Intrinsic detection of motion in segmented sequences

While many motion correction techniques for MRI have been proposed, their use is often limited by increased patient preparation, decreased patient comfort, additional scan time, or the use of specialized sequences not available on many commercial scanners. For this reason, we propose a simple self‐navigating technique designed to detect motion in segmented sequences. We demonstrate that comparing two segments containing adjacent sets of k‐space lines results in an aliased error function. A global shift of the aliased error function indicates the presence of in‐plane rigid‐body translation, while other types of motion are evident in the dispersion or breadth of the error function. Since segmented sequences commonly acquire data in sets of adjacent k‐space lines, this method provides these sequences with an inherent method of detecting object motion. Motion corrupted data can then be reacquired proactively or in some cases corrected or removed retrospectively. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Data convolution and combination operation (COCOA) for motion ghost artifacts reduction

A novel method, data convolution and combination operation, is introduced for the reduction of ghost artifacts due to motion or flow during data acquisition. Since neighboring k‐space data points from different coil elements have strong correlations, a new “synthetic” k‐space with dispersed motion artifacts can be generated through convolution for each coil. The corresponding convolution kernel can be self‐calibrated using the acquired k‐space data. The synthetic and the acquired data sets can be checked for consistency to identify k‐space areas that are motion corrupted. Subsequently, these two data sets can be combined appropriately to produce a k‐space data set showing a reduced level of motion induced error. If the acquired k‐space contains isolated error, the error can be completely eliminated through data convolution and combination operation. If the acquired k‐space data contain widespread errors, the application of the convolution also significantly reduces the overall error. Results with simulated and in vivo data demonstrate that this self‐calibrated method robustly reduces ghost artifacts due to swallowing, breathing, or blood flow, with a minimum impact on the image signal‐to‐noise ratio. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Motion correction using coil arrays (MOCCA) for free‐breathing cardiac cine MRI

In this study, we present a motion correction technique using coil arrays (MOCCA) and evaluate its application in free‐breathing respiratory self‐gated cine MRI. Motion correction technique using coil arrays takes advantages of the fact that motion‐induced changes in k‐space signal are modulated by individual coil sensitivity profiles. In the proposed implementation of motion correction technique using coil arrays self‐gating for free‐breathing cine MRI, the k‐space center line is acquired at the beginning of each k‐space segment for each cardiac cycle with 4 repetitions. For each k‐space segment, the k‐space center line acquired immediately before was used to select one of the 4 acquired repetitions to be included in the final self‐gated cine image by calculating the cross correlation between the k‐space center line with a reference line. The proposed method was tested on a cohort of healthy adult subjects for subjective image quality and objective blood‐myocardium border sharpness. The method was also tested on a cohort of patients to compare the left and right ventricular volumes and ejection fraction measurements with that of standard breath‐hold cine MRI. Our data indicate that the proposed motion correction technique using coil arrays method provides significantly improved image quality and sharpness compared with free‐breathing cine without respiratory self‐gating and provides similar volume measurements compared with breath‐hold cine MRI. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Motion correction using an enhanced floating navigator and GRAPPA operations

A method for motion correction in multicoil imaging applications, involving both data collection and reconstruction, is presented. The floating navigator method, which acquires a readout line off center in the phase‐encoding direction, is expanded to detect translation/rotation and inconsistent motion. This is done by comparing floating navigator data with a reference k‐space region surrounding the floating navigator line, using a correlation measure. The technique of generalized autocalibrating partially parallel acquisition is further developed to correct for a fully sampled, motion‐corrupted dataset. The flexibility of generalized autocalibrating partially parallel acquisition kernels is exploited by extrapolating readout lines to fill in missing “pie slices” of k‐space caused by rotational motion and regenerating full k‐space data from multiple interleaved datasets, facilitating subsequent rigid‐body motion correction or proper weighting of inconsistent data (e.g., with through‐plane and nonrigid motion). Phantom and in vivo imaging experiments with turbo spin‐echo sequence demonstrate the correction of severe motion artifacts. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Navigator accuracy requirements for prospective motion correction

