Gradient Moment Research Articles

Reduced chemical shift-induced phase errors at 3T using novel PC-MRI encoding gradients

Background Cardiovascular MRI benefits from improved SNR-efficiency at ≥3T [1,]2, but is subject to other sources of error, which require careful consideration when transitioning from primary use of 1.5T scanners. For example, chemical shift-induced PC-MRI errors [3] are increased at 3T compared to 1.5T. Chemical shift causes the complex perivascular fat signal to chemically shift into the vessel lumen and superposes with the complex blood (water) signal, thereby corrupting the phase (velocity) estimate. Chemical shift errors can be minimized by increasing the bandwidth (reduces the magnitude of the shifted fat signal), and by using an in-phase TE (TEIN, ensures fat and water are in-phase). Shorter TEs improve SNR, therefore it is advantageous that the minimum TEIN (TEIN,MIN) at 3T is 2.46ms, which is substantially shorter than TEIN,MIN=4.92ms at 1.5T, but such short TEs cannot be attained with conventional flowcompensated/flow-encoded (FCFE) velocity encoding strategies. The objective was to design a velocity encoding strategy void of conventional FCFE gradients that instead achieves through-plane velocity sensitivity using the slice select gradient, which yields a non-zero first gradient moment (M1 )f or the first PC-MRI TR: M1,1=X. The slice-select refocusing gradient (SSRG) lobe is time-shifted for the second TR to produce M1,2=X+Y, such that ΔM1=Y=πg -1 VENC -1 .W e hypothesize that the proposed SSRG velocity encoding scheme, will permit the use of TEIN,MIN for chemical shift insensitive PC-MRI measures at 3T that are both faster and have improved SNR. Methods

Pre-buckling deflection effects on stability of thin-walled beams with open sections

The paper investigates beam lateral buckling stability according to linear and non-linear models. Closed form solutions for single-symmetric cross sections are first derived according to a non-linear model considering flexural-torsional coupling and pre-buckling deformation effects. The closed form solutions are compared to a beam finite element developed in large torsion. Effects of pre-buckling deflection and gradient moment on beam stability are not well known in the literature. The strength of singly symmetric I-beams under gradient moments is particularly investigated. Beams with T and I cross-sections are considered in the study. It is concluded that pre-buckling deflections effects are important for I-section with large flanges and analytical solutions are possible. For beams with T-sections, lateral buckling resistance depends not only on pre-buckling deflection but also on cross section shape, load distribution and buckling modes. Effects of pre-buckling deflections are important only when the largest flange is under compressive stresses and positive gradient moments. For negative gradient moments, all available solutions fail and overestimate the beam strength. Numerical solutions are more powerful. Other load cases are investigated as the stability of continuous beams. Under arbitrary loads, all available solutions fail, and recourse to finite element simulation is more efficient.

LS-SVM based image segmentation using color and texture information

Image segmentation partitions an image into nonoverlapping regions, which ideally should be meaningful for a certain purpose. Automatic segmentation of images is a very challenging fundamental task in computer vision and one of the most crucial steps toward image understanding. In recent years, many image segmentation algorithms have been developed, but they are often very complex and some undesired results occur frequently. In this paper, we present an effective color image segmentation approach based on pixel classification with least squares support vector machine (LS-SVM). Firstly, the pixel-level color feature, Homogeneity, is extracted in consideration of local human visual sensitivity for color pattern variation in HSV color space. Secondly, the image pixel’s texture features, Maximum local energy, Maximum gradient, and Maximum second moment matrix, are represented via Gabor filter. Then, both the pixel-level color feature and texture feature are used as input of LS-SVM model (classifier), and the LS-SVM model (classifier) is trained by selecting the training samples with Arimoto entropy thresholding. Finally, the color image is segmented with the trained LS-SVM model (classifier). This image segmentation not only can fully take advantage of the local information of color image, but also the ability of LS-SVM classifier. Experimental evidence shows that the proposed method has very effective segmentation results and computational behavior, and decreases the time and increases the quality of color image segmentation in comparison with the state-of-the-art segmentation methods recently proposed in the literature.

