Reducing oblique flow effects in interleaved EPI with a centric reordering technique.

Segmented interleaved echo planar imaging offers a fast and efficient approach to magnetic resonance angiography. Unfortunately, this technique is particularly sensitive to oblique flow in the imaging plane. In this work, a mathematical analysis of oblique flow effects for several types of k-space coverage is presented. The conventional linear acquisition scheme, an alternating centric and a nonalternating centric encoding scheme are compared with respect to their flow properties. It is shown both by simulations and imaging experiments that artifacts from oblique in-plane flow are effectively reduced by both centric reordered phase-encoding schemes. The nonalternating centric acquisition scheme is preferred to the alternating centric scheme due to the smoother phase error transition in k-space in the presence of obliquely-angled flow. Magn Reson Med 45:623-629, 2001.     

Band artifacts due to bulk motion.

Band artifacts due to bulk motion were investigated in images acquired with fast gradient echo sequences. A simple analytical calculation shows that the width of the artifacts has a square-root dependence on the velocity of the imaged object, the time taken to acquire each line of k-space and the field of view in the phase-encoding direction. The theory furthermore predicts that the artifact width can be reduced using parallel imaging by a factor equal to the square root of the acceleration parameter. The analysis and results are presented for motion in the phase- and frequency-encoding directions and comparisons are made between sequential and centric ordering. The theory is validated in phantom experiments, in which bulk motion is simulated in a controlled and reproducible manner by rocking the scan table back and forth along the bore axis. Preliminary cardiac studies in healthy human volunteers show that dark bands may be observed in the endocardium in images acquired with nonsegmented fast gradient echo sequences. The fact that the position of the bands changes with the phase-encoding direction suggests that they may be artifacts due to motion of the heart walls during the image acquisition period.     

Reduction in flow artifacts by using interleaved data acquisition in segmented balanced steady-state free precession cardiac MRI.

Balanced steady-state free precession (SSFP) magnetic resonance (MR) imaging is feasible for cine cardiac images because of the high contrast between myocardium and blood pool and robustness to rapid blood flow. Nonetheless, the flow artifacts are often observed because of off-resonance effects and to in-flow effects of the blood flow. Although reshimming the gradients or readjusting the center frequency reduces the artifacts, the technique can be susceptible for respiratory and cardiac motion and operator-dependent. The purpose of this study is to use another MR imaging technique for the reduction in the flow artifacts in the heart: odd-even interleaved data acquisition in segmented balanced SSFP imaging. The flow artifacts in the ventricle, ghost outside the heart, and visualization of the myocardial border were visually compared between sequential and odd-even interleaved k-space data acquisitions in cine balanced SSFP cardiac MR imaging. The odd-even interleaved k-space data acquisition significantly reduced dark flow artifacts in the left ventricle, improved the visualization of the myocardial border, and was easily installed. This imaging technique should be applied to cine segmented balanced SSFP cardiac MR imaging.     

Simulation study for artifacts on Gd-EOB-DTPA-enhanced liver dynamic MR imaging in arterial dominant phase

We performed a simulation for artifacts on liver dynamic MR imaging with the contrast agent gadolinium-ethoxybenzyl (Gd-EOB)-DTPA. The signal enhancement of the image by the contrast agent in the arterial dominant phase was assumed, and the time-enhancement curve was numerically generated. The data in k-space was obtained by the Fourier transform of a liver image. By assuming the scan timing and duration in the time-enhancement curve, the data set of each phase-encoding step in k-space was increased in proportion to the corresponding intensity in the time-enhancement curve. We obtained the simulated image by the Fourier transform of the k-space data, and investigated artifacts in the image. Assuming the use of the centric k-space filling scheme, blurring in the image is found when the scan timing is delayed. When the scan is started in an early timing, we observe the effect of edge enhancement in the image. These artifacts of blurring and edge enhancement are decreased by shortening the scan duration. Assuming the use of the sequential k-space filling scheme, those artifacts are not prominent. The use of the sequential scheme would be effective for the purpose of avoiding the artifacts. It is known that the contrast enhancement would not be sufficient without optimal scan timing; in addition, artifacts should be noted. For basic study of the contrast enhancement and artifacts, our simulation technique based on the time-enhancement curve would be useful.     

