(A) simultaneously acquired navigator for cardiac MRI

Abstract

Cardiac MRI (CMRI) is a safe and non-invasive means by which to visualize the accurate anatomy and function of the heart. However, cardiac and respiratory motion can create motion artifacts in cardiac images. In order to handle cardiac motions, several methods employ the MR signals as navigators for motion synchronization of data acquisitions. Among the various types of navigators, the projection-based navigators are appropriate for cardiac MRI, because they provides spatial-discriminant information and high temporal resolution for motion monitoring. However, in the Cartesian k-space trajectory, the projection-based navigators require additional acquisitions, which are interleaved of acquisitions for imaging data. This procedures decrease the imaging time efficiency. The proposed method simultaneously acquires an image signal and a navigator signal, and separates them in post-processing steps. The conventional SSFP pulse sequence is modified to add an additional RF pulse, whose duration is very short. The additional RF pulse excites a supplementary slice for navigator signal. This sequence enables to obtain navigator signals while maintaining the echo time (TE) of image signals, thereby slightly increasing the repetition time (TR). In post-processing steps, the navigator signals $(S_{nav})$ and image signals $(S-{im})$ are estimated from the acquired data, respectively. By using the alternating phases of the additional RF pulse, image signals and navigator signals could be spatially separated. From Snav and Sim, the motion-gated cardiac images can be obtained by the navigator-based reconstruction method. The cardiac and respiratory motion signals were reliably measured by the proposed method, with the results compared to those of external gating devices. In-vivo experiments demonstrated that the proposed method can provide the cardiac cine images with sufficient image quality levels similar to those realized when using ECG gating with a breath-hold.