Artifact reduction in EPI with phase-encoded reference scan

Referenceless Nyquist ghost correction outperforms standard navigator-based method and improves efficiency of in vivo diffusion tensor cardiovascular magnetic resonance

PURPOSE

The study aims to assess the potential of referenceless methods of EPI ghost correction to accelerate the acquisition of in vivo diffusion tensor cardiovascular magnetic resonance (DT-CMR) data using both computational simulations and data from in vivo experiments.

METHODS

Three referenceless EPI ghost correction methods were evaluated on mid-ventricular short axis DT-CMR spin echo and STEAM datasets from 20 healthy subjects at 3T. The reduced field of view excitation technique was used to automatically quantify the Nyquist ghosts, and DT-CMR images were fit to a linear ghost model for correction.

RESULTS

Numerical simulation showed the singular value decomposition (SVD) method is the least sensitive to noise, followed by Ghost/Object method and entropy-based method. In vivo experiments showed significant ghost reduction for all correction methods, with referenceless methods outperforming navigator methods for both spin echo and STEAM sequences at b = 32, 150, 450, and 600  smm - 2 $$ {\mathrm{smm}}^{-2} $$ . It is worth noting that as the strength of the diffusion encoding increases, the performance gap between the referenceless method and the navigator-based method diminishes.

CONCLUSION

Referenceless ghost correction effectively reduces Nyquist ghost in DT-CMR data, showing promise for enhancing the accuracy and efficiency of measurements in clinical practice without the need for any additional reference scans.

Unsupervised deep learning model for correcting Nyquist ghosts of single-shot spatiotemporal encoding

PURPOSE

To design an unsupervised deep learning (DL) model for correcting Nyquist ghosts of single-shot spatiotemporal encoding (SPEN) and evaluate the model for real MRI applications.

METHODS

The proposed method consists of three main components: (1) an unsupervised network that combines Residual Encoder and Restricted Subspace Mapping (RERSM-net) and is trained to generate a phase-difference map based on the even and odd SPEN images; (2) a spin physical forward model to obtain the corrected image with the learned phase difference map; and (3) cycle-consistency loss that is explored for training the RERSM-net.

RESULTS

The proposed RERSM-net could effectively generate smooth phase difference maps and correct Nyquist ghosts of single-shot SPEN. Both simulation and real in vivo MRI experiments demonstrated that our method outperforms the state-of-the-art SPEN Nyquist ghost correction method. Furthermore, the ablation experiments of generating phase-difference maps show the advantages of the proposed unsupervised model.

CONCLUSION

The proposed method can effectively correct Nyquist ghosts for the single-shot SPEN sequence.

Beat phenomena of oscillating readouts

PURPOSE

To demonstrate slowly varying, erroneous magnetic field gradients for oscillating readouts due to the mechanically resonant behavior of gradient systems.

METHODS

Projections of a static phantom were acquired using a one-dimensional (1D) EPI sequence with varying EPI frequencies ranging from 1121 to 1580 Hz on clinical 3T systems (30 mT/m, 200 T/m/s). Phase due to static B0 inhomogeneities was eliminated by a complex division of two separate scans with different polarities of the EPI readout. The temporal evolution of phase was evaluated and related to the mechanical resonances of the gradient systems derived from the gradient modulation transfer function. Additionally, the impact of temporally varying mechanical resonance effects on EPI was evaluated using an echo-planar spectroscopic imaging sequence.

RESULTS

A beat phenomenon resulting in a slowly varying phase was observed. Its temporal frequency was given by the difference between the EPI frequency and the mechanical resonance frequency of the activated gradient axis. The maximum erroneous, oscillating phase during phase encoding was ±0.5 rad for an EPI frequency of 1281 Hz. Echo-planar spectroscopic imaging images showed the resulting time-dependent stretching/compression of the FOV.

CONCLUSION

Oscillating readouts such as those used in EPI can result in low-frequency, erroneous phase contributions, which are explained by the beat phenomenon. Therefore, EPI phase-correction approaches may need to include beat effects for accurate image reconstruction.

Reconstruction for 7T high-resolution whole-brain diffusion MRI using two-stage N/2 ghost correction and L1-SPIRiT without single-band reference

PURPOSE

To combine a new two-stage N/2 ghost correction and an adapted L1-SPIRiT method for reconstruction of 7T highly accelerated whole-brain diffusion MRI (dMRI) using only autocalibration scans (ACS) without the need of additional single-band reference (SBref) scans.

METHODS

The proposed ghost correction consisted of a 3-line reference approach in stage 1 and the reference-free entropy method in stage 2. The adapted L1-SPIRiT method was formulated within the 3D k-space framework. Its efficacy was examined by acquiring two dMRI data sets at 1.05-mm isotropic resolutions with a total acceleration of 6 or 9 (i.e., 2-fold or 3-fold slice and 3-fold in-plane acceleration). Diffusion analysis was performed to derive DTI metrics and estimate fiber orientation distribution functions (fODFs). The results were compared with those of 3D k-space GRAPPA using only ACS, all in reference to 3D k-space GRAPPA using both ACS and SBref (serving as a reference).

RESULTS

The proposed ghost correction eliminated artifacts more robustly than conventional approaches. Our adapted L1-SPIRiT method outperformed 3D k-space GRAPPA when using only ACS, improving image quality to what was achievable with 3D k-space GRAPPA using both ACS and SBref scans. The improvement in image quality further resulted in an improvement in estimation performances for DTI and fODFs.

CONCLUSION

The combination of our new ghost correction and adapted L1-SPIRiT method can reliably reconstruct 7T highly accelerated whole-brain dMRI without the need of SBref scans, increasing acquisition efficiency and reducing motion sensitivity.

What's New and What's Next in Diffusion MRI Preprocessing

Diffusion MRI (dMRI) provides invaluable information for the study of tissue microstructure and brain connectivity, but suffers from a range of imaging artifacts that greatly challenge the analysis of results and their interpretability if not appropriately accounted for. This review will cover dMRI artifacts and preprocessing steps, some of which have not typically been considered in existing pipelines or reviews, or have only gained attention in recent years: brain/skull extraction, B-matrix incompatibilities w.r.t the imaging data, signal drift, Gibbs ringing, noise distribution bias, denoising, between- and within-volumes motion, eddy currents, outliers, susceptibility distortions, EPI Nyquist ghosts, gradient deviations, B₁ bias fields, and spatial normalization. The focus will be on “what’s new” since the notable advances prior to and brought by the Human Connectome Project (HCP), as presented in the predecessing issue on “Mapping the Connectome” in 2013. In addition to the development of novel strategies for dMRI preprocessing, exciting progress has been made in the availability of open source tools and reproducible pipelines, databases and simulation tools for the evaluation of preprocessing steps, and automated quality control frameworks, amongst others. Finally, this review will consider practical considerations and our view on “what’s next” in dMRI preprocessing.

