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.     

Correction for eddy current induced Bo shifts in diffusion‐weighted echo‐planar imaging

The importance of diffusion‐weighted MRI in the assessment of acute stroke is well‐recognized, and quantitative maps of the apparent diffusion coefficient (ADC) are now widely used. Echo‐planar imaging provides a robust method of acquiring diffusion‐weighted images free of motion artifact. However, initial experience with clinical MRI systems indicates that calculation of artifact‐free ADC maps from a series of echo‐planar diffusion‐weighted images is not necessarily straight‐forward. One of the problems is that frequency shifts resulting from eddy currents can cause misregistration of base diffusion‐weighted images. In this study, an on‐line correction method that overcomes this problem is described, and phantom and human images that demonstrate the validity of the technique are presented. The method uses a non‐phase‐encoded reference scan to correct the phase of each echo in the echo train, and can provide ADC maps that are free of misregistration artifacts, without the need for off‐line postprocessing. Magn Reson Med 41:95‐102, 1999. © 1999 Wiley‐Liss, Inc.     

High resolution diffusion‐weighted imaging using readout‐segmented echo‐planar imaging,parallel imaging and a two‐dimensional navigator‐based reacquisition

Single‐shot echo‐planar imaging (EPI) is well established as the method of choice for clinical, diffusion‐weighted imaging with MRI because of its low sensitivity to the motion‐induced phase errors that occur during diffusion sensitization of the MR signal. However, the method is prone to artifacts due to susceptibility changes at tissue interfaces and has a limited spatial resolution. The introduction of parallel imaging techniques, such as GRAPPA (GeneRalized Autocalibrating Partially Parallel Acquisitions), has reduced these problems, but there are still significant limitations, particularly at higher field strengths, such as 3 Tesla (T), which are increasingly being used for routine clinical imaging. This study describes how the combination of readout‐segmented EPI and parallel imaging can be used to address these issues by generating high‐resolution, diffusion‐weighted images at 1.5T and 3T with a significant reduction in susceptibility artifact compared with the single‐shot case. The technique uses data from a 2D navigator acquisition to perform a nonlinear phase correction and to control the real‐time reacquisition of unusable data that cannot be corrected. Measurements on healthy volunteers demonstrate that this approach provides a robust correction for motion‐induced phase artifact and allows scan times that are suitable for routine clinical application. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Real‐time optical motion correction for diffusion tensor imaging

Head motion is a fundamental problem in brain MRI. The problem is further compounded in diffusion tensor imaging because of long acquisition times, and the sensitivity of the tensor computation to even small misregistration. To combat motion artifacts in diffusion tensor imaging, a novel real‐time prospective motion correction method was introduced using an in‐bore monovision system. The system consists of a camera mounted on the head coil and a self‐encoded checkerboard marker that is attached to the patient's forehead. Our experiments showed that optical prospective motion correction is more effective at removing motion artifacts compared to image‐based retrospective motion correction. Statistical analysis revealed a significant improvement in similarity between diffusion data acquired at different time points when prospective correction was used compared to retrospective correction (P < 0.001). Magn Reson Med, 2010. © 2011 Wiley‐Liss, Inc.     

Robust GRAPPA‐accelerated diffusion‐weighted readout‐segmented (RS)‐EPI

Readout segmentation (RS‐EPI) has been suggested as a promising variant to echo‐planar imaging (EPI) for high‐resolution imaging, particularly when combined with parallel imaging. This work details some of the technical aspects of diffusion‐weighted (DW)‐RS‐EPI, outlining a set of reconstruction methods and imaging parameters that can both minimize the scan time and afford high‐resolution diffusion imaging with reduced distortions. These methods include an efficient generalized autocalibrating partially parallel acquisition (GRAPPA) calibration for DW‐RS‐EPI data without scan time penalty, together with a variant for the phase correction of partial Fourier RS‐EPI data. In addition, the role of pulsatile and rigid‐body brain motion in DW‐RS‐EPI was assessed. Corrupt DW‐RS‐EPI data arising from pulsatile nonlinear brain motion had a prevalence of ～7% and were robustly identified via k‐space entropy metrics. For DW‐RS‐EPI data corrupted by rigid‐body motion, we showed that no blind overlap was required. The robustness of RS‐EPI toward phase errors and motion, together with its minimized distortions compared with EPI, enables the acquisition of exquisite 3 T DW images with matrix sizes close to 512². Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Cardiac motion in diffusion-weighted MRI of the liver: artifact and a method of correction

Purpose:

To characterize cardiac motion artifacts in the liver and assess the use of a postprocessing method to mitigate these artifacts in repeat measurements.