Multidirectional high‐moment encoding in phase contrast MRI

The use of phase contrast MRI to measure vascular flow provides a unique method for acquiring quantitative estimates of flow as well as morphological imaging. The quantitative aspects of phase contrast magnetic resonance angiography (PC-MRA) provide unique relationships between measurement parameters and resulting signal to noise ratio of the velocity measurements. This article introduces a new method to exploit these relationships providing increased efficiency, and therefore, higher vessel conspicuity. Signal to noise ratio gains in high-moment PC-MRA are limited by the ability to unalias phase measurements that fall outside the -π to π interval. Unaliasing phase on a per pixel basis is limited by errors in the measurements due to noise and intravoxel flow distributions. Current dual-VENC methods have been shown to be robust to these errors and provide high velocity to noise ratio gains, however, the collection of a required high-VENC set can be inefficient. The presented method provides more time efficient gains in velocity to noise ratio compared to a dual-VENC approach by eliminating the high-VENC acquisitions and using shared information between nonorthogonal measurements. Simulations, phantom, and in vivo angiography are used to characterize the noise performance of each method. The velocity to noise ratio efficiency of the proposed method is shown to be ∼1.7 times greater than the dual-VENC method at the same gradient moment.

A comparative study of spacecraft attitude determination and estimation algorithms (a cost–benefit approach)

The primary goal of this work is to investigate various performance indices of spacecraft attitude determination and estimation algorithms to perform a cost–benefit analysis. This analysis could help the designer to choose whether to use an attitude determination algorithm or an attitude estimation algorithm. The choice depends mainly on the available computational resources and the required attitude determination accuracy. These performance indices include accuracy, and execution time. In order to determine these performance indices, a test-case spacecraft is utilized. The test-case spacecraft is subject to aerodynamics forces and moments, solar radiation pressure forces and moments, earthʼs oblateness till the fourth order (i.e. J4), residual magnetic dipole moments, and gravity gradient moments. The available spacecraft attitude sensors are magnetometer and gyro.

Magnetization spoiling in radial FLASH contrast‐enhanced MR digital subtraction angiography

To increase the in-plane spatial resolution and image update rates of 2D magnetic resonance (MR) digital subtraction angiography (DSA) pulse sequences to 0.57 × 0.57 mm and 6 frames/sec, respectively, for intracranial vascular disease applications by developing a radial FLASH protocol and to characterize a new artifact, not previously described in the literature, which arises in the presence of such pulse sequences. The pulse sequence was optimized and artifacts were characterized using simulation and phantom studies. With Institutional Review Board (IRB) approval, the pulse sequence was used to acquire time-resolved images from healthy human volunteers and patients with x-ray DSA-confirmed intracranial vascular disease. Artifacts were shown to derive from inhomogeneous spoiling due to the nature of radial waveforms. Gradient spoiling strategies were proposed to eliminate the observed artifact by balancing gradient moments across TR intervals. The resulting radial 2D MR DSA sequence (2.6 sec temporal footprint, 6 frames/sec with sliding window factor 16, 0.57 × 0.57 mm in-plane) demonstrated small vessel detail and corroborated x-ray DSA findings in intracranial vascular imaging studies. Appropriate gradient spoiling in radial 2D MR DSA pulse sequences improves intracranial vascular depiction by eliminating circular banding artifacts. The proposed pulse sequence may provide a useful addition to clinically applied 2D MR DSA scans.

Nelkin scaling for the Burgers equation and the role of high-precision calculations

Nelkin scaling, the scaling of moments of velocity gradients in terms of the Reynolds number, is an alternative way of obtaining inertial-range information. It is shown numerically and theoretically for the Burgers equation that this procedure works already for Reynolds numbers of the order of 100 (or even lower when combined with a suitable extended self-similarity technique). At moderate Reynolds numbers, for the accurate determination of scaling exponents, it is crucial to use higher than double precision. Similar issues are likely to arise for three-dimensional Navier-Stokes simulations.

Prediction of available rotation capacity and ductility of wide-flange beams: Part 1: DUCTROT-M computer program

This paper is a development of the [1], presenting the new improvements in determining the rotation capacity of wide-flange beams. It deals with the available rotation capacity of steel beams, using the local plastic mechanism methodology. For these purposes, a more advanced software was elaborated at the “Politehnica” University of Timisoara, namely DUCTROT-M, substituting the old DUCTROT-96 computer program presented in [1]. The new version considers two different forms the in-plane and out-of-plane plastic mechanisms, as well as the application of gradient or quasi-constant moments. A CD-ROM containing the free DUCTROT-M computer program can be finding in Appendix of [2] or free on the site [3]. Thus, the present paper is focused on the phenomenological aspects of the utilized plastic collapse mechanisms, while the companion paper [41] is devoted to applications in design practice exploiting the new capabilities of the aforementioned current software version.

