Measurements and modeling of long range 3He diffusion in the lung using a slice-washout method

In healthy lung tissue, pulsed-gradient-spin-echo (PGSE) methods reveal apparent diffusion coefficients (ADC) of the order 0.20 cm(2) s(-1): for diffusion times of approximate to 2 ms. For these short diffusion times the ADC is only sensitive to structures approximately (2Dt)(1/2) approximate to 0.6 mm in size. Recent work, using magnetic tagging of the longitudinal magnetization has revealed much smaller ADC values for longer length scales. In this work, the in vivo ADC from within the air-spaces, was measured using a new technique. The signal from a series of images was analyzed from a slice that was repeatedly imaged. Diffusion tends to top-up the non-renewable polarization within the slice, which leads to a non-exponential decay in image signal. Image data were compared to ID finite-difference simulations of diffusion to calculate a long range ADC value. The results yield values of the order 0.034 cm(2) s(-1), which are nearly an order of magnitude smaller than those reported by PGSE measurements at shorter diffusion times. (c) 2005 Elsevier Inc. All rights reserved.