A segmented k-space velocity mapping protocol for quantification of renal artery blood plow during breath-holding

Two Important prerequisites for MR velocity mapping of pulsatile motion are synchronization of the sequence execution to the time course of the flow pattern and robustness toward loss of signal in complex flow fields. Synchronization is normally accomplished by using either prospective ECG triggering or so-called retrospective gating. However, if the studied vessel moves periodically in space as a result of respiratory motion, as in the case of renal arteries, a second synchronization with respect to the vessel motion in space may be necessary. One method to overcome this problem is to use the segmented k-space technique, in which the entire data acquisition can be made within a breath-hold by the sampling of several phase-encoding lines within a small time window during each heart cycle. The aim of this study was to investigate the performance of a segmented k-space velocity mapping protocol for renal artery flow determination. The protocol uses 16 phase-encoding lines per heart beat during 16 heart cycles and gives a temporal velocity resolution of 160 msec. Comparison with a conventional ECG-triggered velocity mapping protocol was made in phantoms as well as in volunteers. In our study, both methods showed sufficient robustness toward complex flow in a phantom model. In comparison with the ECG technique, the segmentation technique reduced vessel blurring and pulsatility artifacts caused by respiratory motion, and average flow values obtained in vivo in the left renal artery agreed between the two methods studied. Although presently hampered by a relatively low temporal resolution, velocity mapping with k-space segmentation In combination with breath-holding will benefit from future increased gradient quality, and we assume that in the future the method will become an attractive choice for noninvasive renal artery flow determination.     

Pulsatile flow artifacts in fast magnetization-prepared sequences

Fast magnetization-prepared magnetic resonance imaging sequences allow clinical acquisitions in about 1 second, with the preparation phase providing the desired contrast. Pulsatile flow artifacts, although reduced by rapid acquisition, can degrade image quality. The authors explore the causes of aortic pulsatile flow artifacts in inversion-recovery-prepared acquisitions of the abdomen, taking into consideration various parameters. The flow signal within an 8-mm-thick section was simulated and subsequently Fourier transformed to determine the location and extent of flow artifacts. Results of simulations were validated with abdominal images of human subjects. Recording all encodings within one cardiac cycle reduced pulsatile flow artifacts in nonsegmented acquisitions with sequential phase-encoding order, regardless of the location of magnetization preparation within the cardiac cycle. In segmented acquisitions, however, the sequential order always increased flow artifacts. To reduce the artifacts in short TI acquisitions, the magnetization should be prepared during diastole. In clinical acquisitions, flow artifacts were further reduced by modifying the phase-encoding scheme.     

MR phase-contrast flow measurement with limited spatial resolution in small vessels: value of model-based image analysis.

Magnetic resonance phase-contrast volume flow rate (VFR) measurement with limited resolution in small vessels is subject to two major sources of error: a) partial volume artifacts, causing systematic overestimation of the VFR, and b) errors related to the selection of vessel pixels [region of interest (ROI)], causing large inter-observer and intra-observer variability. Additionally, limited resolution results in Gibbs-ringing around vessels, which adversely affects VFR determination. In this paper, a semi-automatic model-based method is presented that effectively eliminates errors due to both partial volume effect and Gibbs-ringing and also minimizes errors from variability in the ROI selection. The model assumes a parabolic flow profile and cylindrical vessel geometry, incorporates inflow effects, and takes into account the point-spread function of the acquisition. The method automatically estimates maximum velocity, vessel radius, and VFR. The method is validated in phantoms under various conditions and evaluated in vivo. For small vessels with moderately pulsatile flow, it is demonstrated that accurate VFRs and diameter estimates are obtained, virtually independent of the ROI selection, even in vessels covered by just a few pixels. Compared with conventional VFR analysis, both accuracy and reproducibility improve significantly.     