Prospective motion correction in MRI is becoming increasingly popular to prevent the image artifacts that result from subject motion. Navigator information is used to update the position of the imaging volume before every spin excitation so that lines of acquired k‐space data are consistent. Errors in the navigator information, however, result in residual errors in each k‐space line. This paper presents an analysis linking noise in the tracking system to the power of the resulting image artifacts. An expression is formulated for the required navigator accuracy based on the properties of the imaged object and the desired resolution. Analytical results are compared with computer simulations and experimental data. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Real‐time correction by optical tracking with integrated geometric distortion correction for reducing motion artifacts in functional MRI

Head motion artifacts are a major problem in functional MRI that limit its use in neuroscience research and clinical settings. Real‐time scan‐plane correction by optical tracking has been shown to correct slice misalignment and nonlinear spin‐history artifacts; however, residual artifacts due to dynamic magnetic field nonuniformity may remain in the data. A recently developed correction technique, Phase Labeling for Additional Coordinate Encoding, can correct for absolute geometric distortion using only the complex image data from two echo planar images with slightly shifted k‐space trajectories. An approach is presented that integrates Phase Labeling for Additional Coordinate Encoding into a real‐time scan‐plane update system by optical tracking, applied to a tissue‐equivalent phantom undergoing complex motion and an functional MRI finger tapping experiment with overt head motion to induce dynamic field nonuniformity. Experiments suggest that such integrated volume‐by‐volume corrections are very effective at artifact suppression, with potential to expand functional MRI applications. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Estimation of k‐space trajectories in spiral MRI

For non‐Cartesian data acquisition in MRI, k‐space trajectory infidelity due to eddy current effects and other hardware imperfections will blur and distort the reconstructed images. Even with the shielded gradients and eddy current compensation techniques of current scanners, the deviation between the actual k‐space trajectory and the requested trajectory remains a major reason for image artifacts in non‐Cartesian MRI. It is often not practical to measure the k‐space trajectory for each imaging slice. It has been reported that better image quality is achieved in radial scanning by correcting anisotropic delays on different physical gradient axes. In this article the delay model is applied in spiral k‐space trajectory estimation to reduce image artifacts. Then a novel estimation method combining the anisotropic delay model and a simple convolution eddy current model further reduces the artifact level in spiral image reconstruction. The root mean square error and peak error in both phantom and in vivo images reconstructed using the estimated trajectories are reduced substantially compared to the results achieved by only tuning delays. After a one‐time calibration, it is thus possible to get an accurate estimate of the spiral trajectory and a high‐quality image reconstruction for an arbitrary scan plane. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Compressed‐sensing motion compensation (CosMo): A joint prospective–retrospective respiratory navigator for coronary MRI

Prospective right hemidiaphragm navigator (NAV) is commonly used in free‐breathing coronary MRI. The NAV results in an increase in acquisition time to allow for resampling of the motion‐corrupted k‐space data. In this study, we are presenting a joint prospective–retrospective NAV motion compensation algorithm called compressed‐sensing motion compensation (CosMo). The inner k‐space region is acquired using a prospective NAV; for the outer k‐space, a NAV is only used to reject the motion‐corrupted data without reacquiring them. Subsequently, those unfilled k‐space lines are retrospectively estimated using compressed sensing reconstruction. We imaged right coronary artery in nine healthy adult subjects. An undersampling probability map and sidelobe‐to‐peak ratio were calculated to study the pattern of undersampling, generated by NAV. Right coronary artery images were then retrospectively reconstructed using compressed‐sensing motion compensation for gating windows between 3 and 10 mm and compared with the ones fully acquired within the gating windows. Qualitative imaging score and quantitative vessel sharpness were calculated for each reconstruction. The probability map and sidelobe‐to‐peak ratio show that the NAV generates a random undersampling k‐space pattern. There were no statistically significant differences between the vessel sharpness and subjective score of the two reconstructions. Compressed‐sensing motion compensation could be an alternative motion compensation technique for free‐breathing coronary MRI that can be used to reduce scan time. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Prospective real‐time correction for arbitrary head motion using active markers

