摘要:

AbstractThe authors introduce several sets of time‐efficient gradient waveforms for applying isotropic diffusion weighting in NMR experiments. This creates signal attenuation that depends on the trace of the diffusion tensor and is therefore rotationally invariant. Numerical methods for the calculation of such gradient sets are outlined, and results are shown for isotropic and anisotropic gradient hardware and first order flow moment nulled diffusion weighting gradients. Preliminary experimental results from the human brain validate this new techniqu

ISSN:0740-3194    

DOI:10.1002/mrm.1910340202

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractIn this work, the authors present 3D images acquired from the human head using echo planar encoding for two of the three dimensions ofk‐space. The third dimension ofk‐space is filled by selecting and phase encoding a slab of spins as in conventional 3D steady state (GRASS based) acquisition regimens. Using this approach, a 128 x 64 x 64 3D data matrix could be obtained in 3.4–4.7 sec using effectiveTEvalues of 24 and 34 ms, respectively. High quality 3D images could be acquired once phase ghosts present on 2D images were minimized through proper adjustments of scanner har

ISSN:0740-3194    

DOI:10.1002/mrm.1910340203

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractGRASE (GRadient and Spin Echo) is an echo train imaging technique that combines gradient and RF refocusing. Although overall signal decay is withT2and field inhomogeneity phase errors do not accumulate, the small residual phase errors are periodic with echo number. The echo order described previously eliminates the phase error periodicity inkspace but instead creates periodicity in theT2modulation function that can also cause artifacts. In addition, with this order, the effectiveTEmust be half the echo train time, and asymmetric Fourier sampling is difficult to implement. A new method is described that greatly reduces artifacts due toT2decay, permits greater control ofT2contrast, and lends itself to asymmetric Fourier sampling. Different time segments of the echo train are encoded with different bands of spatial frequency inkspace (hence “kbanding”). Both computer simulations and experimental results demonstrate improvements in GRASE images acquired by this met

ISSN:0740-3194    

DOI:10.1002/mrm.1910340204

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractSpiral scanning gradient waveforms can be optimized with respect to blurring from off‐resonance effects by minimizing the readout time. This is achieved by maximizing the gradient amplitude during the scan so that the edge ofk‐space is reached as quickly as possible. Gradient hardware constraints are incorporated by considering a circuit model for the gradient coil and amplifier. The optimized gradient waveforms are determined by a set of coupled differential equations. The resulting solutions have shorter readout time than solutions that do not consider the circuit mo

ISSN:0740-3194    

DOI:10.1002/mrm.1910340205

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractTo assess the accuracy of quantitative analysis of global and regional wall motion and wall thickening of the left ventricle with cine magnetic resonance (MR), images obtained in eight pigs before and after myocardial infarction were compared with those obtained using gadolinium diethylenetriaminepen‐taacetic acid (Gd‐DTPA)‐enhanced multislice spin‐echo MR imaging and determination of pathology. The region with abnormal wall motion and wall thickening, as determined with cine MR imaging, identified the same region of infarction as indicated by Gd‐DTPA‐enhanced spin‐echo MR imaging and pathology. Within the infarcted region wall motion and wall thickening analyzed with the centerline method were significantly reduced. We conclude that the use of quantitative analysis of cine MR images accurately determines localization and extent of regional left ventricular dysfunction in the infarcted heartin vivo. This analysis using dedicated software including the centerline method allows sequential assessment of regional left ventricular function in normal and inf

ISSN:0740-3194    

DOI:10.1002/mrm.1910340206

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractThe effects of time‐varying readout gradients and data sampling with variable dwell times in magnetic resonance imaging are examined. General reconstruction formulas are given for linear reconstruction with evenk‐space weighting. Closed analytic expressions for estimator variance are given for data sampling during arbitrary gradient waveforms with both uniformkxstep size and nonuniformkxstep size. It is shown that estimator variance increases (the signal‐to‐noise ratio decreases) for nonconstant gradient waveforms. It is also shown that estimator variance is greater for constantk‐space sampling strategies than for constant time sampling at the Nyquist rate. Data collected during a triangular readout gradient waveform, with either constant time or constantk‐space sampling, versus conventional (constant gradient) collection confirms theoretical predictions for estimator variance. The benefits of collecting data while the readout gradient is ramping up from and down to zero ar