Retrospective motion compensation using variable-density spiral trajectories

PURPOSE: To develop a method of retrospectively correcting for motion artifacts using a variable-density spiral (VDS) trajectory. MATERIALS AND METHODS: Each VDS interleaf was designed to adequately sample the same center region of k-space. This central overlapping region can then be used to measure rigid body motion between the acquisition of each VDS interleaf. By applying appropriate phase shifts and rotations of the k-space data, rigid body motion artifacts can be removed, resulting in images with less motion corruption. RESULTS: Both phantom and volunteer experiments are shown, demonstrating the technique's ability to further reduce artifacts in images acquired with an already motion-resistant acquisition trajectory. Registration accuracy is highly dependent on the trajectory design parameters. This space was explored to find an optimal design of VDS trajectories for motion compensation. CONCLUSION: Using appropriately designed VDS trajectories, residual motion artifacts can be significantly reduced by retrospectively correcting for in-plane rigid body motion. An overlapping region of approximately 8% of the central region of k-space and approximately 70 interleaves were found to be near-optimal parameters for retrospective correction using VDS trajectories.     

Centric phase-encoding order in three-dimensional MP-RAGE sequences: application to abdominal imaging.

Three-dimensional (3D) magnetization-prepared rapid gradient-echo imaging has been proposed as a method for improving signal-to-noise ratio (S/N) and contrast-to-noise ratio (C/N) in rapid abdominal imaging. Originally, a standard sequential phase-encoding order was proposed. In the present study, two approaches to a 3D centric phase-encoding order are presented: (a) application of the two-dimensional (2D) centric order to one of the 3D encoding directions, and (b) an interleaved square spiral order, which is the segmented 3D analog of the 2D centric order. With use of simulation, phantom, and volunteer results, the proposed 3D centric methods are compared in terms of S/N, C/N, and artifacts to the 3D sequential method and 2D magnetization-prepared methods. The second centric approach was found to be superior to the first; however, in general, the 3D technique was found to be inferior to the 2D technique for abdominal imaging because of motion artifact in the 3D image set caused by misregistration among the multiple breath holds required.     

Motion correction using the k-space phase difference of orthogonal acquisitions.

Rigid body translations of an object in MRI create image artifacts along the phase-encode (PE) direction in standard 2DFT imaging. If two images are acquired with swapped PE direction, it is possible to determine and correct for arbitrary in-plane translational interview motions in both images directly from phase differences in the k-space acquisitions by solving a large system of linear equations. For example, if one assumes two N x N 2D acquisitions with in-plane translational interview motion, 4N unknown motions may corrupt the two images, but the phase difference at each point in k-space yields a system of N(2) equations in these 4N unknowns. If the acquisitions have orthogonal PE directions, this highly overdetermined system of equations can be solved to provide the motion records, which in turn can be used to correct the motion artifacts in each image. The theory of this orthogonal k-space phase difference (ORKPHAD) technique is described, and results are presented for synthetic and in vivo motion-corrupted data sets. In all cases, the data showed clear improvement of translation-induced artifacts. These methods do not require special pulse sequences and are theoretically generalizable to partial Fourier imaging and 3D acquisitions.     

Comparison of temporal filtering methods for dynamic contrast MRI myocardial perfusion studies.

Dynamic contrast myocardial perfusion studies may benefit from methods that speed up the acquisition. Unaliasing by Fourier encoding the overlaps using the temporal dimension (UNFOLD), and a similar linear interpolation method have been shown to be effective at reducing the number of phase encodes needed for cardiac wall motion studies by using interleaved sampling and temporal filtering. Here such methods are evaluated in cardiac dynamic contrast studies, with particular regard to the effects of the choice of filter and the interframe motion. Four different filters were evaluated using a motion-free canine study. Full k-space was acquired and then downsampled to allow for a measure of truth. The different filters gave nearly equivalent images and quantitative flow estimates compared to full k-space. The effect of respiratory motion on these schemes was graphically depicted, and the performance of the four temporal filters was evaluated in seven human subjects with respiratory motion present. The four filters provided images of similar quality. However, none of the filters were effective at eliminating motion artifacts. Motion registration methods or motion-free acquisitions may be necessary to make these reduced FOV approaches clinically useful.     