Materials and Methods:

Three subjects underwent breathhold diffusion‐weighted (DW) scans consisting of 25 repetitions for three b‐values (0, 500, 1000 sec/mm²). Statistical maps computed from these repetitions were used to assess the distribution and behavior of cardiac motion artifacts in the liver. An objective postprocessing method to reduce the artifacts was compared with radiologist‐defined gold standards.

Results:

Signal dropout is pronounced in areas proximal to the heart, such as the left lobe, but also present in the right lobe and in distal liver segments. The dropout worsens with b‐value and leads to overestimation of the diffusivity. By reference to a radiologist‐defined gold standard, a postprocessing correction method is shown to reduce cardiac motion artifact.

Conclusion:

Cardiac motion leads to significant artifacts in liver DW imaging; we propose a postprocessing method that may be used to mitigate the artifact and is advantageous to standard signal averaging in acquisitions with multiple repetitions. J. Magn. Reson. Imaging 2012;318‐327. © 2011 Wiley Periodicals, Inc.     

3D diffusion tensor MRI with isotropic resolution using a steady‐state radial acquisition

Purpose

To obtain diffusion tensor images (DTI) over a large image volume rapidly with 3D isotropic spatial resolution, minimal spatial distortions, and reduced motion artifacts, a diffusion‐weighted steady‐state 3D projection (SS 3DPR) pulse sequence was developed.

Materials and Methods

A diffusion gradient was inserted in a SS 3DPR pulse sequence. The acquisition was synchronized to the cardiac cycle, linear phase errors were corrected along the readout direction, and each projection was weighted by measures of consistency with other data. A new iterative parallel imaging reconstruction method was also implemented for removing off‐resonance and undersampling artifacts simultaneously.

Results

The contrast and appearance of both the fractional anisotropy and eigenvector color maps were substantially improved after all correction techniques were applied. True 3D DTI datasets were obtained in vivo over the whole brain (240 mm field of view in all directions) with 1.87 mm isotropic spatial resolution, six diffusion encoding directions in under 19 minutes.

Conclusion

A true 3D DTI pulse sequence with high isotropic spatial resolution was developed for whole brain imaging in under 20 minutes. To minimize the effects of brain motion, a cardiac synchronized, multiecho, DW‐SSFP pulse sequence was implemented. Motion artifacts were further reduced by a combination of linear phase correction, corrupt projection detection and rejection, sampling density reweighting, and parallel imaging reconstruction. The combination of these methods greatly improved the quality of 3D DTI in the brain. J. Magn. Reson. Imaging 2009;29:1175–1184. © 2009 Wiley‐Liss, Inc.     

Diffusion tensor imaging (DTI) with retrospective motion correction for large-scale pediatric imaging

Purpose:

To develop and implement a clinical DTI technique suitable for the pediatric setting that retrospectively corrects for large motion without the need for rescanning and/or reacquisition strategies, and to deliver high‐quality DTI images (both in the presence and absence of large motion) using procedures that reduce image noise and artifacts.

Materials and Methods:

We implemented an in‐house built generalized autocalibrating partially parallel acquisitions (GRAPPA)‐accelerated diffusion tensor (DT) echo‐planar imaging (EPI) sequence at 1.5T and 3T on 1600 patients between 1 month and 18 years old. To reconstruct the data, we developed a fully automated tailored reconstruction software that selects the best GRAPPA and ghost calibration weights; does 3D rigid‐body realignment with importance weighting; and employs phase correction and complex averaging to lower Rician noise and reduce phase artifacts. For select cases we investigated the use of an additional volume rejection criterion and b‐matrix correction for large motion.