ISSN:0740-3194    

DOI:10.1002/mrm.1910340207

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractTo achieve correct spatial location of blood vessels, first order gradient moment nulling applied to the phase encoding axes can be used. However, gradient moment nulling prolongs echo time (TE), which may degrade the flow image in regions of complex flow. The fact that abbreviated moment compensated phase‐encoding (AMCPE) can be used to apply partial flow compensation to the phase‐encoding axes to prevent spatial misregistration of vessels without requiring the use of long echo times or using arbitrary chosenTEis demonstrated. AMCPE defines two cutoff lines ink‐space. The flow‐induced phase is completely compensated for values between the cutoff lines and partially compensated beyond the cutoff lines. The AMCPE technique has been tested on both a flow phantom and a human volunteer. The AMCPE images from both thein vivoand thein vitrostudy demonstrate correctly imaged flow. Computer simulations have been performed to analyze the penalty caused by the incomplete flow compensation. The result shows that the ripple artifacts due to the incomplete flow compensation are unobservable when 60%–70% ofk‐space is completely flow

ISSN:0740-3194    

DOI:10.1002/mrm.1910340208

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractA tailored receive‐only coil for cardiac imaging has been designed. The coil consists of two overlapping coil elements and can be used either as a quadrature surface coil or as a phased‐array coil. Through phantom experiments and images of the heart, the authors have shown that the improved cardiac coil provided a signal‐to‐noise ratio 1.6 times higher than a conventional quadrature spine coil, 1.4 times higher than that of a single coil (having the same shape and total dimension), and three times higher than the body coil at the depth of the posterior wall of the heart. The authors have also shown that the cardiac coil improved image quality everywhere in the heart. This coil will enhance routine clinical cardiac studies as well as other examinations such as myocardial perfusion, wall motion, and coronary artery

ISSN:0740-3194    

DOI:10.1002/mrm.1910340209

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractNMR measurements of anisotropic diffusion were studied using a three‐dimensional random‐walk model. It was found that the apparent diffusion coefficient can be expressed in a canonical form as the product of a diagonal matrix, an orthonormal rotation matrix, and a vector representing the encoding magnetic field gradient. The diffusion coefficient can be interpreted as the sum of the corresponding coefficients measured along the principal diffusion axes, weighted by the squares of the directional cosines of the encoding direction with respect to the principal axes. The analysis revealed that determining the orientation of anisotropy, in a cylindrically symmetric system, requires a minimum of four diffusion measurements. A special pulse sequence which minimized gradient cross‐terms and possible restricted diffusion effects was used to characterize diffusion anisotropy in cut chicken gizzards. Diffusion coefficients parallel to the muscle fibers were found to be approximately two to three times larger than those in the transverse direction. Furthermore, the method was successful in detecting the angular change when the sample was rotated by 30°. Results indicate that the proposed approach to measure fiber orientation is valid and may be used to improve the time efficiency of diffusion anisotropy measur

ISSN:0740-3194    

DOI:10.1002/mrm.1910340210

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractImage‐to‐image fluctuation due to physiological motion is a major limitation to the accurate detection of neuronal activity with functional MRI. In this paper, a new and general technique for the estimation and compensation of the physiological effects is presented. By simultaneously monitoring the respiration and heart beat during the acquisition of imaging data, and retrospectively synchronizing the imaging data with physiological activity, physiological effects are estimated and removed. This technique does not rely on the periodicity of the respiration or the heart beat, does not affect the signal changes arising from neuronal activation, and is beneficial to images acquired with any speed. Experimental studies performed with FLASH and EPI sequences have demonstrated that the new technique is effective in reducing physiological fluctuation and improving the sensitivity of functional MRI and is generally applica

ISSN:0740-3194    

DOI:10.1002/mrm.1910340211

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY