Research In Center-Out Echo-Planar Imaging

Echo-planar imaging (EPI), as a fast imaging technique, is widely used in perfusion imaging, diffusion imaging, fMRI, cardiac imaging and other dynamic imaging applications. The full k-space data for a 2D image are sampled in an echo train after a single-shot radiofrequency excitation so that the acquisition time is only tens of microseconds. The echo time (TE), however, is usually long in the conventional EPI as the ky lines are filled from one side of k-space to the other side so that the center k-space (ky=0) data are sampled in the middle of the echo train. The relatively long TE introduces substantial T2 effect in diffusion tensor imaging and arterial spin labeling, and results reduced signal-to-noise ratio (SNR). Partial ky sampling, in which a few ky lines in the low-frequency part at one side of k-space is filled before the filling of the entire ky lines on the other side, is regularly implemented to shorten the TE. However, the minimum TE is limited by the time needed to travel through the overlie ky lines. In this study, a center-out echo-planar-imaging (COEPI) technique is proposed to reduce T2 effect and increase SNR through further shortening the TE. The main contents include:(1) The COEPI technique is developed based on the conventional EPI pulse. The phase-encoding gradients are changed to realize the k-space trajectory that starts from the center and then travels towards both sides of k-space. Partial ky sampling in COEPI is developed with the data in the central region of k-space as well as the data in the peripheral region of one side of k-space sampled for the reduction of the acquisition time.(2) The data reconstruction of COEPI at 1.5T MRI system was studied. The 2D re-gridding algorithm and partial k-space reconstruction based on the character of the conjugate symmetry were applied.(3) The correction method of phase errors between odd and even echoes in COEPI was studied. The data from the reference scan with the phase-encoding gradients turned off was acquired and used for the regression of the phase errors between odd and even echoes. The corresponding ky iines in k-space was then corrected with the regressed phase differences.(4) The correction method of off-resonance effects in COEPI was studied. The frequency-segmented conjugate phase method based on the field inhomogeneity map was applied to diminish the blurring and geometric distortion in off-resonance areas.(5) The relation between the SNR increment of COEPI compared with the conventional EPI and the T2 or T2* value was studied. The SNR values of both phantom and the white matter in human brain at 1.5T were measured. The advantages of reduced T2 effect and increased SNR in COEPI were validated in both spin-echo and gradient-recalled-echo images.