Artifact reduction in moving-table acquisitions using parallel imaging and multiple averages.

Two-dimensional (2D) axial continuously-moving-table imaging has to deal with artifacts due to gradient nonlinearity and breathing motion, and has to provide the highest scan efficiency. Parallel imaging techniques (e.g., generalized autocalibrating partially parallel acquisition GRAPPA)) are used to reduce such artifacts and avoid ghosting artifacts. The latter occur in T(2)-weighted multi-spin-echo (SE) acquisitions that omit an additional excitation prior to imaging scans for presaturation purposes. Multiple images are reconstructed from subdivisions of a fully sampled k-space data set, each of which is acquired in a single SE train. These images are then averaged. GRAPPA coil weights are estimated without additional measurements. Compared to conventional image reconstruction, inconsistencies between different subsets of k-space induce less artifacts when each k-space part is reconstructed separately and the multiple images are averaged afterwards. These inconsistencies may lead to inaccurate GRAPPA coil weights using the proposed intrinsic GRAPPA calibration. It is shown that aliasing artifacts in single images are canceled out after averaging. Phantom and in vivo studies demonstrate the benefit of the proposed reconstruction scheme for free-breathing axial continuously-moving-table imaging using fast multi-SE sequences.     

Off-centered spiral trajectories.

The quality of spiral images depends on the accuracy of the k-space sampling locations. Although newer gradient systems can provide more accurate gradient waveforms, the sampling positions can be significantly distorted by timing misregistration between data acquisition and gradient systems. Even after the timing of data acquisition is tuned, minor residual errors can still cause shading artifacts which are problematic for quantitative MR applications such as phase-contrast flow quantitation. These timing errors can ideally be corrected by measuring the actual k-space trajectory, but trajectory measurement requires additional data acquisition and scan time. Therefore, off-centered spiral trajectories which are more robust against timing errors are proposed and applied to the phase-contrast method. The new trajectories turn shading artifacts into a slowly varying linear phase in reconstructed images without affecting the magnitude of images.     

Improvement of spiral MRI with the measured k-space trajectory

The k-space trajectory of a spiral imaging sequence was measured with a self-encoding technique. The image quality improved dramatically when reconstructed with the measured k-space trajectory. There were substantial artifacts in images reconstructed with the derived k-space trajectory under the assumption of gradient system linearity. The results indicated the non-linearity of the gradient system and the effectiveness of the correction technique.     

Varying kernel-extent gridding reconstruction for undersampled variable-density spirals.

Nonuniform, non-Cartesian k-space trajectories enable fast scanning with reduced motion and flow artifacts. In such cases, the data are usually convolved with a kernel and resampled onto a Cartesian grid before reconstruction. For trajectories such as undersampled variable-density spirals, the mainlobe width of the kernel for undersampled high spatial frequencies has to be larger to limit the amount of aliasing energy. Continuously varying the kernel extent is time consuming. By dividing k-space into several annuli and using appropriate mainlobe widths for each, the aliasing energy and noise can be reduced at the expense of lower resolution towards the edge of the field of view (FOV). Resolution can instead be preserved at the center of the FOV, which is expected to be free of artifacts, without any artifact reduction. The image reconstructed from each annulus can be deapodized separately. The method can be applied to most k-space trajectories used in MRI.     

Direct parallel image reconstructions for spiral trajectories using GRAPPA.

The use of spiral trajectories is an efficient way to cover a desired k-space partition in magnetic resonance imaging (MRI). Compared to conventional Cartesian k-space sampling, it allows faster acquisitions and results in a slight reduction of the high gradient demand in fast dynamic scans, such as in functional MRI (fMRI). However, spiral images are more susceptible to off-resonance effects that cause blurring artifacts and distortions of the point-spread function (PSF), and thereby degrade the image quality. Since off-resonance effects scale with the readout duration, the respective artifacts can be reduced by shortening the readout trajectory. Multishot experiments represent one approach to reduce these artifacts in spiral imaging, but result in longer scan times and potentially increased flow and motion artifacts. Parallel imaging methods are another promising approach to improve image quality through an increase in the acquisition speed. However, non-Cartesian parallel image reconstructions are known to be computationally time-consuming, which is prohibitive for clinical applications. In this study a new and fast approach for parallel image reconstructions for spiral imaging based on the generalized autocalibrating partially parallel acquisitions (GRAPPA) methodology is presented. With this approach the computational burden is reduced such that it becomes comparable to that needed in accelerated Cartesian procedures. The respective spiral images with two- to eightfold acceleration clearly benefit from the advantages of parallel imaging, such as enabling parallel MRI single-shot spiral imaging with the off-resonance behavior of multishot acquisitions.     

