Novel methods and applications of NMR and MRI : low-power RF excitation and hyperpolarized Xenon-129
Year: 2012
Type: dissertation
Abstract: Since their discovery in the middle of the last century, Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) have become an important and very versatile tool in industry, medicine, and basic research. The aim of this work was to explore possible improvements and new applications of novel NMR methods. Frank-Sequence Excitation: Since the beginning of MRI, the strength of the main magnetic field has steadily increased because of the inherently higher signal-to-noise ratio. However, the radio-frequency (rf) power and energy absorbed in the body tissue, quantified scales with the square of the main magnetic field. Therefore, conventional rf-pulse excitation often exceeds legal limitations. This problem could be solved by novel rf-excitation pulse sequences. In the first part of the thesis, a new low-power rf excitation scheme termed Frank-sequence excitation was analyzed in detail, compared to conventional excitation, and was shown to be feasible for MRI. Frank sequences, which have recently been introduced as an excitation scheme for NMR, employ a series of phase-modulated low-amplitude rf pulses with interleaved signal acquisition. Simulations and experiments were performed in order to find and quantify advantages as well as drawbacks of this new technique. These studies revealed that the distribution of total rf energy over a longer period of time allows for significant reduction of peak rf power. The presented findings yield essential information for a deeper understanding of the new pulse scheme and will enhance future developments. Furthermore, the concept of Frank-sequence excitation was employed for MRI for the first time. With purpose-built phantoms, the feasibility of the method in combination with a two-dimensional radial imaging scheme was demonstrated. Together with a low-power slice selection sequence, three-dimensional MRI was successfully performed as well. A comparison with images that were acquired with conventional excitation and space encoding methods showed that the new technique provides a competitive alternative for future MRI applications. Besides offering the advantage of reduced rf power, Frank sequence MRI also provides a new modality to image hard materials and tissues like bones and teeth with short relaxation times in future applications.Hyperpolarized 129Xe NMR and MRI: Another drawback of classical 1H NMR and MRI is the low sensitivity. As a result, different hyperpolarization (HP) techniques have been developed, by which the polarization and consequently the NMR signal of 129Xe can be enhanced by up to five orders of magnitude. Especially in biomedicine, HP 129Xe has been proven a versatile sensor tool for molecular spectroscopy and imaging. As a consequence, the second part of the thesis focused on applications of HP 129Xe for advanced studies of gas dissolution, blood-gas exchange dynamics, and blood rheology. Hyperpolarized 129Xe was employed to analyze gas dissolution by means of hollow-fiber membranes. These so-called xenonizers, which resemble clinical oxygenators, were investigated in detail including a systematical comparison of different fibers. This is of great importance for future HP 129Xe studies as well as for medical applications of the investigated fibers. On the basis of these results, blood-dissolved HP 129Xe was used as a tool to monitor gas exchange in the fiber module online. The signals of HP 129Xe in oxygenated and deoxygenated hemoglobin could be spectroscopically resolved for the first time. Thereby, the oxygen exchange that takes place even under stopped blood-flow conditions could be followed and quantified. Additionally, two-dimensional NMR was used to further analyze the exchange of HP 129Xe between the various sites in the xenonizer. Exchange of xenon with red blood cells was found to not only be significant with the surrounding plasma, but also in a more direct way with the fiber pores. These results form the basis for better understanding of physiological gas-exchange phenomena as well as for xenon anesthesia. Possibilities for further quantitative assessments were presented and discussed together with future experiments to gain more knowledge of fundamental blood-gas dynamics as well as for gas-exchange membrane design. Dissolved HP 129Xe was also applied as a sensor for rheological studies of blood flow in capillaries as well as for first tests in a Couette system. The chemical shift information achieved by HP 129Xe was used as a means of molecular MRI. In combination with 1H velocity MRI, a new method was presented for blood rheology investigations. While dissolved 129Xe signals require further enhancement for applications in complex geometries, results in micro-capillaries already showed very good agreement with expected values. On the basis of the results presented, further studies of blood flow MRI in technical devices will be feasible.
Source: RWTH Publications (RWTH Aachen)
Authors: Nadia Ben Amor, Bernhard Blümich
Language: English
Subfield: Radiology, Nuclear Medicine and Imaging
Field: Medicine
Domain: Health Sciences
Open Access status: green
OpenAlex ID: W2612673561