Selective excitation localized by the Bloch–Siegert shift and a gradient
Jonathan B. Martin, Sai Abitha Srinivas, Christopher E. Vaughn, Heng Sun, Mark A. Griswold, William A. Grissom
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.