23Na imaging was performed on a 7T research scanner (Siemens Healthcare, Erlangen, Germany) using a double-resonant (1H/23Na) quadrature birdcage coil (RAPID Biomedical, Rimpar, Germany). 23Na data was acquired with a density-adapted 3D radial pulse sequence (Nagel et al., 2009 ) with a nominal spatial resolution of (3 mm)3 (Nprojections = 4000; TR/TE = 160 ms / 0.35 ms, Tacq = 10:40 min, Treadout = 10 ms). TE was measured as the time difference between the start of the readout and the center of the rectangular 600 µs RF pulse. Image reconstruction was performed with an iterative 3D Dictionary Learning Compressed Sensing algorithm (3D-DLCS) (Behl et al., 2016 ) (block size B = 3 × 3 × 3; dictionary size D = 80; sample number Nsamp = 500,000; regularization weighting factor µ = 0.5).
Total 23Na concentration was obtained using two reference vials (0.3% and 0.6% NaCl). B1+ and B1− corrections were applied to cope with transmit and receive inhomogeneities. In our case we used the double angle method for the field estimations (Insko and Bolinger, 1993 (link)). Because a birdcage coil was used, the principle of reciprocity can be applied (B1+ = B1−) (Hoult, 2000 (link)).
Total 23Na concentration was obtained using two reference vials (0.3% and 0.6% NaCl). B1+ and B1− corrections were applied to cope with transmit and receive inhomogeneities. In our case we used the double angle method for the field estimations (Insko and Bolinger, 1993 (link)). Because a birdcage coil was used, the principle of reciprocity can be applied (B1+ = B1−) (Hoult, 2000 (link)).