Contrast‐enhanced whole‐heart coronary magnetic resonance angiography at 3 T with radial EPI

Whole‐heart coronary magnetic resonance angiography is a promising method for detecting coronary artery disease. However, the imaging time is relatively long (typically 10–15 min). The goal of this study was to implement a radial echo planar imaging sequence for contrast‐enhanced whole‐heart coronary magnetic resonance angiography, with the aim of combining the scan efficiency of echo planar imaging with the motion insensitivity of radial k‐space sampling. A self‐calibrating phase correction technique was used to correct for off‐resonance effects, trajectory measurement was used to correct for k‐space trajectory errors, and variable density sampling was used in the partition direction to reduce streaking artifacts. Seven healthy volunteers and two patients were scanned with the proposed radial echo planar imaging sequence, and the images were compared with a traditional gradient echo and X‐ray angiography techniques, respectively. Whole‐heart images with the radial EPI technique were acquired with a resolution of 1.0 × 1.0 × 2.0 mm³ in a scan time of 5 min. In healthy volunteers, the average image quality scores and visualized vessel lengths of the RCA and LAD were similar for the radial EPI and gradient echo techniques (P value > 0.05 for all). Anecdotal patient studies showed excellent agreement of the radial EPI technique with X‐ray angiography. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

View angle tilting echo planar imaging for distortion correction

Geometric distortion caused by field inhomogeneity along the phase‐encode direction is one of the most prominent artifacts due to a relatively low effective bandwidth along that direction in magnetic resonance echo planar imaging. This work describes a method for correcting in‐plane image distortion along the phase‐encode direction using a view angle tilting imaging technique in spin‐echo echo planar imaging. Spin‐echo echo planar imaging with view angle tilting uses the addition of gradient blips along the slice‐select direction, concurrently applied with the phase‐encode gradient blips, producing an additional phase. This phase effectively offsets an unwanted phase accumulation caused by field inhomogeneity, resulting in the removal of image distortion along the phase‐encode direction. The proposed method is simple and straightforward both in implementation and application with no scan time penalty. Therefore, it is readily applicable on commercial scanners without having any customized postprocessing. The efficacy of the spin‐echo echo planar imaging with view angle tilting technique in the correction of image distortion is demonstrated in phantom and in vivo brain imaging. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Phase labeling using sensitivity encoding (PLUS): Data acquisition and image reconstruction for geometric distortion correction in EPI

Geometric distortion caused by magnetic field inhomogeneity is generally an inevitable tradeoff for fast MRI acquisitions using echo‐planar imaging. Most of the existing distortion‐correction techniques require separate scans for field maps in order to correct the distortion contained in a measurement. A drawback of these current techniques is that the field map scans and the measurement can capture different patient positions, which invalidates the stationary condition. A new method was developed in this work to correct geometric distortion by using local phase shifts derived directly from the measurement itself, without the need of extra field map scans. This self‐sufficient method takes advantage of parallel imaging and k‐space trajectory modification to produce multiple images from a single acquisition. The measurement is also used to derive sensitivity maps for parallel imaging reconstruction. The derived phase shifts are retrospectively applied to the measurement for correction of geometric distortion in the measurement itself. The proposed method was successfully demonstrated using experimental data from a phantom and a human brain. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

Single‐shot echo‐planar imaging with Nyquist ghost compensation: Interleaved dual echo with acceleration (IDEA) echo‐planar imaging (EPI)

Echo planar imaging (EPI) is most commonly used for blood oxygen level‐dependent fMRI, owing to its sensitivity and acquisition speed. A major problem with EPI is Nyquist (N/2) ghosting, most notably at high field. EPI data are acquired under an oscillating readout gradient and hence vulnerable to gradient imperfections such as eddy current delays and off‐resonance effects, as these cause inconsistencies between odd and even k‐space lines after time reversal. We propose a straightforward and pragmatic method herein termed “interleaved dual echo with acceleration (IDEA) EPI”: two k‐spaces (echoes) are acquired under the positive and negative readout lobes, respectively, by performing phase encoding blips only before alternate readout gradients. From these two k‐spaces, two almost entirely ghost free images per shot can be constructed, without need for phase correction. The doubled echo train length can be compensated by parallel imaging and/or partial Fourier acquisition. The two k‐spaces can either be complex averaged during reconstruction, which results in near‐perfect cancellation of residual phase errors, or reconstructed into separate images. We demonstrate the efficacy of IDEA EPI and show phantom and in vivo images at both 3 T and 7 T. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Reference‐free unwarping of EPI data using dynamic off‐resonance correction with multiecho acquisition (DOCMA)

