A 3D trajectory for undersampling k-space in MRSI applications

Magnetic resonance spectroscopic imaging (MRSI) is a noninvasive technique for producing spatially localized spectra. MRSI presents the important challenge of reducing the scan time while maintaining the spatial resolution. The preferred approach for this is to use time-varying readout gradients to collect the spatial and chemical-shift information. Fast, three-dimensional (3D) spatial encoded methods also reduce the scan time. Despite the existence of several new and faster 3D encoded methods, or k-space trajectories, for magnetic resonance imaging (MRI), only stack of spirals and echo planar have been studied in 3D MRSI. A novel formulation for designing fast, 3D k-space trajectory applicable to 3D MRSI is presented. This approach is simple and consists of rays expanding from the origin of k-space into a revolving sphere, collecting spectral data of all 3D spatial k-space at different times in the same scan. This article describes this new method and presents some results of its application to 3D MRSI. This technique allows some degree of undersampling; hence, it is possible to reconstruct high-quality undersampled spectroscopic imaging in order to recognize different compounds in short scan times. Additionally, the method is tested in regular 3D MRI. This proposed method can also be used for dynamic undersampled imaging.     

In vivo multiecho T2 relaxation measurements using variable TR to decrease scan time

Multiecho T2 relaxation measurements to determine geometric mean T2 (GMT2) and myelin water fraction (MWF) are lengthy, resulting in increased motion artefacts from patient discomfort and reduced patient compliance. The goal of this study was to shorten the acquisition time for multiecho T2 measurements without affecting T1 weighting by varying TR across k-space. Six phantoms and 10 healthy volunteers were imaged with both a constant TR and a variable TR multiecho T2 sequence. T1 weighting was determined by TR at the center of k-space; for variable TR measurement, TR was shortened linearly from the center to the edges of k-space. Phantoms showed excellent agreement for proton density and GMT2 between constant and variable TR measurements. No significant differences were found in proton density or MWF for any of the brain structures between the two measurements. The average GMT2 over all structures between the two experiments was not significantly different. In summary, with the variable TR approach, scan time was reduced by >20%, with minimal loss of image resolution and no significant affect on proton density, MWF or GMT2.     

Truncation artifact reduction in spectroscopic imaging using a dual-density spiral k-space trajectory

Truncation artifacts arise in magnetic resonance spectroscopic imaging (MRSI) of the human brain due to limited coverage of k-space necessitated by low SNR of metabolite signal and limited scanning time. In proton MRSI of the head, intense extra-cranial lipid signals “bleed” into brain regions, thereby contaminating signals of metabolites therein. This work presents a data acquisition strategy for reducing truncation artifact based on extended k-space coverage achieved with a dual-SNR strategy. Using the fact that the SNR in k-space increases monotonically with sampling density, dual-SNR is achieved in an efficient manner with a dual-density spiral k-space trajectory that permits a smooth transition from high density to low density. The technique is demonstrated to be effective in reducing “bleeding” of extra-cranial lipid signals while preserving the SNR of metabolites in the brain.     

Phase-encoding strategies for optimal spatial resolution and T1 accuracy in 3D Look-Locker imaging

The Look-Locker (LL) imaging method provides an accurate and efficient approach for mapping the spin-lattice relaxation time, T(1). However, the same recovery of signal during LL image acquisition required to estimate T(1) also results in unwanted modulation of k-space. This is particularly problematic with 3D LL imaging as the number of phase-encoding steps during the recovery interval (e.g., 16) increases in an effort to reduce imaging times. This modulation of k-space has the effect of introducing a point spread function (PSF), which can lead to either image blurring (if the earlier tip angles are assigned to the centre of k-space) or edge enhancement (if the earlier tip angles are assigned to the edges of k-space), thus corrupting T(1) estimation, particularly for small objects. In this study, the PSF and its effect on the acquired images for four different interleaved phase-encode schemes (centric-in, centric-out, sequential and hybrid-sequential) are simulated for a range of T(1), tip angle and 3D LL acquisition parameters expected in practice. It is shown by simulation and confirmed experimentally in phantoms that a hybrid sequential phase-encoding scheme reduces image blurring while maintaining T(1) accuracy ( approximately 2%) and precision (2%) over a range of object sizes down to 2 pixels (2 mm).     

