Spatiotemporal spiral magnetic resonance imaging

Abstract

ABSTRACT This paper addresses the problem of Magnetic Resonance (MIt) imaging of moving targets using spiral scan echo planar data collection. A system model is developed to interpret the readouts of repeated spiralexcitations via two separate time variables, the slow-time and fast-time. This mathematical model is usedto construct an inversion for forming the time progression of the target image. A method for increasingthe repetition rate of the spiral data collection is presented. Results are provided.Keywords: Magnetic resonance (MR) imaging, spiral scan, moving target imaging I. INTRODUCTION This paper is concerned with spatio-temporal Magnetic Resonance (MR) imaging of moving targets.MR has become a valuable medical imaging modality in which low power radiowaves are used to producehigh resolution and high contrast images of biological structures [1]. The patient's motion, respiratorymotion, and cardiac motion during the MR data acquisition are known to cause ghost-like artifacts andblurring in the reconstructed MR images. Methods to suppress MR motion artifacts via signal processingof the measured MR data and/or the manner the MR data are acquired have been suggested, e.g., [2]-[8].Spiral-scan Echo Planar (SEP) is an MR imaging modality which provides the user with a high-speedMR data collection strategy. This approach was originally introduced by Likes [9] and Ljunggren [10].Cho and his co-workers at the University of California at Irvine [11] provided the first experimental MRreconstruction from SEP data with a constant angular velocity in terms of polar format processing, anddiscussed its advantages. Macovski, Meyer and their co-workers at the Stanford University introducedSEP data with a constant linear velocity [12], which does not contain the redundancy of polar format SEPdata, and have utilized it in various MR imaging problems, e.g., [6],[13]. Other SEP-MR reconstructionmethods have been suggested by Yudilevich and Stark [14], and [17]. In the following discussion, we areconcerned with SEP data with a constant linear velocity.In this paper, a method for SEP-MR imaging of moving targets in time and space is presented. Ourobjective is to utilize the high-speed SEP data collection to construct the time evolution of a movingstructure at a rate which is close to the video rate (30-60 Hz). We begin with a discussion on the principlesand parameters in SEP-MR imaging system (Section II). Next, we examine the information in a SEP-MRdatabase of a time-varying structure (Section III.A). For this, we interpret a repeated set of SEP-MRmeasurements in terms of two separate time variables, the fast-time and slow-time domains.We identify the time progression in the target motion arid repeated SEP excitations via a variablewhich we refer to as the slow-time domain. We identify the readout time for a single SEP excitation bythe fast-time domain. This mathematical view of the repeated SEP-MR system model provides a practicalinversion for these spatio-temporal MR imaging systems. (A similar view of the slow-time and fast-time