Projection reconstruction balanced fast field echo for interactive real-time cardiac imaging.

A balanced fast field echo (FFE) sequence (also referred to as true fast imaging with steady precession (true FISP)), based on projection reconstruction (PR) is evaluated in combination with real-time reconstruction and interactive scanning capabilities for cardiac function studies. Cardiac image sequences obtained with the balanced PR-FFE method are compared with images obtained with a spin-warp (2D Fourier transform (2DFT)) technique. In particular, the representation of motion artifacts in both techniques is investigated. Balanced PR-FFE provides a similar contrast to spin-warp-related techniques, but is less sensitive to motion artifacts. The use of angular undersampling within balanced PR-FFE is examined as a means to increase temporal resolution while causing only minor artifacts. Furthermore, a modification of the profile order allows the reconstruction of PR images at different spatial and temporal resolution levels from the same data. This study shows that balanced PR-FFE is a robust tool for cardiac function studies.     

GRASE (Gradient- and spin-echo) imaging: a novel fast MRI technique.

A fast multi-section MR imaging technique is described. Gradient- and spin-echo (GRASE) imaging utilizes the speed advantages of gradient refocusing while overcoming the image artifacts arising from static field inhomogeneity and chemical shift. Image contrast is determined by the T2 contrast in the Hahn spin echoes. A novel k-space trajectory temporally modulates signals and demodulates artifacts.     

Echo-planar imaging with asymmetric gradient modulation and inner-volume excitation

Single-shot echo-planar imaging is notoriously vulnerable to image artifacts, arising from the necessity of alternate echo time reversal during image reconstruction and from static field inhomogeneity. A technique for overcoming these problems, which further permits imaging on systems with relatively poor gradient waveforms, when data are collected always with the same gradient polarity, is presented. Subsectional and 3D volume imaging are presented as well as a novel phase-correction method for Hermitian symmetry in "half-Fourier" echo-planar imaging.     

View angle tilting echo planar imaging for distortion correction

Geometric distortion caused by field inhomogeneity along the phase‐encode direction is one of the most prominent artifacts due to a relatively low effective bandwidth along that direction in magnetic resonance echo planar imaging. This work describes a method for correcting in‐plane image distortion along the phase‐encode direction using a view angle tilting imaging technique in spin‐echo echo planar imaging. Spin‐echo echo planar imaging with view angle tilting uses the addition of gradient blips along the slice‐select direction, concurrently applied with the phase‐encode gradient blips, producing an additional phase. This phase effectively offsets an unwanted phase accumulation caused by field inhomogeneity, resulting in the removal of image distortion along the phase‐encode direction. The proposed method is simple and straightforward both in implementation and application with no scan time penalty. Therefore, it is readily applicable on commercial scanners without having any customized postprocessing. The efficacy of the spin‐echo echo planar imaging with view angle tilting technique in the correction of image distortion is demonstrated in phantom and in vivo brain imaging. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Removal of local field gradient artifacts in T2*-weighted images at high fields by gradient-echo slice excitation profile imaging

Development of high magnetic field MRI techniques is hampered by the significant artifacts produced by B₀ field inhomogeneities in the excited slices. A technique, gradient-echo slice excitation profile imaging (GESEPI), is presented for recovering the signal lost caused by intravoxel phase dispersion in T₂*-weighted images. This technique superimposes an incremental gradient offset on the slice refocusing gradient to sample Jr-space over the full range of spatial frequencies of the excitation profile. A third Fourier transform of the initial two-dimensional image set generates an image set in which the artifacts produced by the low-order B₀ inhomogeneity field gradients in the sample are separated and removed from the high-order microscopic field gradients responsible for T₂* contrast. Application to high field brain imaging, at 3.0 T for human and at 9.4 T for immature rat imaging demonstrates the significant improvement in quality of the T₂*-weighted contrast images.     

Reduction of flow artifacts in NMR diffusion imaging using view-angle tilted line-integral projection reconstruction

Most of the diffusion imaging techniques employ strong diffusion gradient pulses of long duration in order to achieve appreciable signal attenuation through the diffusion effect. However, these strong and long gradient pulses make the resultant images extremely sensitive to the motion or flow of the object. Fourier imaging, with which most of the current NMR imaging is performed, is especially sensitive to the fluctuating flow and the images are usually obscured by severe flow artifacts smeared in the phase-encoding direction. In this paper, we have proposed a diffusion imaging technique which reduces the flow artifacts by use of the line-integral projection reconstruction (LPR) imaging method. Furthermore, the inhomogeneity artifacts expected to occur in LPR imaging have been corrected by application of the view-angle tilting technique. The pulse sequence of the view-angle tilted LPR diffusion imaging is designed in such a way that it works for both isotropic and anisotropic diffusion. Experimental results are presented along with the experimental procedures.     