Results:

The DTI image reconstruction procedures developed here were extremely robust in correcting for motion, failing on only three subjects, while providing the radiologists high‐quality data for routine evaluation.

Conclusion:

This work suggests that, apart from the rare instance of continuous motion throughout the scan, high‐quality DTI brain data can be acquired using our proposed integrated sequence and reconstruction that uses a retrospective approach to motion correction. In addition, we demonstrate a substantial improvement in overall image quality by combining phase correction with complex averaging, which reduces the Rician noise that biases noisy data. J. Magn. Reson. Imaging 2012;36:961–971. © 2012 Wiley Periodicals, Inc.     

Parallel and partial Fourier imaging with prospective motion correction

Subject motion during scan is a major source of artifacts in MR examinations. Prospective motion correction is a promising technique that tracks subject motion and adjusts the imaging volume in real time; however, additional retrospective correction may be necessary to achieve robust image quality and compatibility with other imaging options. Real‐time realignment of the imaging volume by prospective motion correction changes the coil sensitivity weighting and the field inhomogeneity relative to the imaging volume. This can pose image reconstruction problems with parallel imaging and partial Fourier imaging, which rely on coil sensitivity and image phase information, respectively. This work presents a practical method for reconstructing images acquired using prospective motion correction with parallel imaging and/or partial Fourier imaging. Our proposed approach is data driven and noniterative; data are binned into several position bins based on motion measurements made during the prospective motion correction acquisition and the data in each bin are processed through intrabin operations such as parallel imaging reconstruction (in case of undersampling), phase correction, and coil combination before combination of the position bins. We demonstrate the effectiveness of our technique through simulation studies and in vivo experiments using a prospectively motion‐corrected three‐dimensional fast spin echo sequence. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Whole‐body diffusion‐weighted imaging with a continuously moving table acquisition method: Preliminary results

In this study, a method for whole‐body diffusion‐weighted imaging (wbDWI) during continuous table motion has been developed and implemented on a clinical scanner based on a short‐Tau inversion recovery echo‐planar DWI sequence. Unlike currently available multistation wbDWI, which has disadvantages such as long scanning times, poor image quality, and troublesome data realignment, continuously moving table wbDWI can overcome these technical problems while extending the longitudinal field of view in MRI systems. In continuously moving table wbDWI, images are acquired consecutively at the isocenter of the magnet, having less geometric distortions and various possibilities of spatial and temporal coverage of an extended field of view. The acquired images, together with an apparent diffusion coefficient analysis, show that continuously moving table wbDWI can be used by appropriately adapting the table velocity, scan range, radiofrequency coils, slice resolutions, and spatio‐temporal acquisition schemes according to various clinical demands. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Preterm neonatal diffusion processing using detection and replacement of outliers prior to resampling

In diffusion weighted MRI, subject motion and brain pulsation lead both to signal drop‐outs and image misalignment. Unsedated neonates, with their higher heart rate and propensity for motion are particularly prone to degraded scan quality that impairs diffusion tensor estimation. Retrospective registration and robust estimators are two methods that have previously been demonstrated to address motion and intensity outliers, respectively, in diffusion data. However, when taken together, the resampling of images to correct for misalignment can have the effect of averaging outlier voxels with uncorrupted voxels, thereby making outliers more difficult to detect. This article presents a method to remove outliers prior to resampling while taking misalignment into account so that this averaging of outliers with good data can be avoided. The proposed method is compared to other processing pipelines using simulations and data from unsedated preterm neonates. These results demonstrate advantages to the proposed method, particularly in subjects with high motion. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Super‐resolution for multislice diffusion tensor imaging

