  Purpose: To optimize the choice of the flip-angles of magnetization-prepared
gradient echo (MP-GRE) sequences for improved accuracy, precision, and speed of
3D-T1rho mapping. Methods: We propose a new optimization approach for finding
variable flip-angle (VFA) values that improve MP-GRE sequences used for
3D-T1\r{ho} mapping. This new approach can simultaneously improve the accuracy
and signal-to-noise ratio (SNR) while reducing filtering effects. We
demonstrate the concept in the three different versions of the MP-GRE sequences
that are typically used for 3D-T1rho mapping and evaluate their performance in
model agarose phantoms (n=6) and healthy volunteers (n=5) for knee joint
imaging. We also tested the optimization with sequence parameters targeting
faster acquisitions. Results: Our results show that optimized VFA can improve
the accuracy and the precision of the sequences, seen as a reduction of the
mean of normalized absolute difference (MNAD) from 6~8% to 4~5% in model
phantoms and from 14~22% to 12~14% in the knee joint, and improving SNR from
12~27 to 24~35 in agarose phantoms and 5~13 to 11~16 in healthy volunteers. The
optimization can also compensate for the loss in quality caused by making the
sequence faster. This results in sequence configurations that acquire nearly
twice more data per unit of time with similar SNR and MNAD measurements as
compared to its slower versions. Conclusion: The optimization of the VFA can be
used to increase accuracy and precision, and improve the speed of the typical
imaging sequences used for quantitative 3D-T1rho mapping of the knee joint.
