Zero-gradient-excitation ramped hybrid encoding (zG_RF-RHE) sodium MRI

Purpose: Fast bi-exponential transverse signal decay compounds sodium image quality. This work aims at enhancing image characteristics using a special case of ramped hybrid encoding (RHE). Zero-gradient-excitation (zG_RF)-RHE provides (1) gradient-free excitation for high flip angle, artifact-free excitation profiles and (2) gradient ramping during dead-time for the optimization of encoding time (t_enc) to reduce T₂ ^* signal decay influence during acquisition. Methods: Radial zG_RF-RHE and standard radial UTE were investigated over a range of receiver bandwidths in simulations, phantom and in vivo brain experiments. Central k-space in zG_RF-RHE was acquired through single point measurements at the minimum achievable TE. T₂ ^* blurring artifacts and image SNR and CNR were assessed. Results: zG_RF -RHE enabled 90° flip angle artifact-free excitation, whereas gradient pre-ramping provided greater t_enc efficiency for any readout bandwidths. Experiments confirmed simulation results, revealing sharper edge characteristics particularly at short readout durations (T_RO). Significant SNR improvements of up to 4.8% were observed for longer T_RO. Conclusion: zG_RF -RHE allows for artifact-free high flip angle excitation with time-efficient encoding improving on image characteristics. This hybrid encoding concept with gradient pre-ramping is trajectory independent and can be introduced in any center-out UTE trajectory design.