Robust 2D phase correction for echo planar imaging under a tight field‐of‐view

Nyquist ghost artifacts are a serious issue in echo planar imaging. These artifacts primarily originate from phase difference between even and odd echo images and can be removed or reduced using phase correction methods. The commonly used 1D phase correction can only correct phase difference along readout axis. 2D correction is, therefore, necessary when phase difference presents along both readout and phase encoding axes. However, existing 2D methods have several unaddressed issues that affect their practicality. These issues include uncharacterized noise behavior, image artifact due to unoptimized phase estimation, Gibbs ringing artifact when directly applying to partial k_y data, and most seriously a new image artifact under tight field‐of‐view (i.e., field‐of‐view slightly smaller than object size). All these issues are addressed in this article. Specifically, theoretical analysis of noise amplification and effect of phase estimation error is provided, and tradeoff between noise and ghost is studied. A new 2D phase correction method with improved polynomial fitting, joint homodyne processing and phase correction, compatibility with tight field‐of‐view is then proposed. Various results show that the proposed method can robustly generate images free of Nyquist ghosts and other image artifacts even in oblique scans or when cross‐term eddy current terms are significant. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Phase-correction method for reduction of B0 instability artifacts

View-to-view phase shifts due to B0 instability are corrected by applying a phase correction to the low-spatial-frequency views. The algorithm described herein assumes that the signal intensity in the space outside of an object represents only noise. A set of phases for the low-spatial-frequency views are calculated by minimizing, in the image, the intensity of the induced artifacts in the space outside of the object.     

Two‐dimensional phase correction method for single and multi‐shot echo planar imaging

Ghost artifacts are a serious issue in single and multi‐shot echo planar imaging. Because of these coherent artifacts, it is essential to consistently suppress the ghosts. In this article, we present a phase correction algorithm that achieves excellent ghost suppression for single and multi‐shot echo planar imaging. The phase correction is performed along both the x (read) direction and y (phase) direction. To this end, we apply a double field of view prescan and compute the phase required for ghost suppression. This phase is fitted to a 2D polynomial. The fitted phase is used to correct the echo planar imaging images. The correction algorithm can be used with any readout gradient polarities and any number of shots. A flow chart of the correction method is provided to better clarify the full process. Finally, phantom and volunteer images demonstrate the improvement of artifact suppression obtained with this algorithm over conventional phase correction methods. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Retrospective motion compensation using variable-density spiral trajectories

PURPOSE: To develop a method of retrospectively correcting for motion artifacts using a variable-density spiral (VDS) trajectory. MATERIALS AND METHODS: Each VDS interleaf was designed to adequately sample the same center region of k-space. This central overlapping region can then be used to measure rigid body motion between the acquisition of each VDS interleaf. By applying appropriate phase shifts and rotations of the k-space data, rigid body motion artifacts can be removed, resulting in images with less motion corruption. RESULTS: Both phantom and volunteer experiments are shown, demonstrating the technique's ability to further reduce artifacts in images acquired with an already motion-resistant acquisition trajectory. Registration accuracy is highly dependent on the trajectory design parameters. This space was explored to find an optimal design of VDS trajectories for motion compensation. CONCLUSION: Using appropriately designed VDS trajectories, residual motion artifacts can be significantly reduced by retrospectively correcting for in-plane rigid body motion. An overlapping region of approximately 8% of the central region of k-space and approximately 70 interleaves were found to be near-optimal parameters for retrospective correction using VDS trajectories.     

High spatial resolution EPI using an odd number of interleaves.

