Browsing by Subject "magnetic resonance imaging"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Eight-Channel Head Array and Control System for Parallel Transmit/Receive Magnetic Resonance Imaging(2014-08-11) Moody, KatherineInterest in magnetic resonance imaging (MRI) at high fields strengths (3 Tesla and above) is driven by the associated improvements in signal-to-noise ratio and spectral resolution. In practice, however, technical challenges prevent these benefits from being straightforwardly realized. High fields are associated with an increase in frequency and a decrease in the radiofrequency (RF) wavelength. The shortened wavelength causes potential inhomogeneity in the transmit field of the RF coil, resulting in non-uniform excitations. Susceptibility effects are also more pronounced at high field strengths, and can cause local distortions in the field and create areas of signal dropout. Parallel transmit is one method in development to address these challenges at high fields. Parallel transmit involves using multiple independently driven channels with RF pulses varying in amplitude, phase, and pulse envelope to create desired transmit excitations. Parallel transmit has been implemented to create homogenous transmit patterns and compensate for magnetic susceptibility effects, but despite its proven usefulness, the technology has yet to receive widespread adoption. Few parallel transmit systems exist and little work has been done in the pre-clinical realm. Studies demonstrating the clinical benefits of parallel transmit constitute a gap in the current body of work. This works presents an approach to a parallel transmit array and control system that can be easily and safely integrated on a clinical whole-body scanner. The transmit array was designed for use with ultra-low output impedance amplifiers and demonstrates an array design with a simplified decoupling network augmented by amplifier decoupling in both transmit and receive. The control system was programmed in LabVIEW using off-the-shelf hardware to manage pulse playback, correct transmit chain non-linearities, monitor on-coil waveforms, and drive the transmit hardware. The transmit array was shown with well-isolated patterns, and parallel transmit capability was demonstrated. This work progressed the translation of experimental parallel transmit technology to pre-clinical use.Item Investigation of Cryo-Cooled Microcoils for MRI(2012-10-19) Godley, Richard FranklinWhen increasing magnetic resonance imaging (MRI) resolution into the micron scale, image signal-to-noise ratio (SNR) can be maintained by using small radiofrequency (RF) coils in close proximity to the sample being imaged. Micro-scale RF coils (microcoils) can be easily fabricated on chip and placed adjacent to a sample under test. However, the high series resistance of microcoils limits the SNR due to the thermal noise generated in the copper. Cryo-cooling is a potential technique to reduce thermal noise in microcoils, thereby recovering SNR. In this research, copper microcoils of two different geometries have been cryo-cooled using liquid nitrogen. Quality-factor (Q) measurements have been taken to quantify the reduction in resistance due to cryo-cooling. Image SNR has been compared between identical coils at room temperature and liquid nitrogen temperature. The relationship between the drop in series resistance and the increase in image SNR has been analyzed, and these measurements compared to theory. While cryo-cooling can bring about dramatic increases in SNR, the extremely low temperature of liquid nitrogen is incompatible with living tissue. In general, the useful imaging region of a coil is approximately as deep as the coil diameter, thus cryo-cooling of coils has been limited in the past to larger coils, such that the thickness of a conventional cryostat does not put the sample outside of the optimal imaging region. This research utilizes a scheme of microfluidic cooling (developed in the Texas A&M NanoBio Systems Lab), which greatly reduces the volume of liquid nitrogen required to cryo-cool the coil. Along with a small gas phase nitrogen gap, this eliminates the need for a bulky cryostat. This thesis includes a review of the existing literature on cryo-cooled coils for MRI, as well as a review of planar pair coils and spiral microcoils in MR applications. Our methods of fabricating and testing these coils are described, and the results explained and analyzed. An image SNR improvement factor of 1.47 was achieved after cryo-cooling of a single planar pair coil, and an improvement factor of 4 was achieved with spiral microcoils.Item Magnetic Resonance Pulse Sequences for Fluorine-19(2014-07-11) Terry, RobinCellular therapy is the transplantation of live cells or a cell population in a patient for the treatment of complex diseases. The success of cellular therapy will rely heavily on delivering the cells to their targeted organs or areas of interest. Magnetic resonance imaging (MRI) has the ability to noninvasively track the transplanted cells to ensure they are in the desired destination. Unlike other MRI contrast agents, fluorine-19 has the ability to provide unambiguous cell tracking for two reasons: Fluorinated agents are more readily inert and will not be metabolized quickly so their movement progression can be monitored Additionally, ^(19)F has a limited background MR signal so resulting images will yield positively labeled cells, thus providing successful cell tracking and quantification of cells. The primary objective of this work was to enable the study of ^(19)F MRI on the Siemens MAGNETOM Verio scanner located at the Texas A&M Institute for Preclinical Studies (TIPS) facility at Texas A&M by making the necessary scanner modifications and pulse sequence adaptions. A ^(19)F/^(1)H dual tuned surface coil was purchased from RAPID Biomedical and was used throughout this work. Additionally, pulse sequence modifications to a spin echo sequence and a spoiled gradient echo (called fast low angle shot or FLASH) were made to enable scanning at the fluorine-19 resonant frequency. A series of experiments were performed with the goal of finding the optimal parameters for each sequence. It was found that the spin density, as compared with the T1 and T2 weighted images, resulted in the highest SNR for the spin echo sequence. The FLASH sequence with a small flip angle, low TE, and high TR provided the larger signal for the fluorine-19 images. Additionally, a large voxel size for both sequences provided a detectable and quantifiable signal for this type of functional imaging. T cells were labeled with ^(19)F and imaged to determine sensitivity and labeling efficiency. The goal of this ex vivo study was to obtain a reliable and quantifiable signal and image to be the basis for future in vivo studies. With the completion of this project, Texas A&M Institute for Preclinical Studies will be equipped with the software and knowledge to perform multinuclear MR imaging, specifically of ^(19)F.