Diffusion, relaxation, and magnetic resonance imaging studies of noble gases

Hyperpolarized gas magnetic resonance imaging (HPG-MRI) has recently gained popularity, as its potential for high-resolution imaging is considerable. Because of its novelty, this form of imaging requires investigation of the diffusion-limited spatial resolution, Δr D. In this thesis, a new MRI method for simultaneous measurement of the gaseous diffusion constant and T1 is developed. This method is based on direct observation of diffusive motion. The method\u27s independence of spin-spin relaxation makes it particularly useful for systems with short T2. The value of Δr D is estimated to be ΔrD(3 He) ≈ 130 μm and Δr D(129Xe) ≈ 45 μm. A practical way for noninvasive quantification of the oxygen partial pressure in the lungs is presented. This scheme is based on the demonstration that the relaxation rate of 3He is proportional to the concentration of oxygen (in %): 1/T1 = 0.0079[O2]. The ability of HPG-MRI to generate high-resolution images capable of detecting, diseased regions in the lung is demonstrated. Direct comparison of healthy and emphysematous parenchyma is performed in patients with chronic obstructive pulmonary disease who have undergone unilateral lung transplantation. The phenomenon of gas trapping in emphysematous lung, as detected by HPG-MRI, is also presented. Gas trapping is shown to be a complementary sign of lung malfunction, capable of mapping areas of abnormally high air-resistance on expiration. The possibility of visualizing organs other than the lungs is demonstrated with images of hyperpolarized 3He in human paranasal sinuses. The success of the spin-polarization induced nuclear Overhauser effect (SPINOE) in generating high signal enhancement suggests the feasibility of lung parenchyma imaging based on xenon-tissue polarization transfer. The origins of xenon-proton interaction in water are examined. The existence of a significant NOE between 129Xe and water protons is established. The contribution of the 129Xe-1H dipole-dipole coupling to the overall xenon relaxation is dominant at room temperature, but decreases with increasing temperature. The cross-relaxation is pressure independent in the range 1–10 atm. Calculations demonstrate the feasibility of proton signal enhancement in solutions via xenon-proton SPINOE