Robust autocalibrated structured low-rank EPI ghost correction

PURPOSE

We propose and evaluate a new structured low-rank method for echo-planar imaging (EPI) ghost correction called Robust Autocalibrated LORAKS (RAC-LORAKS). The method can be used to suppress EPI ghosts arising from the differences between different readout gradient polarities and/or the differences between different shots. It does not require conventional EPI navigator signals, and is robust to imperfect autocalibration data.

METHODS

Autocalibrated LORAKS is a previous structured low-rank method for EPI ghost correction that uses GRAPPA-type autocalibration data to enable high-quality ghost correction. This method works well when the autocalibration data are pristine, but performance degrades substantially when the autocalibration information is imperfect. RAC-LORAKS generalizes Autocalibrated LORAKS in two ways. First, it does not completely trust the information from autocalibration data, and instead considers the autocalibration and EPI data simultaneously when estimating low-rank matrix structure. Second, it uses complementary information from the autocalibration data to improve EPI reconstruction in a multi-contrast joint reconstruction framework. RAC-LORAKS is evaluated using simulations and in vivo data, including comparisons to state-of-the-art methods.

RESULTS

RAC-LORAKS is demonstrated to have good ghost elimination performance compared to state-of-the-art methods in several complicated EPI acquisition scenarios (including gradient-echo brain imaging, diffusion-encoded brain imaging, and cardiac imaging).

CONCLUSIONS

RAC-LORAKS provides effective suppression of EPI ghosts and is robust to imperfect autocalibration data.

EPI distortion correction for concurrent human brain stimulation and imaging at 3T

BACKGROUND

Transcranial magnetic stimulation (TMS) can be paired with functional magnetic resonance imaging (fMRI) in concurrent TMS-fMRI experiments. These multimodal experiments enable causal probing of network architecture in the human brain which can complement alternative network mapping approaches. Critically, merely introducing the TMS coil into the scanner environment can sometimes produce substantial magnetic field inhomogeneities and spatial distortions which limit the utility of concurrent TMS-fMRI.

METHOD AND RESULTS

We assessed the efficacy of point spread function corrected echo planar imaging (PSF-EPI) in correcting for the field inhomogeneities associated with a TMS coil at 3 T. In phantom and brain scans, we quantitatively compared the coil-induced distortion artifacts measured in EPI scans with and without PSF correction. We found that the application of PSF corrections to the EPI data significantly improved signal-to-noise and reduced distortions. In phantom scans with the PSF-EPI sequence, we also characterized the temporal profile of dynamic artifacts associated with TMS delivery and found that image quality remained high as long as the TMS pulse preceded the RF excitation pulses by at least 50 ms. Lastly, we validated the PSF-EPI sequence in human brain scans involving TMS and motor behavior as well as resting state fMRI scans.

CONCLUSIONS

Our collective results demonstrate the potential benefits of PSF-EPI for concurrent TMS-fMRI when coil-related artifacts are a concern. The ability to collect high quality resting state fMRI data in the same session as the concurrent TMS-fMRI experiment offers a unique opportunity to interrogate network architecture in the human brain.

k-Space deep learning for reference-free EPI ghost correction

PURPOSE

Nyquist ghost artifacts in echo planar imaging (EPI) are originated from phase mismatch between the even and odd echoes. However, conventional correction methods using reference scans often produce erroneous results especially in high-field MRI due to the nonlinear and time-varying local magnetic field changes. Recently, it was shown that the problem of ghost correction can be reformulated as k-space interpolation problem that can be solved using structured low-rank Hankel matrix approaches. Another recent work showed that data driven Hankel matrix decomposition can be reformulated to exhibit similar structures as deep convolutional neural network. By synergistically combining these findings, we propose a k-space deep learning approach that immediately corrects the phase mismatch without a reference scan in both accelerated and non-accelerated EPI acquisitions.

THEORY AND METHODS

To take advantage of the even and odd-phase directional redundancy, the k-space data are divided into 2 channels configured with even and odd phase encodings. The redundancies between coils are also exploited by stacking the multi-coil k-space data into additional input channels. Then, our k-space ghost correction network is trained to learn the interpolation kernel to estimate the missing virtual k-space data. For the accelerated EPI data, the same neural network is trained to directly estimate the interpolation kernels for missing k-space data from both ghost and subsampling.

RESULTS

Reconstruction results using 3T and 7T in vivo data showed that the proposed method outperformed the image quality compared to the existing methods, and the computing time is much faster.

CONCLUSIONS

The proposed k-space deep learning for EPI ghost correction is highly robust and fast, and can be combined with acceleration, so that it can be used as a promising correction tool for high-field MRI without changing the current acquisition protocol.

Real-Time Filtering with Sparse Variations for Head Motion in Magnetic Resonance Imaging

Estimating a time-varying signal, such as head motion from magnetic resonance imaging data, becomes particularly challenging in the face of other temporal dynamics such as functional activation. This paper describes a new Kalman filter-like framework that includes a sparse residual term in the measurement model. This additional term allows the extended Kalman filter to generate real-time motion estimates suitable for prospective motion correction when such dynamics occur. An iterative augmented Lagrangian algorithm similar to the alterating direction method of multipliers implements the update step for this Kalman filter. This paper evaluates the accuracy and convergence rate of this iterative method for small and large motion in terms of its sensitivity to parameter selection. The included experiment on a simulated functional magnetic resonance imaging acquisition demonstrates that the resulting method improves the maximum Youden's J index of the time series analysis by 2-3% versus retrospective motion correction, while the sensitivity index increases from 4.3 to 5.4 when combining prospective and retrospective correction.

Nyquist ghost correction of breast diffusion weighted imaging using referenceless methods

PURPOSE

Correction of Nyquist ghosts for single-shot spin-echo EPI using the standard 3-line navigator often fails in breast DWI because of incomplete fat suppression, respiration, and greater B₀ inhomogeneity. The purpose of this work is to compare the performance of the 3-line navigator with 4 data-driven methods termed "referenceless methods," including 2 previously proposed in literature, 1 introduced in this work, and finally a combination of all 3, in breast DWI.

METHODS

Breast DWI was acquired for 41 patients with SS SE-EPI. Raw data was corrected offline with the standard 3-line navigator and 4 referenceless methods, which modeled the ghost as a linear phase error and minimized 3 unique cost functions as well as the median solution of all 3. Ghost levels were evaluated based on the signal intensity in the background region, defined by a mask auto-generated from a T₁ -weighted anatomical image. Ghost intensity measurements were fit to a linear mixed model including ghost correction method and b-value as covariates.

