Analysis of EPI phase correction with low flip-angle excitation to reduce the required minimum TE: Application to whole-brain, submillimeter-resolution fMRI at 3 T.

PURPOSE

Echo planar imaging is used widely for its imaging speed. However, its applications often suffer from ghost artifacts. In the community, an approach using three navigator echoes is used commonly for the artifact correction. Although this scheme is effective, as the matrix size increases for high-resolution imaging, the navigator echoes can contribute significantly to increasing the "required minimum TE." To overcome this issue, this work proposes the use of an alternative navigator echo scheme called the "TR-external" scheme.

METHODS

The TR-external scheme reduces the required minimum TE by allocating an  additional excitation loop for the navigator echoes before every main excitation loop. In this work, a detailed analysis on the TR-external scheme was performed to assess its performance in comparison to the standard scheme. Visual fMRI was performed to check the feasibility of using the TR-external scheme for detecting functional signals.

RESULTS

The performance of the TR-external scheme was comparable with that of the standard scheme in terms of the SNR, elimination of ghost artifacts, and the BOLD detection. For a given matrix size (288 × 288), the TR-external scheme allowed a substantially shorter TE (5.94 ms) compared with the standard scheme, which resulted in a higher SNR. Furthermore, this feature enabled the submillimeter-resolution (0.73 × 0.73 mm ) fMRI measurement with a favorable TE (35 ms) at 3 T. The fMRI results revealed that activated voxels are well localized along the cortical ribbon.

CONCLUSION

A TR-external scheme for EPI phase correction was implemented at 3 T. Its feasibility for submillimeter-resolution fMRI was successfully demonstrated.