Robust 2D phase correction for echo planar imaging under a tight field‐of‐view

Nyquist ghost artifacts are a serious issue in echo planar imaging. These artifacts primarily originate from phase difference between even and odd echo images and can be removed or reduced using phase correction methods. The commonly used 1D phase correction can only correct phase difference along readout axis. 2D correction is, therefore, necessary when phase difference presents along both readout and phase encoding axes. However, existing 2D methods have several unaddressed issues that affect their practicality. These issues include uncharacterized noise behavior, image artifact due to unoptimized phase estimation, Gibbs ringing artifact when directly applying to partial k_y data, and most seriously a new image artifact under tight field‐of‐view (i.e., field‐of‐view slightly smaller than object size). All these issues are addressed in this article. Specifically, theoretical analysis of noise amplification and effect of phase estimation error is provided, and tradeoff between noise and ghost is studied. A new 2D phase correction method with improved polynomial fitting, joint homodyne processing and phase correction, compatibility with tight field‐of‐view is then proposed. Various results show that the proposed method can robustly generate images free of Nyquist ghosts and other image artifacts even in oblique scans or when cross‐term eddy current terms are significant. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

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.     

3D isotropic high‐resolution diffusion‐weighted MRI of the whole brain with a motion‐corrected steady‐state free precession sequence

The main obstacle to high‐resolution (<1.5 mm isotropic) 3D diffusion‐weighted MRI is the differential motion‐induced phase error from shot‐to‐shot. In this work, the phase error is addressed with a hybrid 3D navigator approach that corrects motion‐induced phase in two ways. In the first, rigid‐body motion is corrected for every shot. In the second, repeatable nonrigid‐body pulsation is corrected for each portion of the cardiac cycle. These phase error corrections were implemented with a 3D diffusion‐weighted steady‐ state free precession pulse sequence and were shown to mitigate signal dropouts caused by shot‐to‐shot phase inconsistencies compared to a standard gridding reconstruction in healthy volunteers. The proposed approach resulted in diffusion contrast more similar to the contrast observed in the reference echo‐planer imaging scans than reconstruction of the same data without correction. Fractional anisotropy and Color fractional anisotropy maps generated with phase‐corrected data were also shown to be more similar to echo‐planer imaging reference scans than those generated without phase correction. Magn Reson Med 70:466–478, 2013. © 2012 Wiley Periodicals, Inc.     

3D steady‐state diffusion‐weighted imaging with trajectory using radially batched internal navigator echoes (TURBINE)

While most diffusion‐weighted imaging (DWI) is acquired using single‐shot diffusion‐weighted spin‐echo echo‐planar imaging, steady‐state DWI is an alternative method with the potential to achieve higher‐resolution images with less distortion. Steady‐state DWI is, however, best suited to a segmented three‐dimensional acquisition and thus requires three‐dimensional navigation to fully correct for motion artifacts. In this paper, a method for three‐dimensional motion‐corrected steady‐state DWI is presented. The method uses a unique acquisition and reconstruction scheme named trajectory using radially batched internal navigator echoes (TURBINE). Steady‐state DWI with TURBINE uses slab‐selection and a short echo‐planar imaging (EPI) readout each pulse repetition time. Successive EPI readouts are rotated about the phase‐encode axis. For image reconstruction, batches of cardiac‐synchronized readouts are used to form three‐dimensional navigators from a fully sampled central k‐space cylinder. In vivo steady‐state DWI with TURBINE is demonstrated in human brain. Motion artifacts are corrected using refocusing reconstruction and TURBINE images prove less distorted compared to two‐dimensional single‐shot diffusion‐weighted‐spin‐EPI. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

High‐resolution human diffusion tensor imaging using 2‐D navigated multishot SENSE EPI at 7 T

