Lung parenchyma transverse relaxation rates at 0.55 T
Bochao Li, Nam G. Lee, Sophia X. Cui, Krishna S. Nayak
Abstract
Purpose
To determine R2 and R2‘ transverse relaxation rates in healthy lung parenchyma at 0.55T. 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 ECG-triggered, single-slice echo-shifted multi-echo spin echo (ES-MCSE ) pulse sequence with respiratory navigation. Transverse relaxation rates R2 and R2‘ and off-resonance Δf were jointly estimated using nonlinear least-squares estimation. These measurements were compared against R2 estimates from T2-prepared balanced steady-state free precession (T2-Prep bSSFP ) and R2* estimates from multi-echo gradient echo (ME-GRE ), which are widely used but prone to error due to different subvoxel weighting.
Results
The mean R2 and R2‘ values of lung parenchyma obtained from ES-MCSE were 17.3±0.7Hz and 127.5±16.4Hz, respectively. The off-resonance estimates ranged from –60 to 30Hz. The R2 fromT2 -Prep bSSFP was 15.7±1.7Hz and R2* from ME-GRE was 131.2±30.4Hz. 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, while the mean R2’ and R2* estimates from ES-MCSE and ME-GRE were similar to each other across all subjects.
Conclusions
Joint estimation of transverse relaxation rates and off-resonance is feasible at 0.55T with a free-breathing ECG-gated and navigator-gated ES-MCSE sequence. At 0.55T, the mean R2 is similar to the reported at 1.5T, but the mean R2‘ is about 5 to10 times smaller than that reported at 1.5T.