Superbalanced steady state free precession

Steady state free precession (SSFP) signal theory is commonly derived in the limit of quasi-instantaneous radiofrequency (RF) excitation. SSFP imaging protocols, however, are frequently set up with minimal pulse repetition times and RF pulses can thus constitute a considerable amount to the actual pulse repetition time. As a result, finite RF pulse effects can lead to 10-20% signal deviation from common SSFP theory in the transient and in the steady state which may impair the accuracy of SSFP-based quantitative imaging techniques. In this article, a new and generic approach for intrinsic compensation of finite RF pulse effects is introduced. Compensation is based on balancing relaxation effects during finite RF excitation, similar to flow or motion compensation of gradient moments. RF pulse balancing, in addition to the refocusing of gradient moments with balanced SSFP, results in a superbalanced SSFP sequence free of finite RF pulse effects in the transient and in the steady state; irrespective of the RF pulse duration, flip angles, relaxation times, or off-resonances. Superbalancing of SSFP sequences can be used with all quantitative SSFP techniques where finite RF pulse effects are expected or where elongated RF pulses are used.

Susceptibility-resistant variable-flip-angle turbo spin echo imaging for reliable estimation of cortical thickness: A feasibility study

The thickness of the human cerebral cortex, which provides valuable information in the studies of normal and abnormal neuroanatomy, is commonly estimated using high-resolution, volumetric magnetization-prepared rapid gradient echo (MP-RAGE) magnetic resonance imaging due to its strong T1-weighted contrast and high signal-to-noise ratio. However, the accuracy of cortical thickness estimates using MP-RAGE is potentially contaminated by susceptibility-induced signal loss particularly at regions in close proximity to air-filled cavities. The purpose of this work is to investigate the feasibility of susceptibility-resistant variable-flip-angle (VFA) three-dimensional turbo/fast spin echo imaging for reliable estimation of cortical thickness of the human brain, wherein 1) radio-frequency (RF) pulse refocuses susceptibility-induced spin de-phasing, 2) the VFA refocusing pulse train is applied for a tissue-specific prescribed signal evolution along the echo train, 3) the desired T1-weighted contrast is achieved by composite restore pulses at the end of the refocusing pulse train, and 4) blood signals are suppressed using the VFA scheme combined with increasing moments of flow-sensitizing gradients while dura mater signals are attenuated due to short T2 relaxation time, which alleviates potential failure in brain segmentation. Numerical simulations of the Bloch equation are performed in both MP-RAGE and the proposed method for comparison. In vivo studies are performed in 14 healthy volunteers at 3 T. Image processing is then performed using the Freesurfer, resulting in mean and standard deviations of cortical thickness for the entire cortical surfaces. Statistical analysis demonstrates that particularly in the inferior prefrontal and temporal regions heavily affected by susceptibility-induced signal loss conventional MP-RAGE, if compared with the proposed method, significantly under-estimates cortical thickness. It is expected that the proposed pulse sequence, which is resistant to susceptibility-induced signal loss and attenuates the signal intensity of blood and dura mater, can be a potentially promising alternative to conventional MP-RAGE in reliably estimating cortical thickness for the entire brain.

Fast converging with high accuracy estimates of satellite attitude and orbit based on magnetometer augmented with gyro, star sensor and GPS via extended Kalman filter

The primary goal of this work is to extend the work done in, Tamer (2009), to provide high accuracy satellite attitude and orbit estimates needed for imaging purposes and also before execution of spacecraft orbital maneuvers for the next Egyptian scientific satellite. The problem of coarse satellite attitude and orbit estimation based on magnetometer measurements has been treated in the literature. The current research expands the field of application from coarse and slow converging estimates to accurate and fast converging attitude and orbit estimates within 0.1°, and 10m for attitude angles and spacecraft location respectively (1-σ). The magnetometer is used for both spacecraft attitude and orbit estimation, aided with gyro to provide angular velocity measurements, star sensor to provide attitude quaternion, and GPS receiver to provide spacecraft location. The spacecraft under consideration is subject to solar radiation pressure forces and moments, aerodynamics forces and moments, earth’s oblateness till the fourth order (i.e. J4), gravity gradient moments, and residual magnetic dipole moments. The estimation algorithm developed is powerful enough to converge quickly (actually within 10s) despite very large initial estimation errors with sufficiently high accuracy estimates.

