(An) analysis and correction method of artifacts in gradient-echo EPI using spin-echo EPI reference

Abstract

Echo Planar Imaging (EPI) sequence is widely used for various applications which require very fast imaging time such as functional magnetic resonance imaging (fMRI), diffusion-weighted imaging (DWI) and cardiac imaging. In spite of its fast imaging capability, EPI has its limitations in application areas due to two major artifacts of Nyquist ghost artifact and geometric distortion. In order to correct the Nyquist artifact and the geometric distortion, several methods including linear and nonlinear correction methods were developed. Since distortions of the individual echo signal are not considered in the linear correction methods, the ghosts are not properly suppressed especially in high-field MRI, and thus the nonlinear methods are preferred instead. In the nonlinear methods, however, phase errors at certain pixels along the readout (RO) gradient direction may cause streak artifacts. This thesis analyzes the streak artifacts in the gradient-echo (GRE)-EPI images corrected with nonlinear methods, and proposes an artifact correction method that can correct the Nyquist ghost artifact and geometric distortion without causing the streak artifacts in the GRE-EPI. The experimental results confirm that the correction method based on a spin-echo (SE)-EPI reference scan eliminates the N/2 ghost artifact and the streak artifact, even in the presence of moderate field inhomogeneities. In addition, the proposed method corrects the geometric distortion and shows a superior quality improvement compared to Buonocore’s method and Heid’s method. This thesis includes a practical application of the proposed correction method to the simultaneous fMRI and electroencephalography (EEG) measurement for investigating voluntary and inhibited eye blinking.