SAR‐reduced spin‐echo‐based Bloch–Siegert B₁⁺ mapping: BS‐SE‐BURST

Abstract

Fast and accurate B 1 1 mapping is possible using phase-based Bloch-Siegert (BS) methods. Importantly, the off-resonant pulses needed for BS B 1 1 mapping methods can easily be implemented in multiple MR sequences. BS-based B 1 1 mapping has thus been introduced for gradient echo (BS-FLASH), spin-echo (BS-SE), and Carr, Purcell, Meiboom, Gill (CPMG)-based multi-SE and turbo-SE sequences. When using SE and multi-SE/turbo-SE-based BS sequences, however, the high intrinsic specific absorption rates must be considered in clinical situations. This study introduces a fast BS B 1 1 mapping method based on a SE-BURST sequence (BS-SE-BURST). With SE-BURST sequences, multiple low-magnitude excitation pulses are applied prior to the refocusing pulse. Thus, multiple and different phase-encoded echoes can be acquired per excitation cycle. Compared with a SE sequence, this excitation strategy results in a similar signal-to-noise ratio (SNR) per unit time but with reduced specific absorption rate. The proposed BS-SE-BURST sequence was implemented on a conventional 3 T whole body MRI scanner and applied successfully. Magn Reson Med 68:529-536,

A theoretical and experimental comparison of different techniques for B1 mapping at very high fields

“…This study can only cover a small fraction of the published flip angle measurement techniques, with additional sequences and variations of the treated techniques having been published. The results shown here agree in part with the results from earlier studies .…”

Approximate Fourier domain expression for Bloch–Siegert shift

“…Eq. [7] is rewritten as follows: The solution to this first order differential equation can be written as:…”

“…To improve this technique, there have been several studies on the optimization of the sequence and the off-resonance RF pulse shape (3)(4)(5)(6)(7)(8)(9). In Refs.…”

Robust 3D Bloch-Siegert based B1+ mapping using Multi-Echo General Linear Modelling

“…The BSS technique is flexible and can be integrated into a multitude of pulse sequences, such as 2D 5 or 3D gradient echo (GRE) [7][8][9] , Interleaved EPI, spiral GRE 10 and spin echo [11][12][13][14] acquisitions. The BSS technique, as typically implemented, requires two acquisitions with opposite off-resonance frequencies.…”