Self-Correction of B0 Drift and Trajectory Error in Alternating Unbalanced SSFP-FID and SSFP-ECHO (AUSFIDE)-Based 3D Transverse Relaxometry of the Human Brain.

PURPOSE: The alternating unbalanced SSFP (AUSFIDE) pulse sequence, which consists of multi-echo trains of SSFP-FID and SSFP-ECHO modules, enables rapid estimation of R2 and R2', but is prone to errors from temporal drift of the main magnetic field (B0) and k-space trajectory mismatch. Here, we propose a self-correcting AUSFIDE method for joint correction of these two sources of error without requiring additional scans.

METHODS

AUSFIDE signals were modeled in the presence of B0 drift and k-space shifts. To estimate and correct for B0 drift over the entire scan, phase navigators without spatial encoding were inserted before and after each multi-echo train. In addition, the intrinsic phase symmetry between SSFP-FID and SSFP-ECHO signals was exploited, enabling self-calibration of the k-space sampling trajectory. Numerical simulations and phantom experiments were performed for proof-of-concept validation, followed by in vivo brain imaging of 10 healthy subjects on two 3 T scanners. Measured parameters were analyzed in six brain regions, and test-retest reproducibility was evaluated on the two MRI platforms.

RESULTS

Simulations and phantom imaging consistently demonstrated that B0 drift and trajectory errors resulted in image distortions and heavily overestimated R2'. Experiments further revealed that these artifacts and quantification errors were substantially reduced by the proposed self-correction method. Inter-scan agreement of the quantified parameters was also improved after correction on both scanners.

CONCLUSION

The self-corrected AUSFIDE method is able to compensate for errors from B0 drift and k-space trajectory mismatch, and thus is a potentially practical and reliable means for 3D transverse relaxometry.