Elimination of oblique flow artifacts in magnetic resonance imaging.

We present an analysis of how flow oblique to the frequency-encoding direction generates displacement artifacts in MR imaging and show that for flow which has constant velocity between the start of the phase encoding and the center of the echo it is possible to eliminate these artifacts by gradient moment nulling in the phase-encoding direction. However, unlike the standard moment nulling calculations for flow compensating the frequency-encode and slice-selection gradients, the phase-encoding first moment must be nulled specifically with respect to the echo center. Limitations of this method imposed by finite gradient strengths are analyzed. In 3D volume acquisitions with two axes phase encoded it is possible to correct for oblique flow in all directions, and this is demonstrated in images of a human volunteer. Correction for oblique flow displacement artifacts may be particularly useful in quantitative flow and angiographic applications.     

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.     

Correlation of laminated MR appearance of articular cartilage with histology, ascertained by artificial landmarks on the cartilage.

The object of this study was to correlate the laminae of articular cartilage on magnetic resonance (MR) imaging with histologic layers. T1- and fast spin-echo T2-weighted images of articular cartilage with artificial landmarks were obtained under high gradient echo strength (25 mT/m) conditions and a voxel size of 78 x 156 x 2000 microm. Images were also obtained with a) changed frequency-encoding directions; b) changed readout gradient strength; and c) a varied number of phase-encoding steps. T2 mapping was performed with angular variations. Artificial landmarks allowed accurate comparison between the laminae on MR images and the histologic zones. No alterations of the laminae were noted by changing the frequency gradient direction. Altering readout gradient strengths did not show a difference in the thickness of the laminae, and increasing the phase-encoding steps resulted in a more distinct laminated appearance, ruling out chemical shift, susceptibility, and truncation artifacts. The T2 mapping profile showed an anisotropic angular dependency from the magic angle effect. In conclusion, the laminated appearance of articular cartilage on spin-echo and fast spin-echo MR images correlated with the histologic zones rather than MR artifacts.     

Concomitant magnetic-field-induced artifacts in axial echo planar imaging

When a linear magnetic field gradient is used, spatially higher-order magnetic fields are produced to satisfy the Maxwell equations. It has been observed that the higher-order magnetic field produced by the readout gradient causes axial echo planar images acquired with a horizontal solenoid magnet to shift along the phase-encoding direction and lose image intensities. Both the shift and intensity reduction become increasingly severe as the slice offset from the isocenter increases. These phenomena are quantitatively analyzed, and good correlation between experiments and theory has been established. The analysis also predicts a previously unre-ported Nyquist ghost on images with very large slice offsets. This ghost has been verified with computer simulations. Based on the analysis, several methods have been developed to eliminate the image shift, the intensity reduction, and the ghost. Selected methods have been implemented on a commercial scanner and proved effective in removing these image 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.     

The influence of flow and motion in MRI of diffusion using a modified CE-FAST sequence

Severe motion and flow artifacts are a problem in MRI of diffusion in vivo due to the application of strong magnetic field gradients. Here it is shown that image artifacts can be removed by using a modified fast-scan MRI sequence (CE-FAST) in conjunction with averaging of diffusion-weighted images. In phantom studies slow (coherent) flow (less than 1 mm s-1) in the presence of strong diffusion gradients is shown to cause signal losses in diffusion-weighted images that depend on the relative orientations of the flow direction and the diffusion gradient. On the other hand, pulsatile motions of macroscopic dimensions (e.g., 1 mm, 1 Hz, in-plane) lead to smearing and ghosting of signal intensities along the phase-encoding direction of the images. In both phantoms and rabbit brains in vivo motion artifacts were found to be reducible by averaging 8-16 images. Unfortunately, the resulting image contrast no longer represents a "true" diffusion contrast but is affected by additional signal losses due to motion averaging. All experiments were performed on a 40-cm-bore 2.35-T Bruker Medspec system.     