Inhomogeneities of the main magnetic field cause geometric distortion in echo‐planar imaging, a method central to functional MRI. A number of correction methods have been proposed, most of which are based on the acquisition of a fieldmap providing the local offsets to the main magnetic field. Here, accelerated multiecho echo‐planar imaging is used, with echo times short enough to enable the construction of a fieldmap of comparable quality from the data themselves. This way, each volume in a time series can be unwarped using a fieldmap obtained from that volume, avoiding volume‐to‐volume field‐motion interactions that invalidate reference data in conventional approaches that use a single, static, fieldmap. The combination of accelerated acquisition with dynamic distortion correction yields volumes with very low distortion at repetition times similar to conventional echo‐planar imaging. The method is applied to data acquired at 3 and 7 T and is shown to effectively correct image geometry. Furthermore, it is shown that dynamic distortion correction yields better temporal signal stability than correction using a static fieldmap in the presence of subject motion. Magn Reson Med 2012. © 2011 Wiley Periodicals, Inc.     

Distortion correction in EPI at ultra-high-field MRI using PSF mapping with optimal combination of shift detection dimension

Despite its wide use, echo‐planar imaging (EPI) suffers from geometric distortions due to off‐resonance effects, i.e., strong magnetic field inhomogeneity and susceptibility. This article reports a novel method for correcting the distortions observed in EPI acquired at ultra‐high‐field such as 7 T. Point spread function (PSF) mapping methods have been proposed for correcting the distortions in EPI. The PSF shift map can be derived either along the nondistorted or the distorted coordinates. Along the nondistorted coordinates more information about compressed areas is present but it is prone to PSF‐ghosting artifacts induced by large k‐space shift in PSF encoding direction. In contrast, shift maps along the distorted coordinates contain more information in stretched areas and are more robust against PSF‐ghosting. In ultra‐high‐field MRI, an EPI contains both compressed and stretched regions depending on the B₀ field inhomogeneity and local susceptibility. In this study, we present a new geometric distortion correction scheme, which selectively applies the shift map with more information content. We propose a PSF‐ghost elimination method to generate an artifact‐free pixel shift map along nondistorted coordinates. The proposed method can correct the effects of the local magnetic field inhomogeneity induced by the susceptibility effects along with the PSF‐ghost artifact cancellation. We have experimentally demonstrated the advantages of the proposed method in EPI data acquisitions in phantom and human brain using 7‐T MRI. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Removal of olefinic fat chemical shift artifact in diffusion MRI

Diffusion‐weighted (DW) MRI has emerged as a key tool for assessing the microstructure of tissues in healthy and diseased states. Because of its rapid acquisition speed and insensitivity to motion, single‐shot echo‐planar imaging is the most common DW imaging technique. However, the presence of fat signal can severely affect DW‐echo planar imaging acquisitions because of the chemical shift artifact. Fat suppression is usually achieved through some form of chemical shift‐based fat saturation. Such methods effectively suppress the signal originating from aliphatic fat protons, but fail to suppress the signal from olefinic protons. Olefinic fat signal may result in significant distortions in the DW images, which bias the subsequently estimated diffusion parameters. This article introduces a method for removing olefinic fat signal from DW images, based on an echo time‐shifted acquisition. The method is developed and analyzed specifically in the context of single‐shot DW‐echo‐planar imaging, where image phase is generally unreliable. The proposed method is tested with phantom and in vivo datasets, and compared with a standard acquisition to demonstrate its performance. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Correction for EPI distortions using multi-echo gradient-echo imaging.

A novel and effective technique is described for distortion correction in echo planar imaging (EPI) utilizing the field maps derived from multi-echo gradient-echo images. The distortions from different off-resonance related factors such as field inhomogeneity, eddy current effect, radiofrequency pulse frequency offset, and chemical shift effect can be simultaneously reduced to a great extent. With the proposed post-processing algorithm of multi-channel modulation, distortions may be corrected without unwrapping the phase discontinuities in the derived field map, a process that usually restricts the application of other field map-based correction methods. Results from phantom and animal experiments at 4.7 T demonstrate the efficiency of the method in reducing the geometrical distortions in gradient-echo EPI.     

