EPI image reconstruction with correction of distortion and signal losses

PURPOSE: To derive and implement a method for correcting geometric distortions and recovering magnetic resonance imaging (MRI) signal losses caused by susceptibility-induced magnetic field gradients (SFGs) in regions with large static field inhomogeneities in echo-planar imaging (EPI). MATERIALS AND METHODS: Factors to account for field inhomogeneities and SFGs were added in a traditional EPI equation that was a simple Fourier transform (FT) for expressing the actual k-space data of an EPI scan. The inverse calculation of this "distorted EPI" equation was used as a kernel to correct geometric distortions and reductions in intensity during reconstruction. A step-by-step EPI reconstruction method was developed to prevent complicated phase unwrapping problems. Some EPI images of phantom and human brains were reconstructed from standard EPI k-spaces. RESULTS: All images were reconstructed using the proposed multistep method. Geometric distortions were corrected and SFG-induced MRI signal losses were recovered. CONCLUSION: Results suggest that applying our method for reconstructing EPI images to reduce distortions and MRI signal losses is feasible.     

Inversion recovery radial MRI with interleaved projection sets.

The radial trajectory has found applications in cardiac imaging because of its resilience to undersampling and motion artifacts. Recent work has shown that interleaved and weighted radial imaging can produce images with multiple contrasts from a single data set. This feature was investigated for inversion recovery imaging of scar using a radial technique. The 2D radial imaging method was modified to acquire quadruply interleaved projection sets within each acquisition window of the cardiac cycle. These data were reconstructed using k-space weightings that used a smaller segment of the acquisition window for the central k-space data, the determinant of image contrast. This method generates four images with different T1 weightings. The novel approach was compared with noninterleaved radial imaging, interleaved radial without weightings, and Cartesian imaging in simulations, phantoms, and seven subjects with clinical myocardial infarction. The results show that during a typical acquisition window after an inversion pulse, magnetization changes rapidly. The interleaved acquisition provided better image quality than the noninterleaved radial acquisition. Interleaving with weighting provided better quality when the inversion time (TI) was shorter than optimal; otherwise, interleaving without weighting was superior. These methods enable a radial trajectory to be employed in conjunction with preparation pulses for viability imaging.     

Reduction of magnetic field inhomogeneity artifacts in echo planar imaging with SENSE and GESEPI at high field.

Geometric distortion, signal-loss, and image-blurring artifacts in echo planar imaging (EPI) are caused by frequency shifts and T(2)(*) relaxation distortion of the MR signal along the k-space trajectory due to magnetic field inhomogeneities. The EPI geometric-distortion artifact associated with frequency shift can be reduced with parallel imaging techniques such as SENSE, while the signal-loss and blurring artifacts remain. The gradient-echo slice excitation profile imaging (GESEPI) method has been shown to be successful in restoring tissue T(2)(*) relaxation characteristics and is therefore effective in reducing signal-loss and image-blurring artifacts at a cost of increased acquisition time. The SENSE and GESEPI methods are complementary in artifact reduction. Combining these two techniques produces a method capable of reducing all three types of EPI artifacts while maintaining rapid acquisition time.     

Magnetic field shift due to mechanical vibration in functional magnetic resonance imaging.

Mechanical vibrations of the gradient coil system during readout in echo-planar imaging (EPI) can increase the temperature of the gradient system and alter the magnetic field distribution during functional magnetic resonance imaging (fMRI). This effect is enhanced by resonant modes of vibrations and results in apparent motion along the phase encoding direction in fMRI studies. The magnetic field drift was quantified during EPI by monitoring the resonance frequency interleaved with the EPI acquisition, and a novel method is proposed to correct the apparent motion. The knowledge on the frequency drift over time was used to correct the phase of the k-space EPI dataset. Since the resonance frequency changes very slowly over time, two measurements of the resonance frequency, immediately before and after the EPI acquisition, are sufficient to remove the field drift effects from fMRI time series. The frequency drift correction method was tested "in vivo" and compared to the standard image realignment method. The proposed method efficiently corrects spurious motion due to magnetic field drifts during fMRI.     

