Gradient moment smoothing: A new flow compensation technique for multi-shot echo-planar imaging

This work identifies an additional source of phase error across k_y in multi-shot echo-planar imaging resulting from flow or motion along the phase-encoding direction. A velocity-independent flow compensation technique, gradient moment smoothing, is presented that corrects this error by forcing the phase to have smooth quadratic behavior. The correction is implemented, without compromising scan time, by changing the first moment of a bipolar prephaser pulse on a shot-by-shot basis. In phantom and in vivo experiments, gradient moment smoothing effectively eliminates ghosting and signal loss due to phase-encoding flow. When used in conjunction with a “flyback” echo-planar readout, which compensates for flow in the frequency-encoding direction, gradient moment smoothing renders multi-shot echo-planar imaging relatively insensitive to in-plane flow. This can make multi-shot echo-planar imaging a viable technique for accurately imaging in-plane flow and may desensitize it to the otherwise serious problem of in-plane motion.     

Quantitative evaluation of k‐space reordering schemes for compatible dual‐echo arteriovenography (CODEA)

Compatible dual‐echo arteriovenography (CODEA) is a recently developed technique for simultaneous acquisition of time‐of‐flight MR angiogram (MRA) and blood oxygenation level–dependent MR venogram (MRV) using an echo‐specific k‐space reordering scheme. In this study, we evaluated and compared the image quality of CODEA MRA/MRV implemented with two different schemes of echo‐specific k‐space reordering: one along the 1st phase‐encode direction (one‐dimensional) only and the other along both phase‐encode directions (two‐dimensional). Our results showed that use of the two‐dimensional reordering scheme improved contrast‐to‐noise ratio of small arteries by ～8%, although not statistically significant (P > 0.1). Contrast‐to‐noise ratio of the CODEA MRAs was better than that for the non‐CODEA dual‐echo MRA without k‐space reordering (contrast‐to‐noise ratio increased in large arteries by ～10% and small arteries by ～45%; P < 0.1). Contrast‐to‐noise ratio of the CODEA MRAs was comparable with that of the conventional single‐echo MRA for large arteries but reduced by ～20% for small arteries. Contrast‐to‐noise ratio of veins on the CODEA MRVs was equivalent to that of the conventional single‐echo and the non‐CODEA dual‐echo MRVs. However, some veins in the CODEA MRVs showed stronger contrast than those in the single‐echo MRV in relation to the contrast of neighboring arterial signals. Magn Reson Med 63:1404–1410, 2010. © 2010 Wiley‐Liss, Inc.     

Virtual frequency encoding: Achieving symmetry in k-space

Chemical shift artifacts and other off-resonance spatial shifts in 2DFT MRI arise from the linear time dependence in the k-space data in the readout direction. Introduction of a view-dependent time shift of the readout window adds a time dependence to the phase-encoding direction and results in a virtual frequency-encoding direction that is a linear combination of the phase-encode and readout axes. By this method, the readout and phase-encode directions can be made identical in their sensitivity to off-resonance effects and can be arbitrarily swapped with no change in chemical shift or inhomogeneity effects, improving previously reported methods that swap these axes for signal averaging or reduction of motion artifacts.     

Respiratory effects in two-dimensional Fourier transform MR imaging

Respiratory and other regular motions during two-dimensional Fourier transform magnetic resonance imaging produce image artifacts consisting of local blurring and more or less regularly spaced "ghost" images propagating along the direction of the phase-encoding magnetic field gradient. The patterns of these ghost artifacts can be understood in terms of the technique of image production and basic properties of the discrete Fourier transform. This understanding permits, without respiratory gating, production of images of improved quality in body regions in which there is significant respiratory motion. In particular, the ghosts can be maximally separated from the primary image by choosing intervals between phase-encoding gradient pulse increments that are equal to one-half the respiratory period; they can be minimally separated by choosing an interval equal to the respiratory period. Increasing the number of signal averages between each phase-encoding increment decreases the intensity of the ghosts.     

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.     