The combination of parallel imaging with partial Fourier acquisition has greatly improved the performance of diffusion‐weighted single‐shot EPI and is the preferred method for acquisitions at low to medium magnetic field strength such as 1.5 or 3 T. Increased off‐resonance effects and reduced transverse relaxation times at 7 T, however, generate more significant artifacts than at lower magnetic field strength and limit data acquisition. Additional acceleration of k‐space traversal using a multishot approach, which acquires a subset of k‐space data after each excitation, reduces these artifacts relative to conventional single‐shot acquisitions. However, corrections for motion‐induced phase errors are not straightforward in accelerated, diffusion‐weighted multishot EPI because of phase aliasing. In this study, we introduce a simple acquisition and corresponding reconstruction method for diffusion‐weighted multishot EPI with parallel imaging suitable for use at high field. The reconstruction uses a simple modification of the standard sensitivity‐encoding (SENSE) algorithm to account for shot‐to‐shot phase errors; the method is called image reconstruction using image‐space sampling function (IRIS). Using this approach, reconstruction from highly aliased in vivo image data using 2‐D navigator phase information is demonstrated for human diffusion‐weighted imaging studies at 7 T. The final reconstructed images show submillimeter in‐plane resolution with no ghosts and much reduced blurring and off‐resonance artifacts. Magn Reson Med, 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.     

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.     

High resolution diffusion‐weighted imaging using readout‐segmented echo‐planar imaging,parallel imaging and a two‐dimensional navigator‐based reacquisition

Single‐shot echo‐planar imaging (EPI) is well established as the method of choice for clinical, diffusion‐weighted imaging with MRI because of its low sensitivity to the motion‐induced phase errors that occur during diffusion sensitization of the MR signal. However, the method is prone to artifacts due to susceptibility changes at tissue interfaces and has a limited spatial resolution. The introduction of parallel imaging techniques, such as GRAPPA (GeneRalized Autocalibrating Partially Parallel Acquisitions), has reduced these problems, but there are still significant limitations, particularly at higher field strengths, such as 3 Tesla (T), which are increasingly being used for routine clinical imaging. This study describes how the combination of readout‐segmented EPI and parallel imaging can be used to address these issues by generating high‐resolution, diffusion‐weighted images at 1.5T and 3T with a significant reduction in susceptibility artifact compared with the single‐shot case. The technique uses data from a 2D navigator acquisition to perform a nonlinear phase correction and to control the real‐time reacquisition of unusable data that cannot be corrected. Measurements on healthy volunteers demonstrate that this approach provides a robust correction for motion‐induced phase artifact and allows scan times that are suitable for routine clinical application. Magn Reson Med, 2009. © 2009 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.     

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.     

Turboprop+: Enhanced turboprop diffusion‐weighted imaging with a new phase correction

Faster periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) diffusion‐weighted imaging acquisitions, such as Turboprop and X‐prop, remain subject to phase errors inherent to a gradient echo readout, which ultimately limits the applied turbo factor (number of gradient echoes between each pair of radiofrequency refocusing pulses) and, thus, scan time reductions. This study introduces a new phase correction to Turboprop, called Turboprop+. This technique employs calibration blades, which generate 2‐D phase error maps and are rotated in accordance with the data blades, to correct phase errors arising from off‐resonance and system imperfections. The results demonstrate that with a small increase in scan time for collecting calibration blades, Turboprop+ had a superior immunity to the off‐resonance‐related artifacts when compared to standard Turboprop and recently proposed X‐prop with the high turbo factor (turbo factor = 7). Thus, low specific absorption rate and short scan time can be achieved in Turboprop+ using a high turbo factor, whereas off‐resonance related artifacts are minimized. Magn Reson Med 70:497–503, 2013. © 2012 Wiley Periodicals, Inc.     

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.     