A hybrid technique for spectroscopic imaging with reduced truncation artifact

Traditionally, Fourier spectroscopic imaging is associated with a small k-space coverage which leads to truncation artifacts such as "bleeding" and ringing in the resultant image. Because substantial truncation artifacts mainly arise from regions having intense signals, such as the subcutaneous lipid in the head, effective reduction of truncation artifacts can be achieved by obtaining an extended k-space coverage for these regions. In this paper, a hybrid technique which employs phase-encoded spectroscopic imaging (SI) to cover the central portion of the k-space and echo-planar spectroscopic imaging (EPSI) to measure the peripheral portion of the k-space is developed. EPSI, despite its inherently low SNR characteristics, provides a sufficient SNR for outer high-spatial frequency components of the aforementioned high signal regions and supplies an extended k-space coverage of these regions for the reduction of truncation artifacts. The data processing includes steps designed to remove inconsistency between the two types of data and a previously described technique for selectively retaining only outer k-space information for the high signal regions during the reconstruction. Experimental studies, in both phantoms and normal volunteers, demonstrate that the hybrid technique provides significant reduction in truncation artifacts.     

MR image reconstruction of sparsely sampled 3D k-space data by projection-onto-convex sets

In many rapid three-dimensional (3D) magnetic resonance (MR) imaging applications, such as when following a contrast bolus in the vasculature using a moving table technique, the desired k-space data cannot be fully acquired due to scan time limitations. One solution to this problem is to sparsely sample the data space. Typically, the central zone of k-space is fully sampled, but the peripheral zone is partially sampled. We have experimentally evaluated the application of the projection-onto-convex sets (POCS) and zero-filling (ZF) algorithms for the reconstruction of sparsely sampled 3D k-space data. Both a subjective assessment (by direct image visualization) and an objective analysis [using standard image quality parameters such as global and local performance error and signal-to-noise ratio (SNR)] were employed. Compared to ZF, the POCS algorithm was found to be a powerful and robust method for reconstructing images from sparsely sampled 3D k-space data, a practical strategy for greatly reducing scan time. The POCS algorithm reconstructed a faithful representation of the true image and improved image quality with regard to global and local performance error, with respect to the ZF images. SNR, however, was superior to ZF only when more than 20% of the data were sparsely sampled. POCS-based methods show potential for reconstructing fast 3D MR images obtained by sparse sampling.     

Phased array 3D MR spectroscopic imaging of the brain at 7 T

Ultra-high-field 7 T magnetic resonance (MR) scanners offer the potential for greatly improved MR spectroscopic imaging due to increased sensitivity and spectral resolution. Prior 7 T human single-voxel MR Spectroscopy (MRS) studies have shown significant increases in signal-to-noise ratio (SNR) and spectral resolution as compared to lower magnetic fields but have not demonstrated the increase in spatial resolution and multivoxel coverage possible with 7 T MR spectroscopic imaging. The goal of this study was to develop specialized radiofrequency (RF) pulses and sequences for three-dimensional (3D) MR spectroscopic imaging (MRSI) at 7 T to address the challenges of increased chemical shift misregistration, B1 power limitations, and increased spectral bandwidth. The new 7 T MRSI sequence was tested in volunteer studies and demonstrated the feasibility of obtaining high-SNR phased-array 3D MRSI from the human brain.     

Evaluation of motion effects on parallel MR imaging with precalibration

Several parallel imaging techniques such as SMASH, SENSE, k-space inherited parallel acquisition (KIPA) and others use reference (calibration) scans to find the parameters required for image reconstruction. Reference data is used to estimate coil sensitivity profiles for image domain techniques such as SENSE or reconstruction coefficients for k-space domain methods such as SMASH and KIPA. Any motion between the reference and accelerated imaging scans can make the reconstruction coefficients determined from the reference scan data suboptimal, resulting in an artifactual reconstruction. This work aims at comparing the effects of motion on the performance of three parallel imaging methods: SENSE, variable-density SENSE and KIPA, which all require one or more reference scans for calibration.     

