Fast 3D 31P B1+ mapping with a weighted stack of spiral trajectory at 7 T

Purpose Phosphorus MRS (31P MRS) enables noninvasive assessment of energy metabolism, yet its application is hindered by sensitivity limitations. To overcome this, often high magnetic fields are used, leading to challenges such as spatial B1+$$ {\mathrm{B}}_1^{+} $$ inhomogeneity and therefore the need for accurate flip‐angle determination in accelerated acquisitions with short TRs. In response to these challenges, we propose a novel short TR and look‐up table–based double‐angle method for fast 3D 31P B1+$$ {\mathrm{B}}_1^{+} $$ mapping (fDAM). Methods Our method incorporates 3D weighted stack‐of‐spiral gradient‐echo acquisitions and a frequency‐selective pulse to enable efficient B1+$$ {\mathrm{B}}_1^{+} $$ mapping based on the phosphocreatine signal at 7 T. Protocols were optimized using simulations and validated through phantom experiments. The method was validated in the human brain using a 31P 1Ch‐trasmit/32Ch‐receive coil and skeletal muscle using a birdcage 1H/31P volume coil. Results The results of fDAM were compared with the classical DAM. A good correlation (r = 0.95) was obtained between the two B1+$$ {\mathrm{B}}_1^{+} $$ maps. A 3D 31P B1+$$ {\mathrm{B}}_1^{+} $$ mapping in the human calf muscle was achieved in about 10:50 min using a birdcage volume coil, with a 20% extended coverage (number of voxels with SNR > 3) relative to that of the classical DAM (24 min). fDAM also enabled the first full‐brain coverage 31P 3D B1+$$ {\mathrm{B}}_1^{+} $$ mapping in approximately 10:15 min using a 1Ch‐transmit/32Ch‐receive coil. Conclusion fDAM is an efficient method for 31P 3D B1+$$ {\mathrm{B}}_1^{+} $$ mapping, showing promise for future applications in rapid 31P MRSI.