Ghost artifacts in echoplanar imaging (EPI) arise from phase errors caused by differences in eddy currents and gradient ramping during left-to-right traversal of kx(forward echo) versus right-to-left traversal of kx (reverse echo). Reference scans do not always reduce the artifact and may make image quality worse. To eliminate the need for reference scans, a ghost artifact reduction technique based on image phase correction was developed, in which phase errors are directly estimated from images reconstructed separately using only the forward or only the reverse echos. In practice, this technique is applicable only to single-shot EPI that produces only one ghost (shifted 1/2 the field of view from the parent image), because the technique requires that the ghosts do not completely overlap the parent image. For higher spatial resolution, typically an even number of separate k-space traversals (interleaves) are combined to produce one large data set. In this paper, we show that data obtained from an even number of interleaves cannot be combined to produce only one ghost, and image phase correction cannot be applied. We then show that data obtained from an odd number of interleaves can be combined to produce only one ghost, and image phase correction can be applied to reduce ghost intensity significantly. This "odd-number interleaf EPI" provides spatial and temporal resolution tradeoffs that are complementary to, or can replace, those of even-number interleaf EPI. Odd-number interleaf EPI may be particularly useful for MR systems in which reference scans have been unreliable.     

Two‐dimensional phase cycled reconstruction for inherent correction of echo‐planar imaging nyquist artifacts

The inconsistency of k‐space trajectories results in Nyquist artifacts in echo‐planar imaging (EPI). Traditional techniques often only correct for phase errors along the frequency‐encoding direction (one‐dimensional correction), which may leave significant residual artifacts, particularly for oblique‐plane EPI or in the presence of cross‐term eddy currents. As compared with one‐dimensional correction, two‐dimensional (2D) phase correction can be much more effective in suppressing Nyquist artifacts. However, most existing 2D correction methods require reference scans and may not be generally applicable to different imaging protocols. Furthermore, EPI reconstruction with these 2D phase correction methods is susceptible to error amplification due to subject motion. To address these limitations, we report an inherent and general 2D phase correction technique for EPI Nyquist removal. First, a series of images are generated from the original dataset, by cycling through different possible values of phase errors using a 2D reconstruction framework. Second, the image with the lowest artifact level is identified from images generated in the first step using criteria based on background energy in sorted and sigmoid‐weighted signals. In this report, we demonstrate the effectiveness of our new method in removing Nyquist ghosts in single‐shot, segmented and parallel EPI without acquiring additional reference scans and the subsequent error amplifications. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Motion correction with PROPELLER MRI: application to head motion and free-breathing cardiac imaging.

A method for motion correction, involving both data collection and reconstruction, is presented. The PROPELLER MRI method collects data in concentric rectangular strips rotated about the k-space origin. The central region of k-space is sampled for every strip, which (a) allows one to correct spatial inconsistencies in position, rotation, and phase between strips, (b) allows one to reject data based on a correlation measure indicating through-plane motion, and (c) further decreases motion artifacts through an averaging effect for low spatial frequencies. Results are shown in which PROPELLER MRI is used to correct for bulk motion in head images and respiratory motion in nongated cardiac images. Magn Reson Med 42:963-969, 1999.     

Fast method for 1D non-cartesian parallel imaging using GRAPPA.

MRI with non-Cartesian sampling schemes can offer inherent advantages. Radial acquisitions are known to be very robust, even in the case of vast undersampling. This is also true for 1D non-Cartesian MRI, in which the center of k-space is oversampled or at least sampled at the Nyquist rate. There are two main reasons for the more relaxed foldover artifact behavior: First, due to the oversampling of the center, high-energy foldover artifacts originating from the center of k-space are avoided. Second, due to the non-equidistant sampling of k-space, the corresponding field of view (FOV) is no longer well defined. As a result, foldover artifacts are blurred over a broad range and appear less severe. The more relaxed foldover artifact behavior and the densely sampled central k-space make trajectories of this type an ideal complement to autocalibrated parallel MRI (pMRI) techniques, such as generalized autocalibrating partially parallel acquisitions (GRAPPA). Although pMRI can benefit from non-Cartesian trajectories, this combination has not yet entered routine clinical use. One of the main reasons for this is the need for long reconstruction times due to the complex calculations necessary for non-Cartesian pMRI. In this work it is shown that one can significantly reduce the complexity of the calculations by exploiting a few specific properties of k-space-based pMRI.     

