Abstract
Hyperpolarized-gas MRI using 3He or 129Xe is an evolving technology for providing images of the lung with high spatial and temporal resolution. The associated acquisition strategies, such as ventilation imaging and 129Xe dissolved-phase imaging, have great potential to yield a comprehensive morphologic and functional assessment of the lung. The goal of this dissertation was to advance hyperpolarized-gas MRI by addressing three specific needs: (1) robust and rapid calibration of the MR-scanner transmitter; (2) accelerated acquisition of hyperpolarized-gas and proton image sets during a single breath hold; and (3) quantitative regional assessment of 129Xe dissolved in lung tissue and blood.
Subject-specific calibration of the MR-scanner transmitter is an important initial step for every MR examination. However, due to the non-equilibrium nature of hyperpolarized-gas magnetization, techniques established for proton MRI are not applicable for hyperpolarized gases. In this work, we sought to design and implement an optimized phase-based transmitter calibration method that yields accurate results from only a small fraction of the hyperpolarized-gas magnetization, and to validate this method against a commonly-used amplitude-based method in human subjects. To permit integration at the beginning of any imaging pulse sequence, the proposed method requires less than 100-ms, and consumes no more than 5% of the hyperpolarized magnetization. The method yielded results comparable to the reference amplitude-based approach in the presence of B0 and B1 inhomogeneity representative of that encountered in human lung imaging.
Acquisition of 3He and proton (1H) image sets during the same breath hold offers complementary functional and anatomical information, and greatly facilitates quantitative analysis of ventilation defects in the 3He images. Although isotropic 3D image sets are preferred, such a combined 3He /1H acquisition requires a breath-hold duration of 10-20 s (depending on lung dimensions), which may be difficult for subjects with impaired respiratory function. In this work, to accelerate the combined acquisition of 3He and 1H 3D image sets, we incorporated the ideas of compressed sensing to achieve a breath-hold duration which is less than one-half of that required for the conventional approach, without application of multi-coil parallel imaging. The undersampling pattern and reconstruction quality for both 3He and 1H imaging were evaluated using simulations based on fully-sampled data sets and using direct comparison of fully-sampled and undersampled data from the same subject. Finally, the performance of the accelerated and conventional acquisitions in depicting ventilation defects was compared in subjects with pulmonary disease.
Through recent advances in pulse-sequence techniques and 129Xe-polarization technology, acquisition of ventilation and dissolved-phase 129Xe images of the human lung in a single breath hold has been demonstrated. To maximize the information on pulmonary disease offered by dissolved-phase 129Xe, it is important to extend this methodology to permit quantification of the individual dissolved-phase components in the human lung. This project is focused on developing an MRI pulse sequence that permits regional quantification of the tissue (lung parenchyma/plasma) and red blood cell (RBC) fractions of the dissolved 129Xe signal, relative to the associated gas 129Xe signal, from a breath-hold acquisition of less than 20 s. This technique was tested in healthy subjects and validated against spectroscopic measurements of the dissolved-phase 129Xe components in the lung. Finally, a pilot study was performed to compare tissue/RBC fractions in healthy subjects to those in subjects with pulmonary disease.