Echo‐planar imaging with prospective slice‐by‐slice motion correction using active markers

Head motion is a fundamental problem in functional magnetic resonance imaging and is often a limiting factor in its clinical implementation. This work presents a rigid‐body motion correction strategy for echo‐planar imaging sequences that uses micro radiofrequency coil “active markers” for real‐time, slice‐by‐slice prospective correction. Before the acquisition of each echo‐planar imaging‐slice, a short tracking pulse‐sequence measures the positions of three active markers integrated into a headband worn by the subject; the rigid‐body transformation that realigns these markers to their initial positions is then fed back to dynamically update the scan‐plane, maintaining it at a fixed orientation relative to the head. Using this method, prospectively‐corrected echo‐planar imaging time series are acquired on volunteers performing in‐plane and through‐plane head motions, with results demonstrating increased image stability over conventional retrospective image‐realignment. The benefit of this improved image stability is assessed in a blood oxygenation level dependent functional magnetic resonance imaging application. Finally, a non‐rigid‐body distortion‐correction algorithm is introduced to reduce the remaining signal variation. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Real‐time geometric distortion correction for interventional imaging with echo‐planar imaging (EPI)

Many MR‐guided interventional procedures rely on fast imaging sequences for providing images in real‐time with a precise relation between the target position in the image and its true position. Echo‐planar imaging (EPI) methods are very fast but prone to geometric distortions. Here, we propose a correction method designed for real‐time conditions, adapting existing approaches based on dual EPI acquisition with varying echo times. The method is demonstrated with MR‐thermometry for guiding thermal therapies. The proposed approach imposes a small penalty in acquisition speed but adds negligible latency to data processing, an important element for interventions of mobile organs. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Optimizing signal‐to‐noise ratio of high‐resolution parallel single‐shot diffusion‐weighted echo‐planar imaging at ultrahigh field strengths

The potential signal‐to‐noise ratio (SNR) gain at ultrahigh field strengths offers the promise of higher image resolution in single‐shot diffusion‐weighted echo‐planar imaging the challenge being reduced T₂ and T₂* relaxation times and increased B₀ inhomogeneity which lead to geometric distortions and image blurring. These can be addressed using parallel imaging (PI) methods for which a greater range of feasible reduction factors has been predicted at ultrahigh field strengths—the tradeoff being an associated SNR loss. Using comprehensive simulations, the SNR of high‐resolution diffusion‐weighted echo‐planar imaging in combination with spin‐echo and stimulated‐echo acquisition is explored at 7 T and compared to 3 T. To this end, PI performance is simulated for coil arrays with a variable number of circular coil elements. Beyond that, simulations of the point spread function are performed to investigate the actual image resolution. When higher PI reduction factors are applied at 7 T to address increased image distortions, high‐resolution imaging benefits SNR‐wise only at relatively low PI reduction factors. On the contrary, it features generally higher image resolutions than at 3 T due to smaller point spread functions. The SNR simulations are confirmed by phantom experiments. Finally, high‐resolution in vivo images of a healthy volunteer are presented which demonstrate the feasibility of higher PI reduction factors at 7 T in practice. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Geometric distortion correction of high-resolution 3 T diffusion tensor brain images.

Diffusion-weighted images based on echo planar sequences suffer from distortions due to field inhomogeneities from susceptibility differences as well as from eddy currents arising from diffusion gradients. In this paper, a novel approach using nonlinear warping based on optic flow to correct distortions of baseline and diffusion weighted echo planar images (EPI) acquired at 3 T is presented. The distortion correction was estimated by warping the echo planar images to the anatomically correct T2-weighted fast spin echo images (T2-FSE). A global histogram intensity matching of the T2-FSE precedes the base line EPI image distortion correction. A local intensity-matching algorithm was used to transform labeled T2-FSE regions to match intensities of diffusion-weighted EPI images prior to distortion correction of these images. Evaluation was performed using three methods: (i) visual comparison of overlaid contours, (ii) a global mutual information index, and (iii) a local distance measure between homologous points. Visual assessment and the global index demonstrated a decrease in geometrical distortion and the distance measure showed that distortions are reduced to a subvoxel level. In conclusion, the warping algorithm is effective in reducing geometric distortions, enabling generation of anatomically correct diffusion tensor images at 3 T.     

Center‐out echo‐planar spectroscopic imaging with correction of gradient‐echo phase and time shifts

A procedure to prevent the formation of image and spectral Nyquist ghosts in echo‐planar spectroscopic imaging is introduced. It is based on a novel Cartesian center‐out echo‐planar spectroscopic imaging trajectory, referred to as EPSICO, and combined with a correction of the gradient‐echo phase and time shifts. Processing of homogenous sets of forward and reflected echoes is no longer necessary, resulting in an optimized spectral width. The proposed center‐out trajectory passively prevents the formation of Nyquist ghosts by privileging the acquisition of the center k‐space line with forward echoes at the beginning of an echo‐planar spectroscopic imaging dwell time and by ensuring that all k‐space lines and their respective complex conjugates are acquired at equal time intervals. With the proposed procedure, concentrations of N‐acetyl aspartate, creatine, choline, glutamate, and myo‐inositol were reliably determined in human white matter at 3 T. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

High‐resolution fMRI with higher‐order generalized series imaging and parallel imaging techniques (HGS‐parallel)

Purpose

To develop a novel approach for high‐resolution functional MRI (fMRI) using the conventional gradient‐echo sequence.

