Time dependence of ³He diffusion in the human lung: Measurement in the long-time regime using stimulated echoes

A stimulated-echo-based technique was developed to measure the long-time-scale apparent diffusion coefficient (ADC) of hyperpolarized ³He during a single breath-hold acquisition. Computer simulations were used to evaluate the performance of the technique and guide the selection of appropriate parameter values for obtaining accurate ADC values. The technique was used in 10 healthy subjects and two subjects with chronic obstructive pulmonary disease (COPD) to measure the global ADC for diffusion times between a few tenths of a second and several seconds, and to acquire spatial maps of the ADC for a diffusion time of 1.5 s. The reproducibility of the technique and its sensitivity to the direction of diffusion sensitization were also investigated, in healthy subjects, global ADC values decreased by severalfold over the range of diffusion times measured (mean values = 0.039 and 0.023 cm²/s at diffusion times of 0.61 and 1.54 s, respectively). ADC maps were generally uniform, with mean values similar to the corresponding global values. For the two COPD subjects, global ADC values were substantially greater than those of every healthy subject at all diffusion times measured. In addition, regional elevations of ADC values were far more conspicuous on long-time-scale ADC maps than on short-time-scale ADC maps.