Self-navigated motion correction using moments of spatial projections in radial MRI.

Interest in radial MRI (also known as projection reconstruction (PR) MRI) has increased recently for uses such as fast scanning and undersampled acquisitions. Additionally, PR acquisitions offer intrinsic advantages over standard two-dimensional Fourier transform (2DFT) imaging with respect to motion of the imaged object. It is well known that aligning each spatial domain projection's center of mass (calculated using the 0th and 1st moments) to the center of the field of view (FOV) corrects shifts caused by in-plane translation. In this work, a previously unrealized ability to determine the in-plane rotational motion of an imaged object using the 2nd moments of the spatial domain projections in conjunction with a specific projection angle acquisition time order is reported. We performed the correction using only the PR data itself acquired with the newly proposed projection angle acquisition time order. With the proposed view angle acquisition order, the acquisition is "self-navigating" with respect to both in-plane translation and rotation. We reconstructed the images using the aligned projections and detected acquisition angles to significantly reduce image artifacts due to such motion. The theory of the correction technique is described, and its effectiveness is demonstrated in phantom and in vivo experiments.     

Direct FLASH MR imaging of magnetic field inhomogeneities by gradient compensation

MR images based on gradient echoes are sensitive to artifacts caused by inhomogeneities of the static magnetic field. This paper describes the effects of local gradients in rapid FLASH MR images and presents a way of directly imaging affected areas. The idea is to compensate for signal losses due to mutual cancellation of dephased magnetizations by deliberate "misadjustments" of the refocusing part of the slice selection gradient. In contrast to conventional field imaging techniques no three-dimensional data acquisition or subsequent Fourier analysis is required to obtain images at a particular gradient strength. Conventional as well as inhomogeneity compensated FLASH images have been obtained on phantoms and human heads using a 2.35-T 40-cm magnet and a 1.5-T whole-body system, respectively.     

Dynamic MRI with projection reconstruction and KWIC processing for simultaneous high spatial and temporal resolution.

A method for dynamic imaging in MRI is presented that enables the acquisition of a series of images with both high temporal and high spatial resolution. The technique, which is based on the projection reconstruction (PR) imaging scheme, utilizes distinct data acquisition and reconstruction strategies to achieve this simultaneous capability. First, during acquisition, data are collected in multiple undersampled passes, with the view angles interleaved in such a way that those of subsequent passes bisect the views of earlier ones. During reconstruction, these views are weighted according to a previously described k-space weighted image contrast (KWIC) technique that enables the manipulation of image contrast by selective filtering. Unlike conventional undersampled PR methods, the proposed dynamic KWIC technique does not suffer from low image SNR or image degradation due to streaking artifacts. The effectiveness of dynamic KWIC is demonstrated in both simulations and in vivo, high-resolution, contrast-enhanced imaging of breast lesions.     

Measurements of Magnetic Field Variations in the Human Brain Using a 3D-FT Multiple Gradient Echo Technique

A magnetic resonance 3DFT multiple gradient-echo technique was used for measurements of the proton spectrum for each voxel in the measured slice. Water, fat, magnetic field and T₂ distributions in the head of a normal volunteer and a patient with intracerebral hematoma were computed. Magnetic field variations caused by the head were calculated after correction for the static magnetic field inhomogeneity. Large local magnetic field variations up to 3 ppm were found in the human brain near interfaces between air or bone and brain tissues and 0.5 ppm between hematoma and brain tissue. Information about magnetic field variations could be useful for shimming procedures in vivo and for correcting artifacts in imaging and spectroscopy.     

Motion Artifacts in fMRI: Comparison of 2DFT with PR and Spiral Scan Methods

Activation signals based on BOLD contrast changes consequent to neuronal stimulation typically produce cortical intensity differences of < 10% at 1.5T. Hemodynamically driven pulsation of the brain can cause highly pulsatile phase shifts, which in turn result in motion artifacts whose intensity is larger than the activation signals in 2DFT scan methods. This paper presents a theoretical and experimental comparison of the magnitude of such artifacts for 2DFT and two other methods using non-Cartesian k-space trajectories. It is shown that artifacts increase with TR for 2DFT methods, and that projection reconstruction (PR) and spiral methods have significantly reduced artifact intensities, because these trajectories collect low spatial frequencies with every view. The spiral technique is found to be superior in terms of efficiency and motion insensitivity.     

