摘要:

AbstractData from a previously published, multi‐site trial (P. G. Webb, N. Sailasuta, S. J. Kohler, T. Raidy, R. A. Moats, R. E. Hurd, Automated single‐voxel proton MRS: technical development and multisite verification.Magn. Reson. Med.31, 365–373 (1994)) of a fully automatic, single‐voxel, proton spectroscopy package (PROBE/SV, GE Medical Systems) was re‐analyzed in terms of absolute metabolite concentrations using the cerebral water signal as an internal reference. In 100 spectra from parietal white matter in normal volunteers ranging in age from 22 to 34 years at eight sites, overall concentrations of choline (Cho), creatine (Cr), and N‐acetyl‐aspartate (NAA) resonances were found to be 2.00 ± 0.50, 8.43 ± 1.26, and 12.55 ± 1.76 μmol/g wet weight, respectively. These values are in good general agreement with previously published values from quantitative, single‐voxel studies. Metabolite concentrations for NAA, Cr, and Cho across all sites had standard deviations of 14.1%, 14.9%, and 25.1%, respectively. Quantitation of PROBE data sets is routinely possible by using the cerebral water signal as an

ISSN:0740-3194    

DOI:10.1002/mrm.1910360302

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

年代:1996

数据来源: WILEY

摘要:

AbstractMagnetic resonance imaging using laser‐polarized129Xe is a new technique first demonstrated by Albert et al. (Nature 370, 1994) who obtained a129Xe image of an excised mouse lung. This paper describes the factors influencing the accumulation of inhaled, polarized129Xe in human tissue. The resulting model predicts the129Xe magnetization in different tissues as a function of the time from the start of inhalation, the tissue perfusion rate and partition coefficient for xenon, and the relevant T1decay times. The relaxation times of129Xe in biological tissues are not yet known precisely. Substitution of estimated values for these parameters results in an expected signal‐to‐noise ratio (SNR) from polarized129Xe MR in the brain of approximately 2% of the equivalent SNR from prot

ISSN:0740-3194    

DOI:10.1002/mrm.1910360303

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

年代:1996

数据来源: WILEY

摘要:

AbstractAn MR angiographic technique, referred to as 3D TRICKS (3D time‐resolved imaging of contrast kinetics) has been developed. This technique combines and extends to 3D imaging several previously published elements. These elements include an increased sampling rate for lower spatial frequencies, temporal interpolation of k‐space views, and zero‐filling in the slice‐encoding dimension. When appropriately combined, these elements permit reconstruction of a series of 3D image sets having an effective temporal frame rate of one volume every 2‐6 s. Acquiring a temporal series of images offers advantages over the current contrast‐enhanced 3D MRA techniques in that it i) increases the likelihood that an arterial‐only 3D image set will be obtained, ii) permits the passage of the contrast agent to be observed, and iii) allows temporal‐processing techniques to be applied to yield additional information, or improv

ISSN:0740-3194    

DOI:10.1002/mrm.1910360304

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

年代:1996

数据来源: WILEY

摘要:

AbstractA commercial phased‐array multicoil was used to acquire water‐suppressed localized proton spectra of the two major anatomical regions of the prostate. The signal‐to‐noise ratio and spectral resolution allowed identification of peaks from choline and creatine, as well as a major peak from citrate. Quantification of the citrate peak using experimentally determined relaxation parameters with tissue water as an internal concentration reference revealed a marked variability between different volunteers. Nevertheless, in each case, the citrate concentration was up to fourfold greater in the peripheral zone than in the central gland. Furthermore, the difference in citrate concentration between these two regions was positively correlated with the subjects age. The results indicate a consistent difference in cellular function between the major anatomical regions within the prostate and may have important consequences for the application of magnetic resonance spectroscopy to the diagnosis of prostatic pa

ISSN:0740-3194    

DOI:10.1002/mrm.1910360305

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

年代:1996

数据来源: WILEY

摘要:

AbstractA computational strategy for the deconvolution of complex spectra involving scalar multiplet patterns is presented. This approach fits spectra that can be composed of single resonances as well as scalar coupling multiplets for which resonance frequencies, intensities, and lineshape parameters can be optimized. For multiplets, the coupling constant also is optimized. Any external information about the optimizable parameters can be taken into account as external constraints. A lineshape described by absorptive and dispersive Lorentzian and Gaussian contributions and the baseline with up to 40 Fourier and polynomial terms can likewise be optimized. The effectiveness of the procedure is assessed on the basis of computer simulated deconvolutions of a composite of1J(13C−2H) multiplets arising from a mixture of all possible13C−2H isotopomers of deuterated L‐[3‐13C]lactate generated from cell preparations incubated with D‐[1‐13C]glucose in D2O, which was analyzed previously with a manual deconvolution procedure (R. Willem, M. Biesemans, F. Kayser, W. J. Malaisse,Magn. Reson. Med.31, 259–267 (1994)). The use of constraints is shown to lead to an improvement in the results. The fitting strategies and the importance of the baseline as an origin of bias

