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.     

Online motion correction for diffusion-weighted imaging using navigator echoes: Application to RARE imaging without sensitivity loss.

This article describes the first application of true online motion correction to diffusion-weighted RARE imaging. Two orthogonal navigator echoes were acquired and zeroth and first-order phase corrections applied in less than 8 ms between a diffusion-weighted magnetization preparation and data acquisition using the RARE sequence. The zeroth-order phase correction was realized by pulsing the system's B(0)-coil: the first-order error corrected with appropriate magnetic field gradient pulses. Online correction ensured that no irreversible signal loss could occur in the imaging experiment. Diffusion-weighted images of the brain were obtained from healthy volunteers. EGG-triggered acquisition was applied at 400 ms after the R-wave. Data were acquired on a matrix of 256 x 256 with a RARE factor of 16 and a b-value of 804 smm(-2). The images obtained with online motion correction showed a remarkably high image quality, while those acquired without motion correction were severely degraded by artifacts.     

High-resolution DTI of a localized volume using 3D single-shot diffusion-weighted STimulated echo-planar imaging (3D ss-DWSTEPI).

Diffusion tensor MRI (DTI) using conventional single-shot (SS) 2D diffusion-weighted (DW)-EPI is subject to severe susceptibility artifacts. Multishot DW imaging (DWI) techniques can reduce these distortions, but they generally suffer from artifacts caused by motion-induced phase errors. Parallel imaging can also reduce the distortions if the sensitivity profiles of the receiver coils allow a sufficiently high reduction factor for the desired field of view (FOV). A novel 3D DTI technique, termed 3D single-shot STimulated EPI (3D ss-STEPI), was developed to acquire high-resolution DW images of a localized region. The new technique completes k-space acquisition of a limited 3D volume after a single diffusion preparation. Because the DW magnetization is stored in the longitudinal direction until readout, it undergoes T(1) rather than T(2) decay. Inner volume imaging (IVI) is used to limit the imaging volume. This reduces the time required for EPI readout of each complete k(x)-k(y) plane, and hence reduces T(2)(*) decay during the readout and T(1) decay between the readout of each k(z). 3D ss-STEPI images appear to be free of severe susceptibility and motion artifacts. 3D ss-STEPI allows high-resolution DTI of limited volumes of interest, such as localized brain regions, cervical spinal cord, optic nerve, and other extracranial organs.     

PROPELLER EPI: an MRI technique suitable for diffusion tensor imaging at high field strength with reduced geometric distortions.

A technique suitable for diffusion tensor imaging (DTI) at high field strengths is presented in this work. The method is based on a periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) k-space trajectory using EPI as the signal readout module, and hence is dubbed PROPELLER EPI. The implementation of PROPELLER EPI included a series of correction schemes to reduce possible errors associated with the intrinsically higher sensitivity of EPI to off-resonance effects. Experimental results on a 3.0 Tesla MR system showed that the PROPELLER EPI images exhibit substantially reduced geometric distortions compared with single-shot EPI, at a much lower RF specific absorption rate (SAR) than the original version of the PROPELLER fast spin-echo (FSE) technique. For DTI, the self-navigated phase-correction capability of the PROPELLER EPI sequence was shown to be effective for in vivo imaging. A higher signal-to-noise ratio (SNR) compared to single-shot EPI at an identical total scan time was achieved, which is advantageous for routine DTI applications in clinical practice.     

Navigated Diffusion Imaging of Normal and Ischemic Human Brain

The principal barrier to clinical application of diffusion-weighted MR imaging is the severe image degradation caused by patient motion. One way to compensate for motion effects is the use of a “navigator echo” phase correction scheme. In this work, a modification of this technique is introduced, in which the phase correction step is performed in the frequency domain (i.e., after the readout Fourier transform). This significantly improves the robustness of the navigator echo approach and, when combined with cardiac gating, allows diagnostic quality diffusion-weighted images of the brain to be routinely obtained on standard clinical scanner hardware. The technique was evaluated in phantom studies and in 23 humans (3 normal volunteers and 20 patients). Diffusion anisotropy and apparent diffusion coefficient maps were generated from the image data and showed decreased apparent diffusion in acute stroke lesions and, in several cases, increased apparent diffusion in chronic stroke lesions.     

