Lung parenchyma transverse relaxation rates at 0.55 T.

Purpose: To determine R and 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 R and and off-resonance ∆f were jointly estimated using nonlinear least-squares estimation. These measurements were compared against R estimates from T -prepared balanced SSFP (T -Prep bSSFP) and estimates from multi-echo gradient echo, which are used widely but prone to error due to different subvoxel weighting.

Results: The mean R and values of lung parenchyma obtained from ES-MCSE were 17.3 ± 0.7 Hz and 127.5 ± 16.4 Hz (T  = 61.6 ± 1.7 ms;  = 9.5 ms ± 1.6 ms), respectively. The off-resonance estimates ranged from -60 to 30 Hz. The R from T -Prep bSSFP was 15.7 ± 1.7 Hz (T  = 68.6 ± 8.6 ms) and from multi-echo gradient echo was 131.2 ± 30.4 Hz (  = 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 R estimate from T -Prep bSSFP was slightly smaller than that from ES-MCSE, whereas the mean and 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 R of 17.3 Hz is similar to the reported mean R of 16.7 Hz at 1.5 T, but the mean of 127.5 Hz is about 5-10 times smaller than that reported at 1.5 T.