RESULTS

All 4 referenceless methods outperformed the standard 3-line navigator with statistical significance at all 4 b-values tested (b = 0, 100, 600, and 800 s/mm² ).

CONCLUSIONS

Referenceless methods provide a robust way to reduce Nyquist ghosts in breast DWI without the need for any additional calibration scan.

Navigator-Free EPI Ghost Correction With Structured Low-Rank Matrix Models: New Theory and Methods

Structured low-rank matrix models have previously been introduced to enable calibrationless MR image reconstruction from sub-Nyquist data, and such ideas have recently been extended to enable navigator-free echo-planar imaging (EPI) ghost correction. This paper presents a novel theoretical analysis which shows that, because of uniform subsampling, the structured low-rank matrix optimization problems for EPI data will always have either undesirable or non-unique solutions in the absence of additional constraints. This theory leads us to recommend and investigate problem formulations for navigator-free EPI that incorporate side information from either image-domain or k-space domain parallel imaging methods. The importance of using nonconvex low-rank matrix regularization is also identified. We demonstrate using phantom and in vivo data that the proposed methods are able to eliminate ghost artifacts for several navigator-free EPI acquisition schemes, obtaining better performance in comparison with the state-of-the-art methods across a range of different scenarios. Results are shown for both single-channel acquisition and highly accelerated multi-channel acquisition.

Accelerating multi-slice spatiotemporally encoded MRI with simultaneous echo refocusing

Single-shot spatiotemporally encoded (SPEN) ultrafast magnetic resonance imaging (MRI) is of great value to both scientific research and clinical application owing to its capability for delivering MR images with greater robustness to magnetic field inhomogeneity and chemical-shift displacement effects than conventional methods like EPI due to high effective phase-encoded bandwidth. Many SPEN MRI methods have been developed, among which multi-slice SPEN MRI arises as a promising supplement to ultrafast multi-slice sampling. In this work, we propose a new multi-slice SPEN MRI method, termed multi-echo segmented SPEN (ME-SeSPEN) method, which produces multiple images within a single train of echoes and successively samples widely separated slices. The resulting images were reconstructed using de-convolution super-resolved algorithm. The robustness and efficiency of the proposed method were demonstrated by phantom, lemon and in vivo experiments in comparison with spin-echo EPI, spin-echo simultaneous echo refocusing (SER), and segmented SPEN (SeSPEN) MRI. The results indicate that the new method effectively shortens the sampling time (20% reduction practically in comparison with SeSPEN when two slices are simultaneously sampled). ME-SeSPEN also reduces eddy current effects while maintaining the benefits of SPEN MRI, such as similar robustness to field inhomogeneity, spatial resolution and signal-to-noise ratio to SeSPEN MRI. The new method will promote the versatility of multi-slice MRI in practical applications.

Ghost reduction in echo-planar imaging by joint reconstruction of images and line-to-line delays and phase errors

PURPOSE

To correct line-to-line delays and phase errors in echo-planar imaging (EPI).

THEORY AND METHODS

EPI-trajectory auto-corrected image reconstruction (EPI-TrACR) is an iterative maximum-likelihood technique that exploits data redundancy provided by multiple receive coils between nearby lines of k-space to determine and correct line-to-line trajectory delays and phase errors that cause ghosting artifacts. EPI-TrACR was efficiently implemented using a segmented FFT and was applied to in vivo brain data acquired at 7 T across acceleration (1×-4×) and multishot factors (1-4 shots), and in a time series.

RESULTS

EPI-TrACR reduced ghosting across all acceleration factors and multishot factors, compared to conventional calibrated reconstructions and the PAGE method. It also achieved consistently lower ghosting in the time series. Averaged over all cases, EPI-TrACR reduced root-mean-square ghosted signal outside the brain by 27% compared to calibrated reconstruction, and by 40% compared to PAGE.

CONCLUSION

EPI-TrACR automatically corrects line-to-line delays and phase errors in multishot, accelerated, and dynamic EPI. While the method benefits from additional calibration data for initialization, it was not a requirement for most reconstructions. Magn Reson Med 79:3114-3121, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

A Robust Method for Estimating B0 Inhomogeneity Field in the Liver by Mitigating Fat Signals and Phase-Wrapping

We developed an optimized and robust method to estimate liver B0 field inhomogeneity for monitoring and correcting susceptibility-induced geometric distortion in magnetic resonance images for precision therapy. A triple-gradient-echo acquisition was optimized for the whole liver B0 field estimation within a single-exhale breath-hold scan on a 3 T scanner. To eliminate chemical-shift artifacts, fat signals were chosen in-phase between 2 echoes with an echo time difference (ΔTE) of 2.3 milliseconds. To avoid phase-wrapping, other 2 echoes provided a large field dynamic range (1/ΔTE) to cover the B0 field inhomogeneity. In addition, using high parallel imaging factor of 4 and a readout-bandwidth of 1955 Hz/pixel, an ∼18-second acquisition time for breath-held scans was achieved. A 2-step, 1-dimensional regularized method for the ΔB0 field map estimation was developed, tested and validated in phantom and patient studies. Our method was validated on a water phantom with fat components and air pockets; it yielded ΔB0-field maps that had no chemical-shift and phase-wrapping artifacts, and it had a <0.5 mm of geometric distortion near the air pockets. The ΔB0-field maps of the patients' abdominal regions were also free from phase-wrapping and chemical-shift artifacts. The maximum field inhomogeneity was found near the lung–liver interface, up to ∼300 Hz, resulting in ∼2 mm of distortions in anatomical images with a readout-bandwidth of 440 Hz/pixel. The field mapping method in the abdominal region is robust; it can be easily integrated in clinical workflow for patient-based quality control of magnetic resonance imaging geometric integrity.

Robust EPI Nyquist ghost removal by incorporating phase error correction with sensitivity encoding (PEC-SENSE)

PURPOSE

The existing approach of Nyquist ghost correction by parallel imaging in echo planar imaging (EPI) can suffer from image noise amplification. We propose a method that estimates a phase error map from multi-channel data itself and incorporates it into the sensitivity encoding (SENSE) reconstruction for Nyquist ghost correction without compromising the image SNR.

METHODS

This method first reconstructs two ghost-free images from positive and negative echoes using SENSE, respectively, from which the phase error map is computed. This map is then incorporated into the coil sensitivity maps for the negative echo image during the joint SENSE reconstruction of all k-space data to obtain the final ghost-free image. Phantom and in vivo EPI experiments at 7 T and 3 T were performed to evaluate the proposed method.