Independent slab‐phase modulation combined with parallel imaging in bilateral breast MRI

Independent slab‐phase modulation allows three‐dimensional imaging of multiple volumes without encoding the space between volumes, thus reducing scan time. Parallel imaging further accelerates data acquisition by exploiting coil sensitivity differences between volumes. This work compared bilateral breast image quality from self‐calibrated parallel imaging reconstruction methods such as modified sensitivity encoding, generalized autocalibrating partially parallel acquisitions and autocalibrated reconstruction for Cartesian sampling (ARC) for data with and without slab‐phase modulation. A study showed an improvement of image quality by incorporating slab‐phase modulation. Geometry factors measured from phantom images were more homogenous and lower on average when slab‐phase modulation was used for both mSENSE and GRAPPA reconstructions. The resulting improved signal‐to‐noise ratio (SNR) was validated for in vivo images as well using ARC instead of GRAPPA, illustrating average SNR efficiency increases in mSENSE by 5% and ARC by 8% based on region of interest analysis. Furthermore, aliasing artifacts from mSENSE reconstruction were reduced when slab‐phase modulation was used. Overall, slab‐phase modulation with parallel imaging improved image quality and efficiency for 3D bilateral breast imaging. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Correction for geometric distortion and N/2 ghosting in EPI by phase labeling for additional coordinate encoding (PLACE).

Echo-planar imaging (EPI) is vulnerable to geometric distortion and N/2 ghosting. These artifacts can be analyzed with an intuitive k-t space tool, and here we propose a simple method for their correction. In a slightly modified additional EPI acquisition, we sample the k-t space with a shift in k(y) by adding a small area to the phase-encoding (PE) gradient. Physically, the added gradient area creates a relative phase ramp across the object and directly encodes the undistorted original y-coordinate of each voxel into a phase difference between two distorted complex images, in a method called "phase labeling for additional coordinate encoding" (PLACE). The phase information is then used to map the mismapped signals back to their original locations for geometric and intensity correction. Smoothing of expanded complex data matrix effectively reduces noise in the differential phase map and allows subpixel warping. The two acquired images can also be averaged to effectively suppress the N/2 ghost. Efficient correction for both artifacts can be achieved with three acquisitions. These acquisitions can also serve as reference scans to correct for geometric distortion and/or N/2 ghost artifacts on all images in a time series. The technique was successfully demonstrated in phantom and animal studies.     

Combining phase images from multi‐channel RF coils using 3D phase offset maps derived from a dual‐echo scan

A method is presented for the combination of phase images from multi‐channel RF coils in the absence of a volume reference coil. It is based on the subtraction of 3D phase offset maps from the phase data from each coil. Phase offset maps are weighted combinations of phase measurements at two echo times. Multi‐Channel Phase Combination using measured 3D phase offsets (MCPC‐3D) offers a conceptually and computationally simple solution to the calculation of combined phase images. The dual‐echo data required for the phase maps can be intrinsic to the high‐resorlution gradient‐echo scan to be reconstructed (MCPC‐3D‐I). Alternatively, a separate, fast, low‐resolution dual‐echo scan can be used (MCPC‐3D‐II). Both variants are shown to give near perfect phase matching, yielding images with high SNR throughout and high GM‐WM contrast. MCPC‐3D is compared with other reference‐free phase image crombination methods; high‐pass phase filtering, phase difference imaging, and matching using constant offsets (MCPC‐C). Multi‐Channel Phase Combination using measured 3D phase offsets method does not need an overlap between the signals from individual coils and can be used with parallel imaging, making it ideally suited to multi‐channel coils with a large number of elements, and to high and ultra‐high field systems. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Highly k‐t‐space–accelerated phase‐contrast MRI

