Common SENSE (sensitivity encoding using hardware common to all MR scanners): a new method for single-shot segmented echo planar imaging

Reducing ghosting due to k-space discontinuities in fast spin echo (FSE) imaging by a new combination of k-space ordering and parallel imaging

In multi-echo imaging sequences like fast spin echo (FSE), the point spread function (PSF) in the phase encoding direction contains significant secondary peaks (sidebands). This is due to discontinuities in adjacent k-space data obtained at different echo times caused by T(2) decay, and leads to ghosting and hence reduced image quality. Recently, utilising multiple coils for signal reception has become the standard configuration for MR systems due to the additional flexibility that parallel imaging (PI) methods can provide. PI methods generally obtain more data than is required to reconstruct an image. Here, this redundancy in information is exploited to reduce discontinuity-related ghosting in FSE imaging. Adjacent phase encoded k-space lines are acquired at different echo times alternately in the regions of discontinuity (called 'feathering'). This moves the resulting ghost artefacts to the edges of the field of view. This property of the ghost then makes them amenable to removal using PI methods. With 'feathered' array coil data it is possible to reconstruct data over the region of the discontinuity from both echo times. By combining this data, a significant reduction in ghosting can be achieved. We show this approach to be effective through simulated and acquired MRI data.

Doubling the resolution of echo-planar brain imaging by acquisition of two k-space lines per gradient reversal using TRAIL

Single-shot echo-planar imaging (EPI) is an important method for MRI of the brain. A method has been developed to double the resolution of EPI in the phase-encode direction, without requiring increases in the maximum gradient amplitude or slew rate. The new approach is based on an EPI implementation of the TRAIL (two reduced acquisitions interleaved) method, in which two images, acquired in rapid succession, are spatially interleaved. In addition, two lines of k-space are acquired for each reversal of the readout gradient. Two full-length readouts are needed, therefore power deposition is increased and the total acquisition time is doubled compared with conventional EPI. However, the individual readouts do not increase in length, so there is no increase in image blurring, and distortion is halved as a result of the closer temporal spacing of the acquired k-space lines. A correction method is also presented to remove additional potential Nyquist ghosting. The new method is demonstrated in vivo at 4.7 T and could in principle be combined with existing approaches for increasing resolution, such as partial Fourier or parallel imaging.

EPI distortion correction from a simultaneously acquired distortion map using TRAIL

PURPOSE

To develop a method for shot-by-shot distortion correction of single-shot echo-planar imaging (EPI) that is capable of correcting each image individually using a distortion measurement performed during acquisition of the image itself.

MATERIALS AND METHODS

The recently-introduced method known as two reduced acquisitions interleaved (TRAIL) was extended to measure the distribution of the main magnetic field B0 with each shot. This corresponded to a map of distortion, and allowed distortion to be corrected in the acquired images.

RESULTS

Distortion-corrected images were demonstrated in the human brain. The distortion field could be directly visualized using the "stripe" distribution imposed by the TRAIL pulse sequence. This confirmed the success of the correction. Over a time-course measurement of 10 images, variance was reduced by using shot-by-shot distortion correction compared to correction with a constant field map.

CONCLUSION

Shot-by-shot distortion correction may be performed for EPI images acquired using an extension of the TRAIL technique, ensuring that the correction reflects the actual distortion pattern and not merely a previously measured, but possibly no longer valid, distortion field. This avoids errors due to changes in the distortion field or misregistration of a previously measured distortion map resulting from subject motion.