Materials and Methods

Echo‐planar imaging (EPI) techniques have generally been used for fMRI studies due to their fast imaging time. However, it is difficult for studying brain function at the submillimeter level using this sequence. In addition, EPI techniques have some drawbacks, such as Nyquist ghosts and geometric distortions in the reconstructed images, and subsequently require additional postprocessing to reduce these artifacts. One way of solving these problems is to acquire fMRI data by means of a conventional gradient‐echo imaging sequence instead of EPI. To provide a fast imaging time, the proposed method combines higher‐order generalized series (HGS) imaging with a parallel imaging technique which is called the HGS‐parallel technique.

Results

The proposed HGS‐parallel technique achieves a 12.8‐fold acceleration in imaging time without the cost of spatial resolution. The proposed method was verified through the application of fMRI studies on normal subjects.

Conclusion

This study suggests that the proposed method can be used for high‐resolution fMRI studies without the geometric distortion and the Nyquist ghost artifacts compared to EPI. J. Magn. Reson. Imaging 2009;29:924–936. © 2009 Wiley‐Liss, Inc.     

Simultaneous noncontrast angiography and intraPlaque hemorrhage (SNAP) imaging for carotid atherosclerotic disease evaluation

A simultaneous noncontrast angiography and intraplaque hemorrhage (SNAP) MR imaging technique is proposed to detect both luminal stenosis and hemorrhage in atherosclerosis patients in a single scan. Thirteen patients with diagnosed carotid atherosclerotic plaque were admitted after informed consent. All scans were performed on a 3T MR imaging system with SNAP, 2D time‐of‐flight and magnetization‐prepared 3D rapid acquisition gradient echo sequences. The SNAP sequence utilized a phase sensitive acquisition, and was designed to provide positive signals corresponding to intraplaque hemorrhage and negative signals corresponding to lumen. SNAP images were compared to time‐of‐flight images to evaluate lumen size measurements using linear mixed models and the intraclass correlation coefficient. Intraplaque hemorrhage identification accuracy was evaluated by comparing to magnetization‐prepared 3D rapid acquisition gradient echo images using Cohen's Kappa. Diagnostic quality SNAP images were generated from all subjects. Quantitatively, the lumen size measurements by SNAP were strongly correlated (intraclass correlation coefficient = 0.96, P < 0.001) with those measured by time‐of‐flight. For intraplaque hemorrhage detection, strong agreement (κ = 0.82, P < 0.001) was also identified between SNAP and magnetization‐prepared 3D rapid acquisition gradient echo images. In conclusion, a SNAP imaging technique was proposed and shows great promise for imaging both lumen size and carotid intraplaque hemorrhage with a single scan. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Robust GRAPPA‐accelerated diffusion‐weighted readout‐segmented (RS)‐EPI

Readout segmentation (RS‐EPI) has been suggested as a promising variant to echo‐planar imaging (EPI) for high‐resolution imaging, particularly when combined with parallel imaging. This work details some of the technical aspects of diffusion‐weighted (DW)‐RS‐EPI, outlining a set of reconstruction methods and imaging parameters that can both minimize the scan time and afford high‐resolution diffusion imaging with reduced distortions. These methods include an efficient generalized autocalibrating partially parallel acquisition (GRAPPA) calibration for DW‐RS‐EPI data without scan time penalty, together with a variant for the phase correction of partial Fourier RS‐EPI data. In addition, the role of pulsatile and rigid‐body brain motion in DW‐RS‐EPI was assessed. Corrupt DW‐RS‐EPI data arising from pulsatile nonlinear brain motion had a prevalence of ～7% and were robustly identified via k‐space entropy metrics. For DW‐RS‐EPI data corrupted by rigid‐body motion, we showed that no blind overlap was required. The robustness of RS‐EPI toward phase errors and motion, together with its minimized distortions compared with EPI, enables the acquisition of exquisite 3 T DW images with matrix sizes close to 512². Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Spiral water-fat imaging with integrated off-resonance correction on a clinical scanner

Purpose

To integrate water‐fat–resolved spiral gradient‐echo imaging with off‐resonance correction into a clinical MR scanner and to evaluate its basic feasibility and performance.