Image metric-based correction (autocorrection) of motion effects: analysis of image metrics

Magnetic resonance (MR) imaging of the shoulder necessitates high spatial and contrast resolution resulting in long acquisition times, predisposing these images to degradation due to motion. Autocorrection is a new motion correction algorithm that attempts to deduce motion during imaging by calculating a metric that reflects image quality and searching for motion values that optimize this metric. The purpose of this work is to report on the evaluation of 24 metrics for use in autocorrection of MR images of the rotator cuff. Raw data from 164 clinical coronal rotator cuff exams acquired with interleaved navigator echoes were used. Four observers then scored the original and corrected images based on the presence of any motion-induced artifacts. Changes in metric values before and after navigator-based adaptive motion correction were correlated with changes in observer score using a least-squares linear regression model. Based on this analysis, the metric that exhibited the strongest relationship with observer ratings of MR shoulder images was the entropy of the one-dimensional gradient along the phase-encoding direction. We speculate (and show preliminary evidence) that this metric will be useful not only for autocorrection of shoulder MR images but also for autocorrection of other MR exams.     

A B(0) shift correction method based on edge RMS reduction for EPI fMRI

Shifting of echoplanar images (EPI) in the phase-encoding direction during functional magnetic resonance imaging (fMRI) experiments can be observed due to B(0) drift. These shifts can cause artifacts in functional activation maps that can be corrected using a navigator echo (NE) technique, but the NE correction requires pulse sequence modifications not available on many clinical systems. A fast, postprocessing correction method based on edge root-mean-square error reduction (ERMSR) is introduced and shown to provide an equivalent correction. J. Magn. Reson. Imaging 2000;12:956-959.     

Partial k-space reconstruction in single-shot diffusion-weighted echo-planar imaging.

Partial k-space sampling is frequently used in single-shot diffusion-weighted echo-planar imaging (DW-EPI) to reduce the TE and thereby improve the SNR. However, it increases the sensitivity of the technique to bulk rotational motion, which introduces a phase gradient across the tissue that shifts the echo in k-space. If the echo is displaced into the high spatial frequencies, conventional homodyne reconstruction fails, causing intensity oscillations across the image. Zero-padding, on the other hand, compromises the image resolution and may cause truncation artifacts. We present an adaptive version of the homodyne algorithm that detects the location of the echo in k-space and adjusts the center and width of the homodyne filters accordingly. The adaptive algorithm produces artifact-free images when the echo is shifted into the high positive k-space range, and reduces to the standard homodyne algorithm in the absence of bulk motion.     

Brain iron in patients with Parkinson disease: MR visualization using gradient modification

In patients with Parkinson disease, improved visualization of brain iron on a mid-field-strength magnet can be obtained with T2-weighted images and elimination of phase-encoding artifacts. A long echo delay time accentuates the loss of signal from brain iron. However, the long pulse sequence creates phase-encoding artifacts from CSF pulsations at the level of the basal ganglia. These artifacts are eliminated and resolving power increased with additional pulsing in the slice-selective and read gradients. Elimination of motion artifacts enhances visualization of brain iron in three ways: (1) extrapyramidal nuclei containing iron have better definition, (2) abnormalities are better identified, and (3) pseudolesions disappear. Our findings suggest there is significant improvement in the resolving power of brain iron on MR scans made with a mid-field-strength scanner when gradient modification is used.     

Multi-Slice,Breathhold Imaging of the Lung with Submillisecond Echo Times

The signal from the lungs is heavily attenuated by T₂ and T₂* decay in standard MR images of the thorax. The authors utilized the capabilities of a prototype fast gradient system to develop a multi-slice gradient echo sequence that can obtain images with an echo time of 0.7 ms. Images acquired in a single breath-hold are free from respiratory motion artifacts and clearly display signal from lung parenchyma. The use of fast gradients makes short echo times possible without the use of nonstandard RF pulses and spatial encoding techniques and their associated limitations.     