RESULTS

Nyquist ghost was effectively removed in all images even under oblique imaging and poor eddy current conditions. Resulting image signal-to-noise ratio (SNR) was comparable to that by the traditional linear phase error correction method and higher than that by a previous SENSE-based parallel imaging correction approach.

CONCLUSION

The proposed correction method can robustly eliminate Nyquist ghost while preserving the image SNR. This approach requires no additional calibration data beyond standard coil sensitivity maps and can be readily applied to all EPI applications. Magn Reson Med 79:943-951, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Multishot echo-planar MREIT for fast imaging of conductivity, current density, and electric field distributions

PURPOSE

Magnetic resonance electrical impedance tomography (MREIT) sequences typically use conventional spin or gradient echo-based acquisition methods for reconstruction of conductivity and current density maps. Use of MREIT in functional and electroporation studies requires higher temporal resolution and faster sequences. Here, single and multishot echo planar imaging (EPI) based MREIT sequences were evaluated to see whether high-quality MREIT phase data could be obtained for rapid reconstruction of current density, conductivity, and electric fields.

METHODS

A gel phantom with an insulating inclusion was used as a test object. Ghost artifact, geometric distortion, and MREIT correction algorithms were applied to the data. The EPI-MREIT-derived phase-projected current density and conductivity images were compared with simulations and spin-echo images as a function of EPI shot number.

RESULTS

Good agreement among measures in simulated, spin echo, and EPI data was achieved. Current density errors were stable and below 9% as the shot number decreased from 64 to 2, but increased for single-shot images. Conductivity reconstruction relative contrast ratios were stable as the shot number decreased. The derived electric fields also agreed with the simulated data.

CONCLUSIONS

The EPI methods can be combined successfully with MREIT reconstruction algorithms to achieve fast imaging of current density, conductivity, and electric field. Magn Reson Med 79:71-82, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Referenceless one-dimensional Nyquist ghost correction in multicoil single-shot spatiotemporally encoded MRI

Single-shot spatiotemporally encoded (SPEN) MRI is a novel fast imaging method capable of retaining the time efficiency of single-shot echo planar imaging (EPI) but with distortion artifacts significantly reduced. Akin to EPI, the phase inconsistencies between mismatched even and odd echoes also result in the so-called Nyquist ghosts. However, the characteristic of the SPEN signals provides the possibility of obtaining ghost-free images directly from even and odd echoes respectively, without acquiring additional reference scans. In this paper, a theoretical analysis of the Nyquist ghosts manifested in single-shot SPEN MRI is presented, a one-dimensional correction scheme is put forward capable of maintaining definition of image features without blurring when the phase inconsistency along SPEN encoding direction is negligible, and a technique is introduced for convenient and robust correction of data from multi-channel receiver coils. The effectiveness of the proposed processing pipeline is validated by a series of experiments conducted on simulation data, in vivo rats and healthy human brains. The robustness of the method is further verified by implementing distortion correction on ghost corrected data.

Ultrafast multi-slice spatiotemporally encoded MRI with slice-selective dimension segmented

As a recently emerging method, spatiotemporally encoded (SPEN) magnetic resonance imaging (MRI) has a high robustness to field inhomogeneity and chemical shift effect. It has been broadened from single-slice scanning to multi-slice scanning. In this paper, a novel multi-slice SPEN MRI method was proposed. In this method, the slice-selective dimension was segmented to lower the specific absorption rate (SAR) and improve the image quality. This segmented method, dubbed SeSPEN method, was theoretically analyzed and demonstrated with phantom, lemon and in vivo rat brain experiments. The experimental results were compared with the results obtained from the spin-echo EPI, spin-echo SPEN method and multi-slice global SPEN method proposed by Frydman and coauthors (abbr. GlSPEN method). All the SPEN images were super-resolved reconstructed using deconvolution method. The results indicate that the SeSPEN method retains the advantage of SPEN MRI with respect to resistance to field inhomogeneity and can provide better signal-to-noise ratio than multi-slice GlSPEN MRI technique. The SeSPEN method has comparable SAR to the GlSPEN method while the T1 signal attenuation effect is alleviated. The proposed method will facilitate the multi-slice SPEN MRI to scan more slices within one scan with better image quality.

Comprehensive reconstruction of multi-shot multi-channel diffusion data using mussels

Echo planar imaging (EPI)-based magnetic resonance imaging (MRI) data are often corrupted by Nyquist ghost artifacts resulting from odd-even shifts of the EPI read-outs. Algorithms that corrects for the Nyquist ghost artifacts rely on calibration scans that are collected prior to the data acquisition. However, a more complex pattern of ghosting artifacts arises when diffusion-weighted data are acquired using segmented k-space EPI read-outs. The additional under-sampling present in the segmented acquisitions and the inter-shot motion during diffusion weighted acquistion cause ghosting artifacts in addition to the EPI ghosting arising from odd-even shifts. We propose a comprehensive method that can remove the Nyquist-ghosting artifacts as well as the inter-shot motion-induced ghosting artifacts in diffusion weighted images in a single step from partial Fourier data without the need for a calibration scan. We show very high quality diffusion data recovery using the proposed method.

EPI Nyquist ghost and geometric distortion correction by two-frame phase labeling

PURPOSE

To develop a new Nyquist ghost and geometric distortion correction method in echo planar imaging (EPI) using parallel imaging.

METHODS

Two frames of EPI data are acquired with normal and phase-labeled sequence. The phase label is applied by modifying the PE prephase gradient to shift the central echo by one echo spacing. GRAPPA weights are trained from both frames and used to reconstruct images from positive or negative echoes in each frame to remove Nyquist ghost. Geometric distortion is then corrected by the B₀ field map generated from the phase difference between positive and negative echo images. Phantom and in vivo experiments at 7 Tesla (T) and 3T were performed to evaluate the proposed method.

RESULTS

Nyquist ghost was greatly reduced in all images even under oblique imaging and poor eddy current conditions, yielding significant improvements over the existing reference scan and image entropy minimization based methods. Image geometries were fully restored after distortion correction. Phantom results indicated that the signal-to-noise ratio efficiency was largely preserved while fMRI results showed no apparent degradation of temporal resolution.

