A Robust Self-navigation for Respiratory Gating in 3D Radial Ultrashort Echo-time Lung MRI using Concurrent Dephasing and Excitation

Three-dimensional ultrashort echo-time (UTE) imaging with radial k-space acquisition is a well-known MR imaging technique that generates comparable lung images to X-ray and computed tomography (CT). Although researchers have sought to minimize the incidence of motion artifacts, there is still a need to accomplish further reduction of motion artifacts through respiratory gating. In this study, we introduce a robust self-navigation for respiratory gating in 3D radial UTE lung imaging especially based on concurrent dephasing and excitation (CODE). To reduce the baseline fluctuation of self-navigated respiratory signals as well as the dependence on the position of the navigating echoes in the k-space trajectories, both of which originate from varying degrees of steadystate condition outside the fully excited regions of a spin system in CODE-MRI, we proposed a new self-navigation method which applies dual navigating echoes successively in the superior-inferior direction and takes the second navigating echoes for respiratory-motion tracking. The phantom and human experimental results showed that the proposed method successfully suppressed the baseline fluctuations of the navigating-echo signals and the resulting respiratory signals, thereby reducing the respiratory-motion artifacts like blurring in the human lung images thanks to the improved respiratory gating.