Lung parenchyma transverse relaxation rates at 0.55 T

Abstract

Purpose: To determine R2 and (Formula presented.) transverse relaxation rates in healthy lung parenchyma at 0.55 T. This is important in that it informs the design and optimization of new imaging methods for 0.55T lung MRI. Methods: Experiments were performed in 3 healthy adult volunteers on a prototype whole-body 0.55T MRI, using a custom free-breathing electrocardiogram-triggered, single-slice echo-shifted multi-echo spin echo (ES-MCSE) pulse sequence with respiratory navigation. Transverse relaxation rates R2 and (Formula presented.) and off-resonance ∆f were jointly estimated using nonlinear least-squares estimation. These measurements were compared against R2 estimates from T2-prepared balanced SSFP (T2-Prep bSSFP) and (Formula presented.) estimates from multi-echo gradient echo, which are used widely but prone to error due to different subvoxel weighting. Results: The mean R2 and (Formula presented.) values of lung parenchyma obtained from ES-MCSE were 17.3 ± 0.7 Hz and 127.5 ± 16.4 Hz (T2 = 61.6 ± 1.7 ms; (Formula presented.) = 9.5 ms ± 1.6 ms), respectively. The off-resonance estimates ranged from −60 to 30 Hz. The R2 from T2-Prep bSSFP was 15.7 ± 1.7 Hz (T2 = 68.6 ± 8.6 ms) and (Formula presented.) from multi-echo gradient echo was 131.2 ± 30.4 Hz ((Formula presented.) = 8.0 ± 2.5 ms). Paired t-test indicated that there is a significant difference between the proposed and reference methods (p < 0.05). The mean R2 estimate from T2-Prep bSSFP was slightly smaller than that from ES-MCSE, whereas the mean (Formula presented.) and (Formula presented.) estimates from ES-MCSE and multi-echo gradient echo were similar to each other across all subjects. Conclusions: Joint estimation of transverse relaxation rates and off-resonance is feasible at 0.55 T with a free-breathing electrocardiogram-gated and navigator-gated ES-MCSE sequence. At 0.55 T, the mean R2 of 17.3 Hz is similar to the reported mean R2 of 16.7 Hz at 1.5 T, but the mean (Formula presented.) of 127.5 Hz is about 5–10 times smaller than that reported at 1.5 T.