抄録
Magnetic field inhomogeneity causes artifacts in MRI. For example, in single-shot echo-planar imaging (EPI), they often appear as severe geometric distortions along the phase-encoding direction. Sensitivity encoding (SENSE) is useful in reducing the distortion in EPI since it only acquires partial k-space data using multiple receiver channels. In SENSE, a reference scan usually needs to be performed to create a sensitivity profile of each receiver channel. Gradient echo (GRE) sequences are often used in the reference scan. In diffusion-weighted imaging (DWI) using single-shot EPI with SENSE, non-negligible aliasing artifacts often remain in the reconstructed images. We suppose that these artifacts result from misregistration between the reference images acquired using GRE sequences and the DWI acquired using EPI. In this study, we used two types of acquisition methods to create sensitivity profiles, GRE sequences and EPI, and have compared the residual artifacts in the reconstructed images. The sensitivity profiles were created from the data acquired using the GRE and EPI sequences. Artifacts were reduced when EPI sensitivity profiles were used. This resulted from the fact that off-resonance effects, e.g., magnetic field inhomogeneity, susceptibility, and chemical shift, often cause severe image distortion in EPI, and, therefore, there is misregistration between images reconstructed from the data acquired using GRE and EPI sequences. Our study suggests that EPI sensitivity profiles be used when imaging data are acquired using a single-shot EPI with SENSE, although GRE sensitivity profiles have often been used in practice.
Magnetic field inhomogeneity causes artifacts in MRI. For example, in single-shot echo-planar imaging (EPI), they often appear as severe geometric distortions along the phase-encoding direction. Sensitivity encoding (SENSE) is useful in reducing the distortion in EPI since it only acquires partial k-space data using multiple receiver channels. In SENSE, a reference scan usually needs to be performed to create a sensitivity profile of each receiver channel. Gradient echo (GRE) sequences are often used in the reference scan. In diffusion-weighted imaging (DWI) using single-shot EPI with SENSE, non-negligible aliasing artifacts often remain in the reconstructed images. We suppose that these artifacts result from misregistration between the reference images acquired using GRE sequences and the DWI acquired using EPI. In this study, we used two types of acquisition methods to create sensitivity profiles, GRE sequences and EPI, and have compared the residual artifacts in the reconstructed images. The sensitivity profiles were created from the data acquired using the GRE and EPI sequences. Artifacts were reduced when EPI sensitivity profiles were used. This resulted from the fact that off-resonance effects, e.g., magnetic field inhomogeneity, susceptibility, and chemical shift, often cause severe image distortion in EPI, and, therefore, there is misregistration between images reconstructed from the data acquired using GRE and EPI sequences. Our study suggests that EPI sensitivity profiles be used when imaging data are acquired using a single-shot EPI with SENSE, although GRE sensitivity profiles have often been used in practice.
