B1+ mapping near metallic implants using turbo spin echo pulse sequences

Purpose: To propose a B1+ mapping technique for imaging of body parts containing metal hardware, based on magnitude images acquired with turbo spin echo (TSE) pulse sequences. Theory and Methods: To encode the underlying B1+, multiple (two to four) TSE image sets with various excitation and refocusing flip angles were acquired. To this end, the acquired signal intensities were matched to a database of simulated signals which was generated by solving the Bloch equations taking into account the exact sequence parameters. The retrieved B1+ values were validated against gradient-recalled and spin echo dual angle methods, as well as a vendor-provided turboFLASH-based mapping sequence, in gel phantoms and human subjects without and with metal implants. Results: In the absence of metal, phantom experiments demonstrated excellent agreement between the proposed technique using three or four flip angle sets and reference dual angle methods. In human subjects without metal implants, the proposed technique with three or four flip angle sets showed excellent correlation with the spin echo dual angle method. In the presence of metal, both phantoms and human subjects revealed a narrow range of B1+ estimation with the reference techniques, whereas the proposed technique successfully resolved B1+ near the metal. In select cases, the technique was implemented in conjunction with multispectral metal artifact reduction sequences and successfully applied for B1+ shimming. Conclusion: The proposed technique enables resolution of B1+ values in regions near metal hardware, overcoming susceptibility-related and narrow-range limitations of standard mapping techniques.

Optimizing Variable Flip-Angles in Magnetization-Prepared Gradient Echo Sequences for Efficient 3D-T1rho Mapping

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.