Partial k-space reconstruction in single-shot diffusion-weighted echo-planar imaging.

Partial k-space sampling is frequently used in single-shot diffusion-weighted echo-planar imaging (DW-EPI) to reduce the TE and thereby improve the SNR. However, it increases the sensitivity of the technique to bulk rotational motion, which introduces a phase gradient across the tissue that shifts the echo in k-space. If the echo is displaced into the high spatial frequencies, conventional homodyne reconstruction fails, causing intensity oscillations across the image. Zero-padding, on the other hand, compromises the image resolution and may cause truncation artifacts. We present an adaptive version of the homodyne algorithm that detects the location of the echo in k-space and adjusts the center and width of the homodyne filters accordingly. The adaptive algorithm produces artifact-free images when the echo is shifted into the high positive k-space range, and reduces to the standard homodyne algorithm in the absence of bulk motion.     

3D diffusion tensor MRI with isotropic resolution using a steady‐state radial acquisition

Purpose

To obtain diffusion tensor images (DTI) over a large image volume rapidly with 3D isotropic spatial resolution, minimal spatial distortions, and reduced motion artifacts, a diffusion‐weighted steady‐state 3D projection (SS 3DPR) pulse sequence was developed.

Materials and Methods

A diffusion gradient was inserted in a SS 3DPR pulse sequence. The acquisition was synchronized to the cardiac cycle, linear phase errors were corrected along the readout direction, and each projection was weighted by measures of consistency with other data. A new iterative parallel imaging reconstruction method was also implemented for removing off‐resonance and undersampling artifacts simultaneously.

Results

The contrast and appearance of both the fractional anisotropy and eigenvector color maps were substantially improved after all correction techniques were applied. True 3D DTI datasets were obtained in vivo over the whole brain (240 mm field of view in all directions) with 1.87 mm isotropic spatial resolution, six diffusion encoding directions in under 19 minutes.

Conclusion

A true 3D DTI pulse sequence with high isotropic spatial resolution was developed for whole brain imaging in under 20 minutes. To minimize the effects of brain motion, a cardiac synchronized, multiecho, DW‐SSFP pulse sequence was implemented. Motion artifacts were further reduced by a combination of linear phase correction, corrupt projection detection and rejection, sampling density reweighting, and parallel imaging reconstruction. The combination of these methods greatly improved the quality of 3D DTI in the brain. J. Magn. Reson. Imaging 2009;29:1175–1184. © 2009 Wiley‐Liss, Inc.     

Inherently self-calibrating non-Cartesian parallel imaging.

The use of self-calibrating techniques in parallel magnetic resonance imaging eliminates the need for coil sensitivity calibration scans and avoids potential mismatches between calibration scans and subsequent accelerated acquisitions (e.g., as a result of patient motion). Most examples of self-calibrating Cartesian parallel imaging techniques have required the use of modified k-space trajectories that are densely sampled at the center and more sparsely sampled in the periphery. However, spiral and radial trajectories offer inherent self-calibrating characteristics because of their densely sampled center. At no additional cost in acquisition time and with no modification in scanning protocols, in vivo coil sensitivity maps may be extracted from the densely sampled central region of k-space. This work demonstrates the feasibility of self-calibrated spiral and radial parallel imaging using a previously described iterative non-Cartesian sensitivity encoding algorithm.     

Simultaneous phase correction and SENSE reconstruction for navigated multi-shot DWI with non-cartesian k-space sampling.