Time-resolved contrast-enhanced imaging with isotropic resolution and broad coverage using an undersampled 3D projection trajectory.

Time-resolved contrast-enhanced 3D MR angiography (MRA) methods have gained in popularity but are still limited by the tradeoff between spatial and temporal resolution. A method is presented that greatly reduces this tradeoff by employing undersampled 3D projection reconstruction trajectories. The variable density k-space sampling intrinsic to this sequence is combined with temporal k-space interpolation to provide time frames as short as 4 s. This time resolution reduces the need for exact contrast timing while also providing dynamic information. Spatial resolution is determined primarily by the projection readout resolution and is thus isotropic across the FOV, which is also isotropic. Although undersampling the outer regions of k-space introduces aliased energy into the image, which may compromise resolution, this is not a limiting factor in high-contrast applications such as MRA. Results from phantom and volunteer studies are presented demonstrating isotropic resolution, broad coverage with an isotropic field of view (FOV), minimal projection reconstruction artifacts, and temporal information. In one application, a single breath-hold exam covering the entire pulmonary vasculature generates high-resolution, isotropic imaging volumes depicting the bolus passage.     

Single TrAjectory Radial (STAR) imaging.

The number of MRI applications that use radial k-space data acquisition have been increasing because of their inherent robustness to motion-induced reconstruction image artifacts relative to Cartesian acquisition methods. However, images reconstructed from radial data are more prone to image degrading effects due to magnetic field inhomogeneities than images made from Cartesian data. Presented here is a method for acquiring several radial k-space data lines in one trajectory, the Single TrAjectory Radial, or STAR method, that is a variation of radial EPI. The STAR method allows for angular oversampling without the increase in imaging time that occurs with angularly oversampled single line imaging. It is shown that such oversampling potentially reduces the image degrading effect of magnetic field inhomogeneities so that the motion robust features of radial imaging may be realized in a segmented EPI approach.     

Reconstruction of undersampled non-Cartesian data sets using pseudo-Cartesian GRAPPA in conjunction with GROG.

Most k-space-based parallel imaging reconstruction techniques, such as Generalized Autocalibrating Partially Parallel Acquisitions (GRAPPA), necessitate the acquisition of regularly sampled Cartesian k-space data to reconstruct a nonaliased image efficiently. However, non-Cartesian sampling schemes offer some inherent advantages to the user due to their better coverage of the center of k-space and faster acquisition times. On the other hand, these sampling schemes have the disadvantage that the points acquired generally do not lie on a grid and have complex k-space sampling patterns. Thus, the extension of Cartesian GRAPPA to non-Cartesian sequences is nontrivial. This study introduces a simple, novel method for performing Cartesian GRAPPA reconstructions on undersampled non-Cartesian k-space data gridded using GROG (GRAPPA Operator Gridding) to arrive at a nonaliased image. Because the undersampled non-Cartesian data cannot be reconstructed using a single GRAPPA kernel, several Cartesian patterns are selected for the reconstruction. This flexibility in terms of both the appearance and number of patterns allows this pseudo-Cartesian GRAPPA to be used with undersampled data sets acquired with any non-Cartesian trajectory. The successful implementation of the reconstruction algorithm using several different trajectories, including radial, rosette, spiral, one-dimensional non-Cartesian, and zig-zag trajectories, is demonstrated.     

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.     

Consistent non-cartesian off-axis MRI quality: calibrating and removing multiple sources of demodulation phase errors.

The consistency of off-axis MRI with non-Cartesian sequences across a large number of scanners is highly variable. Improper timing alignment of the gradient fields, data acquisition system, and real-time frequency demodulation reference signal, which are necessary for off-axis imaging, is an important source of this variability. In addition, eddy currents and anisotropic gradient delays cause deviations in k-space trajectories that in turn make the demodulation reference signals inaccurate. A method is presented to quickly measure the timing error in the frequency demodulation reference signal and separate it from anisotropic gradient delays. k-Space deviations, as measured with a previous gradient calibration technique, are shown to be a second source of demodulation phase errors that degrade image quality. Using the timing delay and k-space deviations, a retrospective phase correction is applied to each k-space sample before the data are regridded during reconstruction. The timing delays of four MR scanners were measured to be 4.2-7.5 micros below the manufacturer's suggested delay. Significant degradation in 3D radial (3D projection reconstruction (PR)) knee and breast images are retrospectively corrected while a partial prospective correction is applied for spiral imaging. The method allows for more consistent performance of non-Cartesian sequences across multiple scanners without operator intervention.     

Improved k-space trajectory measurement with signal shifting.

