Lung parenchyma transverse relaxation rates at 0.55 T

Abstract

To determine R 2 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 R 2 and R ′ 2 and off-resonance Δf 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 R * 2 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 R ′ 2 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; T ′ 2 = 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 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 R 2 estimate from T 2 -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 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 R ′ 2 of 127.5 Hz is about 5-10 times smaller than that reported at 1.5 T. K E Y W O R D S0.55T MRI, echo-shifted multi-echo spin echo, low-field MRI, lung imaging, transverse relaxation rates, R 2 and R 2 ′ mapping

Submillimeter lung MRI at 0.55 T using balanced steady‐state free precession with half‐radial dual‐echo readout (bSTAR)

“…The bSTAR technique benefits from the intrinsically high SNR offered by bSSFP as well as from the fact that there is very little signal intensity loss in the second echo 27 . However, for short lived T 2 * species, this effect is more pronounced at higher field strength, that is, at 1.5 T where T 2 * in the lung parenchyma is in order of 1–2 ms, compared to 0.55 T where T 2 * in the lung tissue is ˜8 ms 9 …”

“…The bSTAR technique benefits from the intrinsically high SNR offered by bSSFP as well as from the fact that there is very little signal intensity loss in the second echo. 27 However, for short lived T 2 * species, this effect is more pronounced at higher field strength, that is, at 1.5 T where T 2 * in the lung parenchyma is in order of 1-2 ms, compared to 0.55 T where T 2 * in the lung tissue is ∼8 ms. 9 The feasibility of bSTAR was successfully presented at 1.5 T in healthy human subjects 20 and provided free-breathing chest images with 1.2 mm isotropic resolution in <5 min. There scans were performed with undersampling factor of 7.5, which is close to the undersampling factor of 8.3 used in this work for the 12:50 min bSTAR scan.…”

“…Recently, there has been a growing interest in exploring lung MRI on scanner configurations operating at lower field strengths with high performance gradient systems 6–8 . The lower field strength helps to reduce susceptibility artifacts and off‐resonance artifacts, and therefore, provides more favorable transverse relaxation times 9 …”

“…Imaging at high spatial resolution requires lengthening of the readout, which increases TR, and generates unacceptable banding artifacts. However, this constraint can be significantly relaxed when imaging at 0.55 T. Additionally, dual‐echo sampling of bSTAR can benefit from slowly decaying T 2 * species in the lung parenchyma at a lower magnetic field 9 . To this end, in this work we explore the potential of the free‐breathing bSTAR technique for high‐resolution morphologic lung MRI in human subjects using a 0.55 T MR‐scanner equipped with high‐performance gradients.…”

“…[6][7][8] The lower field strength helps to reduce susceptibility artifacts and off-resonance artifacts, and therefore, provides more favorable transverse relaxation times. 9 Despite the aforementioned technical difficulties of pulmonary MRI, significant efforts have been made in recent years to develop dedicated acquisition techniques for studying lung structure and function. In particular, imaging techniques based on UTE or zero echo time (ZTE) have shown promising results for lung imaging at 1.5 T and 3 T. [10][11][12] These techniques use non-Cartesian k-space trajectories, enabling improved sampling of the fast-decaying signal in tissues with very short T 2 *.…”