Optimal control pulse design: Solving low tip angle BIR-4 excitation challenges in X-nuclei spectroscopy

Motivation: Adiabatic BIR-4 pulses, employed for $$$B_1^+$$$ robust spectroscopic excitation, exhibit frequency-dependent modulations in received signals, especially at low tilt angles. These modulations interfere with accurate quantitative analysis. Goal(s): The study aims to examine BIR-4 overshoots at low $$$B_1^+$$$ and introduces optimal control RF pulses to mitigate these issues. Approach: We investigate and compare BIR-4 and optimal control RF pulses in simulations, phantom experiments, and in-vivo 31P spectroscopy. Results: We show that optimal control RF pulse design is imperative for obtaining accurate quantitative data. Optimal control RF pulses have the potential to significantly improve in-vivo 31P magnetic resonance spectroscopy. Impact: Optimal control pulses offer precise excitation, surpassing BIR-4 under low flip angles and challenging transmit conditions. This ensures a stable magnetization steady-state, vital for accurate quantitative analysis in applications such as enzymatic exchange rate measurement via magnetization transfer spectroscopy.