Imaging of complex NMR spectra

The Point Spread Function (PSF) in NMR imaging is the result of both the line broadening due to magnet field inhomogeneity and the intrinsic spectrum of the nucleus at resonance. In the case of proton imaging, the line broadening dominates the small chemical shifts and the spectral lines are not resolved. This is not generally the case with other nuclei having strong chemical shifts and the PSF then has a complex structure. During imaging, the complex PSF is convolved with the spatial distribution of the nucleus at resonance and this leads to halo artifacts which are dependent on the imaging technique employed. The images due to the ensemble of spectral lines can be separated in principle by deconvolution of the data with the PSF before reconstruction. In the special case where the complex PSF is spatially independent, it can be obtained from the Free Induction Decay (FID) data produced in the absence of a spatially encoding gradient field. This technique has been successfully applied to in-vivo imaging of exogenous perfluorocarbon material.     

GRASE (gradient- and spin-echo) MR imaging: a new fast clinical imaging technique

A novel technique of magnetic resonance (MR) imaging, which combines gradient-echo and spin-echo (GRASE) technique, accomplishes T2-weighted multisection imaging in drastically reduced imaging time, currently 24 times faster than spin-echo imaging. The GRASE technique maintains contrast mechanisms, high spatial resolution, and image quality of spin-echo imaging and is compatible with clinical whole-body MR systems without modification of gradient hardware. Image acquisition time is 18 seconds for 11 multisection body images (2,000/80 [repetition time msec/echo time msec]) and 36 seconds for 22 brain images (4,000/104). With a combination of multiple Hahn spin echoes and short gradient-echo trains, the GRASE technique overcomes several potential problems of echo-planar imaging, including large chemical shift, image distortions, and signal loss from field inhomogeneity. Advantages of GRASE over the RARE (rapid acquisition with relaxation enhancement) technique include faster acquisition times and lower deposition of radio-frequency power in the body. Breath holding during 18-second GRASE imaging of the upper abdomen eliminates respiratory-motion artifacts in T2-weighted images. A major improvement in T2-weighted abdominal imaging is suggested.     

Concomitant gradient field effects in balanced steady-state free precession.

Linear magnetic field gradients spatially encode the image information in MRI. Concomitant gradients are undesired magnetic fields that accompany the desired gradients and occur as an unavoidable consequence of Maxwell's equations. These concomitant gradients result in undesired phase accumulation during MRI scans. Balanced steady-state free precession (bSSFP) is a rapid imaging method that is known to suffer from signal dropout from off-resonance phase accrual. In this work it is shown that concomitant gradient phase accrual can induce signal dropout in bSSFP. The spatial variation of the concomitant phase is explored and shown to be a function of gradient strength, slice orientation, phase-encoding (PE) direction, distance from isocenter, and main field strength. The effect on the imaging signal level was simulated and then verified in phantom and in vivo experiments. The nearest signal-loss artifacts occurred in scans that were offset from isocenter along the z direction with a transverse readout. Methods for eliminating these artifacts, such as applying compensatory frequency or shim offsets, are demonstrated. Concomitant gradient artifacts can occur at 1.5T, particularly in high-resolution scans or with additional main field inhomogeneity. These artifacts will occur closer to isocenter at field strengths below 1.5T because concomitant gradients are inversely proportional to the main field strength.     

Fast concomitant gradient field and field inhomogeneity correction for spiral cardiac imaging

Non‐Cartesian imaging provides many advantages in terms of flexibility, functionality, and speed. However, a major drawback to these imaging methods is off‐resonance distortion artifacts. These artifacts manifest as blurring in spiral imaging. Common techniques that remove the off‐resonance field inhomogeneity distortion effects are not sufficient, because the high order concomitant gradient fields are nontrivial for common imaging conditions, such as imaging 5 cm off isocenter in an 1.5 T scanner. Previous correction algorithms are either slow or do not take into account the known effects of concomitant gradient fields along with the field inhomogeneities. To ease the correction, the distortion effects are modeled as a non‐stationary convolution problem. In this work, two fast and accurate postgridding algorithms are presented and analyzed. These methods account for both the concomitant field effects and the field inhomogeneities. One algorithm operates in the frequency domain and the other in the spatial domain. To take advantage of their speed and accuracy, the algorithms are applied to a real‐time cardiac study and a high‐resolution cardiac study. Both of the presented algorithms provide for a practical solution to the off‐resonance problem in spiral imaging. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Motion artifact suppression technique (MAST) for cranial MR imaging: superiority over cardiac gating for reducing phase-shift artifacts

