eIRIS: Eigen-analysis approach for improved spine multi-shot diffusion MRI

Abstract

Image reconstruction using image-space sampling function (IRIS) corrects motion-induced inter-shot phase variations using phase maps from navigator-echo for multi-shot diffusion MRI. However, the bandwidth along the phase-encoding direction of navigator-echo is usually lower than that of image-echo, and thus their geometric distortions may be different. This geometric mismatch is corrected in IRIS by using the B₀ map from an additional scan. In this paper, we present an enhanced IRIS (eIRIS) method that remove the requirement of B₀ map. eIRIS treats shots as virtual coils, and then uses an eigen-analysis-based approach, which is insensitive to geometric mismatch, to estimates coil sensitivity maps containing the inter-shot phase variations. The final image is reconstructed under the framework of SENSE. Simulation, phantom, and cervical spine experiments were performed to evaluate the eIRIS method. The images generated by IRIS without B₀ correction contain severe artifacts. eIRIS obtains results without noticeable artifacts and comparable to those of IRIS with B₀ correction and GRAPPA with a compact kernel (GRAPPA-CK) method. eIRIS slightly outperforms GRAPPA-CK in the terms of normalized root-mean-square error and signal-to-noise ratio. eIRIS has the potential to obtain high-quality diffusion-weighted images and will benefit the research and clinical diagnosis of spinal cord.

Keywords: Diffusion-weighted imaging; ESPIRiT; Multi-shot acquisition; Navigator; Phase correction; Spinal cord.