CONCLUSION

The proposed method provides robust correction of both Nyquist ghost and geometric distortion in EPI, and it is particularly suitable for dynamic EPI applications. Magn Reson Med 77:1749-1761, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Fast and simplified mapping of mean axon diameter using temporal diffusion spectroscopy

Mapping axon diameter is of interest for the potential diagnosis and monitoring of various neuronal pathologies. Advanced diffusion-weighted MRI methods have been developed to measure mean axon diameters non-invasively, but suffer major drawbacks that prevent their direct translation into clinical practice, such as complex non-linear data fitting and, more importantly, long scanning times that are usually not tolerable for most human subjects. In the current study, temporal diffusion spectroscopy using oscillating diffusion gradients was used to measure mean axon diameters with high sensitivity to small axons in the central nervous system. Axon diameters have been found to be correlated with a novel metric, DDR⊥ (the rate of dispersion of the perpendicular diffusion coefficient with gradient frequency), which is a model-free quantity that does not require complex data analyses and can be obtained from two diffusion coefficient measurements in clinically relevant times with conventional MRI machines. A comprehensive investigation including computer simulations and animal experiments ex vivo showed that measurements of DDR⊥ agree closely with histological data. In humans in vivo, DDR⊥ was also found to correlate well with reported mean axon diameters in human corpus callosum, and the total scan time was only about 8 min. In conclusion, DDR⊥ may have potential to serve as a fast, simple and model-free approach to map the mean axon diameter of white matter in clinics for assessing axon diameter changes.

Development of a symmetric echo planar imaging framework for clinical translation of rapid dynamic hyperpolarized 13 C imaging

PURPOSE

To develop symmetric echo planar imaging (EPI) and a reference scan framework for hyperpolarized ¹³ C metabolic imaging.

METHODS

Symmetric, ramp-sampled EPI with partial Fourier reconstruction was implemented on a 3T scanner. The framework for acquiring a reference scan on the ¹ H channel and applied to ¹³ C data was described and validated in both phantoms and in vivo metabolism of [1-¹³ C]pyruvate.

RESULTS

Ramp-sampled, symmetric EPI provided a substantial increase in the signal-to-noise ratio of the phantom experiments. The reference scan acquired on the ¹ H channel yielded ¹³ C phantom images that varied in mean signal intensity <2%, compared with ¹³ C images reconstructed with a reference scan directly measured on the ¹³ C channel. The structural similarity index and dynamic time course from in vivo ¹³ C data further support the application of a ¹ H reference scan to ¹³ C data to mitigate Nyquist ghost artifacts.

CONCLUSION

Ramp-sampled, symmetric EPI with spectral-spatial excitation of a single metabolite provides a fast, robust, and clinically efficacious approach to acquire hyperpolarized ¹³ C dynamic molecular imaging data. The gains of this efficient sampling, combined with partial Fourier methods, enables large matrix sizes required for human studies. Magn Reson Med 77:826-832, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Reference-free single-pass EPI Nyquist ghost correction using annihilating filter-based low rank Hankel matrix (ALOHA)

PURPOSE

MR measurements from an echo-planar imaging (EPI) sequence produce Nyquist ghost artifacts that originate from inconsistencies between odd and even echoes. Several reconstruction algorithms have been proposed to reduce such artifacts, but most of these methods require either additional reference scans or multipass EPI acquisition. This article proposes a novel and accurate single-pass EPI ghost artifact correction method that does not require any additional reference data.

THEORY AND METHODS

After converting a ghost correction problem into separate k-space data interpolation problems for even and odd phase encoding, our algorithm exploits an observation that the differential k-space data between the even and odd echoes is a Fourier transform of an underlying sparse image. Accordingly, we can construct a rank-deficient Hankel structured matrix, whose missing data can be recovered using an annihilating filter-based low rank Hankel structured matrix completion approach.

RESULTS

The proposed method was applied to EPI data for both single and multicoil acquisitions. Experimental results using in vivo data confirmed that the proposed method can completely remove ghost artifacts successfully without prescan echoes.

CONCLUSION

Owing to the discovery of the annihilating filter relationship from the intrinsic EPI image property, the proposed method successfully suppresses ghost artifacts without a prescan step. Magn Reson Med 76:1775-1789, 2016. © 2016 International Society for Magnetic Resonance in Medicine.

New spatiotemporal approaches for fully refocused, multislice ultrafast 2D MRI

PURPOSE

Single-scan multislice acquisition schemes play key roles in magnetic resonance imaging. Central among these "ultrafast" experiments stands echo-planar imaging, a technique that although of optimal sampling is challenged by T2* artifacts. Recent studies described alternatives based on spatiotemporal encoding (SPEN), which are particularly robust if implemented in a "full-refocusing" mode. This work extends this modality from the single-slice acquisitions in which it has hitherto been implemented, by introducing a variety of multislice schemes scanning 3D volumes.

METHODS

Multislice SPEN employing either inversion or stimulated echo pulses and timed to fulfill the demands of full refocusing, are demonstrated. The performance of the ensuing methods was examined in "Hybrid" modalities encoding data in k- and direct-space, in low-specific absorption rate stimulated-echo approaches, and in direct-space SPEN approaches.

RESULTS

When applied in phantoms and in in vivo systems, the ensuing single-shot sequences evidenced similar robustness, sensitivity, and resolution qualities as previously discussed 2D single-slice schemes, while enabling a rapid sampling of the third dimension via multislicing.

CONCLUSION

The unique benefits deriving from fully refocused, multislice, single-scan SPEN sequences were corroborated by phantom tests, as well as by in vivo scans at 3 and 7 T. Low specific absorption rate multislice SPEN variants compatible with human studies were demonstrated.

Imaging of amide proton transfer and nuclear Overhauser enhancement in ischemic stroke with corrections for competing effects

Chemical exchange saturation transfer (CEST) potentially provides the ability to detect small solute pools through indirect measurements of attenuated water signals. However, CEST effects may be diluted by various competing effects, such as non-specific magnetization transfer (MT) and asymmetric MT effects, water longitudinal relaxation (T1 ) and direct water saturation (radiofrequency spillover). In the current study, CEST images were acquired in rats following ischemic stroke and analyzed by comparing the reciprocals of the CEST signals at three different saturation offsets. This combined approach corrects the above competing effects and provides a more robust signal metric sensitive specifically to the proton exchange rate constant. The corrected amide proton transfer (APT) data show greater differences between the ischemic and contralateral (non-ischemic) hemispheres. By contrast, corrected nuclear Overhauser enhancements (NOEs) around -3.5 ppm from water change over time in both hemispheres, indicating whole-brain changes that have not been reported previously. This study may help us to better understand the contrast mechanisms of APT and NOE imaging in ischemic stroke, and may also establish a framework for future stroke measurements using CEST imaging with spillover, MT and T1 corrections.

