MR imaging of apparent 3He gas transport in narrow pipes and rodent airways.

High sensitivity makes hyperpolarized (3)He an attractive signal source for visualizing gas flow with magnetic resonance (MR) imaging. Its rapid Brownian motion, however, can blur observed flow lamina and alter measured diffusion rates when excited nuclei traverse shear-induced velocity gradients during data acquisition. Here, both effects are described analytically, and predicted values for measured transport during laminar flow through a straight, 3.2-mm diameter pipe are validated using two-dimensional (2D) constant-time images of different binary gas mixtures. Results show explicitly how measured transport in narrow conduits is characterized by apparent values that depend on underlying gas dynamics and imaging time. In ventilated rats, this is found to obscure acquired airflow images. Nevertheless, flow splitting at airway branches is still evident and use of 3D vector flow mapping is shown to reveal surprising detail that highlights the correlation between gas dynamics and lung structure.

3D MRI of non-Gaussian (3)He gas diffusion in the rat lung.

In (3)He magnetic resonance images of pulmonary air spaces, the confining architecture of the parenchymal tissue results in a non-Gaussian distribution of signal phase that non-exponentially attenuates image intensity as diffusion weighting is increased. Here, two approaches previously used for the analysis of non-Gaussian effects in the lung are compared and related using diffusion-weighted (3)He MR images of mechanically ventilated rats. One approach is model-based and was presented by Yablonskiy et al., while the other approach utilizes the second order decay contribution that is predicted from the cumulant expansion theorem. Total lung coverage is achieved using a hybrid 3D pulse sequence that combines conventional phase encoding with sparse radial sampling for efficient gas usage. This enables the acquisition of nine 3D images using a total of only approximately 1 L of hyperpolarized (3)He gas. Diffusion weighting ranges from 0 s/cm(2) to 40 s/cm(2). Results show that the non-Gaussian effects of (3)He gas diffusion in healthy rat lungs are directly attributed to the anisotropic geometry of lung microstructure as predicted by the Yablonskiy model, and that quantitative analysis over the entire lung can be reliably repeated in time-course studies of the same animal.

Characterization of bleomycin lung injury by nuclear magnetic resonance: correlation between NMR relaxation times and lung water and collagen content.

The response of the NMR relaxation times (T(1), CPMG T(2), and Hahn T(2)) to bleomycin-induced lung injury was studied in excised, unperfused rat lungs. NMR, histologic, and biochemical (collagen content measurement) analyses were performed 1, 2, 4, and 8 weeks after intratracheal instillation of saline (control lungs) or 10 U/kg bleomycin sulfate. The control lungs showed no important NMR, water content, histologic, or collagen content changes. The spin-spin relaxation times for the fast and intermediate components of the CPMG decay (T(2f) and T(2i), respectively) increased 1 week after bleomycin injury (acute inflammatory stage) and then progressively decreased during the following 2-8 weeks (i.e., with the development of the chronic, fibrotic stage of the injury). The slow component (T(2s)) showed no significant changes. The response of T(1) and the slow component of the Hahn T(2) was, on the whole, similar to that of CPMG T(2f) and T(2i). T(1) changes were very small. Lung water content increased 1 week after injury. Histologic and biochemical assessment of collagen showed that collagen content was close to control at 1 week, but markedly increased at 2, 4, and 8 weeks. T(1) and T(2) data were directly correlated with lung water content and inversely correlated with collagen content. Our results indicate that NMR relaxation time measurements (particularly T(2)) reflect the structural changes associated with bleomycin injury. The prolonged T(2) relaxation times observed in the acute stage are related to the presence of edema, whereas the subsequent decrease in these values marks the stage of the collagen deposition (fibrotic stage). CPMG-T(2) and Hahn-T(2) measurements can be valuable as a potentially noninvasive method for characterizing bleomycin-induced lung injury and pathologically related lung disorders.