Improved TSE imaging at ultrahigh field using nonlocalized efficiency RF shimming and acquisition modes optimized for refocused echoes (AMORE)
Xiaoxuan He, Simon Schmidt, Štefan Zbýň, Tobey Haluptzok, Steen Moeller, Gregory J. Metzger
Abstract
Purpose
To develop and evaluate a novel RF shimming optimization strategy tailored to improve the transmit efficiency in turbo spin echo imaging when performing time-interleaved acquisition of modes (TIAMO) at ultrahigh fields.
Theory and Methods
A nonlocalized efficiency shimming cost function is proposed and extended to perform TIAMO using acquisition modes optimized for refocused echoes (AMORE). The nonlocalized efficiency shimming was demonstrated in brain and knee imaging at 7 Tesla. Phantom and in vivo torso imaging studies were performed to compare the performance between AMORE and previously proposed TIAMO mode optimizations with and without localized constraints in turbo spin echo and gradient echo acquisitions.
Results
The proposed nonlocalized efficiency RF shimming produced a circularly polarized-like field with fewer signal dropouts in the brain and knee. For larger targets, AMORE was used and required a significantly lower transmitter voltage to produce a similar contrast to existing TIAMO mode design approaches for turbo spin echo as well as gradient echo acquisitions. In vivo, AMORE effectively reduced signal dropout in the interior torso while providing more uniform contrast with reduced transmit power. A local constraint further improved performance for a target region while maintaining performance in the larger FOV.
Conclusion
AMORE based on the presented nonlocalized efficiency shimming cost function demonstrated improved contrast and SNR uniformity as well as increased transmit efficiency for both gradient echo and turbo spin echo acquisitions.