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 artifact correction in free‐breathing abdominal MRI using overlapping partial samples to recover image deformations

This article presents a method to reconstruct liver MRI data acquired continuously during free breathing, without any external sensor or navigator measurements. When the deformations associated with k‐space data are known, generalized matrix inversion reconstruction has been shown to be effective in reducing the ghosting and blurring artifacts of motion. This article describes a novel method to obtain these nonrigid deformations. A breathing model is built from a fast dynamic series: low spatial resolution images are registered and their deformations parameterized by overall superior–inferior displacement. The correct deformation for each subset of the subsequent imaging data is then found by comparing a few lines of k‐space with the equivalent lines from a deformed reference image while varying the deformation over the model parameter. This procedure is known as image deformation recovery using overlapping partial samples (iDROPS). Simulations using 10 rapid dynamic studies from volunteers showed the average error in iDROPS‐derived deformations within the liver to be 1.43 mm. A further four volunteers were imaged at higher spatial resolution. The complete reconstruction process using data from throughout several breathing cycles was shown to reduce blurring and ghosting in the liver. Retrospective respiratory gating was also demonstrated using the iDROPS parameterization. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Reduction of ringing and blurring artifacts in fast spin-echo imaging

A simple method was devised to reduce ringing and blurring artifacts caused by discontinuous T2 weighting of k-space data in fast spin-echo magnetic resonance (MR) imaging. The method demodulates the weighting function along the phase-encoding direction by using multiple T2 values derived from a set of non–phase-encoded echoes obtained from an extra excitation. The performance of this method was evaluated by computer simulations and experiments, which confirmed its capability of effectively reducing or, in some cases, even completely removing the ringing and blurring artifacts. The results also show that the proposed method produces better results than other artifact reduction methods. The method is particularly useful at high magnetic field strengths (7.1–9.4 T) and with strong gradients (>20 G/cm) used in MR microscopy, in which the apparent T2 values are short for most tissues. The authors expect that the proposed method will find useful applications in various fast spin echo pulse sequences.     

Effect of and correction for in-plane myocardial motion on estimates of coronary-volume flow rates

The sensitivities of phase-difference (PD) and complex-difference (CD) processing strategies to in-plane motion were examined theoretically and experimentally. Errors in velocity and volume flow rate (VFR) estimates were attributed to (a) motion between different velocity encodings and, in the case of segmented k-space acquisition strategies, (b) motion over the segment duration. PD estimates were found to be insensitive to in-plane motion between velocity encodings, whereas CD VFR estimates were found to be sensitive to this motion. PD estimates, however, were affected by partial volume effects. A corrected CD (CD') scheme was developed that minimizes both partial-volume and in-plane motion effects. Segmented k-space acquisitions with sequential offset and sequential interleaved offset (or centric) phase-encoding schemes were studied. Images obtained using these techniques were found to include both blurring and replication artifacts. The amount of artifact generally increased with the number of views per segment (vps) and the in-plane velocity. PD, CD, and CD' VFR estimates were found to be degraded by these artifacts. The sequential offset phase-encoding scheme generally had acceptable VFR errors (at 4 vps, a CD' VFR error of 7.0%) when averaged over the physiologic range of myocardial motion (>12 cm second^?1); however, larger errors were observed outside this range. VFR estimates obtained using the sequential interleaved phase-encoding scheme at 4 vps were unacceptable. More accurate VFR measurements were obtained using a revised segmented PC strategy, which reversed the order in which the velocity and phase encodings were interleaved. The weighted average CD' VFR error obtained using the revised strategy was 24.5% (for 4 vps). Using displacement information obtained from the two velocity-encoded images, an estimate of the in-plane velocity was obtained and used to correct the acquired data. This decreased the VFR error (weighted average CD' error at 4 vps decreased from 24.5% to ?6.3%); however, the implemented correction algorithm could potentially introduced other artifacts in the images.     

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.     

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.     

