Matching Items (3)
Filtering by
- Genre: Doctoral Dissertation
Description
Magnetic resonance spectroscopic imaging (MRSI) is a non-invasive technique that offers a unique ability to provide the spatial distribution of relevant biochemical compounds (metabolites). The ‘spectrum’ of information provided by MRSI is used as biomarkers for the differential diagnosis of several diseases such as cancer or neurological disorders. Treatment responsive brain tumors can appear similar to non-responsive tumors on conventional anatomical MR images, earlier in the therapy, leading to a poor prognosis for many patients. Biomarkers such as lactate are particularly of interest in the oncological studies of solid tumors to determine their energy metabolism, blood flow, and hypoxia. Despite the capability of nearly all clinical MRI scanners to perform MRSI only limited integration of MRSI into routine clinical studies has occurred to date. The major challenges affecting its true potential are the inherently long acquisition time, low signal-to-noise (SNR) of the signals, overlapping of spectral lines, or the presence of artifacts. The goal of this dissertation work is to facilitate MRSI in routine clinical studies without affecting the current patient throughput.
In this work, the Compressed Sensing (CS) strategy was used to accelerate conventional Point RESolved Spectroscopy (PRESS) MRSI by sampling well below the Shannon-Nyquist limit. Two undersampling strategies, namely the pseudo-random variable density and a novel a priori method was developed and implemented on a clinical scanner. Prospectively undersampled MRSI data was acquired from patients with various brain-related concerns. Spatial-spectral post-processing and CS reconstruction pipeline was developed for multi-channel undersampled data. The fidelity of the CS-MRSI method was determined by comparing the CS reconstructed data to the fully sampled data. Statistical results showed that the a priori approach maintained high spectral fidelity compared to the fully sampled reference for an 80% reduction in scan time. Next, an improvement to the CS-MRSI reconstruction was achieved by incorporating coil sensitivity maps as support in the iterative process. Further, a CS-MRSI-based fast lactate spectroscopic imaging method was developed and implemented to achieve complete water and fat suppression for accurate spatial localization and quantification of lactate in tumors. In vitro phantoms were developed, and the sequence was tested to determine the efficacy of CS-MRSI for low SNR signals, the efficacy of the CS acceleration was determined with statistical analysis.
ContributorsBikkamane Jayadev, Nutandev (Author) / Kodibagkar, Vikram (Thesis advisor) / Chang, John (Committee member) / Robison, Ryan (Committee member) / Smith, Barbara (Committee member) / Sohn, Sung-Min (Committee member) / Arizona State University (Publisher)
Created2021
Description
In 1946 Felix Bloch first demonstrated the phenomenon of nuclear magnetic resonance using continuous-wave signal generation and acquisition. Shortly after in 1966, Richard R. Ernst demonstrated the breakthrough that nuclear magnetic resonance needed to develop into magnetic resonance imaging: the application of Fourier transforms for sensitive pulsed imaging. Upon this discovery, the world of research began to develop high power radio amplifiers and fast radio switches for pulsed experimentation. Consequently, continuous-wave imaging placed on the backburner.Although high power pulses are dominant in clinical imaging, there are unique advantages to low power, continuous-wave pulse sequences that transmit and receive signals simultaneously. Primarily, tissues or materials with short T2 time constants can be imaged and the peak radio power required is drastically reduced.
The fundamental problem with this lies in its nature; the transmitter leaks a strong leakage signal into the receiver, thus saturating the receiver and the intended nuclear magnetic resonance signal is lost noise.
Demonstrated in this dissertation is a multichannel standalone simultaneous transmit and receive (STAR) system with remote user-control that enables continuous- wave full-duplex imaging. STAR calibrates cancellation signals through vector modulators that match the leakage signal of each receiver in amplitude but opposite in phase, therefore destructively interfering the leakage signals. STAR does not require specific imaging coils or console inputs for calibration. It was designed to be general- purpose, therefore integrating into any imaging system. To begin, the user uses an Android tablet to tune STAR to match the Larmor frequency in the bore. Then, the user tells STAR to begin calibration. After self-calibrating, the user may fine-tune the calibration state of the system before enabling a low-power mode for system electronics and imaging may commence. STAR was demonstrated to isolate two receiver coils upwards of 70 dB from the transmit coil and is readily upgradable to enable the use of four receive coils.
Some primary concerns of STAR are the removal of transceivers for multichannel operation, digital circuit noise, external noise, calibration speed, upgradability, and the isolation introduced; all of which are addressed in the proceeding thesis.
ContributorsColwell, Zachary Allen (Author) / Sohn, Sung-Min (Thesis advisor) / Trichopoulos, Georgios (Thesis advisor) / Aberle, James (Committee member) / Sadleir, Rosalind (Committee member) / Arizona State University (Publisher)
Created2023
Description
Magnetic Resonance Imaging has become an increasingly reliable source of medical imaging to obtain high quality detailed images of the human anatomy. Application specific coil or an array of coils when placed closely to the anatomy produces high quality image due to the improved spatial signal to noise ratio. Elastic RF coils have been shown to conform to the shape of the patient’s body and drastically reduce the gap between coil and anatomy. First, a major challenge faced by these elastic RF coils is the changing impedance condition as the coil takes a different shape for every individual. Next, an area that could benefit from the improved image quality and patient comfort that comes from flexible RF coil design is endorectal prostate imaging. Demonstrated in the first part of this dissertation is a modular solution to compensate the impedance mismatch. Standalone Wireless Impedance Matching (SWIM) system is an automatic impedance mismatch compensation system that can function independently of the MR scanner. The matching network consists of a capacitor array with RF switches to electronically cycle through different input impedance conditions. The SWIM system can automatically calibrate an RF coil in 3s with a reflection coefficient of less than -15dB resulting in improved Signal-to-noise ratio (SNR) of the sample image by 12% - 24%, based on sample size, when compared to a loaded coil without retuning.
For the second part, we propose a novel elastic and inflatable RF coil integrated with the SWIM system for endorectal prostate imaging at 9.4T. A silicone polymer substrate filled with liquid metal alloy is designed and fabricated with a cavity to create ii inflation. This inflatable RF coil is combined with the SWIM system to automatically tune and match after inflating the RF coil for individual levels of inflation. The imaging results have shown a ~10%, ~19%, and ~25 % increase in SNR due to inflation of RF coil at different ROIs in the acquired image. Overall, the methods proposed and discussed in this thesis are a step towards a new generation of RF coil systems for both existing applications and upcoming ones.
ContributorsKandala, Sri Kirthi (Author) / Sohn, Sung-Min (Thesis advisor) / Kdibagkar, Vikram (Committee member) / Sadleir, Rosalind J (Committee member) / Beeman, Scott (Committee member) / Trichopoulos, Georgios (Committee member) / Arizona State University (Publisher)
Created2023