Reconstruction of undersampled non-Cartesian data sets using pseudo-Cartesian GRAPPA in conjunction with GROG.

Most k-space-based parallel imaging reconstruction techniques, such as Generalized Autocalibrating Partially Parallel Acquisitions (GRAPPA), necessitate the acquisition of regularly sampled Cartesian k-space data to reconstruct a nonaliased image efficiently. However, non-Cartesian sampling schemes offer some inherent advantages to the user due to their better coverage of the center of k-space and faster acquisition times. On the other hand, these sampling schemes have the disadvantage that the points acquired generally do not lie on a grid and have complex k-space sampling patterns. Thus, the extension of Cartesian GRAPPA to non-Cartesian sequences is nontrivial. This study introduces a simple, novel method for performing Cartesian GRAPPA reconstructions on undersampled non-Cartesian k-space data gridded using GROG (GRAPPA Operator Gridding) to arrive at a nonaliased image. Because the undersampled non-Cartesian data cannot be reconstructed using a single GRAPPA kernel, several Cartesian patterns are selected for the reconstruction. This flexibility in terms of both the appearance and number of patterns allows this pseudo-Cartesian GRAPPA to be used with undersampled data sets acquired with any non-Cartesian trajectory. The successful implementation of the reconstruction algorithm using several different trajectories, including radial, rosette, spiral, one-dimensional non-Cartesian, and zig-zag trajectories, is demonstrated.     

Rapid water and lipid imaging with T2 mapping using a radial IDEAL‐GRASE technique

Three‐point Dixon methods have been investigated as a means to generate water and fat images without the effects of field inhomogeneities. Recently, an iterative algorithm (IDEAL, iterative decomposition of water and fat with echo asymmetry and least squares estimation) was combined with a gradient and spin‐echo acquisition strategy (IDEAL‐GRASE) to provide a time‐efficient method for lipid–water imaging with correction for the effects of field inhomogeneities. The method presented in this work combines IDEAL‐GRASE with radial data acquisition. Radial data sampling offers robustness to motion over Cartesian trajectories as well as the possibility of generating high‐resolution T₂ maps in addition to the water and fat images. The radial IDEAL‐GRASE technique is demonstrated in phantoms and in vivo for various applications including abdominal, pelvic, and cardiac imaging. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Cardiac and respiratory self-gated cine MRI in the mouse: comparison between radial and rectilinear techniques at 7T.

ECG-gated cardiac MRI in the mouse is hindered by many technical difficulties in ECG signal recording inside high magnetic field scanners. The present study proposes a robust rectilinear method of acquiring cardiac and respiratory self-gated cine images in mouse hearts. In this approach, a motion-synchronization MR signal is collected in the center of k-space simultaneously with imaging data in each readout of a nontriggered rectilinear acquisition. This signal is then used for both cardiac and respiratory retrospective gating before cine image reconstruction. The value of this approach for overcoming ECG-gating failure was demonstrated by performing cardiac imaging in eight mice with myocardial infarction. Comparison with an auto-gated radial k-space sampling technique, previously reported for cardiac applications in the mouse, found the rectilinear strategy more robust, thanks to a more reliable self-gating signal, while the radial strategy was less sensitive to motion and flow artifacts.     

K-space in the clinic

Magnetic resonance imaging (MRI) sequences are characterized by both radio frequency (RF) pulses and time-varying gradient magnetic fields. The RF pulses manipulate the alignment of the resonant nuclei and thereby generate a measurable signal. The gradient fields spatially encode the signals so that those arising from one location in an excited slice of tissue may be distinguished from those arising in another location. These signals are collected and mapped into an array called k-space that represents the spatial frequency content of the imaged object. Spatial frequencies indicate how rapidly an image feature changes over a given distance. It is the action of the gradient fields that determines where in the k-space array each data point is located, with the order in which k-space points are acquired being described by the k-space trajectory. How signals are mapped into k-space determines much of the spatial, temporal, and contrast resolution of the resulting images and scan duration. The objective of this article is to provide an understanding of k-space as is needed to better understand basic research in MRI and to make well-informed decisions about clinical protocols. Four major classes of trajectories-echo planar imaging (EPI), standard (non-EPI) rectilinear, radial, and spiral-are explained. Parallel imaging techniques SMASH (simultaneous acquisition of spatial harmonics) and SENSE (sensitivity encoding) are also described.     

