Selective excitation localized by the Bloch–Siegert shift and a B1+ gradient

Abstract

Purpose To perform B1+$$ {B}_1^{+} $$‐selective excitation using the Bloch–Siegert shift for spatial localization. Theory and Methods A B1+$$ {B}_1^{+} $$‐selective excitation is produced by an radiofrequency (RF) pulse consisting of two summed component pulses: an off‐resonant pulse that induces a B1+$$ {B}_1^{+} $$‐dependent Bloch–Siegert frequency shift and a frequency‐selective excitation pulse. The passband of the pulse can be tailored by adjusting the frequency content of the frequency‐selective pulse, as in conventional B0$$ {B}_0 $$ gradient‐localized excitation. Fine magnetization profile control is achieved by using the Shinnar–Le Roux algorithm to design the frequency‐selective excitation pulse. Simulations analyzed the pulses' robustness to off‐resonance, their suitability for multi‐echo spin echo pulse sequences, and how their performance compares to that of rotating‐frame selective excitation pulses. The pulses were evaluated experimentally on a 47.5 mT MRI scanner using an RF gradient transmit coil. Multiphoton resonances produced by the pulses were characterized and their distribution across B1+$$ {B}_1^{+} $$ predicted. Results With correction for varying B1+$$ {B}_1^{+} $$ across the desired profile, the proposed pulses produced selective excitation with the specified profile characteristics. The pulses were robust against off‐resonance and RF amplifier distortion, and suitable for multi‐echo pulse sequences. Experimental profiles closely matched simulated patterns. Conclusion The Bloch–Siegert shift can be used to perform B0$$ {B}_0 $$‐gradient‐free selective excitation, enabling the excitation of slices or slabs in RF gradient‐encoded MRI.

Fast spin‐echo approach for accelerated B1 gradient–based MRI

“…However, a possible extension of this work would be to eliminate the additional pulsed B 0 gradients by using B 1 -selective pulses. [8][9][10][11] Not only would this allow for the removal of an additional gradient but would expand the versatility of the sequence, allowing for multi-slice acquisition schemes that are necessary to improve the sampling efficiency.…”

“…As presented in this work, the FREE‐type sequence initially required the use of two additional pulsed B 0 gradients to perform slice selection and frequency encoding. However, a possible extension of this work would be to eliminate the additional pulsed B 0 gradients by using B 1 ‐selective pulses 8–11 . Not only would this allow for the removal of an additional gradient but would expand the versatility of the sequence, allowing for multi‐slice acquisition schemes that are necessary to improve the sampling efficiency.…”

“…After RFI was first demonstrated by Hoult in his cardinal work on rotating frame zeugmatography, several groups expanded on his work to develop new imaging techniques like transmit array spatial encoding (TRASE), 6 Bloch-Siegert spatial-encoding technique, 7 and B 1 -selective techniques. [8][9][10][11] At these low fields, these RFI-based techniques become significantly more viable as the specific absorption rate (SAR) constraints are reduced. Moreover, modeling of B 1 encoding fields for the purposes of conducting RFI techniques with array coils and parallel transmission becomes more straightforward at lower fields, as the RF wavelength is long relative to the dimensions of the imaged object.…”

“…Further improvements to the portability could be had by coupling these new emerging RF imaging (RFI) techniques with scanners operating in the mid to low field strengths (≤ 1.5 T). After RFI was first demonstrated by Hoult in his cardinal work on rotating frame zeugmatography, several groups expanded on his work to develop new imaging techniques like transmit array spatial encoding (TRASE), 6 Bloch‐Siegert spatial‐encoding technique, 7 and B 1 ‐selective techniques 8–11 …”

Correcting image distortions from a nonlinear B1+$$ {\boldsymbol{B}}_{\mathbf{1}}^{+} $$‐gradient field in frequency‐modulated Rabi‐encoded echoes

“…2 Later versions of RFZ include 2D rotary echoes 3 and echo planar imaging. 4 More recent RFI methods include TRASE, 5 Bloch-Siegert spatial encoding technique (BS-SET), [6][7][8] and parallel transmit RF encoding. 9 Our renewed interest in RFI focuses on the recently developed technique called FREE (frequency-modulated Rabi-encoded echoes), 10 which is less sensitive to resonance offset as compared to some other RFI methods.…”