Single-shot EPI with signal recovery from the susceptibility-induced losses.

A major problem in the gradient-recalled echo-planar imaging (EPI) method that also uses a long echo time (TE) is the severe signal loss in regions with large static field inhomogeneities. These regions include the ventral frontal, medial temporal, and inferior temporal lobes, which experience inhomogeneities induced by susceptibility effects commonly found near air/tissue interfaces. For functional magnetic resonance imaging (fMRI) studies that use both gradient-recalled EPI at relatively long TE and high-field scanners, this signal loss is severe, preventing investigation of certain human cognitive processes that involve these regions, such as memory and attention. Methods have been developed to recover this signal loss; however, most of them require multiple excitations and thus compromise temporal resolution. In this report, a new technique is described which achieves good signal recovery within a single excitation. It is anticipated that this technique will prove useful for fMRI studies in inhomogeneous areas that require high temporal resolution.     

EPI distortion correction from a simultaneously acquired distortion map using TRAIL

PURPOSE: To develop a method for shot-by-shot distortion correction of single-shot echo-planar imaging (EPI) that is capable of correcting each image individually using a distortion measurement performed during acquisition of the image itself. MATERIALS AND METHODS: The recently-introduced method known as two reduced acquisitions interleaved (TRAIL) was extended to measure the distribution of the main magnetic field B0 with each shot. This corresponded to a map of distortion, and allowed distortion to be corrected in the acquired images. RESULTS: Distortion-corrected images were demonstrated in the human brain. The distortion field could be directly visualized using the "stripe" distribution imposed by the TRAIL pulse sequence. This confirmed the success of the correction. Over a time-course measurement of 10 images, variance was reduced by using shot-by-shot distortion correction compared to correction with a constant field map. CONCLUSION: Shot-by-shot distortion correction may be performed for EPI images acquired using an extension of the TRAIL technique, ensuring that the correction reflects the actual distortion pattern and not merely a previously measured, but possibly no longer valid, distortion field. This avoids errors due to changes in the distortion field or misregistration of a previously measured distortion map resulting from subject motion.     

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.     

Improved image reconstruction for partial Fourier gradient-echo echo-planar imaging (EPI).

The partial Fourier gradient-echo echo planar imaging (EPI) technique makes it possible to acquire high-resolution functional MRI (fMRI) data at an optimal echo time. This technique is especially important for fMRI studies at high magnetic fields, where the optimal echo time is short and may not be achieved with a full Fourier acquisition scheme. In addition, it has been shown that partial Fourier EPI provides better anatomic resolvability than full Fourier EPI. However, the partial Fourier gradient-echo EPI may be degraded by artifacts that are not usually seen in other types of imaging. Those unique artifacts in partial Fourier gradient-echo EPI, to our knowledge, have not yet been systematically evaluated. Here we use the k-space energy spectrum analysis method to understand and characterize two types of partial Fourier EPI artifacts. Our studies show that Type 1 artifact, originating from k-space energy loss, cannot be corrected with pure postprocessing, and Type 2 artifact can be eliminated with an improved reconstruction method. We propose a novel algorithm, that combines images obtained from two or more reconstruction schemes guided by k-space energy spectrum analysis, to generate partial Fourier EPI with greatly reduced Type 2 artifact. Quality control procedures for avoiding Type 1 artifact in partial Fourier EPI are also discussed.     

Accelerated point spread function mapping using signal modeling for accurate echo‐planar imaging geometric distortion correction

Echo‐planar imaging is a fast and commonly used magnetic resonance imaging technique with applications in diffusion weighted and functional MRI. Fast data acquisition in echo‐planar imaging is accomplished by the extended readout, which also introduces sensitivity to off‐resonance effects such as amplitude of static (polarizing) field inhomogeneities and eddy‐currents. These off‐resonance effects produce geometric distortions in the corresponding echo‐planar images. To correct for these distortions, an acceleration of point spread function (PSF) acquisition using a special sampling pattern is presented in this work. The proposed technique allows for reliable and fully automated distortion correction of echo‐planar images at a field strength of 3 T. Additionally, a new approach to visualize and determine the distortions in a hybrid (x, y, k_PSF) three‐dimensional space is proposed. The accuracy and robustness of the proposed technique is demonstrated in phantom and in vivo experiments. The accuracy of the presented method here is compared to previous techniques for echo‐planar imaging distortion correction such as PLACE. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Combined prospective and retrospective correction to reduce motion‐induced image misalignment and geometric distortions in EPI

Despite rigid‐body realignment to compensate for head motion during an echo‐planar imaging time‐series scan, nonrigid image deformations remain due to changes in the effective shim within the brain as the head moves through the B₀ field. The current work presents a combined prospective/retrospective solution to reduce both rigid and nonrigid components of this motion‐related image misalignment. Prospective rigid‐body correction, where the scan‐plane orientation is dynamically updated to track with the subject's head, is performed using an active marker setup. Retrospective distortion correction is then applied to unwarp the remaining nonrigid image deformations caused by motion‐induced field changes. Distortion correction relative to a reference time‐frame does not require any additional field mapping scans or models, but rather uses the phase information from the echo‐planar imaging time‐series itself. This combined method is applied to compensate echo‐planar imaging scans of volunteers performing in‐plane and through‐plane head motions, resulting in increased image stability beyond what either prospective or retrospective rigid‐body correction alone can achieve. The combined method is also assessed in a blood oxygen level dependent functional MRI task, resulting in improved Z‐score statistics. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

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.     