A combined analytical solution for chemical exchange saturation transfer and semi-solid magnetization transfer

Off-resonant RF irradiation in tissue indirectly lowers the water signal by saturation transfer processes: on the one hand, there are selective chemical exchange saturation transfer (CEST) effects originating from exchanging endogenous protons resonating a few parts per million from water; on the other hand, there is the broad semi-solid magnetization transfer (MT) originating from immobile protons associated with the tissue matrix with kilohertz linewidths. Recently it was shown that endogenous CEST contrasts can be strongly affected by the MT background, so corrections are needed to derive accurate estimates of CEST effects. Herein we show that a full analytical solution of the underlying Bloch-McConnell equations for both MT and CEST provides insights into their interaction and suggests a simple means to isolate their effects. The presented analytical solution, based on the eigenspace solution of the Bloch-McConnell equations, extends previous treatments by allowing arbitrary lineshapes for the semi-solid MT effects and simultaneously describing multiple CEST pools in the presence of a large MT pool for arbitrary irradiation. The structure of the model indicates that semi-solid MT and CEST effects basically add up inversely in determining the steady-state Z-spectrum, as previously shown for direct saturation and CEST effects. Implications for existing previous CEST analyses in the presence of a semi-solid MT are studied and discussed. It turns out that, to accurately quantify CEST contrast, a good reference Z-value, the observed longitudinal relaxation rate of water, and the semi-solid MT pool size fraction must all be known.

On the origins of chemical exchange saturation transfer (CEST) contrast in tumors at 9.4 T

Chemical exchange saturation transfer (CEST) provides an indirect means to detect exchangeable protons within tissues through their effects on the water signal. Previous studies have suggested that amide proton transfer (APT) imaging, a specific form of CEST, detects endogenous amide protons with a resonance frequency offset 3.5 ppm downfield from water, and thus may be sensitive to variations in mobile proteins/peptides in tumors. However, as CEST measurements are influenced by various confounding effects, such as spillover saturation, magnetization transfer (MT) and MT asymmetry, the mechanism or degree of increased APT signal in tumors is not certain. In addition to APT, nuclear Overhauser enhancement (NOE) effects upfield from water may also provide distinct information on tissue composition. In the current study, APT, NOE and several other MR parameters were measured and compared comprehensively in order to elucidate the origins of APT and NOE contrasts in tumors at 9.4 T. In addition to conventional CEST methods, a new intrinsic inverse metric was applied to correct for relaxation and other effects. After corrections for spillover, MT and T1 effects, corrected APT in tumors was found not to be significantly different from that in normal tissues, but corrected NOE effects in tumors showed significant decreases compared with those in normal tissues. Biochemical measurements verified that there was no significant enhancement of protein contents in the tumors studied, consistent with the corrected APT measurements and previous literature, whereas quantitative MT data showed decreases in the fractions of immobile macromolecules in tumors. Our results may assist in the better understanding of the contrast depicted by CEST imaging in tumors, and in the development of improved APT and NOE measurements for cancer imaging.

Inverse Z-spectrum analysis for spillover-, MT-, and T1 -corrected steady-state pulsed CEST-MRI--application to pH-weighted MRI of acute stroke

Endogenous chemical exchange saturation transfer (CEST) effects are always diluted by competing effects, such as direct water proton saturation (spillover) and semi-solid macromolecular magnetization transfer (MT). This leads to unwanted T2 and MT signal contributions that lessen the CEST signal specificity to the underlying biochemical exchange processes. A spillover correction is of special interest for clinical static field strengths and protons resonating near the water peak. This is the case for all endogenous CEST agents, such as amide proton transfer, -OH-CEST of glycosaminoglycans, glucose or myo-inositol, and amine exchange of creatine or glutamate. All CEST effects also appear to be scaled by the T1 relaxation time of water, as they are mediated by the water pool. This forms the motivation for simple metrics that correct the CEST signal. Based on eigenspace theory, we propose a novel magnetization transfer ratio (MTRRex ), employing the inverse Z-spectrum, which eliminates spillover and semi-solid MT effects. This metric can be simply related to Rex , the exchange-dependent relaxation rate in the rotating frame, and ka , the inherent exchange rate. Furthermore, it can be scaled by the duty cycle, allowing for simple translation to clinical protocols. For verification, the amine proton exchange of creatine in solutions with different agar concentrations was studied experimentally at a clinical field strength of 3 T, where spillover effects are large. We demonstrate that spillover can be properly corrected and that quantitative evaluation of pH and creatine concentration is possible. This proves that MTRRex is a quantitative and biophysically specific CEST-MRI metric. Applied to acute stroke induced in rat brain, the corrected CEST signal shows significantly higher contrast between the stroke area and normal tissue, as well as less B1 dependence, than conventional approaches.

Quantitative magnetization transfer imaging of rodent glioma using selective inversion recovery

Magnetization transfer (MT) provides an indirect means to detect noninvasively variations in macromolecular contents in biological tissues, but, so far, there have been only a few quantitative MT (qMT) studies reported in cancer, all of which used off-resonance pulsed saturation methods. This article describes the first implementation of a different qMT approach, selective inversion recovery (SIR), for the characterization of tumor in vivo using a rodent glioma model. The SIR method is an on-resonance method capable of fitting qMT parameters and T1 relaxation time simultaneously without mapping B0 and B1 , which is very suitable for high-field qMT measurements because of the lower saturation absorption rate. The results show that the average pool size ratio (PSR, the macromolecular pool versus the free water pool) in rat 9 L glioma (5.7%) is significantly lower than that in normal rat gray matter (9.2%) and white matter (17.4%), which suggests that PSR is potentially a sensitive imaging biomarker for the assessment of brain tumor. Despite being less robust, the estimated MT exchange rates also show clear differences from normal tissues (19.7 Hz for tumors versus 14.8 and 10.2 Hz for gray and white mater, respectively). In addition, the influence of confounding effects, e.g. B1 inhomogeneity, on qMT parameter estimates is investigated with numerical simulations. These findings not only help to better understand the changes in the macromolecular contents of tumors, but are also important for the interpretation of other imaging contrasts, such as chemical exchange saturation transfer of tumors.

Referenceless reconstruction of spatiotemporally encoded imaging data: principles and applications to real-time MRI

PURPOSE

Ultrafast sequences based on "Hybrid" spatiotemporal encoding (SPEN) replace echo-planar imaging's phase encoding "blips," while retaining a k-space readout acquisition. Hardware imperfections during acquisition may lead to ghosts and striped artifacts along the SPEN dimension; akin to echo-planar imaging's Nyquist ghosts, but weaker. A referenceless method to eliminate these artifacts in Hybrid SPEN is demonstrated.

THEORY AND METHODS

Owing to its encoding in direct space, rather than reciprocal space, undersampling in SPEN does not generate an echo-planar-imaging-like aliasing, but instead lowers the spatial resolution. Hybrid SPEN data can be split into two undersampled signals: a reference one comprised of the odd-echos, and an even-echo set that has to be "corrected" for consistency with the former. A simple way of implementing such a correction that enables a joint high-resolution reconstruction is proposed.