Various techniques for k-space trajectory measurement have been described in the literature. Self-encoding gradient techniques are time-consuming due to the high number of phase-encoding steps needed. The approach with localized slices is faster, but its use apparently has not been reported in the context of high spatial resolution experiments. Signals associated with high k-space frequencies may then reach low or even zero values, and this may result in errors in the estimate of the trajectories at the k-space periphery. To overcome this problem without increasing the measurement duration of the localized slice method too much, a new approach is proposed in which an addition dephasing gradient applied prior to the gradient to be measured shifts the signal maximum.     

Double average parallel steady-state free precession imaging: optimized eddy current and transient oscillation compensation.

Balanced steady-state free precession (SSFP) imaging is sensitive to off-resonance effects, which can lead to considerable artifacts during a transient phase following magnetization preparation or steady-state interruption. In addition, nonlinear k-space encoding is required if contrast-relevant k-space regions need to be acquired at specific delays following magnetization preparation or for transient artifact reduction in cardiac-gated k-space segmented CINE imaging. Such trajectories are problematic for balanced SSFP imaging due to nonconstant eddy current effects and resulting disruption of the steady state.In this work, a novel acquisition strategy for balanced SSFP imaging is presented that utilizes scan time reduction by parallel imaging for optimized "double average" eddy current compensation and artifact reduction during the transient phase following steady-state storage and magnetization preparation. Double average parallel SSFP imaging was applied to k-space segmented CINE SSFP tagging as well as nongated centrically encoded SSFP imaging. Phantom and human studies exhibit substantial reduction in steady-state storage and eddy current artifacts while maintaining spatial resolution, signal-to-noise ratio, and similar total scan time of a standard SSFP acquisition. The proposed technique can easily be extended to other acquisition schemes that would benefit from nonlinear reordering schemes and/or rely on interruption of the balanced SSFP steady state.     

Optimal gradient waveform design for projection imaging and projection reconstruction echoplanar spectroscopic imaging.

The use of modulated B0 projection gradient waveforms is proposed for magnetic resonance imaging (MRI) and magnetic resonance spectroscopic imaging (MRSI) to shape the k-space sampling density function to match the profile of an applied reconstruction window function. This allows for a time-efficient means of maximizing the signal-to-noise ratio when, for example, a low-pass spatial filter, designed to reduce k-space truncation artifacts and corresponding point-spread function sidelobe energies, is implemented. Both the two-dimensional (2D) and 3D cases are investigated. To create the projection gradient waveforms, a design method is developed that uses nonlinear constrained optimization (NLCO) to minimize the variance of the reconstruction noise. The design is subject to both equality and inequality constraints, which include the maximum gradient magnitude and slew rate. It is shown that NLCO can also be applied to twisting the projection trajectories for purposes of reducing the data acquisition time while still maintaining the desired sampling density. Applications to 1H MRSI are investigated via simulations. Advantages and limitations of the new sampling schemes are discussed.     

Advances in sensitivity encoding with arbitrary k-space trajectories.

New, efficient reconstruction procedures are proposed for sensitivity encoding (SENSE) with arbitrary k-space trajectories. The presented methods combine gridding principles with so-called conjugate-gradient iteration. In this fashion, the bulk of the work of reconstruction can be performed by fast Fourier transform (FFT), reducing the complexity of data processing to the same order of magnitude as in conventional gridding reconstruction. Using the proposed method, SENSE becomes practical with nonstandard k-space trajectories, enabling considerable scan time reduction with respect to mere gradient encoding. This is illustrated by imaging simulations with spiral, radial, and random k-space patterns. Simulations were also used for investigating the convergence behavior of the proposed algorithm and its dependence on the factor by which gradient encoding is reduced. The in vivo feasibility of non-Cartesian SENSE imaging with iterative reconstruction is demonstrated by examples of brain and cardiac imaging using spiral trajectories. In brain imaging with six receiver coils, the number of spiral interleaves was reduced by factors ranging from 2 to 6. In cardiac real-time imaging with four coils, spiral SENSE permitted reducing the scan time per image from 112 ms to 56 ms, thus doubling the frame-rate.     

Bunched phase encoding (BPE): a new fast data acquisition method in MRI.

