Hua Guo1
1Center for Biomedical Imaging Research, School of Biomedical Engineering, Tsinghua University, Beijing, China
Synopsis
Keywords: Image acquisition: Fast imaging, Image acquisition: Sequences, Physics & Engineering: Physics
EPI is one of the most important MRI techniques, widely used in functional MRI and DWI. Unlike conventional Cartesian sampling methods, EPI can cover k-space in just a single or a limited number of excitations. This distinctive sampling approach also results in a unique signal sampling acceleration mechanism, differentiating it from traditional techniques. This lecture will introduce how parallel imaging is used in EPI. We will begin by discussing the use of conventional parallel imaging for EPI, followed by an introduction of simultaneous multislice imaging. Finally, we will delve into undersampling strategies in 3D EPI for fMRI and DWI.
Introduction and Background
EPI
has been the primary imaging sequence used for functional MRI and diffusion-weighted
imaging (DWI) due to its high sampling efficiency and insensitivity to motion. Unlike
traditional Cartesian sampling methods, EPI can cover k-space in a single or a
limited number of excitations. This
distinctive sampling approach also results in the unique image character,
including geometric distortions, low spatial resolution and blurring artifacts.
To improve image quality and sampling
efficiency of EPI, tremendous efforts have been devoted and some of the parallel
imaging techniques have been adopted for EPI. Parallel imaging for 2D EPI
EPI usually has low bandwidth along the
phase encoding (PE) direction, which serves as the primary reason for image
distortion along the PE dimension. Parallel imaging, such as sensitivity
encoding (SENSE)(1) and generalized
autocalibrating partially parallel acquisition (GRAPPA)(2), can increase the
effective bandwidth along the PE direction by accelerating k-space traversal. Thus
parallel imaging is mainly used for suppression of image distortion, signal dropout and
blurring in EPI. In practice, the acceleration factor R is
typically set as 2 or 3 for EPI due to the signal-to-noise
ratio (SNR) penalty, g-factor-induced noise amplification, and image artifacts.
To improve acquisition efficiency,
simultaneous multi-slice (SMS) EPI(3), is also widely used for EPI. SMS uses
RF pulses with multiband excitation, and thus enables the simultaneous excitation
of multiple, spatially distinct slices within a single repetition time. To
separate these overlapped slices, the aforementioned parallel imaging techniques
is used. Blipped-controlled aliasing (blipped-CAIPI), which evolved from
CAIPRINHA(4), adjusts the relative positions of
simultaneously excited slices. To achieve this, Blipped-CAIPI uses additional phase
encoding gradients (blips) along the slice direction during the EPI readout(5). The
shift improves slice separation and enables much higher acceleration factors
while reducing
the noise amplification and g-factor penalties.Parallel imaging for 3D EPI
3D EPI has been gradually
adopted in fMRI(6) and provides improved performance than
the traditional 2D EPI, including but not limited to increased temporal
signal-to-noise ratio (tSNR). CAIPIRINHA is used for 3D EPI sampling
straightforwardly to increase the sampling efficiency and image quality. Like 3D
fMRI, 3D EPI is also used for diffusion MRI. In particular, multi-slab 3D EPI(7) or simultaneous multi-slab (SMSlab)(8,9) has been developed for high-resolution submillimeter
isotropic DWI. In this lecture, parallel imaging-accelerated 3D EPI techniques,
especially 3D DWI, will also be given.Acknowledgements
No acknowledgement found.References
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