Patient motion during an MRI exam can result in major degradation of image quality, and is of increasing concern due to the aging population and its associated diseases. This work presents a general strategy for real‐time, intraimage compensation of rigid‐body motion that is compatible with multiple imaging sequences. Image quality improvements are established for structural brain MRI acquired during volunteer motion. A headband integrated with three active markers is secured to the forehead. Prospective correction is achieved by interleaving a rapid track‐and‐update module into the imaging sequence. For every repetition of this module, a short tracking pulse‐sequence remeasures the marker positions; during head motion, the rigid‐body transformation that realigns the markers to their initial positions is fed back to adaptively update the image‐plane—maintaining it at a fixed orientation relative to the head—before the next imaging segment of k‐space is acquired. In cases of extreme motion, corrupted lines of k‐space are rejected and reacquired with the updated geometry. High‐precision tracking measurements (0.01 mm) and corrections are accomplished in a temporal resolution (37 ms) suitable for real‐time application. The correction package requires minimal additional hardware and is fully integrated into the standard user interface, promoting transferability to clinical practice. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

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.     

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.     

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.     

IIR GRAPPA for parallel MR image reconstruction

Accelerated parallel MRI has advantage in imaging speed, and its image quality has been improved continuously in recent years. This paper introduces a two‐dimensional infinite impulse response model of inverse filter to replace the finite impulse response model currently used in generalized autocalibrating partially parallel acquisitions class image reconstruction methods. The infinite impulse response model better characterizes the correlation of k‐space data points and better approximates the perfect inversion of parallel imaging process, resulting in a novel generalized image reconstruction method for accelerated parallel MRI. This k‐space‐based reconstruction method includes the conventional generalized autocalibrating partially parallel acquisitions class methods as special cases and has a new infinite impulse response data estimation mechanism for effective improvement of image quality. The experiments on in vivo MRI data show that the proposed method significantly reduces reconstruction errors compared with the conventional two‐dimensional generalized autocalibrating partially parallel acquisitions method, particularly at the high acceleration rates. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Application of k‐space energy spectrum analysis for inherent and dynamic B0 mapping and deblurring in spiral imaging

Spiral imaging is vulnerable to spatial and temporal variations of the amplitude of the static magnetic field (B₀) caused by susceptibility effects, eddy currents, chemical shifts, subject motion, physiological noise, and system instabilities, resulting in image blurring. Here, a novel off‐resonance correction method is proposed to address these issues. A k‐space energy spectrum analysis algorithm is first applied to inherently and dynamically generate a B₀ map from the k‐space data at each time point, without requiring any additional data acquisition, pulse sequence modification, or phase unwrapping. A simulated phase evolution rewinding algorithm and an automatic residual deblurring algorithm are then used to correct for the blurring caused by both spatial and temporal B₀ variations, resulting in a high spatial and temporal fidelity. This method is validated against conventional B₀ mapping and deblurring methods, and its advantages for dynamic MRI applications are demonstrated in functional MRI studies. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Motion correction in periodically‐rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) and turboprop MRI

Periodically‐rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) and Turboprop MRI are characterized by greatly reduced sensitivity to motion, compared to their predecessors, fast spin‐echo (FSE) and gradient and spin‐echo (GRASE), respectively. This is due to the inherent self‐navigation and motion correction of PROPELLER‐based techniques. However, it is unknown how various acquisition parameters that determine k‐space sampling affect the accuracy of motion correction in PROPELLER and Turboprop MRI. The goal of this work was to evaluate the accuracy of motion correction in both techniques, to identify an optimal rotation correction approach, and determine acquisition strategies for optimal motion correction. It was demonstrated that blades with multiple lines allow more accurate estimation of motion than blades with fewer lines. Also, it was shown that Turboprop MRI is less sensitive to motion than PROPELLER. Furthermore, it was demonstrated that the number of blades does not significantly affect motion correction. Finally, clinically appropriate acquisition strategies that optimize motion correction are discussed for PROPELLER and Turboprop MRI. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Inverse reconstruction method for segmented multishot diffusion‐weighted MRI with multiple coils

