Phase-encoded xSPEN: A novel high-resolution volumetric alternative to RARE MRI

Purpose: To develop a rapid, non-CPMG high-resolution volumetric imaging approach, exhibiting a speed and in-plane resilience to field inhomogeneities comparable to RARE/turbo-spin-echo (TSE) while endowed with unique downsampling characteristics.

Methods: A multi-scan extension of cross-term spatiotemporal encoding (xSPEN) is introduced and analyzed. The method simultaneously yields k_y /k_z data containing low and high frequency components, as well as transposed, low-resolution z/y images. This dual k-/spatial-domain information is captured by a multi-scan procedure that phase-encodes k_y while simultaneously slice-selecting z. A reconstruction scheme converting this information into high resolution 3D images with fully multiplexed volumetric coverage is introduced and exemplified.

Results: Phase-encoded xSPEN was tested by human brain imaging at sub-mm resolutions. The method exceeded 2D TSE's sensitivity by factors of ≈3-4, while providing similar resolution and SNR as 3D TSE in ≈50% acquisition times. The method's contrast is dominated by T₂ and is free from "bright-fat" effects associated to spin-echo trains. Further acceleration is enabled by the method's downsampling abilities. Tradeoffs between encoding time, number of measurements, spatial resolution, SNR, and artifact levels are also laid out.

Conclusion: A new MRI strategy is introduced delivering high in- and through-plane resolutions while enjoying full Fourier multiplexing, leading to fast acquisitions with high SNR.