Variable field of view for spatial resolution improvement in continuously moving table magnetic resonance imaging.

An approach is described in which the field of view (FOV) along the Y (right/left) phase encoding direction can be dynamically altered during a continuously moving table (CMT) coronal acquisition for extended FOV MRI. We hypothesize that with this method, regions of the anatomy exhibiting significantly different lateral widths can be imaged with a matching local FOV(Y), thereby improving local lateral spatial resolution. k-space raw data from the variable-FOV CMT acquisition do not allow simple Fourier reconstruction due to the presence of a mixture of phase encodes sampled at different Deltak(Y) intervals. In this work, we employ spline interpolation to reregister the mixed data set onto a uniformly sampled k-space grid. Using this interpolation scheme, we present phantom and peripheral contrast-enhanced MR angiography results demonstrating an approximate 45% improvement in local lateral spatial resolution for continuously moving table acquisitions.     

A flexible view ordering technique for high-quality real-time 2DFT MR fluoroscopy.

A method to tailor the view order to the reconstruction cycle is introduced for real-time MRI. It is well known that view sharing and oversampling central k-space views can improve the temporal resolution of gradient-echo pulse sequences. By ordering phase-encodes to synchronize k-space acquisition with the reconstruction cycle, apparent temporal resolution can match the frame rate with as few as one-fourth of the phase-encodes sampled per reconstruction. Spatial resolution is maintained by periodically updating high spatial frequencies. In addition to apparent temporal resolution, three other criteria for real-time imaging are identified and evaluated: display latency, dispersion, and frame-to-frame consistency. Latency is minimized by ordering views in a reverse-centric manner within each reconstruction interval, sampling high-energy views immediately prior to beginning reconstruction. Dispersion is kept low and consistent by synchronizing acquisition and reconstruction, thus avoiding poorly timed reconstruction instances. Real-time implementation demonstrates pulsatile time-of-flight blood signal enhancement in humans.     

Novel reconstruction method for three‐dimensional axial continuously moving table whole‐body magnetic resonance imaging featuring autocalibrated parallel imaging GRAPPA

Continuously moving table MR imaging has been successfully evaluated for whole‐body tumor staging and metastasis screening. In previous studies it was demonstrated that three‐dimensional (3D) slab‐selective excitation with lateral readout can provide very efficient k‐space coverage when the longitudinal field of view (FOV) is limited. To reduce respiratory artifacts, data acquisition in the thoracoabdominal region of the patient typically must be performed during one single breath hold. This consequently restricts acquisition time and thus spatial resolution. In this work, a novel reconstruction method is introduced for axial 3D moving table data acquisition with lateral readout. The method features table position correction completely in k‐space and is compatible with autocalibrated parallel imaging (GRAPPA). Parallel imaging can be applied to increase spatial resolution while maintaining the breath‐holding time. A sophisticated protocol for whole‐body moving table MRI was developed. The impact of gradient nonlinearity on the featured imaging method was evaluated in phantom and volunteer experiments. Finally, the protocol was optimized toward minimizing residual artifacts. Moving table whole‐body MRI with lateral readout was performed in 5 healthy volunteers and was compared with lateral readout data acquired with a GRAPPA accelerated protocol providing increased spatial resolution. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

TSE with average-specific phase encoding ordering for motion detection and artifact suppression