Each k‐space segment in multishot diffusion‐weighted MRI is affected by a different spatially varying phase which is caused by unavoidable motions and amplified by the diffusion‐encoding gradients. A proper image reconstruction therefore requires phase maps for each segment. Such maps are commonly derived from two‐dimensional navigators at relatively low resolution but do not offer robust solutions. For example, phase variations in diffusion‐weighted MRI of the brain are often characterized by high spatial frequencies. To overcome this problem, an inverse reconstruction method for segmented multishot diffusion‐weighted MRI is described that takes advantage of the full k‐space data acquired from multiple receiver coils. First, the individual coil sensitivities are determined from the non–diffusion‐weighted acquisitions by regularized nonlinear inversion. These coil sensitivities are then used to estimate accurate motion‐associated phase maps for each segment by iterative linear inversion. Finally, the coil sensitivities and phase maps serve to reconstruct artifact‐free images of the object by iterative linear inversion, taking advantage of the data of all segments. The efficiency of the new method is demonstrated for segmented diffusion‐weighted stimulated echo acquisition mode MRI of the human brain. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

SPIRiT: Iterative self‐consistent parallel imaging reconstruction from arbitrary k‐space

A new approach to autocalibrating, coil‐by‐coil parallel imaging reconstruction, is presented. It is a generalized reconstruction framework based on self‐consistency. The reconstruction problem is formulated as an optimization that yields the most consistent solution with the calibration and acquisition data. The approach is general and can accurately reconstruct images from arbitrary k‐space sampling patterns. The formulation can flexibly incorporate additional image priors such as off‐resonance correction and regularization terms that appear in compressed sensing. Several iterative strategies to solve the posed reconstruction problem in both image and k‐space domain are presented. These are based on a projection over convex sets and conjugate gradient algorithms. Phantom and in vivo studies demonstrate efficient reconstructions from undersampled Cartesian and spiral trajectories. Reconstructions that include off‐resonance correction and nonlinear ?₁‐wavelet regularization are also demonstrated. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Fast,simple gradient delay estimation for spiral MRI

Timing delays between data acquisition and gradient transmission result in image degradation. This is especially true in spiral MRI, where delays can alter data in a nonuniform manner, generating significant artifact in the reconstructed data. The many methods that exist to mitigate these delays or measure the k‐space coordinates require long measurement times, complicated analysis, specialized phantoms or hardware, or significant changes to the sequence of interest. A fast and simple method is proposed to measure delays on each gradient channel. It requires only minimal modification to an existing spiral sequence and can be used to measure independent delays on three gradient channels and any scan subject within six sequence repetition times. The effectiveness and accuracy of this method are analyzed. Magn Reson Med 63:1683–1690, 2010. © 2010 Wiley‐Liss, 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.     

Quantitative T*2‐mapping based on multi‐slice multiple gradient echo flash imaging: Retrospective correction for subject motion effects

Numerous clinical and research applications for quantitative mapping of the effective transverse relaxation time T*₂ have been described. Subject motion can severely deteriorate the quality and accuracy of results. A correction method for T*₂ maps acquired with multi‐slice multiple gradient echo FLASH imaging is presented, based on acquisition repetition with reduced spatial resolution (and consequently reduced acquisition time) and weighted averaging of both data sets, choosing weighting factors individually for each k‐space line to reduce the influence of motion. In detail, the procedure is based on the fact that motion artifacts reduce the correlation between acquired and exponentially fitted data. A target data set is constructed in image space, choosing the data yielding best correlation from the two acquired data sets. The k‐space representation of the target is subsequently approximated as linear combination of original raw data, yielding the required weighting factors. As this method only requires a single acquisition repetition with reduced spatial resolution, it can be employed on any clinical system offering a suitable sequence with export of modulus and phase images. Experimental results show that the method works well for sparse motion, but fails for strong motion affecting the same k‐space lines in both acquisitions. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.