Free-Breathing Magnetization Transfer Imaging of the Lung at 0.55 T Using bSTAR

To develop a clinically applicable method for magnetization transfer (MT) imaging of the lung at 0.55 T.

MT imaging of the chest was explored in healthy volunteers at 0.55 T using a self-gated 3D half-radial dual-echo balanced steady-state free precession sequence (bSTAR), a 2D multi-slice gradient echo method (GRE) acquired in end-expiratory breath-hold, and a self-gated 3D half-radial single-echo ultra-short TE (UTE) approach. MT contrast relied on RF pulse prolongation for bSTAR and on pulsed off-resonance irradiation for UTE and GRE. Data reconstruction from the self-gated scans was performed offline and for the end-expiratory tidal phase using a compressed sensing algorithm. MT ratio (MTR) maps were derived from an MT-weighted and a non-MT-weighted scan and took 8:32 min for bSTAR, 17:16 min for UTE, and 0:48 min for GRE. In addition, patients with pulmonary diseases underwent bSTAR MTR imaging.

MT imaging was successfully performed in all volunteers with highly similar average MTR values for the lung of 28.8 pu (UTE), 29.4 pu (GRE), and 30.7 pu (bSTAR). In terms of resolution, however, bSTAR clearly outperformed both UTE and GRE variants. Finally, MTR imaging with bSTAR demonstrated high reproducibility in volunteers and showed substantially different MTR values for patients with various pulmonary diseases.

At 0.55 T, MT-sensitized bSTAR offers in vivo high-resolution free-breathing MTR imaging of the entire lung in clinically acceptable scan times and shows high reproducibility. Initial results suggest that MTR imaging at 0.55 T may potentially serve as a noninvasive biomarker for investigating and differentiating pulmonary diseases.