Off-resonance image artifacts in interleaved-EPI and GRASE pulse sequences

Echo-time shifting (ETS) is used in GRASE and interleaved-EPI sequences to improve the phase evolutions for off-resonance signal sources. However, even with ETS the phase evolutions still exhibit discontinuities. In this work, we extend previous studies of ETS by quantitatively evaluating the magnitude and form of the image artifacts that result from these phase discontinuities. The functional form of the phase evolution is used to derive the general conditions under which artifacts are expected. The artifacts for two sequence structures are then evaluated as a function of off-resonance frequency and data sampling period by calculating point spread functions and simulated images. It was found that even when ETS is used to improve the phase evolutions, periodic phase discontinuities may degrade image quality by producing ghosting artifacts of edges. These artifacts are similar to those that commonly occur with periodic motion. From our results recommendations are derived for limiting the ghosting artifacts.     

Verification and evaluation of internal flow and motion. True magnetic resonance imaging by the phase gradient modulation method

We report qualitative and quantitative evaluation and verification studies of the bipolar phase gradient modulation method for true MR imaging of internal flow and motion velocities. Velocity encoding modulations provide speed-of-motion and direction-sensitive images using special phase-sensitive reconstructions. True motion MR imaging does not depend upon subject parameters, T1 or T2, nor upon selective active-volume time-of-flight calculations, nor is it limited strictly to fluid-flow velocities. Conventional MR sequences often induce strong accidental phase gradient modulations that can cause severe artifacts in conventional MR scans and limit the useful sensitivities of true motion MR. Multiple steps of velocity encoding allow resolution of separate elements of the velocity spectrum, and enable suppression of all such phase-artifact difficulties. Some view-to-view phase inconsistencies are intrinsic to the subject being scanned, e.g., strong motion variations during the heart cycle; limitations due to such effects require external modifications in the scanning, such as cardiac gating. Since conventional density information remains in the data, independent of velocity encoding modulations, we suggest a multiple encoding sequence and saving the MR raw data. These evaluations and verifications demonstrate exciting potential in clinical application for the phase gradient modulation method of true flow and motion MR imaging.     

High spatial resolution EPI using an odd number of interleaves.

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

Experiments for two MR imaging theories of motion phase sensitivity

Two theories of motion-sensitive phase shifts in magnetic resonance (MR) imaging result in different mathematical predictions of the observed effects of gradient modulation-induced motion artifacts. The consequences are critical for gradient waveform designed to minimize motion artifact contaminations from time-dependent motion sensitivity. To resolve this discrepancy with a test case (the monopolar waveform of a commonly used, discretely pulsed encoding phase gradient), computer integration of the fundamental Bloch equations for MR imaging with motion was performed. Simulation images for constant and erratic motion showed almost complete agreement with the predictions of the transport integral solutions for motion phase sensitivity; the artifact was solely time-of-flight oblique flow misregistration. Conventional method-of-moments gradient moment nulling compensations produced greater motion artifacts in experiments than did use of no waveform compensation at all. Transport equation solutions implied second-integral zeroing instead; these modifications eliminated the artifacts.     

Artifacts and pitfalls in diffusion MRI

Although over the last 20 years diffusion MRI has become an established technique with a great impact on health care and neurosciences, like any other MRI technique it remains subject to artifacts and pitfalls. In addition to common MRI artifacts, there are specific problems that one may encounter when using MRI scanner gradient hardware for diffusion MRI, especially in terms of eddy currents and sensitivity to motion. In this article we review those artifacts and pitfalls on a qualitative basis, and introduce possible strategies that have been developed to mitigate or overcome them.     

Flow compensation in balanced SSFP sequences.

In balanced steady-state free precession (b-SSFP) sequences, uncompensated first-order moments of encoding gradients induce a nonconstant phase evolution for moving spins within the excitation train, resulting in signal loss and image artifacts. To restore these flow-related phase perturbations, "pairing" of consecutive phase-encoding (PE) steps is compared with a fully flow-compensated sequence using compensating gradient waveforms along all three encoding directions. In volunteer studies, the quality of images acquired with the "pairing" technique was comparable to that of images obtained with the fully flow-compensated technique, regardless of the selected view-ordering scheme used for data acquisition. Nevertheless, the results of phantom experiments indicate that the pairing technique becomes ineffective at flow velocities exceeding roughly 0.5-1 m/s. Consequently, the additional scan time required to null the first gradient moments in a flow-compensated b-SSFP sequence makes the "pairing" technique preferable for applications in which slow to moderate flow velocities can be expected.     

