Ghost artifact reduction for echo planar imaging using image phase correction

An algorithm is described for reducing ghost artifacts in echo planar imaging (EPI) using phase corrections derived from images reconstructed using only even or odd k-space lines. The N/2 ghost, that arises principally from time-reversal of alternate k-space lines, was significantly reduced by this algorithm without the need for a calibration scan. In images obtained in eight subjects undergoing EPI for auditory functional MRI (fMRI) experiments, N/2 ghost intensity was reduced from 10.3% – 2.1% (range: 7.9–14.1%) to 4.5% ± 0.2% (range: 4.1–4.9%) of parent image intensity, corresponding to a percent reduction in ghost intensity of 54% ± 9% (range: 43–65%), and the algorithm restored this intensity to the parent image. It provided a significant improvement in image appearance, and increased the correlation coefficients related to neural activation in functional MRI studies. The algorithm provided reduction of artifacts from all polynomial orders of spatial phase errors in both spatial directions. The algorithm did not eliminate N/2 ghost intensity contributed by field inhomogeneities, susceptibility, or chemical shift.     

Quantification and reduction of ghosting artifacts in interleaved echo-planar imaging

A mathematical analysis of ghosting artifacts often seen in interleaved echo-planar images (EPI) is presented. These artifacts result from phase and amplitude discontinuities between lines of k-space in the phase-encoding direction, and timing misregistrations from system filter delays. Phase offsets and time delays are often measured using “reference” scans, to reduce ghosting through post-processing. From the expressions describing ghosting artifacts, criteria were established for reducing ghosting to acceptable levels., Subsequently, the signal-to-noise ratio (SNR) requirements for estimation of time delays and phase offsets, determined from reference scans, was evaluated to establish the effect of estimation emor on artifact reduction for interleaved EPI. Artifacts resulting from these effects can be reduced to very low levels when appropriate reference scan estimation is used. This has important implications for functional MRI (fMRI) and applications involving small changes in signal intensity.     

Ineffectiveness of averaging for reducing motion artifacts in half-Fourier MR imaging.

Two data sets for half-Fourier imaging (HFI) can be collected in the same time as one data set for conventional full Fourier imaging (FFI). The hypothesis is that averaging twice as much data in HFI does not make ghost artifacts caused by motion have less signal intensity than in FFI. This hypothesis was tested with images of a human subject by measuring the standard deviation within regions of interest containing ghosts. The control experiment involved measuring the standard deviation on images from the same data reconstructed with FFI. The images were formed after averaging of one to eight data sets from a collection of nine data sets acquired sequentially. Background ghosts or those in other regions of low intensity were less intense on images from HFI after twice as much averaging as in FFI, but this was not the case for ghosts superimposed on anatomic structures. This observation is explained by showing that an image obtained by means of FFI can be expressed in terms of two images obtained by means of HFI applied to the top and bottom halves of the data. The use of HFI to allow twice as much averaging without prolonging data acquisition time is not advantageous for reducing ghost artifacts caused by motion.     

Invited. MRI of laser-induced interstitial thermal injury in an in vivo animal liver model with histologic correlation

Laser-induced interstitial thermal therapy (LITT) is a preferred method of minimally invasive therapy. MRI is a noninvasive method by which to monitor the thermal effects of LITT. To properly control such effects, changes in MRI parameters during and after LITT should be correlated with changes in the tissue. T1-weighted fast spin echo (FSE) MRI (1 image/10 seconds) at 1.5 T monitored LITT in vivo in rabbit liver (n = 6) using an interstitial bare delivery fiber (600-μm diameter; 3.0 W; 1,064 nm; 150 seconds). During laser irradiation, MRI signal intensity decreased around the fiber tip; after irradiation, this hypointensity proved reversible and permanent lesions were evident. The lesions had hyperintense margins that were brighter than surrounding normal tissue (P < .001); the tissue in these bright regions was mapped to tissue necrosis characterized by the presence of thermally damaged ghost red blood cells amid generally normal hepatocytes. T1-FSE identified the spatial extent of the LITT lesions.     