Modeling and numerical simulation of linear and nonlinear spacecraft attitude dynamics and gravity gradient moments: A comparative study

In this paper linear and nonlinear models of spacecraft attitude dynamics equations and gravity gradient moments are investigated. In addition, effects of gravity gradient moments on attitude dynamics of the satellite are studied. The purpose of this paper is to present a comparison between nonlinear and linear models of spacecraft attitude dynamics and gravity gradient moments in order to determine divergence of linear approximation from the nonlinear model. Simulation results indicate that designer of spacecraft attitude control subsystem should be meticulous in applying linear approximation of equations especially in low earth orbits. Consequently, finding an upper bound for small angle to keep the linear model valid and precise enough would be a vital part of using linear approximation. Results supported by numerical examples demonstrate various features of this study.

Phase enhancement for time-of-flight and flow-sensitive black-blood MR angiography

A magnitude-based MR angiography method of standard time-of-flight (TOF) employing a three-dimensional gradient-echo sequence with flow rephasing is widely used. A recently proposed flow-sensitive black-blood (FSBB) method combining three-dimensional gradient-echo sequence with a flow-dephasing gradient and a hybrid technique, called hybrid of opposite-contrast, allow depiction of smaller blood vessels than does standard TOF. To further enhance imaging of smaller vessels, a new enhancement technique combining phase with magnitude is proposed. Both TOF and FSBB pulse sequences were used with only 0th-order gradient moment nulling, and suitable dephasing gradients were added to increase the phase shift introduced mainly by flow. Magnitude-based vessel-to-background contrast-to-noise ratios in TOF and FSBB were further enhanced to increase the dynamic range between positive and negative signals through the use of cosine-function-based filters for white- and black-blood imaging. The proposed phase-enhancement processing both improved visualization of slow-flow vessels in the brains of volunteer subjects with shorter echo time in TOF, FSBB, and hybrid of opposite-contrast and reduced wraparound artifacts with smaller b values without sacrificing vessel-to-background contrast in FSBB. This method of enhancement processing has excellent potential to become clinically useful.

Determination of the optimal first-order gradient moment for flow-sensitive dephasing magnetization-prepared 3D noncontrast MR angiography

Flow-sensitive dephasing (FSD) magnetization preparation has been developed for black-blood vessel wall MRI and noncontrast MR angiography. The first-order gradient moment, m(1) , is a measure of the flow-sensitization imparted by an FSD preparative module. Determination of the optimal m(1) for each individual is highly desirable for FSD-prepared MR angiography. This work developed a 2D m(1)-scouting method that evaluates a range of m(1) values for their effectiveness in blood signal suppression in a single scan. The feasibility of using the 2D method to predict blood signal suppression in 3D FSD-prepared imaging was validated on a flow phantom and the popliteal arteries of 5 healthy volunteers. Excellent correlation of the blood signal measurements between the 2D scouting and 3D FSD imaging was obtained. Therefore, the optimal m(1) determined from the 2D m(1)-scouting scan may be directly translated to 3D FSD-prepared imaging. In vivo studies of additional 10 healthy volunteers and 2 patients have demonstrated the proposed method can help significantly improve the signal performance of FSD MR angiography, indicating its potential to enhance diagnostic confidence. Further systematic studies in patients are warranted to evaluate its clinical value.

Real-time feedback optimization of z-shim gradient for automatic compensation of susceptibility-induced signal loss in EPI

Signal loss in gradient-echo echo planar imaging (GE-EPI) due to susceptibility-induced magnetic field inhomogeneity makes it difficult to assess the blood oxygenation level-dependent (BOLD) effect in fMRI investigations. The z-shim method that applies an additional gradient moment is one of the more popular methods of compensating for GE-EPI signal loss. However, this method requires a calibration sweep scan and post-processing to identify the optimal z-shim gradients, which slows down fMRI experiments. This study attempts to decrease the calibration time by introducing a real-time feedback framework. Creating a feedback loop between the image processing and the GE-EPI pulse sequence converts the calibration of z-shim gradients to an optimization problem, which can be accelerated by local search methods. This study proposes an interleaved scan that allows the simultaneous optimization of two z-shim gradient moments and allocates sufficient processing time for networking and computation. The z-shim compensated images obtained by the proposed real-time method are comparable to those created by the sweep method. The optimization procedure for obtaining negative and positive gradient moments generally requires about twenty GE-EPI repetitions. In conclusion, the proposed z-shim method includes an automated real-time framework to achieve a significant reduction in susceptibility-induced signal loss in GE-EPI with a minimal increase in calibration time. The proposed procedure is fully automatic and compatible with conventional GE-EPI and can thus serve as a pre-adjustment module in EPI-based fMRI researches.