RESULTS

The referenceless algorithm is demonstrated with various examples, including oblique scans, large in vivo datasets from real-time dynamic contrast-enhanced perfusion experiments, and human brain imaging.

CONCLUSIONS

The referenceless correction enables robust single-scan imaging under changing conditions-such as patient motion and changes in shimming over time-without the need of ancillary navigators. This opens new options for real-time MRI and interactive scanning.

Diffusion weighted MRI by spatiotemporal encoding: analytical description and in vivo validations

Diffusion-weighted (DW) MRI is a powerful modality for studying microstructure in normal and pathological tissues. The accuracy derived from DW MRI depends on the acquisition of quality images, and on a precise assessment of the b-values involved. Conventional DW MRI tends to be of limited use in regions suffering from large magnetic field or chemical shift heterogeneities, which severely distort the MR images. In this study we propose novel sequences based on SPatio-temporal ENcoding (SPEN), which overcome such shortcomings owing to SPEN's inherent robustness to offsets. SPEN, however, relies on the simultaneous application of gradients and radiofrequency-swept pulses, which may impart different diffusion weightings along the spatial axes. These will be further complicated in DW measurements by the diffusion-sensitizing gradients, and will in general lead to complex, spatially-dependent b-values. This study presents a formalism for analyzing these diffusion-weighted SPEN (dSPEN) data, which takes into account the concomitant effects of adiabatic pulses, of the imaging as well as diffusion gradients, and of the cross-terms between them. These analytical b-values derivations are subject to experimental validations in phantom systems and ex vivo spinal cords. Excellent agreement is found between the theoretical predictions and these dSPEN experiments. The ensuing methodology is then demonstrated by in vivo mapping of diffusion in human breast - organs where conventional k-space DW acquisition methods are challenged by both field and chemical shift heterogeneities. These studies demonstrate the increased robustness of dSPEN vis-à-vis comparable DW echo planar imaging, and demonstrate the value of this new methodology for medium- or high-field diffusion measurements in heterogeneous systems.

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.

3D phase unwrapping using global expected phase as a reference: application to MRI global shimming

MRI phase data often suffers from phase wrapping (i.e., phase may be discontinuous by 2π jumps). Numerous MRI phase unwrapping strategies were developed in the past using a criterion based on phase information of local or neighboring voxels. In this study, an alternative and novel three dimensional phase unwrapping strategy is introduced. This method considers the global character of the phase distribution and utilizes continuous trigonometric functions to construct an expected phase map as an unwrapping reference, which is then used to guide the phase correction of every individual voxel. The original phase is estimated by analyzing the derivative of the wrapped phase image. Simulations of various phase wrapped situations were performed and this new method was also used for an in vivo application (i.e., MRI automatic global shimming). Both simulated and experimental results demonstrate that our proposed method is more reliable and robust than traditional algorithms at obtaining correct phase maps, especially in regions of low-signal and air cavities, such as the abdomen and pelvis.

Functional MRI using super-resolved spatiotemporal encoding

Recently, new ultrafast imaging sequences such as rapid acquisition by sequential excitation and refocusing (RASER) and hybrid spatiotemporal encoding (SPEN) magnetic resonance imaging (MRI) have been proposed, in which the phase encoding of conventional echo planar imaging (EPI) is replaced with a SPEN. In contrast to EPI, SPEN provides significantly higher immunity to frequency heterogeneities including those caused by B(0) inhomogeneities and chemical shift offsets. Utilizing the inherent robustness of SPEN, it was previously shown that RASER can be used to successfully perform functional MRI (fMRI) experiments in the orbitofrontal cortex--a task which is challenging using EPI due to strong magnetic susceptibility variation near the air-filled sinuses. Despite this superior performance, systematic analyses have shown that, in its initial implementation, the use of SPEN was penalized by lower signal-to-noise ratio (SNR) and higher radiofrequency power deposition as compared to EPI-based methods. A recently developed reconstruction algorithm based on super-resolution principles is able to alleviate both of these shortcomings; the use of this algorithm is hereby explored within an fMRI context. Specifically, a series of fMRI measurements on the human visual cortex confirmed that the super-resolution algorithm retains the statistical significance of the blood oxygenation level dependent (BOLD) response, while significantly reducing the power deposition associated with SPEN and restoring the SNR to levels that are comparable with those of EPI.

A correction method for streak artifacts in gradient-echo EPI using spin-echo EPI reference data

OBJECTIVE

To analyze the streak artifacts in a gradient-echo echo planar imaging (GE-EPI) sequence and to propose a correction method for the Nyquist ghost artifacts that does not cause streak artifacts in the GE-EPI imaging.

MATERIALS AND METHODS

Several GE-EPI imaging experiments with various reference scans, using both GE-EPI and SE-EPI scan data, were performed to analyze the streak artifacts and to investigate the spin dephasing phenomena of the GE-EPI reference scan. In addition, the analysis based on the spin dephasing was undertaken in order to demonstrate that the SE-EPI reference data can be used for the correction of the GE-EPI main scan data.

RESULTS

The experimental results confirmed that the improvement of the reference data using either signal averaging or a large flip angle cannot guarantee perfect correction of the streak artifact if the noise is not completely removed. Due to the main field inhomogeneity, the spins of the GE-EPI reference data were dephased in multiple echo signals. The proposed correction method, which uses a SE-EPI reference scan for the GE-EPI images, eliminates the N/2 ghost artifacts without producing streak artifacts.

CONCLUSION

It is believed that the proposed phase error correction scheme can improve the EPI performance in high field MRIs with higher magnetic field inhomogeneities.

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.

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.

Two-dimensional phase cycled reconstruction for inherent correction of echo-planar imaging Nyquist artifacts

The inconsistency of k-space trajectories results in Nyquist artifacts in echo-planar imaging (EPI). Traditional techniques often only correct for phase errors along the frequency-encoding direction (one-dimensional correction), which may leave significant residual artifacts, particularly for oblique-plane EPI or in the presence of cross-term eddy currents. As compared with one-dimensional correction, two-dimensional (2D) phase correction can be much more effective in suppressing Nyquist artifacts. However, most existing 2D correction methods require reference scans and may not be generally applicable to different imaging protocols. Furthermore, EPI reconstruction with these 2D phase correction methods is susceptible to error amplification due to subject motion. To address these limitations, we report an inherent and general 2D phase correction technique for EPI Nyquist removal. First, a series of images are generated from the original dataset, by cycling through different possible values of phase errors using a 2D reconstruction framework. Second, the image with the lowest artifact level is identified from images generated in the first step using criteria based on background energy in sorted and sigmoid-weighted signals. In this report, we demonstrate the effectiveness of our new method in removing Nyquist ghosts in single-shot, segmented and parallel EPI without acquiring additional reference scans and the subsequent error amplifications.

