Lung parenchyma transverse relaxation rates at 0.55 T

Abstract

Purpose: To determine R₂ and $R_2^{\text{'}}$ 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 R₂ and $R_2^{\text{'}}$ 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 $R_2^{\text{'}}$ 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; $T_2^{\text{'}}$ = 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 $R_2^{\text{'}}$ 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 $R_2^{\text{'}}$ 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.

FIGURE 1

Illustration of a free‐breathing electrocardiogram (ECG)–triggered and respiratory‐gated single‐slice echo‐shifted multi‐echo spin echo (ES‐MCSE). A diaphragm motion curve indicates various respiratory phases. The acceptance window of a respiratory motion navigator is chosen at end‐expiratory phase, and ECG triggering is used to acquire data at end‐diastole with a trigger delay time. An imaging module consists of eight readouts per phase‐encoding step, and five echo shifts (−4, −2, 0, 2, and 4 ms) are demonstrated with shifted readout gradients in different colors. Abbreviations: PE, phase‐encoding; RO, readout; SS, single slice

FIGURE 2

Estimated signal evolutions are shown for two representative on‐resonance cases: ${R_2/R}_2^{\prime }$ = 14/60 Hz (A) and 20/110 Hz (B). Standard of R ₂ and $R_2^{\prime }$ estimates from ES‐MCSE using seven spin echo times and five echo shifts. A simulated noiseless signal evolution (blue), noisy samples (dots in different colors), and the aggregate SD of R ₂ (C) and $R_2^{\prime }$ (D) for a pair of R ₂ and $R_2^{\prime }$ are calculated from 10 000 noise realizations across all $\Delta \mathit{f}$ from −30 to 30 Hz

FIGURE 3

Representative ES‐MCSE images of seven spin echo times and five echo shifts. The images at the first TE (20 ms) were discarded (not shown) due to stimulated‐echo contamination. A signal evolution of 35 samples was used for pixelwise parameter estimation to yield R ₂ and $R_2^{\prime }$ maps

FIGURE 4

In vivo parameter maps estimated from ES‐MCSE. The R ₂, $R_2^{\prime }$, and $\mathrm{\Delta }f$ maps within region of interests for 3 healthy subjects were estimated from EC‐MCSE. Images were acquired at middiastole and end‐expiratory phase, and of a coronal slice that intersects the descending aorta