Decoupled automated rotational and translational registration for functional MRI time series data: The dart registration algorithm

A rapid, in-plane image registration algorithm that accurately estimates and corrects for rotational and translational motion is described. This automated, one-pass method achieves its computational efficiency by decoupling the estimation of rotation and translation, allowing the application of rapid cross-correlation and cross-spectrum techniques for the determination of displacement parameters. k-space regridding and modulation techniques are used for image correction as alternatives to linear interpolation. The performance of this method was analyzed with simulations and echo-planar image data from both phantoms and human subjects. The processing time for image registration on a Hewlett-Packard 735/125 is 7.5 s for a 128 × 128 pixel image and 1.7 s for a 64 × 64 pixel image. Imaging phantom data demonstrate the accuracy of the method (mean rotational error, ?0.09°; standard deviation = 0.17°; range, ?0.44° to + 0.31°; mean translational error = ?0.035 pixels; standard deviation = 0.054 pixels; range, ?0.16 to + 0.06 pixels). Registered human functional imaging data demonstrate a significant reduction in motion artifacts such as linear trends in pixel time series and activation artifacts due to stimulus-correlated motion. The advantages of this technique are its noniterative one-pass nature, the reduction in image degradation as compared to previous methods, and the speed of computation.     

Augmented generalized SENSE reconstruction to correct for rigid body motion.

The correction of motion artifacts continues to be a significant problem in MRI. In the case of uncooperative patients, such as children, or patients who are unable to remain stationary, the accurate determination and correction of motion artifacts becomes a very important prerequisite for achieving good image quality. The application of conventional motion-correction strategies often produces inconsistencies in k-space data. As a result, significant residual artifacts can persist. In this work a formalism is introduced for parallel imaging in the presence of motion. The proposed method can improve overall image quality because it diminishes k-space inconsistencies by exploiting the complementary image encoding capacity of individual receiver coils. Specifically, an augmented version of an iterative SENSE reconstruction is used as a means of synthesizing the missing data in k-space. Motion is determined from low-resolution navigator images that are coregistered by an automatic registration routine. Navigator data can be derived from self-navigating k-space trajectories or in combination with other navigation schemes that estimate patient motion. This correction method is demonstrated by interleaved spiral images collected from volunteers. Conventional spiral scans and scans corrected with proposed techniques are shown, and the results illustrate the capacity of this new correction approach.     

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.     

Strategies to improve contrast in turboFLASH imaging: reordered phase encoding and k-space segmentation.

TurboFLASH (fast low-angle shot) sequences enable the acquisition of an image in a fraction of a second. However, unique to T1-weighted ultrafast imaging, the magnetization variation during image acquisition can produce artifacts along the phase-encoding direction. In this study, the signal behavior and nature of these artifacts were analyzed with various acquisition schemes to improve image contrast. The magnetization variation during image acquisition and its filtering effect on the image were simulated for three different approaches to T1-weighted turboFLASH imaging: standard turboFLASH with (a) monotonically ascending phase-encoding steps, (b) reordered phase encoding, and (c) k-space segmentation. Each of the modified data acquisition schemes has advantages. However, for subsecond imaging, reordered phase encoding produced improved image contrast over that of standard turboFLASH, and segmented k-space imaging gave superior tissue contrast compared with that of both standard and reordered turboFLASH, with imaging time that permits breath-hold studies.     

Multiecho segmented EPI with z-shimmed background gradient compensation (MESBAC) pulse sequence for fMRI.