Data convolution and combination operation (COCOA) for motion ghost artifacts reduction

A novel method, data convolution and combination operation, is introduced for the reduction of ghost artifacts due to motion or flow during data acquisition. Since neighboring k‐space data points from different coil elements have strong correlations, a new “synthetic” k‐space with dispersed motion artifacts can be generated through convolution for each coil. The corresponding convolution kernel can be self‐calibrated using the acquired k‐space data. The synthetic and the acquired data sets can be checked for consistency to identify k‐space areas that are motion corrupted. Subsequently, these two data sets can be combined appropriately to produce a k‐space data set showing a reduced level of motion induced error. If the acquired k‐space contains isolated error, the error can be completely eliminated through data convolution and combination operation. If the acquired k‐space data contain widespread errors, the application of the convolution also significantly reduces the overall error. Results with simulated and in vivo data demonstrate that this self‐calibrated method robustly reduces ghost artifacts due to swallowing, breathing, or blood flow, with a minimum impact on the image signal‐to‐noise ratio. Magn Reson Med, 2010. © 2010 Wiley‐Liss, 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.     

High‐field diffusion MR histology: Image‐based correction of eddy‐current ghosts in diffusion‐weighted rapid acquisition with relaxation enhancement (DW‐RARE)

High‐resolution, diffusion‐weighted (DW) MR microscopy is gaining increasing acceptance as a nondestructive histological tool for the study of fixed tissue samples. Spin‐echo sequences are popular for high‐field diffusion imaging due to their high tolerance to B₀ field inhomogeneities. Volumetric DW rapid acquisition with relaxation enhancement (DW‐RARE) currently offers the best tradeoff between imaging efficiency and image quality, but is relatively sensitive to residual eddy‐current effects on the echo train phase, resulting in encoding direction‐dependent ghosting in the DW images. We introduce two efficient, image‐based phase corrections for ghost artifact reduction in DW‐RARE of fixed tissue samples, neither of which require navigator echo acquisition. Both methods rely on the phase difference in k‐space between the unweighted reference image and a given DW image and assume a constant, per‐echo phase error arising from residual eddy‐current effects in the absence of sample motion. Significant qualitative and quantitative ghost artifact reductions are demonstrated for individual DW and calculated diffusion tensor images. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

Robust EPI Nyquist ghost elimination via spatial and temporal encoding

Nyquist ghosts are an inherent artifact in echo planar imaging acquisitions. An approach to robustly eliminate Nyquist ghosts is presented that integrates two previous Nyquist ghost correction techniques: temporal domain encoding (phase labeling for additional coordinate encoding: PLACE and spatial domain encoding (phased array ghost elimination: PAGE). Temporal encoding modulates the echo planar imaging acquisition trajectory from frame to frame, enabling one to interleave data to remove inconsistencies that occur between sampling on positive and negative gradient readouts. With PLACE, one can coherently combine the interleaved data to cancel residual Nyquist ghosts. If the level of ghosting varies significantly from image to image, however, the signal cancellation that occurs with PLACE can adversely affect SNR‐sensitive applications such as perfusion imaging with arterial spin labeling. This work proposes integrating PLACE into a PAGE‐based reconstruction process to yield significantly better Nyquist ghost correction that is more robust than PLACE or PAGE alone. The robustness of this method is demonstrated in the presence of magnetic field drift with an in‐vivo arterial spin labeling perfusion experiment. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Minimization of Nyquist ghosting for echo‐planar imaging at ultra‐high fields based on a “negative readout gradient” strategy

Purpose:

To improve the traditional Nyquist ghost correction approach in echo planar imaging (EPI) at high fields, via schemes based on the reversal of the EPI readout gradient polarity for every other volume throughout a functional magnetic resonance imaging (fMRI) acquisition train.

Materials and Methods:

An EPI sequence in which the readout gradient was inverted every other volume was implemented on two ultrahigh‐field systems. Phantom images and fMRI data were acquired to evaluate ghost intensities and the presence of false‐positive blood oxygenation level‐dependent (BOLD) signal with and without ghost correction. Three different algorithms for ghost correction of alternating readout EPI were compared.

Results:

Irrespective of the chosen processing approach, ghosting was significantly reduced (up to 70% lower intensity) in both rat brain images acquired on a 9.4T animal scanner and human brain images acquired at 7T, resulting in a reduction of sources of false‐positive activation in fMRI data.

Conclusion:

It is concluded that at high B₀ fields, substantial gains in Nyquist ghost correction of echo planar time series are possible by alternating the readout gradient every other volume. J. Magn. Reson. Imaging 2009;30:1171–1178. © 2009 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.     