PURPOSE: To detect motion-corrupted measurements in multi-average turbo-spin-echo (TSE) acquisitions and reduce motion artifacts in reconstructed images. MATERIALS AND METHODS: An average-specific phase encoding (PE) ordering scheme was developed for multi-average TSE sequences in which each echo train is assigned a unique PE pattern for each pre-averaged image (PAI). A motion detection algorithm is developed based on this new PE ordering to identify which echo trains in which PAIs are motion-corrupted. The detected PE views are discarded and replaced by uncorrupted k-space data of the nearest PAI. Both phantom and human studies were performed to investigate the effectiveness of motion artifact reduction using the proposed method. RESULTS: Motion-corrupted echo trains were successfully detected in all phantom and human experiments. Significant motion artifact suppression has been achieved for most studies. The residual artifacts in the reconstructed images are mainly caused by residual inconsistencies that remain after the corrupted k-space data is corrected. CONCLUSION: The proposed method combines a novel data acquisition scheme, a robust motion detection algorithm, and a simple motion correction algorithm. It is effective in reducing motion artifacts for images corrupted by either bulk motion or local motion that occasionally happens during data acquisition.     

SMASH navigators.

The additional data acquired when using multiple receiver coils is commonly used to improve SNR or reduce acquisition times. It may also be used to remove image artifacts by selectively replacing corrupt data. In the present study, a correction scheme is presented based on simultaneous acquisition of spatial harmonics (SMASH) that enables detection and correction of motion artifacts caused by 2D translations. Newly measured data is compared with predictions from previously measured data by making negative and positive spatial harmonics. Differences are attributed to motion occurring in the interval between the acquisition of separate phase encode lines and correction parameters are determined. Two types of rigid body motion are considered: 1) object and coil array move, and 2) object only moves, since each causes different phase errors in k-space. Simulation, phantom, and volunteer experiments demonstrate the validity of the technique.     

Scatter correction for clinical cone beam CT breast imaging based on breast phantom studies

In flat-panel detector-based cone beam CT breast imaging (CBCTBI) systems, scattering is an important factor that degrades image quality. It is not practical to measure the scattering profiles of a breast for all view angles in a patient study, but it is possible to develop a method to estimate the scattering profiles based on information acquired from breast phantom studies. A new scattering correction method is proposed for clinical CBCTBI in this study. The scattering profiles of three anthropomorphic uncompressed breast phantoms of different sizes were thoroughly investigated, and the results indicated that though phantom size differed, the scattering profiles were mainly determined by local breast diameters, which are the approximate diameters of coronal slices that are perpendicular to the nipple-to-chestwall direction. Thus for scattering correction purposes it is possible to establish a relationship between location breast diameters and local scattering profiles, namely the fitted smooth curves of scatter-to-primary ratios (SPR) and normalized scattered radiations (NSR). In clinical CBCTBI studies, after the local breast diameters are sampled and measured on projection images, the scattering image for every projection image can be generated based on the established relationship, and the projection images can be corrected using either the SPR based method or the NSR based method. Phantom studies and clinical studies showed that both the SPR and NSR methods are able to correct cupping artifacts and reduce reconstruction error. The SPR method does not increase tissue contrast or noise while the NSR method increases both.     

Improving MR image quality in the presence of motion by using rephasing gradients

Numerous techniques exist for suppressing ghosting artifacts due to respiratory motion on MR images. Although such methods can remove coherent ghosting artifacts, motion during gradient pulses also leads to poor image quality. This is due to phase variations at the echo caused by changes in velocity from one phase-encoding view to the next. The effect becomes severe for long sampling times and long TE values and can lead to low estimates of T2. We discuss general, robust modifications of the standard gradient or spin-echo sequences by using rephasing gradients that force the phase of constant-velocity moving spins to be zero at the echo. These sequences lead to a significant reduction in motion artifacts and hence improvement in image quality. They can be applied to multislice, multiecho, water/fat, and gating schemes as well. Since motion problems are universal, it would appear that these modified sequences should come into common usage for MR imaging.     

