Magnetization tagging decay to measure long-range (3)He 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 T(1) and over correspondingly long distances. Magnetization tagging is used here with hyperpolarized (3)He 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).