Browsing by Subject "Contrast"

The purpose of this thesis is to report technological developments in contrast mechanisms for MRI. The search for new forms of contrast is on-going, with the hope that new contrast mechanisms and new contrast agents will provide unique insights into various molecular processes and disease states. In this thesis, we will describe new contrast mechanisms developed by manipulating the inherent physics of the system, as well as the development of exogenous contrast agents. More specifically, we will describe the application of iMQCs (intermolecular multiple quantum coherences) to thermometry and structural imaging, and the unique information provided from these studies. We will also describe methods for migrating iMQC-based pulse sequences from a Bruker research console onto a clinical GE console, thus enabling the application of iMQCs to humans. We will describe the development of hyperpolarized contrast agents which have the potential to provide an unprecedented level of molecular contrast to MRI and the development of techniques to enhance the lifetime of these hyperpolarized contrast agents. Finally, we will discuss a new type of T2 -weighted imaging which significantly improves the refocusing of CPMG-type sequences.

The goal of this research was to investigate the feasibility of using a weighted dual-energy subtraction method in dedicated breast CT using quasi-monochromatic x-ray beams, to better distinguish soft tissues in the breast. This research used computer simulations and measurements, includes development of protocols and phantoms, and yields quantitative results and physical components. Quasi-monochromatic beams were created using different specially processed, hermetically sealed ultra-thick K-edge filters yielding a maximum mean energy difference of 7 keV; also, different kVp potentials on the x-ray tube were used with higher atomic number filters, yielding a maximum mean energy difference of 11 keV. A cylindrical phantom containing muscle tissue equivalent material, glandular tissue equivalent material, polyethylene, and acrylic was filled with methanol (adipose tissue equivalent) was developed and scanned to investigate dual-energy contrast enhancement of the different materials. The CT scans were acquired using a dual-modality SPECT-CT system for dedicated breast imaging. The weighted dual-energy subtraction method is adapted from dual-energy x-ray absorptiometry, where a high and low energy CT scan is acquired using two different ultra-thick k-edge filters (or tube potentials) and the data is reconstructed. Region of interest values are obtained from each image, which is then multiplied by a weighted value (k), and the higher energy image is subtracted from the lower energy image to achieve contrast enhancement. The k-values were calculated from the ratio of measured attenuation coefficients for the material to be subtracted. In a projection-noise normalized acquisition scenario, it was found that weighted dual-energy subtraction with quasi-monochromatic beams might not be feasible under the current circumstances due to the especially high noise (poor contrast to noise ratios) and poor contrast. While this was not an optimized scenario, the approach does have promise, indicating that more investigation is needed.

Clutter is a problematic noise artifact in a variety of ultrasound applications. Clinical tasks complicated by the presence of clutter include detecting cancerous lesions in abdominal organs (e.g. livers, bladders) and visualizing endocardial borders to assess cardiovascular health. In this dissertation, an analytical expression for contrast loss due to clutter is derived, clutter is quantified in abdominal images, and sources of abdominal clutter are identified. Novel clutter reduction methods are also presented and tested in abdominal and cardiac images.

One of the novel clutter reduction methods is Short-Lag Spatial Coherence (SLSC) imaging. Instead of applying a conventional delay-and-sum beamformer to measure the amplitude of received echoes and form B-mode images, the spatial coherence of received echoes are measured to form SLSC images. The world's first SLSC images of simulated, phantom, and in vivo data are presented herein. They demonstrate reduced clutter and improved contrast, contrast-to-noise, and signal-to-noise ratios compared to conventional B-mode images. In addition, the resolution characteristics of SLSC images are quantified and compared to resolution in B-mode images.

A clinical study with 14 volunteers was conducted to demonstrate that SLSC imaging offers 19-33% improvement in the visualization of endocardial borders when the quality of B-mode images formed from the same echo data was poor. There were no statistically significant improvements in endocardial border visualization with SLSC imaging when the quality of matched B-mode images was medium to good.