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

Abstract

Purpose: To perform $B_1^{+}$ -selective excitation using the Bloch-Siegert shift for spatial localization.

Theory and methods: A $B_1^{+}$ -selective excitation is produced by an radiofrequency (RF) pulse consisting of two summed component pulses: an off-resonant pulse that induces a $B_1^{+}$ -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 $B_0$ 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 $B_1^{+}$ predicted.

Results: With correction for varying $B_1^{+}$ 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 $B_0$ -gradient-free selective excitation, enabling the excitation of slices or slabs in RF gradient-encoded MRI.

Keywords: Bloch-Siegert shift; RF pulse design; RF-encoded MRI; low-field MRI; multiphoton; selective RF excitation.