Fat/water separation using a concentric rings trajectory

The concentric rings two‐dimensional (2D) k‐space trajectory enables flexible trade‐offs between image contrast, signal‐to‐noise ratio (SNR), spatial resolution, and scan time. However, to realize these benefits for in vivo imaging applications, a robust method is desired to deal with fat signal in the acquired data. Multipoint Dixon techniques have been shown to achieve uniform fat suppression with high SNR‐efficiency for Cartesian imaging, but application of these methods for non‐Cartesian imaging is complicated by the fact that fat off‐resonance creates significant blurring artifacts in the reconstruction. In this work, two fat–water separation algorithms are developed for the concentric rings. A retracing design is used to sample rings near the center of k‐space through multiple revolutions to characterize the fat–water phase evolution difference at multiple time points. This acquisition design is first used for multipoint Dixon reconstruction, and then extended to a spectroscopic approach to account for the trajectory's full evolution through 3D k‐t space. As the trajectory is resolved in time, off‐resonance effects cause shifts in frequency instead of spatial blurring in 2D k‐space. The spectral information can be used to assess field variation and perform robust fat–water separation. In vivo experimental results demonstrate the effectiveness of both algorithms. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

POCS‐enhanced correction of motion artifacts in parallel MRI

A new method for correction of MRI motion artifacts induced by corrupted k‐space data, acquired by multiple receiver coils such as phased arrays, is presented. In our approach, a projections onto convex sets (POCS)‐based method for reconstruction of sensitivity encoded MRI data (POCSENSE) is employed to identify corrupted k‐space samples. After the erroneous data are discarded from the dataset, the artifact‐free images are restored from the remaining data using coil sensitivity profiles. The error detection and data restoration are based on informational redundancy of phased‐array data and may be applied to full and reduced datasets. An important advantage of the new POCS‐based method is that, in addition to multicoil data redundancy, it can use a priori known properties about the imaged object for improved MR image artifact correction. The use of such information was shown to improve significantly k‐space error detection and image artifact correction. The method was validated on data corrupted by simulated and real motion such as head motion and pulsatile flow. Magn Reson Med 63:1104–1110, 2010. © 2010 Wiley‐Liss, Inc.     

A generalized k-sampling scheme for 3D fast spin echo

The phase-encoding scheme can significantly affect the quality of fast spin-echo (FSE) images because the echo amplitude is modulated as a function of the echo position in k-space. The effects of the modulation in two-dimensional FSE imaging include ghosting and blurring artifacts and resolution loss in the phase-encoding (PE) direction. In 3D FSE imaging, the use of two PE directions presents the opportunity for improved PE schemes. A new scheme for assignment of echoes to views in 3D FSE, termed generalized, has been developed. This scheme distributes T(2) effects along both PE directions, allowing considerable flexibility in the selection of blurring artifact appearance. In a set of simulations, phantom experiments, and in vivo experiments, the performance of the generalized PE scheme for 3D FSE imaging was compared with the performance of existing PE schemes. The results demonstrate that the generalized PE scheme can be used to reduce blurring artifacts greatly relative to other PE techniques that are presently in use. This approach to PE can be used to manipulate the blurring artifact appearance and to optimize acquisition time.     

Abbreviated moment-compensated phase encoding

To achieve correct spatial location of blood vessels, first order gradient moment nulling applied to the phase encoding axes can be used. However, gradient moment nulling prolongs echo time (TE), which may degrade the flow image in regions of complex flow. The fact that abbreviated moment compensated phase-encoding (AMCPE) can be used to apply partial flow compensation to the phase-encoding axes to prevent spatial misregistration of vessels without requiring the use of long echo times or using arbitrary chosen TE is demonstrated. AMCPE defines two cutoff lines in k-space. The flow-induced phase is completely compensated for values between the cutoff lines and partially compensated beyond the cutoff lines. The AMCPE technique has been tested on both a flow phantom and a human volunteer. The AMCPE images from both the in vivo and the in vitro study demonstrate correctly imaged flow. Computer simulations have been performed to analyze the penalty caused by the incomplete flow compensation. The result shows that the ripple artifacts due to the incomplete flow compensation are unobservable when 60%–70% of k-space is completely flow compensated.     

