Single breath-hold volumetric lung imaging at 0.55T using stack-of-spiral (SoS) out-in balanced SSFP

To develop a robust single breath-hold approach for volumetric lung imaging at 0.55T.

A balanced-SSFP (bSSFP) pulse sequence with 3D stack-of-spiral (SoS) out-in trajectory for volumetric lung imaging at 0.55T was implemented. With 2.7× undersampling, the pulse sequence enables imaging during a 17-s breath-hold. Image reconstruction is performed using 3D SPIRiT and 3D l1-Wavelet regularizations. In two healthy volunteers, single breath-hold SoS out-in bSSFP was compared against stack-of-spiral UTE (spiral UTE) and half-radial dual-echo bSSFP (bSTAR), based on signal intensity (SI), blood-lung parenchyma contrast, and image quality. In six patients with pathologies including lung nodules, fibrosis, emphysema, and air trapping, single breath-hold SoS out-in and bSTAR were compared against low-dose computed tomography (LDCT).

SoS out-in bSSFP achieved 2-mm isotropic resolution lung imaging with a single breath-hold duration of 17 s. SoS out-in (2-mm isotropic) provided higher lung parenchyma and blood SI and blood-lung parenchyma contrast compared to spiral UTE (2.4 × 2.4 × 2.5 mm³) and bSTAR (1.6-mm isotropic). When comparing SI normalized by voxel size, SoS out-in has lower lung parenchyma signal, higher blood signal, and a higher blood-lung parenchyma contrast compared to bSTAR. In patients, SoS out-in bSSFP was able to identify lung fibrosis and lung nodules of size 4 and 8 mm, and breath-hold bSTAR was able to identify lung fibrosis and 8 mm nodules.

Single breath-hold volumetric lung imaging at 0.55T with 2-mm isotropic spatial resolution is feasible using SoS out-in bSSFP. This approach could be useful for rapid lung disease screening, and in cases where free-breathing respiratory navigated approaches fail.