Phase-navigated multi-shot acquisition and parallel imaging are two techniques that have been applied to diffusion-weighted imaging (DWI) to diminish distortions and to enhance spatial resolution. Specifically, sensitivity encoding (SENSE) has been combined with single-shot echo planar imaging (EPI). Thus far, it has been difficult to apply parallel imaging methods, like SENSE, to multi-shot DWI because motion-induced phase error varies from shot to shot and interferes with sensitivity encoding. Although direct phase subtraction methods have been introduced to correct this phase error, they generally are not suitable for SENSE reconstruction, and they cannot remove all the motion artifacts even if the phase error is fully known. Here, an effective algorithm is proposed to correct the motion-induced phase error using an iterative reconstruction. In this proposed conjugate-gradient (CG) algorithm, the phase error is treated as an image encoding function. Given the complex perturbation terms, diffusion-weighted images can be reconstructed using an augmented sensitivity map. The mathematical formulation and image reconstruction procedures of this algorithm are similar to the SENSE reconstruction. By defining a dynamic composite sensitivity, the CG phase correction method can be conveniently incorporated with SENSE reconstruction for the application of multi-shot SENSE DWI. Effective phase correction and multi-shot SENSE DWI (R = 1 to 3) are demonstrated on both simulated and in vivo data acquired with PROPELLER and SNAILS.     

Direct parallel image reconstructions for spiral trajectories using GRAPPA.

The use of spiral trajectories is an efficient way to cover a desired k-space partition in magnetic resonance imaging (MRI). Compared to conventional Cartesian k-space sampling, it allows faster acquisitions and results in a slight reduction of the high gradient demand in fast dynamic scans, such as in functional MRI (fMRI). However, spiral images are more susceptible to off-resonance effects that cause blurring artifacts and distortions of the point-spread function (PSF), and thereby degrade the image quality. Since off-resonance effects scale with the readout duration, the respective artifacts can be reduced by shortening the readout trajectory. Multishot experiments represent one approach to reduce these artifacts in spiral imaging, but result in longer scan times and potentially increased flow and motion artifacts. Parallel imaging methods are another promising approach to improve image quality through an increase in the acquisition speed. However, non-Cartesian parallel image reconstructions are known to be computationally time-consuming, which is prohibitive for clinical applications. In this study a new and fast approach for parallel image reconstructions for spiral imaging based on the generalized autocalibrating partially parallel acquisitions (GRAPPA) methodology is presented. With this approach the computational burden is reduced such that it becomes comparable to that needed in accelerated Cartesian procedures. The respective spiral images with two- to eightfold acceleration clearly benefit from the advantages of parallel imaging, such as enabling parallel MRI single-shot spiral imaging with the off-resonance behavior of multishot acquisitions.     

Inherent correction of motion-induced phase errors in multishot spiral diffusion-weighted imaging

Multishot spiral imaging is a promising alternative to echo‐planar imaging for high‐resolution diffusion‐weighted imaging and diffusion tensor imaging. However, subject motion in the presence of diffusion‐weighting gradients causes phase inconsistencies among different shots, resulting in signal loss and aliasing artifacts in the reconstructed images. Such artifacts can be reduced using a variable‐density spiral trajectory or a navigator echo, however at the cost of a longer scan time. Here, a novel iterative phase correction method is proposed to inherently correct for the motion‐induced phase errors without requiring any additional scan time. In this initial study, numerical simulations and in vivo experiments are performed to demonstrate that the proposed method can effectively and efficiently correct for spatially linear phase errors caused by rigid‐body motion in multishot spiral diffusion‐weighted imaging of the human brain. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Accelerating MR diffusion tensor imaging via filtered reduced-encoding projection-reconstruction.

