Regional anisotropy of airspace orientation in the lung as assessed with hyperpolarized helium‐3 diffusion MRI

Purpose: To evaluate regional anisotropy of lung-airspace orientation by assessing the dependence of helium-3 (He-3) apparent diffusion coefficient (ADC) values on the direction of diffusion sensitization at two field strengths. Materials and Methods: Hyperpolarized He-3 diffusion-weighted magnetic resonance imaging (MRI) of the lung was performed at 0.43T and 1.5T in 12 healthy volunteers. A gradient-echo pulse sequence was used with a bipolar diffusion-sensitization gradient applied separately along three orthogonal directions. ADC maps, median ADC values, and signal-to-noise ratios were calculated from the diffusion-weighted images. Two readers scored the ADC maps for increased values at lung margins, major fissures, or within focal central regions. Results: ADC values were found to depend on the direction of diffusion sensitization (P < 0.01, except for craniocaudal vs. anteroposterior directions at 1.5T) and were increased at the lateral and medial surfaces for left-right diffusion sensitization (12 of 12 subjects); at the apex and base (9 of 12), and along the major fissure (8 of 12), for craniocaudal diffusion sensitization; and at the most anterior and posterior lung (10 of 12) for anteroposterior diffusion sensitization. Median ADC values at 0.43T (0.201 +/- 0.017, left-right; 0.193 +/- 0.019, craniocaudal; and 0.187 +/- 0.017 cm(2)/s, anteroposterior) were slightly lower than those at 1.5T (0.205 +/- 0.017, 0.197 +/- 0.017 and 0.194 +/- 0.016 cm(2)/s, respectively; P < 0.05). Conclusion: These findings indicate that diffusion-weighted hyperpolarized He-3 MRI can detect regional anisotropy of lung-airspace orientation, including that associated with preferential orientation of terminal airways near pleural surfaces.