Motion artifact suppression technique (MAST) for cranial MR imaging: superiority over cardiac gating for reducing phase-shift artifacts

A motion artifact suppression technique (MAST) has recently been developed that significantly reduces motion artifacts in conventional 2DFT imaging. The technique utilizes modifications of slice-select and read gradient waveforms to eliminate velocity, acceleration, and pulsatility phase shifts that occur between the 90 degrees pulse and data collection. T2-weighted cranial MAST images were rated visually superior to cardiac-gated images by two experienced neuroradiologists in 13 of 15 cases and in 14 of 15 cases, respectively (p less than 0.001). Quantitative signal-to-noise comparisons for six brain regions in each patient confirmed the visually apparent superiority of MAST, especially for imaging the brainstem and subarachnoid cisterns (p = 0.02). Improvements in signal-to-noise ratios of up to 43% were obtained when using MAST instead of cardiac gating. MAST or a similar technique has the potential to render cardiac gating obsolete as a method for reducing flow-related artifacts in cranial MR imaging.     

Body MRI artifacts in clinical practice: A physicist's and radiologist's perspective

The high information content of MRI exams brings with it unintended effects, which we call artifacts. The purpose of this review is to promote understanding of these artifacts, so they can be prevented or properly interpreted to optimize diagnostic effectiveness. We begin by addressing static magnetic field uniformity, which is essential for many techniques, such as fat saturation. Eddy currents, resulting from imperfect gradient pulses, are especially problematic for new techniques that depend on high performance gradient switching. Nonuniformity of the transmit radiofrequency system constitutes another source of artifacts, which are increasingly important as magnetic field strength increases. Defects in the receive portion of the radiofrequency system have become a more complex source of problems as the number of radiofrequency coils, and the sophistication of the analysis of their received signals, has increased. Unwanted signals and noise spikes have many causes, often manifesting as zipper or banding artifacts. These image alterations become particularly severe and complex when they are combined with aliasing effects. Aliasing is one of several phenomena addressed in our final section, on artifacts that derive from encoding the MR signals to produce images, also including those related to parallel imaging, chemical shift, motion, and image subtraction. J. Magn. Reson. Imaging 2013;38:269–287. © 2013 Wiley Periodicals, Inc.     

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.     

Compensation for effects of linear motion in MR imaging

Various compensation methods for different types of motion during MR image acquisition have been proposed. Presented here is a postprocessing scheme for eliminating artifacts due to linear, intra-slice motion of known velocity. The data for each phase encoding or "view" acquired from a moving object are shown to differ from those which would be measured from the stationary object by a phase factor which depends on the object's displacement from a reference point. This derivation is then used to propose a correction scheme for linear motion in which all phase encodings measured at different positions of the moving object are collapsed onto the same reference position. After subsequent reconstruction, the object appears perfectly "focused." By selection of different reference positions, the method permits positioning of the compensated object as desired within the field of view of the image. This property allows the method to be extended to create sequences of corrected images with smooth object motion between frames of the sequence. The basic correction scheme and its variations were tested experimentally in phantom studies with velocities as large as 8 cm/s.     

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.     

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.     

Reduction of flow artifacts in NMR diffusion imaging using view-angle tilted line-integral projection reconstruction

Most of the diffusion imaging techniques employ strong diffusion gradient pulses of long duration in order to achieve appreciable signal attenuation through the diffusion effect. However, these strong and long gradient pulses make the resultant images extremely sensitive to the motion or flow of the object. Fourier imaging, with which most of the current NMR imaging is performed, is especially sensitive to the fluctuating flow and the images are usually obscured by severe flow artifacts smeared in the phase-encoding direction. In this paper, we have proposed a diffusion imaging technique which reduces the flow artifacts by use of the line-integral projection reconstruction (LPR) imaging method. Furthermore, the inhomogeneity artifacts expected to occur in LPR imaging have been corrected by application of the view-angle tilting technique. The pulse sequence of the view-angle tilted LPR diffusion imaging is designed in such a way that it works for both isotropic and anisotropic diffusion. Experimental results are presented along with the experimental procedures.     

A preliminary study of the effects of trigger timing on diffusion tensor imaging of the human spinal cord

BACKGROUND AND PURPOSE: Diffusion tensor and diffusion-weighted spinal cord imaging remain relatively unexplored techniques despite demonstrations that such images can be obtained and may yield clinically relevant findings. In this study, we examined the temporal dynamics of spinal cord motion and their impact on diffusion tensor image quality. METHODS: Four healthy volunteers underwent phase contrast-based velocity mapping and segmented echo-planar diffusion tensor scans of the cervical spinal cord. Regions of interest in the cord were used to identify the temporal patterns of motion. The delay of data acquisition after the cardiac trigger was varied to correspond to either quiescence or motion of the cord. RESULTS: The cervical spinal cord consistently displayed maximal velocities in the range of 0.5 cm/s and accelerations of up to 25 cm/s(2). In both these respects, the cervical cord values were greater than those of the medulla. Despite this pronounced motion, approximately 40% of the cardiac cycle can be described as relatively calm, with absolute velocities and accelerations less than 20% of the maximum values. Confining image acquisition to this window reduced ghosting artifacts and increased the consistency with which the dominant direction of diffusion was along the rostral-caudal axis in both gray and white matter of the spine. Preliminary clinical application and fiber tracking in pathologic cases was feasible, and alterations of the diffusion properties by multiple sclerosis lesions, tumor, and syringomyelia were seen. CONCLUSIONS: Acquiring DTI data during the quiescent phase of spinal cord motion can reduce ghosting artifacts and improve fiber tracking.     

