Speck, Oliver

PurposeSevere geometric distortions induced by tissue susceptibility, water–fat chemical shift, and eddy currents pose a substantial obstacle in single‐shot EPI, especially for high‐resolution imaging at ultrahigh field. View angle tilting (VAT)‐EPI can mitigate in‐plane distortion. However, the accompanied strong image blurring prevented its widespread applications. On the other hand, point‐spread function mapping (PSF)‐EPI can correct distortion and blurring accurately but requires prolonged scan time. We present fused VAT‐PSF‐EPI and possibilities for acceleration.MethodsMR signal equations were explicitly derived to quantify image blurring in VAT‐EPI and the maximum acceleration capacity in VAT‐PSF‐EPI. To validate the theoretical prediction, phantom measurements with varying in‐plane parallel imaging factors, slice thicknesses, and RF pulses were conducted at 7 Tesla. In addition, in vivo human brain scans were acquired with T2 and diffusion weighting to assess distortion and blurring correction.ResultsVAT can effectively suppress distortion, and the introduced image blurring is corrected through PSF encoding. Up to fourfold acceleration (only 5 shots) in VAT‐PSF‐EPI was achieved compared with standard PSF‐EPI without VAT. VAT‐induced signal loss was mitigated by adjusting the sequence parameters and EPI resolution. In vivo T2‐weighted EPI data with 1.4 mm3 resolution demonstrate immunity to water–fat chemical shift‐induced distortion. Very high‐spatial resolution diffusion‐weighted EPI (0.7 × 0.7 × 2.8 mm3 and 1.2 mm3) demonstrates the immunity to eddy current‐induced distortion.ConclusionVAT‐PSF‐EPI is a novel spin‐echo EPI‐based sequence for fast high‐resolution diffusion imaging at ultrahigh field.

PurposeTo develop a reconstruction pipeline that intrinsically accounts for both simultaneous multislice echo planar imaging (SMS‐EPI) reconstruction and dynamic slice‐specific Nyquist ghosting correction in time‐series data.MethodsAfter 1D slice‐group average phase correction, the separate polarity (i.e., even and odd echoes) SMS‐EPI data were unaliased by slice GeneRalized Autocalibrating Partial Parallel Acquisition. Both the slice‐unaliased even and odd echoes were jointly reconstructed using a model‐based framework, extended for SMS‐EPI reconstruction that estimates a 2D self‐phase map, corrects dynamic slice‐specific phase errors, and combines data from all coils and echoes to obtain the final images.ResultsThe percentage ghost‐to‐signal ratios (%GSRs) and its temporal variations for MB3Ry2 with a field of view/4 shift in a human brain obtained by the proposed dynamic 2D and standard 1D phase corrections were 1.37 ± 0.11 and 2.66 ± 0.16, respectively. Even with a large regularization parameter λ applied in the proposed reconstruction, the smoothing effect in fMRI activation maps was comparable to a very small Gaussian kernel size 1 × 1 × 1 mm3.ConclusionThe proposed reconstruction pipeline reduced slice‐specific phase errors in SMS‐EPI, resulting in reduction of GSR. It is applicable for functional MRI studies because the smoothing effect caused by the regularization parameter selection can be minimal in a blood‐oxygen‐level–dependent activation map.

Purpose To describe a model‐based reconstruction strategy for single‐shot echo planar imaging (EPI) that intrinsically accounts for k‐space nonuniformity, Nyquist ghosting, and geometric distortions during rather than before or after image reconstruction. Methods Ramp sampling and inhomogeneous B0 field‐induced distortion cause the EPI samples to lie on a non‐Cartesian grid, thus requiring the nonuniform fast Fourier transform. Additionally, a 2D Nyquist ghost phase correction without the need for extra navigator acquisition is included in the proposed reconstruction. Coil compression is also incorporated to reduce the computational load. The proposed method is applied to phantom and human brain MRI data. Results The results demonstrate that Nyquist ghosting and geometric distortions are reduced by the proposed reconstruction. The proposed 2D phase correction is superior to a conventional 1D correction. The reductions of both artifacts lead to improved temporal signal‐to‐noise ratio (tSNR). The virtual coil results suggest that the processing time can be reduced by up to 75%, with a mean tSNR loss of only 3.2% when using 8‐virtual instead of 32‐physical coils for twofold undersampled data. Conclusion The proposed reconstruction improves the quality (ghosting, geometry, and tSNR) of EPI without requiring calibration data for Nyquist ghost correction. Magn Reson Med 78:2250–2264, 2017. © 2017 International Society for Magnetic Resonance in Medicine.