Ghost phase cancellation with phase-encoding gradient modulation

Motion artifacts are a dominant cause of magnetic resonance image quality degradation. Periodic or nearly periodic motion results in image replicates of the moving structures in spin-warp Fourier imaging. The replicates, or ghosts, propagate in the image in the phase encoding, or y, direction. These ghosted images can be considered to consist of the time-averaged spin density I₀and a ghost mask g. A set of J ghosted images I_j may be acquired in which the ghost mask is intentionally phase shifted by varying amounts relative to I₀ with interleaved acquisitions that have shifted phaseencoding orders or by acquiring multiple images during a single readout period in the presence of an oscillating phase-encoding gradient. The resulting complex images I_j have the same time-averaged spin density I₀ but have ghost contributions g_j that, on a pixel-by-pixel basis, trace part of a circle around I₀. The source images I_j can then be used to estimate I₀. Simulations and experiments with the phase-encoding gradient modulation method show good general ghost suppression for a variety of quasi-periodic motion sources including both respiratory-type artifacts and flow artifacts. The primary limitation of the method is the need for rapid gradient switching.     

Diffusion imaging with prospective motion correction and reacquisition

A major source of artifacts in diffusion‐weighted imaging is subject motion. Slow bulk subject motion causes misalignment of data when more than one average or diffusion gradient direction is acquired. Fast bulk subject motion can cause signal dropout artifacts in diffusion‐weighted images and results in erroneous derived maps, e.g., fractional anisotropy maps. To address both types of artifacts, a fully automatic method is presented that combines prospective motion correction with a reacquisition scheme. Motion correction is based on the prospective acquisition correction method modified to work with diffusion‐weighted data. The images to reacquire are determined automatically during the acquisition from the imaging data, i.e., no extra reference scan, navigators, or external devices are necessary. The number of reacquired images, i.e., the additional scan duration can be adjusted freely. Diffusion‐weighted prospective acquisition correction corrects slow bulk motion well and reduces misalignment artifacts like image blurring. Mean absolute residual values for translation and rotation were <0.6 mm and 0.5°. Reacquisition of images affected by signal dropout artifacts results in diffusion maps and fiber tracking free of artifacts. The presented method allows the reduction of two types of common motion related artifacts at the cost of slightly increased acquisition time. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

MR imaging in the presence of vascular stents: A systematic assessment of artifacts for various stent orientations, sequence types, and field strengths

A systematic evaluation of the potential quality of magnetic resonance images recorded in the presence of metallic stents was performed on a low-field open imager operating at 0.2 T and on a high-field closed unit operating at 1.0 T. Eight different stent types were examined by two-dimensional gradient-echo sequences with echo times of 4 and 10 msec and by a fast spin-echo technique. In addition, a three-dimensional gradient-echo sequence was applied with an echo time of 2.4 msec. A set of sequence and slice parameters was used on both scanners. Thus, artifacts due to susceptibility effects depending on the magnetic field strength could be distinguished from radiofrequency shielding effects in the lumen of the stents (independent of the field strength). Nine different orthogonal orientations of the stent axis and the image (in terms of slice, read, and phase-encoding direction) were tested, and the artifacts (extension of signal void and visibility of the lumen) were compared. The optimal strategy for visualization of vascular and perivascular regions outside the stents was fast spin-echo imaging with the stent axis and read direction parallel to the static field. Susceptibility-induced signal void in gradient-echo images was minimal using the three-dimensional approach. Increased transmitter amplitudes above usual values provided clearly improved insight in the lumen using gradient-echo sequences.     

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.     

Correction of vibration artifacts in DTI using phase-encoding reversal (COVIPER)

Diffusion tensor imaging is widely used in research and clinical applications, but still suffers from substantial artifacts. Here, we focus on vibrations induced by strong diffusion gradients in diffusion tensor imaging, causing an echo shift in k-space and consequential signal-loss. We refined the model of vibration-induced echo shifts, showing that asymmetric k-space coverage in widely used Partial Fourier acquisitions results in locally differing signal loss in images acquired with reversed phase encoding direction (blip-up/blip-down). We implemented a correction of vibration artifacts in diffusion tensor imaging using phase-encoding reversal (COVIPER) by combining blip-up and blip-down images, each weighted by a function of its local tensor-fit error. COVIPER was validated against low vibration reference data, resulting in an error reduction of about 72% in fractional anisotropy maps. COVIPER can be combined with other corrections based on phase encoding reversal, providing a comprehensive correction for eddy currents, susceptibility-related distortions and vibration artifact reduction.     

