Lung parenchyma transverse relaxation rates at 0.55 T

Bochao Li; Nam G Lee; Sophia X Cui; Krishna S Nayak

doi:10.1002/mrm.29541

Lung parenchyma transverse relaxation rates at 0.55 T

Magn Reson Med. 2023 Apr;89(4):1522-1530. doi: 10.1002/mrm.29541. Epub 2022 Nov 20.

Authors

Bochao Li¹, Nam G Lee¹, Sophia X Cui², Krishna S Nayak^{1

3}

Affiliations

¹ Department of Biomedical Engineering, Viterbi School of Engineering, University of Southern California, California, Los Angeles, USA.
² Siemens Medical Solutions USA, Los Angeles, California, USA.
³ Ming Hsieh Department of Electrical and Computer Engineering, Viterbi School of Engineering, University of Southern California, California, Los Angeles, USA.

Abstract

Purpose: To determine R₂ and $R_{2}^{'}$ 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 $R_{2}^{'}$ 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 $R_{2}^{*}$ estimates from multi-echo gradient echo, which are used widely but prone to error due to different subvoxel weighting.

Results: The mean R₂ and $R_{2}^{'}$ 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; $T_{2}^{'}$ = 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 $R_{2}^{*}$ from multi-echo gradient echo was 131.2 ± 30.4 Hz ( $T_{2}^{*}$ = 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 $R_{2}^{'}$ and $R_{2}^{*}$ 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 $R_{2}^{'}$ of 127.5 Hz is about 5-10 times smaller than that reported at 1.5 T.

Keywords: 0.55T MRI; R2 and R2′ mapping; echo-shifted multi-echo spin echo; low-field MRI; lung imaging; transverse relaxation rates.

Publication types

Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

Adult
Electrocardiography
Humans
Lung / diagnostic imaging
Magnetic Resonance Imaging* / methods
Respiration*