ISSN:0740-3194    

DOI:10.1002/mrm.1910360306

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

年代:1996

数据来源: WILEY

摘要:

AbstractDivergencies between chemical shift measurements of temperature and directly measured values using optical sensors have been studiedin vivoin human peripheral muscle with the assistance of a variety of experimental and theoretical techniques. These include the modeling of both thermal and susceptibility changes using two‐ and three‐dimensional finite element methods, as well as the use of multi‐wavelength near infrared observations. The conclusion of these studies is that a simple temperature calibration is not accessible, with results affected by the complex response of the tissue i

ISSN:0740-3194    

DOI:10.1002/mrm.1910360307

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

年代:1996

数据来源: WILEY

摘要:

AbstractA homogeneous static (Bo) magnetic field is required for many NMR experiments such as echo planar imaging, localized spectroscopy, and spiral scan imaging. Although semi‐automated techniques have been described to improve the Bofield homogeneity, none has been applied to thein vivoheart. The acquisition of cardiac field maps is complicated by motion, blood flow, and chemical shift artifact from epicardial fat. To overcome these problems, an ungated three‐dimensional (3D) chemical shift image (CSI) was collected to generate a time and motion‐averaged Bofield map. Boheterogeneity in the heart was minimized by using a previous algorithm that solves for the optimal shim coil currents for an input field map, using up to third‐order current‐bounded shims (1). The method improved the Bohomogeneity of the heart in all 11 normal volunteers studied. After application of the algorithm to the unshimmed cardiac field maps, the standard deviation of proton frequency decreased by 43%, the magnitude1H spectral linewidth decreased by 24%, and the peak‐peak gradient decreased by 35%. Simulations of the high‐order (second‐ and third‐order) shims in Bofield correction of the heart show that high order shims are important, resulting for nearly half of the improvement in homogeneity for several subjects. The T2* of the left ventricular anterior wall before and after field correction was determined at 4.0 Tesla. Finally, results show that cardiac shimming is of benefit in cardiac31P NMR spectroscopy and cardiac ec

ISSN:0740-3194    

DOI:10.1002/mrm.1910360308

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

年代:1996

数据来源: WILEY

摘要:

AbstractNew centric phase encoding orders are introduced for transient state three‐dimensional MR imaging. Unlike previous centric orders, these variants are not fixed with respect to differences in the relativeyandzfields of view, but are tailored to account for the differential degrees of gradient dephasing in thekyandkzphase encoding directions and thus maintain the highest weighting for the most central k‐space values. The new centric orders are compared with previous methods in both simulation and experimental magnetization‐prepared MR angiography of the carotid and renal arteries. In comparison with other tested centric orders, the new methods provided better retention of true vessel shape caused by isotropic blurring across the phase encoding plane. In addition, the new methods provided increased contrast, with the degree of improvement over previous methods depending on vessel orientation, size, and degree of i

ISSN:0740-3194    

DOI:10.1002/mrm.1910360309

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

年代:1996

数据来源: WILEY

摘要:

AbstractThe diffusion characteristics of water in brain white matter were studied in patients with benign and secondary progressive multiple sclerosis (MS), and also in normal controls. In the MS patients, both lesions and normal‐appearing white matter (NAWM) were examined to assess whether pathological differences might be evident from the diffusion behavior. A volume‐selective technique was used to reduce data acquisition time and improve the reliability and precision of the measurements. This also allowed the time‐dependence of apparent diffusion coefficients to be assessed. While lesions from both patient groups showed an elevated diffusion coefficient, no differences between the two groups were found. In addition, NAWM was elevated for both patient groups compared with the control group, although this was only statistically significant for patients with a benign disease course. The degree of elevation of the diffusion coefficient within the individual lesions measured was not related to the disability of the patient. Pathological differences between lesions in patients with different disease courses, if they exist, have not been detected in this study of brain water diff

ISSN:0740-3194    

DOI:10.1002/mrm.1910360310

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

年代:1996

数据来源: WILEY

摘要:

AbstractOptimal control theory has been applied in the past for the design of RF pulses for selective excitation. This was the outcome of having established the controllability of the MR spin system for the selective excitation problem. “Minimum distance” was the main formulation used for the solution. Because of their robust behavior in the presence of inhomogeneous RF fields, adiabatic pulses play an important role in spin inversion and excitation. In this study, we present a method for incorporating adiabaticity into the optimal control problem by enhancing the cost functional with an appropriate term. Two different types of adiabatic terms are proposed. Furthermore, two methods are used to solve the optimal control problem, namely the Hamiltonian approach and the solution by mathematical programming. Design examples include both a frequency selective pulse for performing fat suppression by inversion and a regular inversion pulse. It is shown that, in the course of optimization, the pulse designer can trade‐off slice resolution against pulse adiabat

ISSN:0740-3194    

DOI:10.1002/mrm.1910360311

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

年代:1996

数据来源: WILEY