Artifacts and signal loss due to flow in the presence of Bo inhomogeneity

An in vitro study was performed to investigate the effects of B_o inhomogeneity on magnetic resonance images of flow. Controlled inhomogeneity gradients (G₁) were applied and the magnitude of the artifacts produced was quantified for different echo delay times (TE). Both steady and pulsatile flows were examined. In the presence of an inhomogeneity gradient, signal loss is apparent if the flow is pulsatile and/or if the slice thickness is large. The signal loss increases with increasing TE and G₁. With pulsatile flow, ghosting artifacts are also generated. These increase in intensity with increasing TE and G₁. In vivo, field inhomogeneity due to susceptibility variations is large enough to produce these effects. Representative time-of-flight images obtained of a normal volunteer with two different TEs demonstrate the effect in vivo. Flow-related signal loss and artifacts, therefore, increase with increasing TE independent of the moments of the applied gradients.     

Motion artifact correction in free‐breathing abdominal MRI using overlapping partial samples to recover image deformations

This article presents a method to reconstruct liver MRI data acquired continuously during free breathing, without any external sensor or navigator measurements. When the deformations associated with k‐space data are known, generalized matrix inversion reconstruction has been shown to be effective in reducing the ghosting and blurring artifacts of motion. This article describes a novel method to obtain these nonrigid deformations. A breathing model is built from a fast dynamic series: low spatial resolution images are registered and their deformations parameterized by overall superior–inferior displacement. The correct deformation for each subset of the subsequent imaging data is then found by comparing a few lines of k‐space with the equivalent lines from a deformed reference image while varying the deformation over the model parameter. This procedure is known as image deformation recovery using overlapping partial samples (iDROPS). Simulations using 10 rapid dynamic studies from volunteers showed the average error in iDROPS‐derived deformations within the liver to be 1.43 mm. A further four volunteers were imaged at higher spatial resolution. The complete reconstruction process using data from throughout several breathing cycles was shown to reduce blurring and ghosting in the liver. Retrospective respiratory gating was also demonstrated using the iDROPS parameterization. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Image-based EPI ghost correction using an algorithm based on projection onto convex sets (POCS).

This work describes the use of a method, based on the projection onto convex sets (POCS) algorithm, for reduction of the N/2 ghost in echo-planar imaging (EPI). In this method, ghosts outside the parent image are set to zero and a model k-space is obtained from the Fourier transform (FT) of the resulting image. The zeroth- and first-order phase corrections for each line of the original k-space are estimated by comparison with the corresponding line in the model k-space. To overcome problems of phase wrapping, the first-order phase corrections for the lines of the original k-space are estimated by registration with the corresponding lines in the model k-space. It is shown that applying these corrections will result in a reduction of the ghost, and that iterating the process will result in a convergence towards an image in which the ghost is minimized. The method is tested on spin-echo EPI data. The results show that the method is robust and remarkably effective, reducing the N/2 ghost to a level nearly comparable to that achieved with reference scans.     

Respiratory effects in two-dimensional Fourier transform MR imaging

Respiratory and other regular motions during two-dimensional Fourier transform magnetic resonance imaging produce image artifacts consisting of local blurring and more or less regularly spaced "ghost" images propagating along the direction of the phase-encoding magnetic field gradient. The patterns of these ghost artifacts can be understood in terms of the technique of image production and basic properties of the discrete Fourier transform. This understanding permits, without respiratory gating, production of images of improved quality in body regions in which there is significant respiratory motion. In particular, the ghosts can be maximally separated from the primary image by choosing intervals between phase-encoding gradient pulse increments that are equal to one-half the respiratory period; they can be minimally separated by choosing an interval equal to the respiratory period. Increasing the number of signal averages between each phase-encoding increment decreases the intensity of the ghosts.     

Concomitant magnetic-field-induced artifacts in axial echo planar imaging

When a linear magnetic field gradient is used, spatially higher-order magnetic fields are produced to satisfy the Maxwell equations. It has been observed that the higher-order magnetic field produced by the readout gradient causes axial echo planar images acquired with a horizontal solenoid magnet to shift along the phase-encoding direction and lose image intensities. Both the shift and intensity reduction become increasingly severe as the slice offset from the isocenter increases. These phenomena are quantitatively analyzed, and good correlation between experiments and theory has been established. The analysis also predicts a previously unre-ported Nyquist ghost on images with very large slice offsets. This ghost has been verified with computer simulations. Based on the analysis, several methods have been developed to eliminate the image shift, the intensity reduction, and the ghost. Selected methods have been implemented on a commercial scanner and proved effective in removing these image artifacts.     