The purpose of this study was to combine a recently introduced spatiotemporal parallel imaging technique, PEAK‐GRAPPA (parallel MRI with extended and averaged generalized autocalibrating partially parallel acquisition), with two‐dimensional (2D) cine phase‐contrast velocity mapping. Phase‐contrast MRI was applied to measure the blood flow in the thoracic aorta and the myocardial motion of the left ventricle. To evaluate the performance of different reconstruction methods, fully acquired k‐space data sets were used to compare conventional parallel imaging using GRAPPA with reduction factors of R = 2–6 and PEAK‐GRAPPA as well as sliding window reconstruction with reduction factors R = 2–12 (net acceleration factors up to 5.2). PEAK‐GRAPPA reconstruction resulted in improved image quality with considerably reduced artifacts, which was also supported by error analysis. To analyze potential blurring or low‐pass filtering effects of spatiotemporal PEAK‐GRAPPA, the velocity time courses of aortic flow and myocardial tissue motion were evaluated and compared with conventional image reconstructions. Quantitative comparisons of blood flow velocities and pixel‐wise correlation analysis of velocities highlight the potential of PEAK‐GRAPPA for highly accelerated dynamic phase‐contrast velocity mapping. Magn Reson Med 60:1169–1177, 2008. © 2008 Wiley‐Liss, Inc.     

High spatial resolution EPI using an odd number of interleaves.

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

X‐PROP: A fast and robust diffusion‐weighted propeller technique

Diffusion‐weighted imaging (DWI) has shown great benefits in clinical MR exams. However, current DWI techniques have shortcomings of sensitivity to distortion or long scan times or combinations of the two. Diffusion‐weighted echo‐planar imaging (EPI) is fast but suffers from severe geometric distortion. Periodically rotated overlapping parallel lines with enhanced reconstruction diffusion‐weighted imaging (PROPELLER DWI) is free of geometric distortion, but the scan time is usually long and imposes high Specific Absorption Rate (SAR) especially at high fields. TurboPROP was proposed to accelerate the scan by combining signal from gradient echoes, but the off‐resonance artifacts from gradient echoes can still degrade the image quality. In this study, a new method called X‐PROP is presented. Similar to TurboPROP, it uses gradient echoes to reduce the scan time. By separating the gradient and spin echoes into individual blades and removing the off‐resonance phase, the off‐resonance artifacts in X‐PROP are minimized. Special reconstruction processes are applied on these blades to correct for the motion artifacts. In vivo results show its advantages over EPI, PROPELLER DWI, and TurboPROP techniques. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Single‐shot 3D GRASE with cylindrical k‐space trajectories

Development of GRASE (gradient‐ and spin‐echo) pulse sequences for single‐shot 3D imaging has been motivated by physiologic studies of the brain. The duration of echo‐planar imaging (EPI) subsequences between RF refocusing pulses in the GRASE sequence is determinant of image distortions and susceptibility artifacts. To reduce these artifacts the regular Cartesian trajectory is modified to a circular trajectory in 2D and a cylindrical trajectory in 3D for reduced echo train time. Incorporation of “fly‐back” trajectories lengthened the time of the subsequences and proportionally increased susceptibility artifact but the unipolar readout gradients eliminate all ghost artifacts. The modified cylindrical trajectory reduced susceptibility artifact and distortion artifact while raising the signal‐to‐noise ratio in both phantom and human brain images. Magn Reson Med 60:976–980, 2008. © 2008 Wiley‐Liss, Inc.     

Generalized GRAPPA operators for wider spiral bands: Rapid self‐calibrated parallel reconstruction for variable density spiral MRI

A rapid and self‐calibrated parallel imaging reconstruction method is proposed for undersampled variable density spiral datasets. A set of generalized GRAPPA for wider readout line operators are used to expand each acquired spiral line into a wider spiral band, therefore fulfilling Nyquist sampling criterion throughout the k‐space. The calibration of generalized GRAPPA for wider readout line operators is performed using the fully sampled central k‐space region. The resulting generalized GRAPPA for wider readout line operator weights are adaptively regularized to minimize the error in the newly‐generated data at different k‐space locations. Simulation and experimental results demonstrate that the technique can be used either to achieve a significant acceleration and/or to reduce off‐resonance artifacts due to a shorten readout duration. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.