Fast low-specific absorption rate B0 -mapping along projections at high field using two-dimensional radiofrequency pulses.
OBJECTIVE
At 7 Tesla (T), conventional static field (B0 ) projection mapping techniques, e.g., FASTMAP, FASTESTMAP, lead to elevated specific absorption rates (SAR), requiring longer total acquisition times (TA). In this work, the series of adiabatic pulses needed for slab selection in FASTMAP is replaced by a single two-dimensional radiofrequency (2D-RF) pulse to minimize TA while ensuring equal shimming performance.


METHODS
Spiral gradients and 2D-RF pulses were designed to excite thin slabs in the small tip angle regime. The corresponding selection profile was characterized in phantoms and in vivo. After optimization of the shimming protocol, the spectral linewidths obtained after 2D localized shimming were compared with conventional techniques and published values from (Emir et al NMR Biomed 2012;25:152-160) in six different brain regions.


RESULTS
Results on healthy volunteers show no significant difference (P > 0.5) between the spectroscopic linewidths obtained with the adiabatic (TA = 4 min) and the new low-SAR and time-efficient FASTMAP sequence (TA = 42 s). The SAR can be reduced by three orders of magnitude and TA accelerated six times without impact on the shimming performances or quality of the resulting spectra.


CONCLUSIONS
Multidimensional pulses can be used to minimize the RF energy and time spent for automated shimming using projection mapping at high field.
CIBM, EPFL, Lausanne, Switzerland.
Olivier
O
Reynaud
Daniel
D
Gallichan
Benoit
B
Schaller
Rolf
R
Gruetter
D065638:Absorption, Radiation; D000328:Adult; D000465:Algorithms; D001921:Brain / Q000378:metabolism*; D005260:Female; D006801:Humans; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008297:Male; D011846:Radio Waves; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted; D014018:Tissue Distribution; D055815:Young Adult
2D-RF pulse; FASTESTMAP; FASTMAP; SAR; projection mapping; spiral