A new fast data acquisition method, "Bunched Phase Encoding" (BPE), is presented. In conventional rectilinear data acquisition, only a readout gradient (and no phase encoding gradient) is applied when k-space data are acquired. Reduction of the number of phase encoding lines by increasing the phase encoding step size often leads to aliasing artifacts. Papoulis's generalized sampling theory asserts that in some cases aliasing artifact-free signals can be reconstructed even if the Nyquist criterion is violated in some regions of the Fourier domain. In this study, Papoulis's theoretical construct is exploited to reduce the number of acquired phase encoding lines. To achieve this, k-space data are sampled along a "zigzag" trajectory during each readout; samples are acquired at a sampling frequency higher than that of the normal rectilinear acquisition. The total number of TR cycles and, hence, the total scan time can be reduced. The resultant signal-to-noise ratio (SNR) often varies across the reconstructed image when using the BPE technique, and the image SNR depends on the reconstruction method. This work is comparable to a gradient based version of parallel imaging. Evidence suggests it may serve as the basis for new opportunities for fast data acquisition in MRI.     

Spectroscopic imaging using concentrically circular echo-planar trajectories in vivo

An alternative to the standard echo-planar spectroscopic imaging technique is presented, spectroscopic imaging using concentrically circular echo-planar trajectories (SI-CONCEPT). In contrast to the conventional chemical shift imaging data, the sampled data from each set of concentric rings were regridded into Cartesian space. Usage of concentric k-space trajectories has the advantage of requiring significantly reduced slew rates than echo-planar spectroscopic imaging, allowing for the collection of higher spectral bandwidths and opening the door for high-bandwidth echo-planar styled spectroscopic imaging at higher magnetic fields. Before two-dimensional spatial and one-dimensional spectral encoding, the volume of interest was localized using the standard point-resolved spectroscopy sequence. The feasibility of using concentric k-space trajectories is demonstrated, and the spatial profiles and representative spectra are compared with the standard echo-planar spectroscopic imaging technique in a gray matter phantom containing metabolites at physiological concentrations and healthy human brain in vivo. The symmetric nature of the concentric circles also reduces the number of required excitations for a given resolution by a factor of two. Possible artifacts and limitations are discussed.     

Direct imaging of magnetic field gradients by group spin-echo selection.

A new image processing method for single-echo gradient echo imaging is presented which extracts local phase gradient information by k-space filtering instead of by phased reconstruction and spatial differentiation. It is shown that local phase gradient directions and semiquantitative local phase gradient magnitudes can be directly measured, even in regions where phased image reconstruction suffers from multiple phase foldovers due to strong phase modulations. The directional information thus obtained can be used as a reference to identify and correct phase modulation foldovers in phase maps which may be computed from the same raw data. The method is applied here to measure static magnetic field gradients and illustrates fundamental k-space signal properties of gradient echo imaging. Based on this concept, image artifacts caused by conventional strong k-space filtering in gradient echo imaging are discussed.     

Generalized self-navigated motion detection technique: Preliminary investigation in abdominal imaging.

Patient motion remains a primary obstacle to diagnostic image quality, especially in the abdomen, despite the existence of various motion artifact reduction techniques. This work presents a self-navigated motion detection method that can be generalized for most pulse sequences and k-space trajectories. Motion information is extracted directly from raw MR data, requiring no additional gradient or RF pulses, no physiologic monitoring equipment, and minimal--if any--impact on scan time. Initial feasibility results with a two-dimensional gradient echo sequence are shown in phantom studies and in vivo volunteer abdominal studies, demonstrating the sensitivity of the method to both respiratory motion and cardiovascular pulsatility. Prospectively gated images were acquired using the self-navigated data to synchronize image acquisition with motion. These preliminary results suggest that the self-navigated method is a promising technique for reducing motion artifacts in clinical abdominal and cardiac applications.     

Fast 3D imaging using variable-density spiral trajectories with applications to limb perfusion.

Variable-density k-space sampling using a stack-of-spirals trajectory is proposed for ultra fast 3D imaging. Since most of the energy of an image is concentrated near the k-space origin, a variable-density k-space sampling method can be used to reduce the sampling density in the outer portion of k-space. This significantly reduces scan time while introducing only minor aliasing artifacts from the low-energy, high-spatial-frequency components. A stack-of-spirals trajectory allows control over the density variations in both the k(x)-k(y) plane and the k(z) direction while fast k-space coverage is provided by spiral trajectories in the k(x)-k(y) plane. A variable-density stack-of-spirals trajectory consists of variable-density spirals in each k(x)-k(y) plane that are located in varying density in the k(z) direction. Phantom experiments demonstrate that reasonable image quality is preserved with approximately half the scan time. This technique was then applied to first-pass perfusion imaging of the lower extremities which demands very rapid volume coverage. Using a variable-density stack-of-spirals trajectory, 3D images were acquired at a temporal resolution of 2.8 sec over a large volume with a 2.5 x 2.5 x 8 mm(3) spatial resolution. These images were used to resolve the time-course of muscle intensity following contrast injection.