Magnetization Tagging Decay to Measure Long-Range ³He Diffusion in Healthy and Emphysematous Canine Lungs

Spatial modulation (tagging) of the longitudinal magnetization allows diffusive displacements to be measured over times approximately as long as T₁ and over correspondingly long distances. Magnetization tagging is used here with hyperpolarized ³He gas in canine lungs with unilateral elastase-induced emphysema. A new scheme for analyzing images subsequent to tagging determines the spatially-resolved fractional modulation and its decay rate, using a sliding window. The diffusivity so determined over seconds and centimeter lengths, D_sec, is smaller in all cases than the diffusivity measured over milliseconds and hundreds of microns, D _msec (in healthy lungs, this ratio is about 0.1). While D _msec is sensitive to lung microstructure on the alveolar level, D_sec reflects airway connectivity and provides new information on lung structure. The results show substantial increases in D_sec in the lungs of four dogs with clear evidence of emphysema. For these dogs, the fractional increase in long-range diffusivity D_sec in the emphysematous lungs was greater than that in short-range diffusivity D _msec.