A MultiEcho Segmented EPI with z-shimmed BAckground gradient Compensation (MESBAC) pulse sequence is proposed and validated for functional MRI (fMRI) study in regions suffering from severe susceptibility artifacts. This sequence provides an effective tradeoff between spatial and temporal resolution and reduces image distortion and signal dropout. The blood oxygenation level-dependent (BOLD)-weighted fMRI signal can be reliably obtained in the region of the orbitofrontal cortex (OFC). To overcome physiological motion artifacts during prolonged multisegment EPI acquisition, two sets of navigator echoes were acquired in both the readout and phase-encoding directions. Ghost artifacts generally produced by single-shot EPI acquisition were eliminated by separately placing the even and odd echoes in different k-space trajectories. Unlike most z-shim methods that focus on increasing temporal resolution for event-related functional brain mapping, the MESBAC sequence simultaneously addresses problems of image distortion and signal dropout while maintaining sufficient temporal resolution. The MESBAC sequence will be particularly useful for pharmacological and affective fMRI studies in brain regions such as the OFC, nucleus accumbens, amygdala, parahippocampus, etc.     

Floating navigator echo (FNAV) for in-plane 2D translational motion estimation.

A modification of the classical navigator echo (NAV) technique is presented whereby both 2D translational motion components are computed from a single navigator line. Instead of acquiring the NAV at the center of the k-space, a kx line is acquired off-center in the phase-encoding (ky) direction as a floating NAV (FNAV). It is shown that the translational motion in both the readout and phase-encoding directions can be computed from this line. The algorithm used is described in detail and verified experimentally. The new technique can be readily implemented to replace classic NAV in MRI sequences, with little to no additional cost or complexity. The new method can help suppress 2D translational motion and provide more accurate motion estimates for other motion-suppression techniques, such as the diminishing variance algorithm.     

Inversion recovery radial MRI with interleaved projection sets.

The radial trajectory has found applications in cardiac imaging because of its resilience to undersampling and motion artifacts. Recent work has shown that interleaved and weighted radial imaging can produce images with multiple contrasts from a single data set. This feature was investigated for inversion recovery imaging of scar using a radial technique. The 2D radial imaging method was modified to acquire quadruply interleaved projection sets within each acquisition window of the cardiac cycle. These data were reconstructed using k-space weightings that used a smaller segment of the acquisition window for the central k-space data, the determinant of image contrast. This method generates four images with different T1 weightings. The novel approach was compared with noninterleaved radial imaging, interleaved radial without weightings, and Cartesian imaging in simulations, phantoms, and seven subjects with clinical myocardial infarction. The results show that during a typical acquisition window after an inversion pulse, magnetization changes rapidly. The interleaved acquisition provided better image quality than the noninterleaved radial acquisition. Interleaving with weighting provided better quality when the inversion time (TI) was shorter than optimal; otherwise, interleaving without weighting was superior. These methods enable a radial trajectory to be employed in conjunction with preparation pulses for viability imaging.     

Effective blood signal suppression using double inversion-recovery and slice reordering for multislice fast spin-echo MRI and its application in simultaneous proton density and T2 weighted imaging

PURPOSE: To design a multislice double inversion-recovery fast spin-echo (FSE) sequence, with k-space reordered by inversion time at slice position (KRISP) technique, to produce black-blood vessel wall magnetic resonance imaging (MRI). MATERIALS AND METHODS: In this sequence, central k-space sampling for each slice is required at inversion time (TI) of the blood signal. To fill the entire k-space, the peripheral lines are obtained less or greater the TI and using a rotating slice order. Blood flow signal suppression was first evaluated using a phantom. Simulation studies were used to investigate FSE image quality. The final sequence was then applied to the rabbit abdominal aorta MRI at 4.7 T. RESULTS: In the flow phantom study, artifacts from slow-flowing water were substantially reduced by the KRISP technique; residual water spins were dephased by the strong phase-encoding gradient required for peripheral k-space. These dephased spins flowed into the slice plane where the center of k-space was being acquired at the TI of the flowing water signal. Multislice black-blood MR images were successfully obtained in the rabbit abdomen using the sequence with the k-trajectory optimized by the simulation study. CONCLUSION: The KRISP technique was effective both in multislice double inversion-recovery FSE and in blood signal suppression.     

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.