Diffusion weighted magnetic resonance images are often acquired with single shot multislice imaging sequences, because of their short scanning times and robustness to motion. To minimize noise and acquisition time, images are generally acquired with either anisotropic or isotropic low resolution voxels, which impedes subsequent posterior image processing and visualization. In this article, we propose a super‐resolution method for diffusion weighted imaging that combines anisotropic multislice images to enhance the spatial resolution of diffusion tensor data. Each diffusion weighted image is reconstructed from a set of arbitrarily oriented images with a low through‐plane resolution. The quality of the reconstructed diffusion weighted images was evaluated by diffusion tensor metrics and tractography. Experiments with simulated data, a hardware DTI phantom, as well as in vivo human brain data were conducted. Our results show a significant increase in spatial resolution of the diffusion tensor data while preserving high signal to noise ratio. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

3D steady‐state diffusion‐weighted imaging with trajectory using radially batched internal navigator echoes (TURBINE)

While most diffusion‐weighted imaging (DWI) is acquired using single‐shot diffusion‐weighted spin‐echo echo‐planar imaging, steady‐state DWI is an alternative method with the potential to achieve higher‐resolution images with less distortion. Steady‐state DWI is, however, best suited to a segmented three‐dimensional acquisition and thus requires three‐dimensional navigation to fully correct for motion artifacts. In this paper, a method for three‐dimensional motion‐corrected steady‐state DWI is presented. The method uses a unique acquisition and reconstruction scheme named trajectory using radially batched internal navigator echoes (TURBINE). Steady‐state DWI with TURBINE uses slab‐selection and a short echo‐planar imaging (EPI) readout each pulse repetition time. Successive EPI readouts are rotated about the phase‐encode axis. For image reconstruction, batches of cardiac‐synchronized readouts are used to form three‐dimensional navigators from a fully sampled central k‐space cylinder. In vivo steady‐state DWI with TURBINE is demonstrated in human brain. Motion artifacts are corrected using refocusing reconstruction and TURBINE images prove less distorted compared to two‐dimensional single‐shot diffusion‐weighted‐spin‐EPI. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Real‐time motion correction for high‐resolution larynx imaging

Motion—both rigid‐body and nonrigid—is the main limitation to in vivo, high‐resolution larynx imaging. In this work, a new real‐time motion compensation algorithm is introduced. Navigator data are processed in real time to compute the displacement information, and projections are corrected using phase modulation in k‐space. Upon automatic feedback, the system immediately reacquires the data most heavily corrupted by nonrigid motion, i.e., the data whose corresponding projections could not be properly corrected. This algorithm overcomes the shortcomings of the so‐called diminishing variance algorithm by combining it with navigator‐based rigid‐body motion correction. Because rigid‐body motion correction is performed first, continual bulk motion no longer impedes nor prevents the convergence of the algorithm. Phantom experiments show that the algorithm properly corrects for translations and reacquires data corrupted by nonrigid motion. Larynx imaging was performed on healthy volunteers, and substantial reduction of motion artifacts caused by bulk shift, swallowing, and coughing was achieved. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

High‐resolution human diffusion tensor imaging using 2‐D navigated multishot SENSE EPI at 7 T

The combination of parallel imaging with partial Fourier acquisition has greatly improved the performance of diffusion‐weighted single‐shot EPI and is the preferred method for acquisitions at low to medium magnetic field strength such as 1.5 or 3 T. Increased off‐resonance effects and reduced transverse relaxation times at 7 T, however, generate more significant artifacts than at lower magnetic field strength and limit data acquisition. Additional acceleration of k‐space traversal using a multishot approach, which acquires a subset of k‐space data after each excitation, reduces these artifacts relative to conventional single‐shot acquisitions. However, corrections for motion‐induced phase errors are not straightforward in accelerated, diffusion‐weighted multishot EPI because of phase aliasing. In this study, we introduce a simple acquisition and corresponding reconstruction method for diffusion‐weighted multishot EPI with parallel imaging suitable for use at high field. The reconstruction uses a simple modification of the standard sensitivity‐encoding (SENSE) algorithm to account for shot‐to‐shot phase errors; the method is called image reconstruction using image‐space sampling function (IRIS). Using this approach, reconstruction from highly aliased in vivo image data using 2‐D navigator phase information is demonstrated for human diffusion‐weighted imaging studies at 7 T. The final reconstructed images show submillimeter in‐plane resolution with no ghosts and much reduced blurring and off‐resonance artifacts. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

