B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping near metallic implants using turbo spin echo pulse sequences.

Abstract

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.