Single-Shot and Segmented EPI Ghost Artifacts Removal with Two-Dimensional Phase Correction

Introduction Asymmetry between odd and even echoes generates Nyquist ghost artifact in a single-shot EPI , and further enhances multiple ghost artifacts in a segmented EPI. These ghost artifacts not only severely degrade EPI image quality, but may lead to incorrect ADC values in EPI based diffusion measurements (1). Methods currently available for odd-even-echo asymmetry compensation include one-dimensional phase correction using non-phase encoded or phase encoded reference scans (2,3), pulse sequence calibration (4), and comparison of odd and even echoes (5). We report a new and effective method of ghost artifact reduction for both single-shot and segmented EPI. The phase errors induced by odd-even-echo asymmetry are first measured with the phaseencoded reference scan sequence proposed by Hu (3). Instead of correcting phase errors along only the readout direction as in Hu’s method, the phase correction in our new method is performed along both readout and phase directions. The phase variations due to Bo field inhomogeneity and anisotropic eddy current effects can be further suppressed, and therefore the new method has a much better tolerance to field inhomogeneity. Methods SinpIe-shot EPI EPI phase errors originating from asymmetry between odd and even echoes are first measured with the pulse sequence previously reported by Hu (3), as shown in Fig. la. By extracting only even-echo Icy lines, two new sets of k-space data and their corresponding Fourier-transformed phase images (Fig. lb) can be generated. The complex ratio of the two phase images (Fig. lc) accounts for the phase errors due to odd-evenecho asymmetry, and will be used to correct subsequent dynamic EPI scans. The EPI k-space data to be corrected is decomposed and reconstructed as odd-eCho only and even-echo only complex images (Fig. 2a). The EPI image with the lowest Nyquist ghost artifact can be obtained from summation of the odd-echo image and the even-echo image corrected with the derived twodimensional phase map, as indicated by Fig. 2b.