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

Abstract

Purpose: To evaluate regional anisotropy of lung-airspace orientation by assessing the dependence of helium-3 ((3) He) apparent diffusion coefficient (ADC) values on the direction of diffusion sensitization at two field strengths.

Materials and methods: Hyperpolarized (3) He 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 (3) He MRI can detect regional anisotropy of lung-airspace orientation, including that associated with preferential orientation of terminal airways near pleural surfaces.

Keywords: anisotropy; diffusion-weighted MRI; hyperpolarized gas; lung.

Figure 1

Median ADC values from each subject (open circles) and the mean ± standard deviation of median ADC values (closed circles) at 0.43T and 1.5T with the diffusion-sensitization gradient applied in the left-right (LR), craniocaudal (CC) or anteroposterior (AP) direction. Median ADC values were significantly different between the two field strengths for each of the three diffusion-sensitization directions (*p < 0.05). (The lines connecting open circles indicate median ADC values for the same subject.)

Figure 2

Representative coronal ³He ADC maps obtained at 0.43T (a) and 1.5T (b) with the diffusion-sensitization gradient applied in the left-right (LR, phase-encoding), craniocaudal (CC, readout) or anteroposterior (AP, slice-select) direction. The white arrowheads indicate increased ADC values at the lateral surfaces for left-right sensitization. The blue circles indicate a region with relatively higher ADC values for left-right sensitization and relatively lower ADC values for craniocaudal sensitization.

Figure 3

Magnified sections of the right lung from representative coronal ³He ADC maps obtained at 0.43T with the diffusion-sensitization gradient applied in the left-right (LR), craniocaudal (CC) or anteroposterior (AP) direction. The linear structure indicated by the arrow corresponds to the major fissure of the right lung. (See Fig. 2 for colorbar.)