Wavefront sensorless adaptive optics: a general model-based approach

Wavefront sensorless adaptive optics (AO) systems have been widely studied in recent years. To reach optimum results, such systems require an efficient correction method. In this paper, a general model-based correction method for a wavefront sensorless AO system is presented. The general model-based approach is set up based on a relationship wherein the second moments (SM) of the wavefront gradients are approximately proportionate to the FWHM of the far-field intensity distribution. The general model-based method is capable of taking various common sets of functions as predetermined bias functions and correcting the aberrations by using fewer photodetector measurements. Numerical simulations of AO corrections of various random aberrations are performed. The results show that the Strehl ratio is improved from 0.07 to about 0.90, with only N + 1 photodetector measurement for the AO correction system using N aberration modes as the predetermined bias functions.

The Effects of 5f Localization on Magnetic Properties of UAl3

The structural, electronic, and magnetic properties of UAl3 have been calculated using density functional theory by the Wien2k package within LDA, GGA, LDA + U, and GGA + U approaches. The total energy calculations indicate that at zero pressure the ferromagnetic phase is the most stable phase. The energy band calculation and the density of state curves indicate that the localization of 5f electron and spin orbit coupling have a considerable effect on electronic properties of the UAl3 compound. The calculations of the electric field gradient, magnetic moment, and optical properties of this compound have been performed under the presence and absence of spin orbit coupling. The contribution of different orbitals to the EFG shows that the strongest anisotropy in the charge distribution is due to the electrons in p orbitals. The electric field gradient and magnetic moment as a function of pressure have been investigated in the presence and absence of spin orbit coupling.

Flow compensation for the fast spin echo triple-echo Dixon sequence

The fast spin echo (FSE) triple-echo Dixon (FTED) sequence uses bipolar triple-echo readout during each echo-spacing period of FSE to collect all the images necessary for Dixon water and fat separation in a single scan. In comparison to other FSE implementations of the Dixon technique, the triple echo readout used in FTED incurs minimal deadtime in the pulse sequence design and thus greatly enhances the overall scan efficiency. A potential drawback of FTED is that the time dependence of the gradient moment along the frequency encode direction becomes more complicated than in FSE and flow compensation based on the gradient moment (GM) nulling is difficult to achieve. In this work, the first order GM along the frequency encode direction of FTED was examined and two different methods to minimize the GM were proposed. The first method nulls the GM at all the locations of the refocusing radiofrequency pulses so that the Carr-Purcell Meiboom-Gill condition is always maintained. The second method minimizes the GM of the spin echo component of the FSE signal at the echo locations. The efficacy of both methods in reducing the first order GM and flow-related artefacts was demonstrated both in phantom and in images in vivo.

Optimized EPI for fMRI using a slice-dependent template-based gradient compensation method to recover local susceptibility-induced signal loss

Most functional magnetic resonance imaging (fMRI) experiments use gradient-echo echo planar imaging (GE EPI) to detect the blood oxygenation level-dependent (BOLD) effect. This technique may fail in the presence of anatomy-related susceptibility-induced field gradients in the human head. In this work, we present a novel 3D compensation method in combination with a template-based correction that can be optimized over particular regions of interest to recover susceptibility-induced signal loss without acquisition time penalty. Based on an evaluation of B(0) field maps of eight subjects, slice-dependent gradient compensation moments are derived for maximal BOLD sensitivity in two compromised regions: the orbitofrontal cortex and the amygdala areas. A modified EPI sequence uses these additional gradient moments in all three imaging directions. The method is compared to non-compensated, template-based and subject-specific correction gradients and also in a breath-holding experiment. The slice-dependent gradient compensation method significantly improves signal intensity/BOLD sensitivity by about 35/43% in the orbitofrontal cortex and by 17/30% in the amygdala areas compared to a conventional acquisition. Template-based correction and subject-specific correction perform equally well. The BOLD sensitivity in the breath hold experiment is effectively increased in compensated regions. The new method addresses the problem of susceptibility-induced signal loss, without compromising temporal resolution. It can be used for event-related functional experiments without requiring additional subject-specific calibration or calculation time.

MRI Pulse Sequence Design With First-Order Gradient Moment Nulling in Arbitrary Directions by Solving a Polynomial Program

We suggest a polynomial program for the calculation of optimized gradient waveforms for magnetic resonance tomography pulse sequences. Such non-linear mathematical programs can describe gradient system capabilities, meet k-space trajectory specifications, and capture sequence timing conditions. Moreover they allow the incorporation of gradient moment nulling constraints in one or several arbitrary spatial directions, which can reduce flow motion artifacts in the images. We report first experiences in solving such automatic pulse sequence design programs with the interior point solver Ipopt.