Common SENSE (sensitivity encoding using hardware common to all MR scanners): a new method for single-shot segmented echo planar imaging.

A new method is presented that enables image acquisition to be segmented into two readouts. This is achieved using a new pulse sequence that creates two components of magnetization with different spatial profiles. Each component of the magnetization is measured in one of the readouts. This produces two images with complimentary "sensitivity profiles" and near identical contrast. The images can be acquired with a reduced data matrix that corresponds to shorter periods of data acquisition. The reduced matrix images are then combined to produce a full matrix image using reconstruction methods previously applied to images from multiple RF coils in the sensitivity encoding (SENSE) technique.The most promising application for this technique is in improving the performance of echo planar imaging (EPI) at high field. In this application, common SENSE obtains two segments of data in a single excitation of the magnetization (i.e., two readouts are performed per shot). The combination of these segments in image space avoids the difficulties normally associated with segmented EPI methods, namely, increased ghosting from discontinuities in the k-space data. The main advantages are a reduction in distortion and blurring. Common SENSE is compatible with parallel imaging and partial Fourier methods.     

k-space undersampling in PROPELLER imaging.

PROPELLER MRI (periodically rotated overlapping parallel lines with enhanced reconstruction) provides images with significantly fewer B(0)-related artifacts than echo-planar imaging (EPI), as well as reduced sensitivity to motion compared to conventional multiple-shot fast spin-echo (FSE). However, the minimum imaging time in PROPELLER is markedly longer than in EPI and 50% longer than in conventional multiple-shot FSE. Often in MRI, imaging time is reduced by undersampling k-space. In the present study, the effects of undersampling on PROPELLER images were evaluated using simulated and in vivo data sets. Undersampling using PROPELLER patterns with reduced number of samples per line, number of lines per blade, or number of blades per acquisition, while maintaining the same k-space field of view (FOV(k)) and uniform sampling at the edges of FOV(k), reduced imaging time but led to severe image artifacts. In contrast, undersampling by means of removing whole blades from a PROPELLER sampling pattern that sufficiently samples k-space produced only minimal image artifacts, mainly manifested as blurring in directions parallel to the blades removed, even when reducing imaging time by as much as 50%. Finally, undersampling using asymmetric blades and taking advantage of Hermitian symmetries to fill-in the missing data significantly reduced imaging time without causing image artifacts.     

Radial GRASE: implementation and applications.

RAD-GRASE is an MRI sequence that combines radial (RAD) k-space scanning with the gradient and spin-echo (GRASE) technique. RAD-GRASE has the advantages of all radial data acquisition methods in that it can reduce motion sensitivity and correct motion-induced data errors, which can be exploited to achieve high-resolution diffusion-weighted imaging (DWI). One can obtain different types of image contrast, including DWI, T(1), T(2), and T(2)*, in RAD-GRASE by controlling the magnetization preparation and sequence timing. Moreover, because there is oversampling of the low spatial frequencies inherent to radial sequences, partial data reconstruction can be used to achieve multiple forms of image contrast from a single acquired data set, and to generate parametric image maps of equilibrium magnetization, T(2), and T(2) (dagger). The RAD-GRASE technique can also be used to achieve fat-suppressed and/or separated fat and water images by choosing the appropriate timing parameters.     

