T2-weighted Half Fourier Echo Planar Imaging

Kim, C Y; Kim, H J; Ahn, C B

Abstract

PURPOSE: T2-weighted half Fourier Echo Planar Imaging (T2HEPI) method is proposed to reduce measurement time of existing EPI by a factor of 2. In addition, high T2 contrast is obtained for clinical applications. High resolution single-shot EPI images with T2 contrast are obtained with 128x128 matrix size by the proposed method.
MATERIALS AND METHODS
In order to reduce measurement time in EPI, half Fourier space is measured, and rest of half Fourier data is obtained by conjugate symmetric filling. Thus high resolution single shot EPI image with 128x128 matrix size is obtained with 64 echoes. By the arrangement of phase encoding gradients, high T2 weighted images are obtained. The acquired data in k-space are shifted if there exists residual gradient field due to eddy current along phase encoding gradient, which results in a serious problem in the reconstructed image. The residual field is estimated by the correlation coefficient between the echo signal for dc and the corresponding reference data acquired during the pre-scan. Once the residual gradient field is properly estimated, it can be removed by the adjustment of initial phase encoding gradient field between 90 degrees and 180 degrees rf pulses.
RESULTS
The suggested T2HEPI is implemented in a 1.0 Tesla whole body MRI system. Experiments are done with the effective echo times of 72ms and 96ms with single shot acquisitions. High resolution(128x128) volunteer head images with high T2 contrast are obtained in a single scan by the proposed method.
CONCLUSION
Using the half Fourier technique, higher resolution EPI images are obtained with matrix size of 128x128 in a single scan. Furthermore T2 contrast is controlled by the effective echo time. Since the suggested method can be implemented by software alone (pulse sequence and corresponding tuning and reconstruction algorithms) without addition of special hardware, it can be widely used in existing MRI systems.