Dynamic MRI of benign and malignant testicular lesions: preliminary observations

Sonography and color-coded Doppler sonography are the methods of first choice in diagnosing pathological changes of the testes. The MRI technique was tested using a dynamic technique to evaluate the possibility of differentiating between benign and malignant testicular lesions. The testes of 20 volunteers and of 15 patients were examined in a field strength of 1.5 Tesla (Philips Gyroscan S 15) with a multislice T 1-weighted fast field echo (FFE) sequence before and every minute after the injection of 0.1 mmol/kg Gd-DTPA. The histopathological evaluation after the MRI examination revealed five seminomas, three mixed tumors, one teratocarcinoma, one embryonal carcinoma, one Sertoli tumor, two dermoid cysts, one chronic epididymitis/fibrosis and one necrosis/atrophy. In malignant lesions the maximum increase of signal intensity (SI) after injecting contrast medium was significantly higher (p < 0.001) compared with normal testes, whereas benign lesions showed a lower or similar maximum relative signal change compared with the volunteer group. If these early observations can be confirmed in a larger number of patients, dynamic MRI seems to be able to differentiate between benign and malignant testicular lesions. Dynamic MRI might become an additional tool with high diagnostic accuracy that is independent of the examiner.     

Off-resonance image artifacts in interleaved-EPI and GRASE pulse sequences

Echo-time shifting (ETS) is used in GRASE and interleaved-EPI sequences to improve the phase evolutions for off-resonance signal sources. However, even with ETS the phase evolutions still exhibit discontinuities. In this work, we extend previous studies of ETS by quantitatively evaluating the magnitude and form of the image artifacts that result from these phase discontinuities. The functional form of the phase evolution is used to derive the general conditions under which artifacts are expected. The artifacts for two sequence structures are then evaluated as a function of off-resonance frequency and data sampling period by calculating point spread functions and simulated images. It was found that even when ETS is used to improve the phase evolutions, periodic phase discontinuities may degrade image quality by producing ghosting artifacts of edges. These artifacts are similar to those that commonly occur with periodic motion. From our results recommendations are derived for limiting the ghosting artifacts.     

Multi-gradient echo with susceptibility inhomogeneity compensation (MGESIC): Demonstration of fMRI in the olfactory cortex at 3.0 T

Short image acquisition times and sensitivity to magnetic susceptibility favor the use of gradient echo imaging methods in functional MRI (fMRI). However, magnetic susceptibility effects attributed to air-tissue interfaces also lead to severe signal loss in images of the large inferior frontal and lateral temporal cortices of the human brain, which renders these regions inaccessible to fMRI. The signal loss is caused by the local field gradients in the slice selection direction. A multigradient echo with magnetic susceptibility inhomogeneity compensation method (MGESIC) is proposed to overcome this problem. The MGESIC method effectively corrects the susceptibility artifacts and maintains the advantages of gradient echo methods to both BOLD sensitivity and fast image acquisition. The effectiveness of the MGESIC method is demonstrated by fMRI experimental results within the olfactory cortex.     

The effect of quality control on the function of magnetic resonance imaging (MRI), using American College of Radiology (ACR) phantom

Objective

To study image quality of MRI scanner using the American College of Radiology (ACR) phantom.

Single scan PC‐MRI by alternating the velocity encoding gradient polarity between phase encoding steps

The purpose of this study was to develop a faster approach to phase contrast magnetic resonance imaging. This article proposes a phase contrast imaging scheme called single scan phase contrast in which the polarity of the velocity‐encoding gradient is alternated between phase encoding steps. In single scan phase contrast, ghost images due to moving spins form. The signal intensity of the ghost images is modulated by the sine of the motion‐induce phase shift. Prior to image acquisition, the region of interest containing moving spins is identified, and the field of view is configured so to avoid overlap between the object in the image and the ghost image(s) due to motion in the region of interest. The image values of the region of interest and the ghost image are used to quantify velocity. At best, single scan phase contrast reduces the total acquisition time by a factor of two when compared to phase contrast. In this study, single scan phase contrast is validated against phase contrast in phantom and in vivo. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.