Multipoint mapping for imaging of semi-solid materials

Multipoint k-space mapping is a hybrid between constant-time (single-point mapping) and spin-warp imaging, involving sampling of a k-line segment of r points per TR cycle. In this work the method was implemented for NMR imaging of semi-solid materials on a 400 MHz micro-imaging system and two different k-space sampling strategies were investigated to minimize the adverse effects from relaxation-induced k-space signal modulation. Signal attenuation from T(2) decay results in artifacts whose nature depends on the k-space sampling strategy. The artifacts can be minimized by increasing the readout gradient amplitude, by PSF deconvolution or by oversampling in readout direction. Finally, implementation of a T(2) selective RF excitation demonstrates the feasibility of obtaining short-T(2) contrast even in the presence of tissues with long-T(2). The method's potential is illustrated with 3D proton images of short-T(2) materials such as synthetic polymers and bone.     

Absolute Quantification Using Turbo Spectroscopic Imaging in Multiple Sclerosis Patients

One of the drawbacks of scanning patients using multiple-voxel spectroscopic imaging is the long acquisition time. This is especially true when one is interested in obtaining absolute metabolite concentrations which requires acquisition of unsuppressed water spectra in addition to the suppressed spectra. In our experiment, turbo spectroscopic imaging (TSI) method with acquisition of three echoes per excitation was applied to reduce scanning time without lowering the spatial resolution. In 15 relapsing-remitting multiple sclerosis patients (mean age 37.07 years, mean disease duration 7.67 years), an MRSI scan at the level of centrum semiovale was obtained. The scan time was approximately 7 min including the unsuppressed spectra. Tissue water was used as an internal concentration reference to obtain absolute metabolite concentrations of N-acetyl-aspartate (NAA), creatine (Cr), and choline (Cho). The peak areas were corrected for differences in transversal and longitudinal relaxation times and a water concentration of 55.5 M was assumed. A three-dimensional high-resolution T ₁ scan was acquired and used to segment tissue in gray matter (GM), white matter (WM), and cerebrospinal fluid using FSL’S FAST segmentation method (a software library of the automated segmentation tool by the Center of Functional MRI of the Brain, Oxford, UK). Finally, a regression analysis was employed to address the metabolite concentrations and ratios in GM and WM, respectively. Our study shows that the metabolite concentrations (NAA, Cho, Cr) and metabolite ratios (NAA/Cr and Cho/Cr) in GM and WM obtained using the methods discussed earlier are comparable to the results found in other studies of similar patient groups. It also shows that TSI method can be used to obtain the absolute metabolite ratios in a reasonable scan time.     

Optimal k-space sampling for single point imaging of transient systems

A novel approach for sampling k-space in a pure phase encoding imaging sequence is presented using the Single Point Imaging (SPI) technique. The sequence is optimised with respect to the achievable Signal-to-Noise ratio (SNR) for a given time interval via selective sparse k-space sampling, dictated by prior knowledge of the overall object of interest's shape. This allows dynamic processes featuring short T(2)( *) NMR signal to be more readily followed, in our case the absorption of moisture by a cereal-based wafer material. Further improvements in image quality are also shown via the use of complete sampling of k-space at the start or end of the series of imaging experiments; followed by subsequent use of this data for un-sampled k-space points as opposed to zero filling.     

Motion artifact reduction technique for dual-contrast FSE imaging

There is considerable similarity between proton density-weighted (PDw) and T2-weighted (T2w) images acquired by dual-contrast fast spin-echo (FSE) sequences. The similarity manifests itself in image space as consistency between the phases of PDw and T2w images and in k-space as correspondence between PDw and T2w k-space data. A method for motion artifact reduction for dual-contrast FSE imaging has been developed. The method uses projection onto convex sets (POCS) formalism and is based on image space phase consistency and the k-space similarity between PDw and T2w images. When coupled with a modified dual-contrast FSE phase encoding scheme the method can yield considerable artifact reduction, as long as less than half of the acquired data is corrupted by motion. The feasibility and efficiency of the developed method were demonstrated using phantom and human MRI data.     

