Spatiotemporal Encoding With Nonlinear Gradient Hardware Using Pulseq: From Principles to Practical Demonstration

PURPOSE: Provide the theoretical foundation and the first practical demonstration of spatiotemporal encoding (SPEN) using additional nonlinear gradient hardware. METHODS: The quadratic phase profile can be generated either by a chirped-RF pulse combined with a constant gradient or, directly, by a quadratic gradient pulse. Both a conventional chirped-RF and a novel SPEN method using a custom-built matrix gradient coil for quadratic phase generation were implemented and integrated into a spin-echo echo-planar-imaging (SE-EPI) sequence using Pulseq. The methods were compared through phantom imaging experiments performed on a 3T MRI system. RESULTS: The required quadratic phase profile for SPEN was successfully generated using the nonlinear gradient coil, resulting in images of comparable quality. This quadratic gradient-based approach was achieved while exploiting the advantages of SPEN and overcoming current SAR and minimal TE limitations arising from the use of chirped-RF pulses. CONCLUSION: The generation of the SPEN-defining quadratic phase using nonlinear gradients is an advantageous alternative to conventional methods. This approach enables improved clinical applicability of SPEN, particularly for 3D and high-field MRI, by mitigating critical safety and timing limitations. Additionally, an implementation of the conventional method is provided open-source to support further research.