Respiratory reordered UNFOLD perfusion imaging

PURPOSE: To propose a respiratory reordered UNFOLD (RR-UNFOLD) imaging sequence to significantly reduce the amount of k-space data required for first-pass MR myocardial perfusion imaging. MATERIALS AND METHODS: Rapid acquisition of high-resolution imaging data is essential to detailed quantitative analysis of first-pass myocardial perfusion. Existing MR sequences have explored the full capacity of the imaging hardware to reduce the acquisition window within each cardiac cycle while maintaining the desired spatial resolution. Further improvement in perfusion imaging will require a more efficient use of the information content of the k-space data. The method uses prospective diaphragmatic navigator echoes to ensure that temporal filtering of UNFOLD is carried out on a series of images that are spatially registered. An adaptive real-time rebinning algorithm is developed for the creation of static image subseries related to different levels of respiratory motion. Issues concerning the temporal smoothing of tracer kinetic signals are discussed, and a solution based on oversampling of the central k-space is provided. The method is assessed in 10 normal subjects without the administration of contrast agent, and further validated by administration of Gd-DTPA in 10 patients at rest. RESULTS: The results of this study show that RR-UNFOLD significantly extends the applicability of UNFOLD to perfusion imaging, which yields a 40% reduction in image artifact when the same amount of k-space information is used. CONCLUSION: The scan efficiency achieved can be used in combination with MR hardware improvements for extending the three-dimensional spatial coverage and shortening the data acquisition window to provide detailed information on regional myocardial perfusion abnormalities.     

Homodyne reconstruction and IDEAL water-fat decomposition.

Multipoint water-fat separation methods have received renewed interest because they provide uniform separation of water and fat despite the presence of B0 and B1 field inhomogeneities. Unfortunately, full-resolution reconstruction of partial k-space acquisitions has been incompatible with these methods. Conventional homodyne reconstruction and related algorithms are commonly used to reconstruct partial k-space data sets by exploiting the Hermitian symmetry of k-space in order to maximize the spatial resolution of the image. In doing so, however, all phase information of the image is lost. The phase information of complex source images used in a water-fat separation acquisition is necessary to decompose water from fat. In this work, homodyne imaging is combined with the IDEAL (iterative decomposition of water and fat with echo asymmetry and least squares estimation) method to reconstruct full resolution water and fat images free of blurring. This method is extended to multicoil steady-state free precession and fast spin-echo applications and examples are shown.     

Radial single-shot STEAM MRI.

Rapid MR imaging using the stimulated echo acquisition mode (STEAM) technique yields single-shot images without any sensitivity to resonance offset effects. However, the absence of susceptibility-induced signal voids or geometric distortions is at the expense of a somewhat lower signal-to-noise ratio than EPI. As a consequence, the achievable spatial resolution is limited when using conventional Fourier encoding. To overcome the problem, this study combined single-shot STEAM MRI with radial encoding. This approach exploits the efficient undersampling properties of radial trajectories with use of a previously developed iterative image reconstruction method that compensates for the incomplete data by incorporating a priori knowledge. Experimental results for a phantom and human brain in vivo demonstrate that radial single-shot STEAM MRI may exceed the resolution obtainable by a comparable Cartesian acquisition by a factor of four.     

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

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

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

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

Implementation and assessment of diffusion-weighted partial Fourier readout-segmented echo-planar imaging

Single-shot echo-planar imaging has been used widely in diffusion magnetic resonance imaging due to the difficulties in correcting motion-induced phase corruption in multishot data. Readout-segmented EPI has addressed the multishot problem by introducing a two-dimensional nonlinear navigator correction with online reacquisition of uncorrectable data to enable acquisition of high-resolution diffusion data with reduced susceptibility artifact and T*(2) blurring. The primary shortcoming of readout-segmented EPI in its current form is its long acquisition time (longer than similar resolution single-shot echo-planar imaging protocols by approximately the number of readout segments), which limits the number of diffusion directions. By omitting readout segments at one side of k-space and using partial Fourier reconstruction, readout-segmented EPI imaging times could be reduced. In this study, the effects of homodyne and projection onto convex sets reconstructions on estimates of the fractional anisotropy, mean diffusivity, and diffusion orientation in fiber tracts and raw T(2)- and trace-weighted signal are compared, along with signal-to-noise ratio results. It is found that projections onto convex sets reconstruction with 3/5 segments in a 2 mm isotropic diffusion tensor image acquisition and 9/13 segments in a 0.9 × 0.9 × 4.0 mm(3) diffusion-weighted image acquisition provide good fidelity relative to the full k-space parameters. This allows application of readout-segmented EPI to tractography studies, and clinical stroke and oncology protocols.     

