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NMR Biomed. 2013 Sep;26(9):1070-8. doi: 10.1002/nbm.2920. Epub 2013 Jan 28.

Improved Bloch-Siegert based B1 mapping by reducing off-resonance shift.

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  • 1Advanced MRI Section, Laboratory of Functional and Molecular Imaging, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892, USA.


An MRI method based on the Bloch-Siegert (BS) shift phenomenon was recently proposed as a fast and precise way to map a radio frequency (RF) transmit field (B1(+) field). For MRI at high field, the mapping sensitivity of this approach was limited by tissue heating associated with a BS irradiation pulse. To mitigate this, we investigated the possibility of lowering the off-resonance frequency of this pulse since theoretical analysis indicated that the sensitivity of Bloch-Siegert based B1(+) mapping could be substantially improved when irradiating closer to resonance. Using optimized irradiation pulse shape and gradient crushers to minimize direct excitation effects, in vivo experiments on human brains at 7 T confirmed improved sensitivity with this approach. Improved sensitivity translated into an 80% reduction in B1(+) estimation errors without increasing tissue heating. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.


B1 mapping; flip angle; high field MRI; transmit sense; wavelength effects

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