Cardiac cine parallel imaging on a 0.7T open system.

BACKGROUND: Parallel imaging can be applied to cardiac imaging with a cylindrical MRI (magnetic resonance imaging) apparatus. Studies of open MRI, however, are few. This study sought to achieve cardiac cine parallel imaging (or RAPID, for "rapid acquisition through parallel imaging design") with an open 0.7T MRI apparatus. MATERIALS AND METHODS: Imaging time was shortened in all slice directions with the use of a dedicated four-channel RF receiving coil comprising solenoid coils and butterfly coils. Coil shape was designed through an RF-coil simulation that considered biological load. The auto-calibration of a 0.7T open MRI apparatus incorporated a modified image-domain reconstruction algorithm. Cine images were obtained with a BASG, or balanced SARGE (steady-state acquisition with rewound gradient echo), sequence. Image quality was evaluated with cylindrical phantoms and five healthy volunteers. RESULTS: Multi-slice phantom images showed no visible artifacts. Cine images taken under breath-hold with an acceleration factor of two were evaluated carefully. With auto-calibration, the images revealed no visible unfolded artifacts or motion artifacts. RAPID thus improved the acquisition speed, time resolution, and spatial resolution of short-axis, long-axis, and four-chamber images. CONCLUSION: The use of a dedicated RF coil enabled cardiac cine RAPID to be performed with an open MRI apparatus.     

Concomitant gradient field effects in balanced steady-state free precession.

Linear magnetic field gradients spatially encode the image information in MRI. Concomitant gradients are undesired magnetic fields that accompany the desired gradients and occur as an unavoidable consequence of Maxwell's equations. These concomitant gradients result in undesired phase accumulation during MRI scans. Balanced steady-state free precession (bSSFP) is a rapid imaging method that is known to suffer from signal dropout from off-resonance phase accrual. In this work it is shown that concomitant gradient phase accrual can induce signal dropout in bSSFP. The spatial variation of the concomitant phase is explored and shown to be a function of gradient strength, slice orientation, phase-encoding (PE) direction, distance from isocenter, and main field strength. The effect on the imaging signal level was simulated and then verified in phantom and in vivo experiments. The nearest signal-loss artifacts occurred in scans that were offset from isocenter along the z direction with a transverse readout. Methods for eliminating these artifacts, such as applying compensatory frequency or shim offsets, are demonstrated. Concomitant gradient artifacts can occur at 1.5T, particularly in high-resolution scans or with additional main field inhomogeneity. These artifacts will occur closer to isocenter at field strengths below 1.5T because concomitant gradients are inversely proportional to the main field strength.     

Magnetic resonance imaging artifacts: mechanism and clinical significance

Many types of artifacts may occur in magnetic resonance imaging. These artifacts may be related to extrinsic factors such as patient motion or metallic artifacts; they may be due specifically to the MR system such as power gradient drop off and chemical shift artifacts; they may occur as a consequence of general image processing techniques, as in the case of truncation artifacts and aliasing. Change in patient position, pulse sequence, or other imaging variables may improve some artifacts. Although reduction of some artifacts may require a service engineer, the radiologist has the responsibility to recognize MR imaging problems. The radiologist's knowledge of MR imaging artifacts is important to the continued maintenance of high image quality and is essential if one is to avoid confusing artifactual appearances with pathology.     

Encoding of electrophysiology and other signals in MR images

PURPOSE: To develop a gradient insensitive, generic technique for recording of non-MR signals by use of surplus scanner bandwidth. MATERIALS AND METHODS: Relatively simple battery driven hardware is used to transform one or more signals into radio waves detectable by the MR scanner. Similar to the "magstripe" technique used for encoding of soundtracks in motion pictures, the electrical signals are in this way encoded as artifacts appearing in the MR images or spectra outside the region of interest. The encoded signals are subsequently reconstructed from the signal recorded by the scanner. RESULTS: Electrophysiological (EP) eye and heart muscular recording (electrooculography [EOG] and electrocardiography [ECG]) during fast echo planar imaging (EPI) is demonstrated with an expandable, modular 8-channel prototype implementation. The gradient artifacts that would normally be dominating EOG are largely eliminated. CONCLUSION: The method provides relatively inexpensive sampling with inherent microsecond synchronization and it reduces gradient artifacts in physiological recordings significantly. When oversampling is employed, the method is compatible with all MR reconstruction and postprocessing techniques.