Fast 3D ³¹P B 1 + mapping with a weighted stack of spiral trajectory at 7 T

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 $B_1^{+}$ 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 $B_1^{+}$ mapping (fDAM).

Our method incorporates 3D weighted stack-of-spiral gradient-echo acquisitions and a frequency-selective pulse to enable efficient $B_1^{+}$ 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.

The results of fDAM were compared with the classical DAM. A good correlation (r = 0.95) was obtained between the two $B_1^{+}$ maps. A 3D ³¹P $B_1^{+}$ 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 $B_1^{+}$ mapping in approximately 10:15 min using a 1Ch-transmit/32Ch-receive coil.

fDAM is an efficient method for ³¹P 3D $B_1^{+}$ mapping, showing promise for future applications in rapid ³¹P MRSI.