B1 mapping by Bloch-Siegert shift.
B1 mapping by Bloch-Siegert shift.
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DOI：
10.1002/mrm.22357
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发表时间：
2010-05
影响因子：
3.3
通讯作者：
Vogell, Mika W.
中科院分区：
文献类型：
--
作者：
Sacolick, Laura I.;Wiesinger, Florian;Hancu, Ileana;Vogell, Mika W.
A novel method for B1+ field mapping based on the Bloch-Siegert shift is presented. Unlike conventionally applied double-angle or other signal magnitude-based methods it encodes the B1 information into signal phase, resulting in important advantages in terms of acquisition speed, accuracy and robustness. The Bloch Siegert frequency shift is caused by irradiating with an off-resonance RF pulse following conventional spin excitation. When applying the off-resonance RF in the kHz range, spin nutation can be neglected and the primarily observed effect is a spin precession frequency shift. This shift is proportional to the square of the RF field magnitude B12. Adding gradient image encoding following the off-resonance pulse allows one to acquire spatially resolved B1 maps. The frequency shift from the Bloch-Siegert effect gives a phase shift in the image that is proportional to B12. The phase difference of two acquisitions, with the RF pulse applied at two frequencies symmetrically around the water resonance is used to eliminate undesired off-resonance effects due to B0 inhomogeneity and chemical shift. In-vivo Bloch Siegert B1 mapping with 25 seconds / slice is demonstrated to be quantitatively comparable to a 21 minute double-angle map. As such this method enables robust, high resolution B1+ mapping in a clinically acceptable time frame.
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