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

Provide the theoretical foundation and the first practical demonstration of spatiotemporal encoding (SPEN) using additional nonlinear gradient hardware.

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.

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.

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.