Rat Brain Global Ischemia-Induced Diffusion Changes Revisited: Biophysical Modeling of the Water and NAA MR "Diffusion Signal".

PURPOSE: To assess changes in intracellular diffusion as a mechanism for the reduction in water ADC that accompanies brain injury. Using NAA as a marker of neuronal cytoplasmic diffusion, NAA diffusion was measured before and after global ischemia (immediately postmortem) in the female Sprague-Dawley rat. METHODS: Diffusion-weighted PRESS spectra, with diffusion encoding in a single direction, were acquired from large voxels of rat brain gray matter in vivo and postischemia employing either pairs of pulsed half-sine-shaped gradients (in vivo and postischemia, bmax  = 19 ms/µm2 ) or sinusoidal oscillating gradients (in vivo only) with frequencies of 99.2-250 Hz. A 2D randomly oriented cylinder (neurite) model gave estimates of longitudinal and transverse diffusivities (DL and DT , respectively). In this model, DL represents the "free" diffusivity of NAA, whereas DT reflects highly restricted diffusion. Using oscillating gradients, the frequency dependence of DT [DT (ω)] gave estimates of the cylinder (axon/dendrite) radius. RESULTS: A 10% decrease in DL,NAA followed global ischemia, dropping from 0.391 ± 0.012 µm2 /ms to 0.350 ± 0.009 µm2 /ms. Modeling DT,NAA (ω) provided an estimate of the neurite radius of 1.0 ± 0.6 µm. CONCLUSION: Whereas the increase in apparent intraneuronal viscosity suggested by changes in DL,NAA may contribute to the overall reduction in water ADC associated with brain injury, it is not sufficient to be the sole explanation. Estimates of neurite radius based on DT (ω) were consistent with literature values.

3He diffusion MRI of the lung.

RATIONALE AND OBJECTIVES: MR imaging of the restricted diffusion of laser-polarized 3He gas provides unique insights into the changes in lung microstructure in emphysema. RESULTS: We discuss measurements of ventilation (spin density), mean diffusivity, and the anisotropy of diffusion, which yields the mean acinar airway radius. In addition, the use of spatially modulated longitudinal magnetization allows diffusion to be measured over longer distances and times, with sensitivity to collateral ventilation paths. Early results are also presented for spin density and diffusivity maps made with a perfluorinated inert gas, C3F8. METHODS: Techniques for purging and imaging excised lungs are discussed.