A motion artifact suppression technique (MAST) has recently been developed that significantly reduces motion artifacts in conventional 2DFT imaging. The technique utilizes modifications of slice-select and read gradient waveforms to eliminate velocity, acceleration, and pulsatility phase shifts that occur between the 90 degrees pulse and data collection. T2-weighted cranial MAST images were rated visually superior to cardiac-gated images by two experienced neuroradiologists in 13 of 15 cases and in 14 of 15 cases, respectively (p less than 0.001). Quantitative signal-to-noise comparisons for six brain regions in each patient confirmed the visually apparent superiority of MAST, especially for imaging the brainstem and subarachnoid cisterns (p = 0.02). Improvements in signal-to-noise ratios of up to 43% were obtained when using MAST instead of cardiac gating. MAST or a similar technique has the potential to render cardiac gating obsolete as a method for reducing flow-related artifacts in cranial MR imaging.     

Quantitative sodium imaging with a flexible twisted projection pulse sequence

The quantification of sodium MR images from an arbitrary intensity scale into a bioscale fosters image interpretation in terms of the spatially resolved biochemical process of sodium ion homeostasis. A methodology for quantifying tissue sodium concentration using a flexible twisted projection imaging sequence is proposed that allows for optimization of tradeoffs between readout time, signal‐to‐noise ratio efficiency, and sensitivity to static field susceptibility artifacts. The gradient amplitude supported by the slew rate at each k‐space radius regularizes the readout gradient waveform design to avoid slew rate violation. Static field inhomogeneity artifacts are corrected using a frequency‐segmented conjugate phase reconstruction approach, with field maps obtained quickly from coregistered proton imaging. High‐quality quantitative sodium images have been achieved in phantom and volunteer studies with real isotropic spatial resolution of 7.5 × 7.5 × 7.5 mm³ for the slow T₂ component in ～8 min on a clinical 3‐T scanner. After correcting for coil sensitivity inhomogeneity and water fraction, the tissue sodium concentration in gray matter and white matter was measured to be 36.6 ± 0.6 μmol/g wet weight and 27.6 ± 1.2 μmol/g wet weight, respectively. Magn Reson Med 63:1583–1593, 2010. © 2010 Wiley‐Liss, Inc.     

Elimination of magnetic field foldover artifacts in MR images

Foldover artifacts arise when the same imaging frequency occurs both at a desired location within a slice and at another location within the sensitive region of the radiofrequency (RF) coil. Foldover artifacts can be caused by nonlinearity in the gradient system and by inhomogeneity in B(0). This study investigates an approach in which an extra RF receiver coil and a postprocessing method are used to identify and remove foldover artifacts.     

Phase Constrained Encoding (PACE): A Technique for MRI in Large Static Field Inhomogeneities

In spin echo imaging, magnetization is assigned to a location defined by its frequency of rotation. In the presence of a static magnetic field inhomogeneity, however, this location does not correspond to the true location of the magnetization. This paper describes a magnetic resonance imaging technique called phase constrained encoding (PACE) that assigns magnetization to its true location through the use of a spin echotrain and alternating readout gradients. Small artifactual sidebands occur in the point spread function but can be minimized or eliminated using higher gradient strengths, more echoes, and/or additional acquisitions. Implementation of a simple version of this technique confirms simulations.     

Reconstruction of MRI data encoded with arbitrarily shaped,curvilinear, nonbijective magnetic fields

A basic framework for image reconstruction from spatial encoding by curvilinear, nonbijective magnetic encoding fields in combination with multiple receivers is presented. The theory was developed in the context of the recently introduced parallel imaging technique using localized gradients (PatLoc) approach. In this new imaging modality, the linear gradient fields are generalized to arbitrarily shaped, nonbijective spatial encoding magnetic fields, which lead to ambiguous encoding. Ambiguities are resolved by adaptation of concepts developed for parallel imaging. Based on theoretical considerations, a practical algorithm for Cartesian trajectories is derived in the case that the conventional gradient coils are replaced by coils for PatLoc. The reconstruction method extends Cartesian sensitivity encoding (SENSE) reconstruction with an additional voxelwise intensity‐correction step. Spatially varying resolution, signal‐to‐noise ratio, and truncation artifacts are described and analyzed. Theoretical considerations are validated by two‐dimensional simulations based on multipolar encoding fields and they are confirmed by applying the reconstruction algorithm to initial experimental data. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.