Accelerated free-breathing 3D whole-heart magnetic resonance angiography with a radial phyllotaxis trajectory,compressed sensing,and curvelet transform

PurposeTo develop and validate an accelerated free-breathing 3D whole-heart magnetic resonance angiography (MRA) technique using a radial k-space trajectory with compressed sensing and curvelet transform.MethodA 3D radial phyllotaxis trajectory was implemented to traverse the centerline of k-space immediately before the segmented whole-heart MRA data acquisition at each cardiac cycle. The k-space centerlines were used to correct the respiratory-induced heart motion in the acquired MRA data. The corrected MRA data were then reconstructed by a novel compressed sensing algorithm using curvelets as the sparsifying domain. The proposed 3D whole-heart MRA technique (radial CS curvelet) was then prospectively validated against compressed sensing with a conventional wavelet transform (radial CS wavelet) and a standard Cartesian acquisition in terms of scan time and border sharpness.ResultsFifteen patients (females 10, median age 34-year-old) underwent 3D whole-heart MRA imaging using a standard Cartesian trajectory and our proposed radial phyllotaxis trajectory. Scan time for radial phyllotaxis was significantly shorter than Cartesian (4.88 ± 0.86 min. vs. 6.84 ± 1.79 min., P-value = 0.004). Radial CS curvelet border sharpness was slightly lower than Cartesian and, for the majority of vessels, was significantly better than radial CS wavelet (P-value < 0.050).ConclusionThe proposed technique of 3D whole-heart MRA acquisition with a radial CS curvelet has a shorter scan time and slightly lower vessel sharpness compared to the Cartesian acquisition with radial profile ordering, and has slightly better sharpness than radial CS wavelet. Future work on this technique includes additional clinical trials and extending this technique to 3D cine imaging.     

Centric scan SPRITE magnetic resonance imaging: optimization of SNR, resolution, and relaxation time mapping

Two strategies for the optimization of centric scan SPRITE (single point ramped imaging with T1 enhancement) magnetic resonance imaging techniques are presented. Point spread functions (PSF) for the centric scan SPRITE methodologies are numerically simulated, and the blurring manifested in a centric scan SPRITE image through PSF convolution is characterized. Optimal choices of imaging parameters and k-space sampling scheme are predicted to obtain maximum signal-to-noise ratio (SNR) while maintaining acceptable image resolution. The point spread function simulation predictions are verified experimentally. The acquisition of multiple FID points following each RF excitation is described and the use of the Chirp z-Transform algorithm for the scaling of field of view (FOV) of the reconstructed images is illustrated. Effective recombination of the rescaled images for SNR improvement and T*2 mapping is demonstrated.     

Reduced aliasing artifacts using shaking projection k-space sampling trajectory

Radial imaging techniques, such as projection-reconstruction （PR）, are used in magnetic resonance imaging （MRI） for dynamic imaging, angiography, and short-T2 imaging. They are less sensitive to flow and motion artifacts, and support fast imaging with short echo times. However, aliasing and streaking artifacts are two main sources which degrade radial imaging quality. For a given fixed number of k-space projections, data distributions along radial and angular directions will influence the level of aliasing and streaking artifacts. Conventional radial k-space sampling trajectory introduces an aliasing artifact at the first principal ring of point spread function （PSF）. In this paper, a shaking projection （SP） k-space sampling trajectory was proposed to reduce aliasing artifacts in MR images. SP sampling trajectory shifts the projection alternately along the k-space center, which separates k-space data in the azimuthal direction. Simulations based on conventional and SP sampling trajectories were compared with the same number projections. A significant reduction of aliasing artifacts was observed using the SP sampling trajectory. These two trajectories were also compared with different sampling frequencies. ASP trajectory has the same aliasing character when using half sampling frequency （or half data） for reconstruction. SNR comparisons with different white noise levels show that these two trajectories have the same SNR character. In conclusion, the SP trajectory can reduce the aliasing artifact without decreasing SNR and also provide a way for undersampling recon- struction. Furthermore, this method can be applied to three-dimensional （3D） hybrid or spherical radial k-space sampling for a more efficient reduction of aliasing artifacts.     

