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 . Magn Reson Med 51:1002–1008, 2004. © 2004 Wiley‐Liss, Inc.