A simple method of generating variable T1 contrast images using temporally reordered phase encoding

A rapid method of generating T1 images by means of an inversion pulse followed by a fast low-angle imaging experiment is presented. The T1 contrast is manipulated by a temporal reordering of the phase-encoding gradient, resulting in an almost completely free choice of T1 contrast, without any extra restriction on the size of the data acquisition matrix. The intrinsic rapidity of the sequence renders it insensitive to motion artifacts and lends itself to multipoint T1 calculations. The method is particularly well suited to high-field imaging.     

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.     

Overcoming motion in abdominal MR imaging

Anatomic structures that move periodically during the acquisition of data for an MR image become multiple ghosts in the phase-encoding direction. There is a constant spacing in pixels between consecutive ghosts, which is equal to the number of cycles of motion that occurred during the acquisition of data. The intensity of ghosts depends on the intensity of the moving structure and the number of pixels over which the motion occurred. No single method is completely satisfactory at suppressing motion artifacts. The major attributes and limitations of each method are summarized in Table 2, with plus (+) signs denoting merit. Theoretically, some methods perform better in reducing the intensity of ghosts and restoring the image intensity to its proper place. This certainly is not the final criterion, however. Some methods reduce the blurring in addition to suppressing the ghosts, or they suppress ghosts without prolonging the time for imaging. Certain methods also reduce ghosts from other kinds of motion. It is very appealing for a method to function without monitoring. The success of monitoring often depends too much on the cooperation of both the patient and technologist. The theoretical performance, attributes, and deficiencies of the various methods have been combined into a subjective overall rating in the last column of Table 2. All of the methods can be effective under the appropriate circumstances. Moreover, the methods are not mutually exclusive. It is advantageous, therefore, to combine methods to achieve even greater suppression. For example, physical restraint can be used for all but the most uncooperative patients. Most imaging techniques can be designed with gradients that rephase the signals from moving structures. Then other methods, such as averaging or reordering, can be applied as necessary. Fortunately, there are effective motion artifact suppression methods, even though not all are widely available yet on commercial equipment. Consistent suppression of motion artifacts will enhance the quality of MR images. Elimination of motion artifacts will improve the capability of MR to detect lesions and will provide a higher standard of performance for MR in the body.     

Motion‐adaptive spatio‐temporal regularization for accelerated dynamic MRI

Accelerated magnetic resonance imaging techniques reduce signal acquisition time by undersampling k‐space. A fundamental problem in accelerated magnetic resonance imaging is the recovery of quality images from undersampled k‐space data. Current state‐of‐the‐art recovery algorithms exploit the spatial and temporal structures in underlying images to improve the reconstruction quality. In recent years, compressed sensing theory has helped formulate mathematical principles and conditions that ensure recovery of (structured) sparse signals from undersampled, incoherent measurements. In this article, a new recovery algorithm, motion‐adaptive spatio‐temporal regularization, is presented that uses spatial and temporal structured sparsity of MR images in the compressed sensing framework to recover dynamic MR images from highly undersampled k‐space data. In contrast to existing algorithms, our proposed algorithm models temporal sparsity using motion‐adaptive linear transformations between neighboring images. The efficiency of motion‐adaptive spatio‐temporal regularization is demonstrated with experiments on cardiac magnetic resonance imaging for a range of reduction factors. Results are also compared with k‐t FOCUSS with motion estimation and compensation—another recently proposed recovery algorithm for dynamic magnetic resonance imaging. Magn Reson Med 70:800–812, 2013. © 2012 Wiley Periodicals, Inc.     

Rapid local rectangular views and magnifications: Reduced phase encoding of orthogonally excited spin echoes

A method is described for rapid, artifact-free imaging and magnification of small regions within a larger sample. This combines a rectangular window, reconstructed from a reduced number of phase-encoding steps, and confinement of spin echoes to a similar rectangular strip by orthogonal π/2 and π excitations. Phase encoding is along the width of the strip (along Y). Off-center strips are excited by offsetting the Y slice-selecting gradient, and the reconstruction window is kept coincident with excitation by similarly offsetting the Y phase-encoding gradient. The excited strip is centered in the reconstruction window by setting the radiofrequency transmitter on resonance. The method is shown to be useful for long narrow structures such as the spine where the acquisition time is reduced by over a factor of 5 determined by the image aspect ratio. © 1988 Academic Press, Inc.