B₁ mapping by Bloch‐Siegert shift

Abstract

A novel method for amplitude of radiofrequency field (B þ 1 ) mapping based on the Bloch-Siegert shift is presented. Unlike conventionally applied double-angle or other signal magnitude-based methods, it encodes the B 1 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 radiofrequency pulse following conventional spin excitation. When applying the off-resonance radiofrequency in the kilohertz 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 magnitude of B 2 1 . Adding gradient image encoding following the off-resonance pulse allows one to acquire spatially resolved B 1 maps. The frequency shift from the Bloch-Siegert effect gives a phase shift in the image that is proportional to B 2 1 . The phase difference of two acquisitions, with the radiofrequency pulse applied at two frequencies symmetrically around the water resonance, is used to eliminate undesired off-resonance effects due to amplitude of static field inhomogeneity and chemical shift. In vivo Bloch-Siegert B 1 mapping with 25 sec/ slice is demonstrated to be quantitatively comparable to a 21-min double-angle map. As such, this method enables robust, high-resolution B A wide variety of amplitude of radiofrequency (RF) field (B 1 ) mapping methods has been developed to date; however, no single one has emerged yet in widespread application. B 1 mapping is used in diverse applications in MR, including transmit gain adjustment to produce specific flip-angle RF pulses, design of multitransmit channel RF pulses (1-3), T 1 mapping and other quantitative MRI (4), and chemical shift imaging.Generally, B 1 mapping methods fall into two classes: signal magnitude or signal phase based. The majority of B 1 mapping methods depend on changes in signal magnitude based on RF flip angle. Existing methods in this category include fitting progressively increasing flip angles (5), stimulated echoes (6), image signal ratios (7-10), signal null at certain flip angles (11), and comparison of spin echo and stimulated echo signals (12).These methods suffer from combinations of the following problems: T 1 dependence; long acquisition times, mainly from acquiring many images and/or a long pulse repetition time (TR) to mitigate the T 1 dependence; inability to use some of these methods with slice selection or in a multislice acquisition; inaccuracy over a large range of B 1 , especially at low flip angles or flip angles close to 90 or 180 ; and large RF power deposition in the case of B 1 mapping sequences based on large flip angles or reset pulses to mitigate the T 1 dependence.There are far fewer phase-based B 1 mapping methods. One method by Morrell (13) uses the phase accrued from a 2a-a flip angle sequence to determine B 1 . This method has the same long TR requirement as the signal magnitude-based sequences, although it ...

Automatic brain segmentation using fractional signal modeling of a multiple flip angle, spoiled gradient-recalled echo acquisition

“…For example, whole-brain FRASIER data acquisition has previously been performed in 4 min 32 s by applying Look-Locker read-out [7], and SPGR-SEG could be accelerated by using ultra-short TR (e.g., TurboFLASH) combined with faster B1 + mapping methods (e.g., actual flip-angle imaging [30] or Bloch-Siegert shift [31]). …”

Improved encoding strategy for CPMG‐based Bloch‐Siegert B mapping

“…Following (1), if v off ) gB 1 , the phase introduced by an off-resonant BS pulse can be written as:…”

“…The line v off indicates the different off-resonance BS pulse signs used for the different sequences. a: BE-SE sequence (1,4). b: BS-TSE/MSE sequence (4).…”

“…BS-B þ 1 mapping previously required two independent BS experiments with inverted off-resonant (6v off ) BS pulse frequencies (1,4). This work proposes a modification of the BS encoding strategy for CPMG-based BS MSE/TSE imaging.…”

A Boundary-Value-Free Reconstruction Method for Magnetic Resonance Electrical Properties Tomography Based on the Neumann-Type Integral Formula over a Circular Region

“…MREPT reconstructs EP distributions from magnetic field data distorted by the loaded object using the so-called B1 mapping technique [8]- [10]. The interaction between electromagnetic field transceived by MR radio frequency (RF) coils and EPs are described by the following time-harmonic Maxwell equations:…”