Selective excitation localized by the Bloch-Siegert shift and a B 1 + gradient.

PURPOSE: To perform B 1 + $$ {B}_1^{+} $$ -selective excitation using the Bloch-Siegert shift for spatial localization. THEORY AND METHODS: A B 1 + $$ {B}_1^{+} $$ -selective excitation is produced by an radiofrequency (RF) pulse consisting of two summed component pulses: an off-resonant pulse that induces a B 1 + $$ {B}_1^{+} $$ -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 $$ {B}_0 $$ 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 + $$ {B}_1^{+} $$ predicted.
RESULTS: With correction for varying B 1 + $$ {B}_1^{+} $$ 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 $$ {B}_0 $$ -gradient-free selective excitation, enabling the excitation of slices or slabs in RF gradient-encoded MRI.