On the application of ultra-fast RARE experiments.

The ultra-fast application of the RARE experiment is described in detail, with special emphasis on its multifarious applications with preparation experiments that produce transverse magnetization. The factors affecting the temporal evolution of the magnetization during the experiment are described, and the implications for the slice profile when using a Gaussian refocusing pulse are experimentally examined. The choice of phase-encoding scheme for use with preparation experiments is discussed, as is the use of various phase-encoding schemes to reduce line broadening in the phase-encoding direction if a number of averages are acquired. An explanation for the decomposition of the echo are into two components if the read gradient is imbalanced is given, and the experimental conditions necessary for the coherent addition of these two echo groups are described. An alternative sequence that removes one of these groups from the acquisition window is proposed. The sensitivity of the sequence to flow and motion is investigated, and the drastic loss of signal in this situation explained. The in vivo and in vitro application of preparation experiments leading to the accurate measurement of T1, T2, diffusion constant, and magnetization transfer characteristics is presented. The implementation of zoom-imaging using spin- and stimulated-echo preparation is described, and 3D in vivo spin-echo zoom images are presented. Simple phantom experiments demonstrating the feasibility of chemical-shift selective and spectroscopic imaging are also given.     

Phase-Encode reordering to minimize errors caused by motion

A new method for suppressing the effects of motion in MR images by reordering the acquisition of k space has been developed. Existing reordering methods suffer from image blurring. The method presented here applies specifically to translation along the phase-encoding direction, in which case it reduces both ghosting and blurring. The method is conceptually similar to a linear phase shift of k space, except that it has the advantage of not corrupting stationary structures inadvertently. The method is intended for anatomic sites in which substantial translational motion occurs along the phase-encoding direction, such as the cranial-caudal motion of the liver and kidneys. The reordering method is motivated from an analysis of factors affecting the severity of motion artifacts. The theory behind the reordering method is described and validated experimentally by imaging a moving phantom.     

Fast, three-dimensional free-breathing MR imaging of myocardial infarction: a feasibility study.

Imaging delayed hyperenhancement of myocardial infarction is most commonly performed using an inversion recovery (IR) prepared 2D breathhold segmented k-space gradient echo (FGRE) sequence. Since only one slice is acquired per breathhold in this technique, 12-16 successive breathholds are required for complete anatomical coverage of the heart. This prolongs the overall scan time and may be exhausting for patients. A navigator-echo gated, free-breathing, 3D FGRE sequence is proposed that can be used to acquire a single slab covering the entire heart with high spatial resolution. The use of a new variable sampling in time (VAST) acquisition scheme enables the entire 3D volume to be acquired in 1.5-2 min, minimizing artifacts from bulk motion and diaphragmatic drift and contrast variations due to contrast media washout.     

Artifacts induced by concomitant magnetic field in fast spin-echo imaging

It has been observed that fast spin-echo (FSE) images with a large field of view (>40 cm) in certain directions exhibit unusual ghosting artifacts that cannot be eliminated with existing ghost removal methods. These artifacts have been related to a higher-order magnetic field perturbation (known as the concomitant field, or Maxwell field) concomitant to the linear i-maging gradient, in accordance with the Maxwell equations Several methods have been developed to eliminate or minimize the effects of the concomitant magnetic field by redesigning the FSE pulse sequences. In the slice-selection direction, the gradient waveforms are made symmetrical about the refocusing RF pulses wherever possible. Surrounding the first refocusing pulse, such symmetry cannot be achieved due to the slice-refocusing gradient, which is often combined with the left crusher. In this case, it is shown how crusher gradients can be reshaped to nullify the phase due to the concomitant field. In the phase-encoding direction, the gradient amplitude is reduced and its duration is prolonged. Artifacts due to the readout gradient are eliminated by reshaping the prephasing lobe, while keeping its area fixed. In all the three directions, the gradient waveforms are adjusted so that they have minimal overlap. Selected methods have been implemented on a clinical scanner, and typically reduce the ghost intensities in phantom and human images by a factor of 3.