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, Dsec, 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, Dsec reflects airway connectivity and provides new information on lung structure. The results show substantial increases in Dsec in the lungs of four dogs with clear evidence of emphysema. For these dogs, the fractional increase in long-range diffusivity Dsec in the emphysematous lungs was greater than that in short-range diffusivity D msec.
- Magnetization tagging