Proton magnetic resonance of lung
Creators: name: Estilaei, Mohammadreza
Publisher: name: University of British Columbia
Year: 2009

description: A major problem currently in magnetic resonance imaging is the paucity of specificity and accurate quantification of NMR parameters for clinical use. If the lack of specificity is eliminated and accurate quantification is achieved, it would help physicians and radiologists to employ current therapies more effectively. Proton nuclear magnetic resonance ¹H NMR was used to investigate the entire signal from excised lung tissue. The free induction decay signal contained a motionally restricted component which decayed in a few 10's of μs and a mobile component which persisted about 10 ms or longer. The motionally restricted component was characterized by the second moment of its lineshape which had an average value of 3.42 ± (0.25) x 10[superscript 9]s[superscript]-2. This value was about 1/3 of the rigid lattice M[subscript 2] value, indicating that long macromolecules undergo considerable anisotropic motion on the NMR timescale. The mobile component of the lung was characterized by its T[subscript 2] relaxation times which relate to the microscopic tissue environment. Due to the inhomogeneous nature of the structure and biochemical composition of lung, a smooth T[subscript 2] distribution was assumed. The mobile signal consistently showed four resolvable components of T[subscript 2] range: 2-6, 10-40, 80-110, and 190-400 ms. The 2-6 ms component was present in a fully dehydrated preparation and was therefore assigned to a non-aqueous lung constituent. Collagen is a major protein present in lung tissue and has high tensile strength, rigidity, and binding affinity for water. For this reason, the dependence of the second moment, T[subscript 2] relaxation times, and T[subscript 2] relaxation amplitudes on collagen content were studied. To determine the lung wet/dry ratio, the hydrogen content per unit mass for lung parenchyma and water were estimated in two ways: 1) on the basis of chemical content and 2) on the basis of comparison of restricted and mobile signals to the gravimetric (G) water content for a lung sample studied at a wide range of water contents. Lung Wet/Dry weight ratios were estimated from the free induction decays and compared with gravimetric measurements. The ratio of (Wet/Dry)NMR/(Wet/Dry)G was 1.00 ± (0.08) and 1.00 ± (0.05) for the two methods of estimation. The water content measurements were validated and T[subscript 2] distributions were determined in inflated, deflated, and perfused lungs on a clinical 1.5 T MRI scanner. The mean difference between the gravimetric and MRI water contents was — 4.1g ± 7.6% and an excellent linear correlation squared (R² = 0.98) was observed between the two independent measurements. A voxel-by-voxel investigation of the T[subscript 2] distribution in inflated lung was particularly informative. Plotting the global geometric mean T[subscript 2] versus lung-water density in inflated lung helped to differentiate two distinct regions separated by the lung water density of about 0.4 g/ml. A spherical shell model was tailored to characterize the susceptibility-induced magnetic field gradients in inflated lung and a simulation was performed to assess the effect of diffusion alone on the T[subscript 2] decay curve. This approach demonstrated that the multiexponential nature of the T[subscript 2] distribution was largely due to diffusion of water molecules in the magnetic field gradients. This study also enabled measurements of the inherent T[subscript 2] relaxation. In addition, it was found that the inherent T[subscript 2] relaxation was dependent upon lung water density. The estimation of the magnetic field gradients facilitated measurement of the apparent diffusion coefficient by collecting images at a fixed imaging time using a multiecho pulse sequence with different echo spacing. The apparent diffusion coefficient decreased from about 1.1 x 10[superscript -5]cm²/s to 1.7 x 10[superscript -6]cm²/s as diffusion time increased from 12 to 60 ms. [Scientific formulae used in this abstract could not be reproduced.]