibility contrast in MRI arising from arbitrarily shaped structures. However, the FPM has low computational efficiency in simulating water diffusion, especially for complex three-dimensional structures that mimic tissue. In this work, an improved computational approach that combines the FPM with a matrix-based finite difference method (FDM), termed the Finite Perturber Finite Difference Method (FPFDM), was developed to more efficiently investigate the biophysical basis of DSC-MRI data and its sensitivity to vascular geometry and contrast agent (CA) distribution within tissue. The application of the FPFDM to the physiological and physical conditions encountered in a typical DSC-MRI brain tumor study enabled the derivation of a new DSC-MRI metric, termed the Transverse Relaxivity at Tracer Equilibrium (TRATE), which we propose specifically reports on tumor cellular properties. Computational FPFDM studies revealed that TRATE depends on cellular density, size, shape and spatial distribution. To validate the in vivo sensitivity of TRATE it was evaluated in two animal brain tumor models, the 9L and C6, which have varying cellular characteristics. The TRATE values were also compared to measures of the apparent diffusion coefficient (ADC), the CA transfer constant (Ktrans), the extravascular extracellular volume fraction (ve) and histological data. The TRATE values in 9L tumors were significantly higher than those in C6 tumors, a finding that reflects the histologically confirmed higher cell density in 9L tumors and lower cellular density. A voxel-wise comparison of TRATE with ADC, ve, and Ktrans maps showed low spatial correlation, indicating it is providing unique and complementary information on tumor status. In summary, the studies described herein highlight the value of pairing computational and experimental advancements in order to enhance our characterization of DSC-MRI contrast mechanisms and how such mechanisms can be leveraged to derive new non-invasive metrics for evaluating brain tumors
Studies Of Proton Nuclear Magnetic Resonance Relaxation In Human Cortical Bone: From Ex Vivo Spectroscopy To Clinical Imaging Methods
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