Gaussian process modeling for image distortion correction in echo planar imaging.

An enhanced method for correction of image distortion due to B(0)-field inhomogeneities in echo planar imaging (EPI) is presented. The algorithm is based on the measurement of the point spread function (PSF) associated with each image voxel using a reference scan. The expected distortion map in the phase encode direction is then estimated using a nonparametric inference algorithm known as Gaussian process modeling. The algorithm is shown to be robust to the presence of regions of low signal-to-noise in the image and large inhomogeneities.     

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.     

Toward a practical protocol for human optic nerve DTI with EPI geometric distortion correction

Purpose

To develop a practical protocol for diffusion tensor imaging (DTI) of the human optic nerve with echo planar imaging (EPI) geometric distortion correction.

Materials and Methods

A conventional DTI protocol was modified to acquire images with fat and cerebrospinal fluid (CSF) suppression and field inhomogeneity maps of contiguous coronal slices covering the whole brain. The technique was applied to healthy volunteers and multiple sclerosis patients with and without a history of unilateral optic neuritis. DTI measures and optic nerve tractography before and after geometric distortion correction were compared. Diffusion measures from left and right or from affected and unaffected eyes in different subject cohorts were reported.

Results

The image geometry after correction closely resembled reference anatomical images. Optic nerve tractography became feasible after distortion correction. The diffusion measures from the healthy volunteers were in good agreement with the literature. Statistically significant differences were found in the fractional anisotropy and orthogonal eigenvalues between affected and unaffected eyes in optic neuritis patients with poor recovery. The diffusion measures before and after geometric distortion correction were not significantly different. For cohorts without optic neuritis, the difference between diffusion measures from left and right eyes was not statistically significant.

Conclusion

The proposed technique could provide a practical DTI protocol to study the human optic nerve. J. Magn. Reson. Imaging 2009;30:699–707. © 2009 Wiley‐Liss, Inc.     

Correction of spatial distortion in EPI due to inhomogeneous static magnetic fields using the reversed gradient method

PURPOSE: To derive and implement a method for correcting spatial distortion caused by in vivo inhomogeneous static magnetic fields in echo-planar imaging (EPI). MATERIALS AND METHODS: The reversed gradient method, which was initially devised to correct distortion in images generated by spin-warp MRI, was adapted to correct distortion in EP images. This method provides point-by-point correction of distortion throughout the image. EP images, acquired with a 3 T MRI system, of a phantom and a volunteer's head were used to test the correction method. RESULTS: Good correction was observed in all cases. Spatial distortion in the uncorrected images ranged up to 4 pixels (12 mm) and was corrected successfully. CONCLUSION: The correction was improved by the application of a nonlinear interpolation scheme. The correction requires that two EP images be acquired at each slice position. This increases the acquisition time, but an improved signal-to-noise ratio (SNR) is seen in the corrected image. The local SNR gain decreases with increasing distortion. In many EPI acquisition schemes, multiple images are averaged at each slice position to increase the SNR; in such cases the reversed gradient correction method can be applied with no increase in acquisition duration.     

Removal of EPI Nyquist ghost artifacts with two-dimensional phase correction.

Odd-even echo inconsistencies result in Nyquist ghost artifacts in the reconstructed EPI images. The ghost artifacts reduce the image signal-to-noise ratio and make it difficult to correctly interpret the EPI data. In this article a new 2D phase mapping protocol and a postprocessing algorithm are presented for an effective Nyquist ghost artifacts removal. After an appropriate k-space data regrouping, a 2D map accurately encoding low- and high-order phase errors is derived from two phase-encoded reference scans, which were originally proposed by Hu and Le (Magn Reson Med 36:166-171;1996) for their 1D nonlinear correction method. The measured phase map can be used in the postprocessing algorithm developed to remove ghost artifacts in subsequent EPI experiments. Experimental results from phantom, animal, and human studies suggest that the new technique is more effective than previously reported methods and has a better tolerance to signal intensity differences between reference and actual EPI scans. The proposed method may potentially be applied to repeated EPI measurements without subject movements, such as functional MRI and diffusion coefficient mapping.     

Image distortion correction in EPI: comparison of field mapping with point spread function mapping.

Echo-planar imaging (EPI) can provide rapid imaging by acquiring a complete k-space data set in a single acquisition. However, this approach suffers from distortion effects in geometry and intensity, resulting in poor image quality. The distortions, caused primarily by field inhomogeneities, lead to intensity loss and voxel shifts, the latter of which are particularly severe in the phase-encode direction. Two promising approaches to correct the distortion in EPI are field mapping and point spread function (PSF) mapping. The field mapping method measures the field distortions and translates these into voxel shifts, which can be used to assign image intensities to the correct voxel locations. The PSF approach uses acquisitions with additional phase-encoding gradients applied in the x, y, and/or z directions to map the 1D, 2D, or 3D PSF of each voxel. These PSFs encode the spatial information about the distortion and the overall distribution of intensities from a single voxel. The measured image is the convolution of the undistorted density and the PSF. Measuring the PSF allows the distortion in geometry and intensity to be corrected. This work compares the efficacy of these methods with equal time allowed for field mapping and PSF mapping.     

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.