Rapid Free-Breathing and Automated 2D Shimming of the Lung at 3T

To develop and validate a rapid, fully automated method for shimming of the lung.

For field mapping of the lung, a custom 2D RF spoiled gradient echo sequence was used offering sub-millisecond echo times. At 3T, four echo-shifted coronal images were acquired with a minimum TE of 0.57 ms and an inter-echo spacing of 0.1 ms. Imaging was performed in free-breathing using three repetitions with a total scan time of 4.2 s. Subsequently, motion correction, field mapping, lung segmentation, and lung shim currents were calculated inline. Finally, the derived shim currents were saved locally, accessible by any custom sequence for application during scan preparation. The shimming method's performance was compared against the vendor's default settings in five healthy volunteers for functional lung imaging with matrix pencil decomposition using balanced steady-state free precession (bSSFP).

Field mapping and shimming of the lung was successfully performed in all volunteers and derivation of shim currents took less than 10s. At 3T, subject-specific shimming reduced the mean frequency offset in the lung by up to 45 Hz and the frequency range (max-min) by up to 180 Hz. Overall, this improved bSSFP signal homogeneity, resulting in more uniform functional images.

This work demonstrates a robust, automated shimming solution for lung imaging at 3T, easily integrable into clinical workflows. The technique significantly enhances image quality and reliability for high-field, bSSFP-based pulmonary imaging. At lower field strengths, the method can possibly relax TR constraints, reducing SAR and peripheral nerve stimulation.