Lung parenchyma transverse relaxation rates at 0.55 T

PURPOSE: To determine R (2) and [Formula: see text] 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 (2) and [Formula: see text] and off‐resonance ∆f were jointly estimated using nonlinear least‐squares estimation. These measurements were compared against R (2) estimates from T(2)‐prepared balanced SSFP (T(2)‐Prep bSSFP) and [Formula: see text] estimates from multi‐echo gradient echo, which are used widely but prone to error due to different subvoxel weighting. RESULTS: The mean R (2) and [Formula: see text] values of lung parenchyma obtained from ES‐MCSE were 17.3 ± 0.7 Hz and 127.5 ± 16.4 Hz (T(2) = 61.6 ± 1.7 ms; [Formula: see text] = 9.5 ms ± 1.6 ms), respectively. The off‐resonance estimates ranged from −60 to 30 Hz. The R (2) from T(2)‐Prep bSSFP was 15.7 ± 1.7 Hz (T(2) = 68.6 ± 8.6 ms) and [Formula: see text] from multi‐echo gradient echo was 131.2 ± 30.4 Hz ([Formula: see text] = 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 (2) estimate from T(2)‐Prep bSSFP was slightly smaller than that from ES‐MCSE, whereas the mean [Formula: see text] and [Formula: see text] 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 (2) of 17.3 Hz is similar to the reported mean R (2) of 16.7 Hz at 1.5 T, but the mean [Formula: see text] of 127.5 Hz is about 5–10 times smaller than that reported at 1.5 T.