Unbalanced SSFP for super‐resolution in MRI
Peter J. Lally, Paul M. Matthews, Neal K. Bangerter
Abstract
Purpose
To achieve rapid, low specific absorption rate (SAR) super‐resolution imaging by exploiting the characteristic magnetization off‐resonance profile in SSFP.
Theory and Methods
In the presented technique, low flip angle unbalanced SSFP imaging is used to acquire a series of images at a low nominal resolution that are then combined in a super‐resolution strategy analogous to non‐linear structured illumination microscopy. This is demonstrated in principle via Bloch simulations and synthetic phantoms, and the performance is quantified in terms of point‐spread function (PSF) and SNR for gray and white matter from field strengths of 0.35T to 9.4T. A k‐space reconstruction approach is proposed to account for B0 effects. This was applied to reconstruct super‐resolution images from a test object at 9.4T.
Results
Artifact‐free super‐resolution images were produced after incorporating sufficient preparation time for the magnetization to approach the steady state. High‐resolution images of a test object were obtained at 9.4T, in the presence of considerable B0 inhomogeneity. For gray matter, the highest achievable resolution ranges from 3% of the acquired voxel dimension at 0.35T, to 9% at 9.4T. For white matter, this corresponds to 3% and 10%, respectively. Compared to an equivalent segmented gradient echo acquisition at the optimal flip angle, with a fixed TR of 8 ms, gray matter has up to 34% of the SNR at 9.4T while using a ×10 smaller flip angle. For white matter, this corresponds to 29% with a ×11 smaller flip angle.
Conclusion
This approach achieves high degrees of super‐resolution enhancement with minimal RF power requirements.