Short TE in vivo (1)H MR spectroscopic imaging at 1.5 T: acquisition and automated spectral analysis

Spectral analysis of short TE in vivo proton magnetic resonance spectroscopic imaging (MRSI) data are complicated by the presence of spectral overlap, low signal to noise and uncharacterized signal contributions. In this study, it is shown that an automated data analysis method can be used to generate metabolite images from MRSI data obtained from human brain at TE = 25 ms and 1.5 T when optimized pulse sequences and a priori metabolite knowledge are used. The analysis approach made use of computer simulation methods to obtain a priori spectral information of the metabolites of interest and utilized a combination of parametric spectral modeling and non-parametric signal characterization for baseline fitting. This approach was applied to data from optimized PRESS-SI and multi-slice spin-echo SI acquisitions, for which sample spectra and metabolite images are shown.     

Cardiac-induced physiological noise in 3D gradient echo brain imaging: effect of k-space sampling scheme

The physiological noise in 3D image acquisition is shown to depend strongly on the sampling scheme. Five sampling schemes are considered: Linear, Centric, Segmented, Random and Tuned. Tuned acquisition means that data acquisition at k-space positions k and -k are separated with a specific time interval. We model physiological noise as a periodic temporal oscillation with arbitrary spatial amplitude in the physical object and develop a general framework to describe how this is rendered in the reconstructed image. Reconstructed noise can be decomposed in one component that is in phase with the signal (parallel) and one that is 90° out of phase (orthogonal). Only the former has a significant influence on the magnitude of the signal. The study focuses on fMRI using 3D EPI. Each k-space plane is acquired in a single shot in a time much shorter than the period of the physiological noise. The above mentioned sampling schemes are applied in the slow k-space direction and noise propagates almost exclusively in this direction. The problem then, is effectively one-dimensional. Numerical simulations and analytical expressions are presented. 3D noise measurements and 2D measurements with high temporal resolution are conducted. The measurements are performed under breath-hold to isolate the effect of cardiac-induced pulsatile motion. We compare the time-course stability of the sampling schemes and the extent to which noise propagates from a localized source into other parts of the imaging volume. Tuned and Linear acquisitions perform better than Centric, Segmented and Random.     

3D-snapshot flash NMR imaging of the human heart

SNAPSHOT-FLASH is a recently developed, ultrafast imaging technique, based on conventional FLASH imaging. The application of this new variant to 3D imaging allows the acquisition of a 128 x 128 x 32 data set in 12.5 seconds without triggering, or for cardiac imaging with gating within 32 heartbeats. Compared to standard 3D-FLASH this is 128 times faster, because triggering is only required when the 3D phase-encoding gradient is incremented. The method depicts for the first time fast three-dimensional views of the human heart without motional artifacts. The images are spin-density weighted. Using suitable prepulses any desired T1- or T2-contrast may be achieved. The generation of 3D movies is possible without an increase of the total scan time.     

Three dimensional k-space trajectory design using genetic algorithms

Image quality and total scan time in MRI are determined in large part by the trajectory employed to sample the Fourier space. Each trajectory has different properties like coverage of k-space, scan time, sensitivity to off-resonance conditions, etc. These properties are often contradictory, therefore a universal optimal trajectory does not exist and ultimately, it will depend on the image characteristics sought. Most trajectories used today are designed based on intuition and k-space analysis more than with optimization methods. This work presents a 3D k-space trajectory design method based on Genetic Algorithm optimization. Genetic Algorithms have been chosen because they are particularly good for searching large solution spaces. They emulate the natural evolutionary process allowing better offsprings to survive. The objective function searches the maximum of the trajectory's k-space coverage subject to hardware constraints for a fixed scanning time using the trajectory's torsion as its optimization variable.The method proved to be effective for generating k-space trajectories. They are compared with well-established trajectories. The results of simulated experiments show that they can be appropriate for image acquisition under certain special conditions, like off-resonance and undersampling. This design method can be extended to include other objective functions for different behaviors.