MR diffusion tensor imaging (DTI) is a promising tool for characterizing the microstructure of ordered tissues. However, its practical applications are hampered by relatively low signal-to-noise-ratio and spatial and temporal resolution. Reduced-encoding imaging (REI) via k-space sharing with constrained reconstruction has previously been shown to be effective for accelerating DTI, although the implementation was based on rectilinear k-space sampling. Due to the intrinsic oversampling of central k-space and allowance for isotropic downsampling, projection-reconstruction (PR) imaging may be better suited for REI. In this study, regularization procedures, including radial filtering and baseline signal correction to adequately reconstruct reduced encoded PR imaging data, are investigated. The proposed filtered reduced-encoding projection-reconstruction (FREPR) technique is applied to DTI tissue fiber orientation and fractional anisotropy (FA) measurements. Results show that FREPR offers improved reconstructions of the reduced encoded images and on an equal total scan-time basis provides more accurate fiber orientation and FA measurements compared to rectilinear k-space sampling-based REI methods or a control experiment consisting of only fully encoded images. These findings suggest a potentially significant role of FREPR in accelerating repeated imaging and improving the data acquisition-time efficiency of DTI experiments.     

High-resolution diffusion-weighted 3D MRI, using diffusion-weighted driven-equilibrium (DW-DE) and multishot segmented 3D-SSFP without navigator echoes.

In this work we report on the development of a novel technique for high-resolution diffusion-weighted (DW) MRI based upon 3D steady-state free precession (3D-SSFP). First the 3D-SSFP acquisition was segmented (each segment consisting of a series of RF pulses and gradient-recalled echoes), and then DW-driven equilibrium (DE) was inserted between each segment. The in-plane imaging matrix was typically 256 x 192 or 256 x 160, which resulted in high-resolution DW images. The DW-DE segmented SSFP signal was contaminated by the non-DW magnetization, which recovered and contributed signal during the readout train (T(1) contamination). Center-out slice encoding was used to place the greatest diffusion weighting at the center of k-space. A numerical simulation and supporting experiments were performed to evaluate the relationship of the transverse magnetization to imaging parameters, such as the b-value, echo-train length (ETL), echo-train (group) repetition time (TR(g)), and RF excitation TR (Delta t). Both the numerical simulation and the experiments suggested that the effect of T(1) contamination would be reduced with a longer TR(g), smaller b-value, shorter ETL, and center-out slice phase encoding. Phase errors caused by microscopic motions during the diffusion gradients were converted into amplitude errors by the tip-up pulse at the end of the diffusion-weighting segment. As a result, small bulk motions, such as CSF pulsation, did not cause motion-related ghosting artifacts, which would be typical in images from other multishot DWI techniques. This technique can be used for high-resolution DWI of nonbrain anatomies.     

Retrospective motion compensation using variable-density spiral trajectories

PURPOSE: To develop a method of retrospectively correcting for motion artifacts using a variable-density spiral (VDS) trajectory. MATERIALS AND METHODS: Each VDS interleaf was designed to adequately sample the same center region of k-space. This central overlapping region can then be used to measure rigid body motion between the acquisition of each VDS interleaf. By applying appropriate phase shifts and rotations of the k-space data, rigid body motion artifacts can be removed, resulting in images with less motion corruption. RESULTS: Both phantom and volunteer experiments are shown, demonstrating the technique's ability to further reduce artifacts in images acquired with an already motion-resistant acquisition trajectory. Registration accuracy is highly dependent on the trajectory design parameters. This space was explored to find an optimal design of VDS trajectories for motion compensation. CONCLUSION: Using appropriately designed VDS trajectories, residual motion artifacts can be significantly reduced by retrospectively correcting for in-plane rigid body motion. An overlapping region of approximately 8% of the central region of k-space and approximately 70 interleaves were found to be near-optimal parameters for retrospective correction using VDS trajectories.     

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.     

Augmented generalized SENSE reconstruction to correct for rigid body motion.

