title: Efficient Bloch-Siegert B1 (+) mapping using spiral and echo-planar readouts.
abstract: The Bloch-Siegert (B-S) B1 (+) mapping technique is a fast, phase-based method that is highly SAR limited especially at 7T, necessitating the use of long repetition times. Spiral and echo-planar readouts were incorporated in a gradient-echo based B-S sequence to reduce specific absoprtion rate (SAR) and improve its scan efficiency. A novel, numerically optimized 4 ms B-S off-resonant pulse at + 1960 Hz was used to increase sensitivity and further reduce SAR compared with the conventional 6 ms Fermi B-S pulse. Using echo-planar and spiral readouts, scan time reductions of 8-16 were achieved. By reducing the B-S pulse width by a factor of 1.5, SAR was reduced by a factor of 1.5 and overall sensitivity was increased by a factor of 1.33 due to the nearly halved resonance offset of the new B-S pulse. This was validated on phantoms and volunteers at 7 T.
authors: Department of Radiology, Stanford University School of Medicine, Stanford, California, USA.
authors: Manojkumar
authors: M
authors: Saranathan
authors: Mohammad Mehdi
authors: MM
authors: Khalighi
authors: Gary H
authors: GH
authors: Glover
authors: Prachi
authors: P
authors: Pandit
authors: Brian K
authors: BK
authors: Rutt
mesh_terms: D000465:Algorithms; D001921:Brain / Q000033:anatomy & histology*; D001931:Brain Mapping / Q000379:methods*; D017352:Echo-Planar Imaging / Q000379:methods*; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: B1+ mapping; Bloch-Siegert method; echo-planar readout; spiral readout
references: PLoS One. 2012;7(3):e32379
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