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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.