The fast spiral-SelMQC technique for in vivo MR spectroscopic imaging of polyunsaturated fatty acids in human breast tissue

The selective multiple-quantum coherence transfer method has been applied to image polyunsaturated fatty acids (PUFA) distributions in human breast tissues in vivo for cancer detection, with complete suppression of the unwanted lipid and water signals in a single scan. The Cartesian k-space mapping of PUFA in vivo using the selective multiple-quantum coherence transfer (Sel-MQC) chemical shift imaging (CSI) technique, however, requires excessive MR scan time. In this article, we report a fast Spiral-SelMQC sequence using a rapid spiral k-space sampling scheme. The Spiral-SelMQC images of PUFA distribution in human breast were acquired using two-interleaved spirals on a 3 T GE Signa magnetic resonance imaging scanner. Approximately 160-fold reduction of acquisition time was observed as compared with the corresponding selective multiple-quantum coherence transfer CSI method with an equivalent number of scans, permitting acquisition of high-resolution PUFA images in minutes. The reconstructed Spiral-SelMQC PUFA images of human breast tissues achieved a sub-millimeter resolution of 0.54 × 0.54 or 0.63 × 0.63 mm(2) /pixel for field of view = 14 or 16 cm, respectively. The Spiral-SelMQC parameters for PUFA detection were optimized in 2D selective multiple-quantum coherence transfer experiments to suppress monounsaturated fatty acids and other lipid signals. The fast in vivo Spiral-SelMQC imaging method will be applied to study human breast cancer and other human diseases in extracranial organs.     

Off-resonance artifacts correction with convolution in k-space (ORACLE)

Off-resonance artifacts hinder the wider applicability of echo-planar imaging and non-Cartesian MRI methods such as radial and spiral. In this work, a general and rapid method is proposed for off-resonance artifacts correction based on data convolution in k-space. The acquired k-space is divided into multiple segments based on their acquisition times. Off-resonance-induced artifact within each segment is removed by applying a convolution kernel, which is the Fourier transform of an off-resonance correcting spatial phase modulation term. The field map is determined from the inverse Fourier transform of a basis kernel, which is calibrated from data fitting in k-space. The technique was demonstrated in phantom and in vivo studies for radial, spiral and echo-planar imaging datasets. For radial acquisitions, the proposed method allows the self-calibration of the field map from the imaging data, when an alternating view-angle ordering scheme is used. An additional advantage for off-resonance artifacts correction based on data convolution in k-space is the reusability of convolution kernels to images acquired with the same sequence but different contrasts.     

High-resolution diffusion-weighted imaging with interleaved variable-density spiral acquisitions

PURPOSE: To develop a multishot magnetic resonance imaging (MRI) pulse sequence and reconstruction algorithm for diffusion-weighted imaging (DWI) in the brain with submillimeter in-plane resolution. MATERIALS AND METHODS: A self-navigated multishot acquisition technique based on variable-density spiral k-space trajectory design was implemented on clinical MRI scanners. The image reconstruction algorithm takes advantage of the oversampling of the center k-space and uses the densely sampled central portion of the k-space data for both imaging reconstruction and motion correction. The developed DWI technique was tested in an agar gel phantom and three healthy volunteers. RESULTS: Motions result in phase and k-space shifts in the DWI data acquired using multishot spiral acquisitions. With the two-dimensional self-navigator correction, diffusion-weighted images with a resolution of 0.9 x 0.9 x 3 mm3 were successfully obtained using different interleaves ranging from 8-32. The measured apparent diffusion coefficient (ADC) in the homogenous gel phantom was (1.66 +/- 0.09) x 10(-3) mm2/second, which was the same as measured with single-shot methods. The intersubject average ADC from the brain parenchyma of normal adults was (0.91 +/- 0.01) x 10(-3) mm2/second, which was in a good agreement with the reported literature values. CONCLUSION: The self-navigated multishot variable-density spiral acquisition provides a time-efficient approach to acquire high-resolution diffusion-weighted images on a clinical scanner. The reconstruction algorithm based on motion correction in the k-space data is robust, and measured ADC values are accurate and reproducible.