Materials and Methods

Three‐point chemical shift imaging was implemented with forward and strongly T₂*‐weighted reverse spiral sampling and with off‐resonance correction after water–fat separation. It was applied in a volunteer study on single breathhold abdominal imaging, which included a brief comparison with Cartesian sampling.

Results

Water‐fat–resolved, off‐resonance–corrected forward and reverse three‐dimensional interleaved spiral imaging was found to be feasible on a clinical MR scanner with only minor changes to the existing data acquisition and reconstruction, and to provide good image quality. Three‐point chemical shift encoded data thus support both, water–fat separation and off‐resonance correction with high accuracy.

Conclusion

The combination of chemical shift encoding and appropriate postprocessing could pave the way for water‐fat–resolved spiral imaging in clinical applications. J. Magn. Reson. Imaging 2010;32:1262–1267. © 2010 Wiley‐Liss, Inc.     

Point spread function mapping with parallel imaging techniques and high acceleration factors: fast, robust, and flexible method for echo-planar imaging distortion correction.

Echo-planar imaging (EPI) is an ultrafast magnetic resonance (MR) imaging technique prone to geometric distortions. Various correction techniques have been developed to remedy these distortions. Here improvements of the point spread function (PSF) mapping approach are presented, which enable reliable and fully automated distortion correction of echo-planar images at high field strengths. The novel method is fully compatible with EPI acquisitions using parallel imaging. The applicability of parallel imaging to further accelerate PSF acquisition is shown. The possibility of collecting PSF data sets with total acceleration factors higher than the number of coil elements is demonstrated. Additionally, a new approach to visualize and interpret distortions in the context of various imaging and reconstruction methods based on the PSF is proposed. The reliable performance of the PSF mapping technique is demonstrated on phantom and volunteer scans at field strengths of up to 4 T.     

High efficiency multishot interleaved spiral‐in/out: Acquisition for high‐resolution BOLD fMRI

Growing demand for high spatial resolution blood oxygenation level dependent (BOLD) functional magnetic resonance imaging faces a challenge of the spatial resolution versus coverage or temporal resolution tradeoff, which can be addressed by methods that afford increased acquisition efficiency. Spiral acquisition trajectories have been shown to be superior to currently prevalent echo‐planar imaging in terms of acquisition efficiency, and high spatial resolution can be achieved by employing multiple‐shot spiral acquisition. The interleaved spiral in/out trajectory is preferred over spiral‐in due to increased BOLD signal contrast‐to‐noise ratio (CNR) and higher acquisition efficiency than that of spiral‐out or noninterleaved spiral in/out trajectories (Law & Glover. Magn Reson Med 2009; 62:829–834.), but to date applicability of the multishot interleaved spiral in/out for high spatial resolution imaging has not been studied. Herein we propose multishot interleaved spiral in/out acquisition and investigate its applicability for high spatial resolution BOLD functional magnetic resonance imaging. Images reconstructed from interleaved spiral‐in and ‐out trajectories possess artifacts caused by differences in T2* decay, off‐resonance, and k‐space errors associated with the two trajectories. We analyze the associated errors and demonstrate that application of conjugate phase reconstruction and spectral filtering can substantially mitigate these image artifacts. After applying these processing steps, the multishot interleaved spiral in/out pulse sequence yields high BOLD CNR images at in‐plane resolution below 1 × 1 mm while preserving acceptable temporal resolution (4 s) and brain coverage (15 slices of 2 mm thickness). Moreover, this method yields sufficient BOLD CNR at 1.5 mm isotropic resolution for detection of activation in hippocampus associated with cognitive tasks (Stern memory task). The multishot interleaved spiral in/out acquisition is a promising technique for high spatial resolution BOLD functional magnetic resonance imaging applications. Magn Reson Med 70:420–428, 2013. © 2012 Wiley Periodicals, Inc.     

Two‐dimensional phase correction method for single and multi‐shot echo planar imaging

Ghost artifacts are a serious issue in single and multi‐shot echo planar imaging. Because of these coherent artifacts, it is essential to consistently suppress the ghosts. In this article, we present a phase correction algorithm that achieves excellent ghost suppression for single and multi‐shot echo planar imaging. The phase correction is performed along both the x (read) direction and y (phase) direction. To this end, we apply a double field of view prescan and compute the phase required for ghost suppression. This phase is fitted to a 2D polynomial. The fitted phase is used to correct the echo planar imaging images. The correction algorithm can be used with any readout gradient polarities and any number of shots. A flow chart of the correction method is provided to better clarify the full process. Finally, phantom and volunteer images demonstrate the improvement of artifact suppression obtained with this algorithm over conventional phase correction methods. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.