Development of an echo-shifted, multi-echo, gradient-echo sequence for T* quantification of slow-relaxing water pools.

Purpose: Although conventional multi-echo gradient-echo (GRE) sequences effectively quantify short and intermediate T* in brain tissue, and general interest in cerebrospinal fluid (CSF) is growing due to its association with the glymphatic system, quantifying T* in CSF remains underexplored. Accurate quantification of the slow-relaxing water pools requires imaging at long echo times, significantly increasing acquisition time. This study proposes a novel sequence capable of quantifying the entire range of T* without prolonged acquisition time, mapping T* in both CSF and brain tissue.

Methods: The proposed echo-shifted, multi-echo GRE (ES-mGRE) combines the conventional multi-echo GRE sequence with an echo-shifting technique. Additional gradients are introduced, producing echoes in the next sub-repetition time interval.

Results: ES-mGRE generates artifact-free images at both short and long echo times without extending acquisition time. Increasing the area of the additional gradients enhances diffusion sensitivity, allowing simultaneous quantification of T* and D in CSF. The mean T* of white matter and gray matter is 55.9 ms and 51.5 ms at 3 T, respectively. The mean T* in the ventricles is 234.5 ms. The simultaneously quantified mean D value of 3.07 μm/ms is closely aligned with the reference diffusivity.

Conclusion: We demonstrate that the proposed ES-mGRE sequence can effectively quantify the T* of both CSF and brain tissue while also providing simultaneous diffusion information.