Contrast-enhanced whole-heart coronary magnetic resonance angiography at 3 T using interleaved echo planar imaging

OBJECTIVES

The goal of this work was to reduce the scan time of contrast-enhanced whole-heart coronary magnetic resonance angiography (MRA) by using a gradient echo interleaved echo planar imaging (GRE-EPI) sequence at 3 T field strength.

MATERIALS AND METHODS

A GRE-EPI sequence was optimized to acquire contrast-enhanced whole-heart coronary MRA at 3 T. First-order phase correction was used for alignment of the odd and even echoes in the GRE-EPI echo train. Single and dual reference scan techniques for estimation of the linear phase correction parameters were evaluated using both phantom and volunteer studies. The GRE-EPI readout was combined with parallel imaging for a further reduction in scan time. To avoid image distortions, calibration signals for coil sensitivity estimation were acquired in a separate low resolution GRE scan before the whole-heart GRE-EPI scan. Eight healthy volunteers were scanned with the optimized contrast-enhanced GRE-EPI sequence. GRE-EPI images were acquired during slow infusion (0.3 mL/s) of 0.1 mmol/kg body weight of Gd-BOPTA. For comparison purposes, the same 8 volunteers were scanned again in a separate scan session using a traditional GRE sequence with double the dose (0.2 mmol/kg body weight) of the same contrast agent with the same injection rate. The contrast-enhanced GRE-EPI and contrast-enhanced GRE techniques were compared in terms of relative signal-to-noise ratio (rSNR), relative contrast-to-noise ratio (rCNR), image quality scores, and visualized vessel lengths.

RESULTS

Both, phantom and volunteer studies demonstrated that the dual reference scan phase correction technique was a key step for obtaining satisfactory image quality using GRE-EPI at 3 T. Whole-heart coronary MRA with a spatial resolution of 1.0 x 1.0 x 2.0 mm3 was acquired with the GRE-EPI sequence in an average scan time of 2.5 +/- 0.6 minutes, compared with 8.6 +/- 2.7 minutes for the GRE technique. The GRE-EPI technique had lower rCNR compared with the GRE sequence. The image quality and coronary artery visualization with the GRE-EPI technique were adequate, and there was no statistically significant difference in the image quality scores, rSNR, and visualized coronary artery lengths between the GRE-EPI and GRE techniques.

CONCLUSIONS

Contrast-enhanced whole-heart coronary MRA using the GRE-EPI technique resulted in excellent delineation of all the major coronary arteries and compared with current GRE techniques demonstrated a factor of 2 reduction in contrast agent dose and a factor of 3 reduction in scan time.

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.

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 B(0) fields, substantial gains in Nyquist ghost correction of echo planar time series are possible by alternating the readout gradient every other volume.

Influence of selecting EPI readout-encoding bandwidths on arterial spin labeling perfusion MRI

OBJECT

The objective of this study was to investigate effects of varying readout bandwidths on the arterial spin labeling (ASL)-perfusion MRI measurements at a high magnetic field MRI system.

MATERIALS AND METHODS

Brain perfusion studies were performed on nine volunteers (four males, five females) using flow sensitive alternating inversion recovery (FAIR) ASL single-shot echo-planar imaging (EPI)-MRI. To investigate EPI bandwidth effects on the time-series perfusion-weighted imaging (PWI) data, two regions-of-interest (ROI) were placed outside the brain to determine the level of noise and another ROI inside the brain to determine the level of signal. Coefficients of variations (CoV) were calculated for the time-series PWI data. One-way analysis of variance (ANOVA) was used to investigate voxel-wise differences in the time-series PWI data between two different bandwidth values.

RESULTS

At the level of ROI, there was no significant effect of changing EPI bandwidths on the time-series PWI data in any of the volunteers (P > 0.031). In contrast, CoV values over the dynamic PWI data varied with depending on selecting EPI bandwidths and voxel-based tests showed that N2 ghosting, modulated by EPI bandwidth, can appear in some brain regions, especially in areas that overlap with the spatial distribution of N2 ghosting artifacts.

CONCLUSIONS

Although N2 ghosting can be reduced by adjusting the bandwidth of EPI on the time-series of PWI data, the effects cannot be entirely eliminated. In particular, N2 ghosting can bias CBF quantification if EPI control scans to determine the equilibrium-state signal are confounded by N2 ghosting. Therefore, careful tuning of the bandwidth of EPI is necessary to avoid artifacts in the ASL signal from N2-ghosting.

Snapshot gradient-recalled echo-planar images of rat brains at long echo time at 9.4 T

With improved B 0 homogeneity along with satisfactory gradient performance at high magnetic fields, snapshot gradient-recalled echo-planar imaging (GRE-EPI) would perform at long echo times (TEs) on the order of T2*, which intrinsically allows obtaining strongly T2*-weighted images with embedded substantial anatomical details in ultrashort time. The aim of this study was to investigate the feasibility and quality of long TE snapshot GRE-EPI images of rat brain at 9.4 T. When compensating for B 0 inhomogeneities, especially second-order shim terms, a 200 x 200 microm2 in-plane resolution image was reproducibly obtained at long TE (>25 ms). The resulting coronal images at 30 ms had diminished geometric distortions and, thus, embedded substantial anatomical details. Concurrently with the very consistent stability, such GRE-EPI images should permit to resolve functional data not only with high specificity but also with substantial anatomical details, therefore allowing coregistration of the acquired functional data on the same image data set.

Doubling the resolution of echo-planar brain imaging by acquisition of two k-space lines per gradient reversal using TRAIL

Single-shot echo-planar imaging (EPI) is an important method for MRI of the brain. A method has been developed to double the resolution of EPI in the phase-encode direction, without requiring increases in the maximum gradient amplitude or slew rate. The new approach is based on an EPI implementation of the TRAIL (two reduced acquisitions interleaved) method, in which two images, acquired in rapid succession, are spatially interleaved. In addition, two lines of k-space are acquired for each reversal of the readout gradient. Two full-length readouts are needed, therefore power deposition is increased and the total acquisition time is doubled compared with conventional EPI. However, the individual readouts do not increase in length, so there is no increase in image blurring, and distortion is halved as a result of the closer temporal spacing of the acquired k-space lines. A correction method is also presented to remove additional potential Nyquist ghosting. The new method is demonstrated in vivo at 4.7 T and could in principle be combined with existing approaches for increasing resolution, such as partial Fourier or parallel imaging.