Estimating Absolute-Phase Maps Using ESPIRiT and Virtual Conjugate Coils

TL;DR

This paper introduces a novel ESPIRiT-based approach that estimates coil sensitivities including absolute phase, enhancing the robustness and accuracy of phase-constrained MRI image reconstruction.

Contribution

It presents a new method combining ESPIRiT with virtual conjugate coils to explicitly compute phase-inclusive coil sensitivities for improved MRI reconstruction.

Findings

Accurate sensitivity maps including low-resolution phase were estimated.

High-frequency phase variations can be detected with additional maps.

Incorporating these maps improves phase-constrained imaging robustness.

Abstract

Purpose: To develop an ESPIRiT-based method to estimate coil sensitivities with image phase as a building block for efficient and robust image reconstruction with phase constraints. Theory and Methods: ESPIRiT is a new framework for calibration of the coil sensitivities and reconstruction in parallel Magnetic Resonance Imaging (MRI). Applying ESPIRiT to a combined set of physical and virtual conjugate coils (VCC-ESPIRiT) implicitly exploits conjugate symmetry in k-space similar to VCC-GRAPPA. Based on this method, a new post-processing step is proposed for the explicit computation of coil sensitivities that include the absolute phase of the image. The accuracy of the computed maps is directly validated using a test based on projection onto fully sampled coil images and also indirectly in phase-constrained parallel-imaging reconstructions. Results: The proposed method can estimate accurate sensitivities which include low-resolution image phase. In case of high-frequency phase variations VCC-ESPIRiT yields an additional set of maps that indicates the existence of a high-frequency phase component. Taking this additional set of maps into account can improve the robustness of phase-constrained parallel imaging. Conclusion: The extended VCC-ESPIRiT is a useful tool for phase-constrained imaging.