Inherent correction of motion-induced phase errors in multishot spiral diffusion-weighted imaging

Multishot spiral imaging is a promising alternative to echo‐planar imaging for high‐resolution diffusion‐weighted imaging and diffusion tensor imaging. However, subject motion in the presence of diffusion‐weighting gradients causes phase inconsistencies among different shots, resulting in signal loss and aliasing artifacts in the reconstructed images. Such artifacts can be reduced using a variable‐density spiral trajectory or a navigator echo, however at the cost of a longer scan time. Here, a novel iterative phase correction method is proposed to inherently correct for the motion‐induced phase errors without requiring any additional scan time. In this initial study, numerical simulations and in vivo experiments are performed to demonstrate that the proposed method can effectively and efficiently correct for spatially linear phase errors caused by rigid‐body motion in multishot spiral diffusion‐weighted imaging of the human brain. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Removal of EPI Nyquist ghost artifacts with two-dimensional phase correction.

Odd-even echo inconsistencies result in Nyquist ghost artifacts in the reconstructed EPI images. The ghost artifacts reduce the image signal-to-noise ratio and make it difficult to correctly interpret the EPI data. In this article a new 2D phase mapping protocol and a postprocessing algorithm are presented for an effective Nyquist ghost artifacts removal. After an appropriate k-space data regrouping, a 2D map accurately encoding low- and high-order phase errors is derived from two phase-encoded reference scans, which were originally proposed by Hu and Le (Magn Reson Med 36:166-171;1996) for their 1D nonlinear correction method. The measured phase map can be used in the postprocessing algorithm developed to remove ghost artifacts in subsequent EPI experiments. Experimental results from phantom, animal, and human studies suggest that the new technique is more effective than previously reported methods and has a better tolerance to signal intensity differences between reference and actual EPI scans. The proposed method may potentially be applied to repeated EPI measurements without subject movements, such as functional MRI and diffusion coefficient mapping.     

Accelerating MRI by skipped phase encoding and edge deghosting (SPEED).

A fast imaging method called skipped phase encoding and edge deghosting (SPEED) is introduced. The k-space is sparsely sampled into three interleaved datasets, each with a skip-size N and a relative shift in phase encoding (PE). These datasets are separately reconstructed by 2DFT and edge-enhanced by a differential filter in the PE direction, resulting in edge maps with phase-shifted aliasing ghosts. The sparseness of edges reduces the chance of ghost overlapping. Typical ghosted-edge maps can be adequately modeled with only two dominating ghost layers that are resolved from a set of three equations using least-square error minimization, yielding N ghost maps of different orders that can be registered and averaged into a single deghosted-edge map for noise and artifact reduction. Finally, the deghosted-edge map is transformed into a deghosted image by an inverse filter. A few central k-space lines are collected without PE skip to aid the inverse filtering. SPEED has been demonstrated by in vivo data to reduce scan time considerably without noticeable artifacts. It has various potential applications, such as MR angiography (MRA), where the signal itself is sparse. As an independent method, SPEED can be combined with other fast imaging methods for further acceleration.     

Coil-by-coil image reconstruction with SMASH.

The SiMultaneous Acquisition of Spatial Harmonics (SMASH) technique uses linear combinations of undersampled datasets from the component coils of an RF coil array to reconstruct fully sampled composite datasets in reduced imaging times. In previously reported implementations, SMASH reconstructions were designed to reproduce the images that would otherwise be obtained by simple sums of fully gradient encoded component coil images. This strategy has left SMASH images vulnerable to phase cancellation artifacts when the sensitivities of RF coil array elements are not suitably phase-aligned. In fully gradient encoded imaging schemes these artifacts can be eliminated using a variety of methods for combining the individual coil images, including matched filter combinations as well as sum of squares combinations. Until now, these reconstruction schemes have been unavailable to SMASH reconstructions as SMASH produced a final composite image directly from the raw component coil k-space datasets. This article demonstrates a modification to SMASH that allows reconstruction of a full set of accelerated individual component coil images by fitting component coil sensitivity functions to a complete set of spatial harmonics tailored for each coil in the array. Standard component coil combinations applied to the individual reconstructed images produce final composite images free of phase cancellation artifacts.