X‐PROP: A fast and robust diffusion‐weighted propeller technique

Diffusion‐weighted imaging (DWI) has shown great benefits in clinical MR exams. However, current DWI techniques have shortcomings of sensitivity to distortion or long scan times or combinations of the two. Diffusion‐weighted echo‐planar imaging (EPI) is fast but suffers from severe geometric distortion. Periodically rotated overlapping parallel lines with enhanced reconstruction diffusion‐weighted imaging (PROPELLER DWI) is free of geometric distortion, but the scan time is usually long and imposes high Specific Absorption Rate (SAR) especially at high fields. TurboPROP was proposed to accelerate the scan by combining signal from gradient echoes, but the off‐resonance artifacts from gradient echoes can still degrade the image quality. In this study, a new method called X‐PROP is presented. Similar to TurboPROP, it uses gradient echoes to reduce the scan time. By separating the gradient and spin echoes into individual blades and removing the off‐resonance phase, the off‐resonance artifacts in X‐PROP are minimized. Special reconstruction processes are applied on these blades to correct for the motion artifacts. In vivo results show its advantages over EPI, PROPELLER DWI, and TurboPROP techniques. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Turboprop+: Enhanced turboprop diffusion‐weighted imaging with a new phase correction

Faster periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) diffusion‐weighted imaging acquisitions, such as Turboprop and X‐prop, remain subject to phase errors inherent to a gradient echo readout, which ultimately limits the applied turbo factor (number of gradient echoes between each pair of radiofrequency refocusing pulses) and, thus, scan time reductions. This study introduces a new phase correction to Turboprop, called Turboprop+. This technique employs calibration blades, which generate 2‐D phase error maps and are rotated in accordance with the data blades, to correct phase errors arising from off‐resonance and system imperfections. The results demonstrate that with a small increase in scan time for collecting calibration blades, Turboprop+ had a superior immunity to the off‐resonance‐related artifacts when compared to standard Turboprop and recently proposed X‐prop with the high turbo factor (turbo factor = 7). Thus, low specific absorption rate and short scan time can be achieved in Turboprop+ using a high turbo factor, whereas off‐resonance related artifacts are minimized. Magn Reson Med 70:497–503, 2013. © 2012 Wiley Periodicals, Inc.     

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.     

Correction of high-order eddy current induced geometric distortion in diffusion-weighted echo-planar images.

Diffusion-weighted images acquired with the echo-planar imaging technique are highly sensitive to eddy current induced geometric distortions that vary with the magnitude and direction of the diffusion sensitizing gradients. Such distortions cause misalignment of images acquired with different diffusion strengths and orientations. This in turn can result in errors when calculating maps of the apparent diffusion coefficient and diffusion tensor. Previous correction methods either require separate calibration data or only deal with low-order errors. In this study, we demonstrate a method that can correct for higher-order errors. The method relies on collecting pairs of images with diffusion sensitizing gradients reversed. This paired data are first corrected for shifts and linear distortion and then combined to cancel higher-order errors. All acquired data contribute to the final results. The method has been tested by simulation, on phantoms, on adult volunteers, and on neonatal brain examinations.     

Prospective motion correction with continuous gradient updates in diffusion weighted imaging

Despite the existence of numerous motion correction methods, head motion during MRI continues to be a major source of artifacts and can greatly reduce image quality. This applies particularly to diffusion weighted imaging, where strong gradients are applied during long encoding periods. These are necessary to encode microscopic movements. However, they also make the technique highly sensitive to bulk motion. In this work, we present a prospective motion correction method where all applied gradients are adjusted continuously to compensate for changes of the object position and ensure the desired phase evolution in the image coordinate frame. Additionally, in phantom experiments this new technique is used to reproduce motion artifacts with high accuracy by changing the position of the imaging frame relative to the measured object. In vivo measurements demonstrate the validity of the new correction method.