The correction of motion artifacts continues to be a significant problem in MRI. In the case of uncooperative patients, such as children, or patients who are unable to remain stationary, the accurate determination and correction of motion artifacts becomes a very important prerequisite for achieving good image quality. The application of conventional motion-correction strategies often produces inconsistencies in k-space data. As a result, significant residual artifacts can persist. In this work a formalism is introduced for parallel imaging in the presence of motion. The proposed method can improve overall image quality because it diminishes k-space inconsistencies by exploiting the complementary image encoding capacity of individual receiver coils. Specifically, an augmented version of an iterative SENSE reconstruction is used as a means of synthesizing the missing data in k-space. Motion is determined from low-resolution navigator images that are coregistered by an automatic registration routine. Navigator data can be derived from self-navigating k-space trajectories or in combination with other navigation schemes that estimate patient motion. This correction method is demonstrated by interleaved spiral images collected from volunteers. Conventional spiral scans and scans corrected with proposed techniques are shown, and the results illustrate the capacity of this new correction approach.     

In vivo diffusion tensor imaging of the human prostate.

This study demonstrates the feasibility of in vivo prostate diffusion tensor imaging (DTI) in human subjects. We implemented an EPI-based diffusion-weighted (DW) sequence with seven-direction diffusion gradient sensitization, and acquired DT images from six subjects using cardiac gating with a phased-array prostate surface coil operating in a linear mode. We calculated two indices to quantify diffusion anisotropy. The direction of the eigenvector corresponding to the leading eigenvalue was displayed by means of a color-coding scheme. The average diffusion values of the prostate peripheral zone (PZ) and central gland (CG) were 1.95 +/- 0.08 x 10(-3) mm2 s and 1.53 +/- 0.34 x 10(-3) mm2 s, respectively. The average fractional anisotropy (FA) values for the PZ and CG were 0.46 +/- 0.04 and 0.40 +/- 0.08, respectively. The diffusion ellipsoid in prostate tissue was anisotropic and approximated a prolate model, as shown in the color maps of the anisotropy. Consistent with the tissue architecture, the prostate fiber orientations were predominantly in the superior-inferior (SI) direction for both the PZ and CG. This study shows the feasibility of in vivo DTI and establishes normative DT values for six subjects.     

Novel interleaved spiral imaging motion correction technique using orbital navigators.

Although spiral imaging seldom produces apparent artifacts related to flow, it remains sensitive to rapid object motion. In this article, a new correction method is presented for rapid rigid body motion in interleaved spiral imaging. With this technique, an identical circular navigator k-space trajectory is linked to each spiral trajectory. Data inconsistency due to both rotation and translation among spiral interleaves can be corrected by evaluating the magnitudes and phases of the data contained in the navigator "ring." Further, it is difficult to create a frequency field map for off-resonance correction when an object moves during a scan, because there is motion-dependent misregistration between the two images acquired with different TEs. However, this difficulty can be overcome by combining the motion-correction method with a recently proposed technique (off-resonance correction using variable-density spirals (ORC-VDS)), thereby enabling both motion compensation and off-resonance correction with no additional scanning.     

Improvement of spiral MRI with the measured k-space trajectory

The k-space trajectory of a spiral imaging sequence was measured with a self-encoding technique. The image quality improved dramatically when reconstructed with the measured k-space trajectory. There were substantial artifacts in images reconstructed with the derived k-space trajectory under the assumption of gradient system linearity. The results indicated the non-linearity of the gradient system and the effectiveness of the correction technique.     

Single breath-hold whole-heart MRA using variable-density spirals at 3T.

Multislice breath-held coronary imaging techniques conventionally lack the coverage of free-breathing 3D acquisitions but use a considerably shorter acquisition window during the cardiac cycle. This produces images with significantly less motion artifact but a lower signal-to-noise ratio (SNR). By using the extra SNR available at 3 T and undersampling k-space without introducing significant aliasing artifacts, we were able to acquire high-resolution fat-suppressed images of the whole heart in 17 heartbeats (a single breath-hold). The basic pulse sequence consists of a spectral-spatial excitation followed by a variable-density spiral readout. This is combined with real-time localization and a real-time prospective shim correction. Images are reconstructed with the use of gridding, and advanced techniques are used to reduce aliasing artifacts.