摘要:

AbstractStrategies to optimize flip angles for chemical shift selective fat suppression are discussed. Mathematical models for fat suppression in spoiled gradient recalled acquisition, spin echo, and RARE, which incorporate steady state conditions and multiple spectral components of fat, are developed. The optimal suppression flip angle is found to be larger than that determined with a single fat component model by more than 10° due to contributions from unflipped components such as olefinic and glycerol protons that lie outside the suppression band

ISSN:0740-3194    

DOI:10.1002/mrm.1910400402

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

年代:1998

数据来源: WILEY

摘要:

AbstractIn this paper, a series of diffusion‐weighted fast spin‐echo (FSE) sequences with a new motion correction scheme are introduced. This correction scheme is based on the navigator echo technique. Unlike conventional spin‐echo imaging, motion correction for FSE is complicated by the phase oscillation between odd‐numbered and even‐numbered echoes and the complex phase relationship between spin echo and stimulated echo components. In our approach, incoherent phase shifting due to motion is monitored by consecutive acquisition of two navigator echoes, which provide information on both inter‐echo and intra‐echo train phase shifts. Applications to both phantom andin vivostudies

ISSN:0740-3194    

DOI:10.1002/mrm.1910400403

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

年代:1998

数据来源: WILEY

摘要:

AbstractMany pathophysiological processes in the myocardium are in close relation to changes of the regional blood volume and regional myocardial blood flow or perfusion. Only few methods exist to obtain quantitative values for these parameters. Quantitative regional blood volume (RBV) studies in rat myocardium are presented using snapshot fast low angle shot (FLASH) inversion recoveryT1measurements with two different blood pool contrast agents, gadolinium diethylenetriamin‐opentaacetic acid (Gd‐DTPA) albumin and Gd‐DTPA polyly‐sine. In contrast to previous attempts, each snapshot FLASH image acquisition was ECG‐triggered under breathhold conditions. To measure relaxation times shorter than a heart cycle, eachT1sequence was repeated two times with different delays between inversion pulse and first image acquisition. The experiments were performed on a Bruker Biospec 70/21 using a homogeneous transmitter coil and a circularly polarized surface receiver coil, a special ECG trigger unit, and a respirator that is controlled by the pulse program. Based on a fast exchange model RBVmmaps were calculated from the relaxation time maps for different concentrations of the two blood pool contrast agents. A significant dependence of the RBV, values on bloodT1was found. This is in accordance with a model that has been developed recently relating the dependence of RBVmonT1of blood to perfusion. For Gd‐DTPA albumin, the application of the model to the experimental data yields realistic values for RBV and perfusion. The values, which are in accordance with literature data, were obtained at highest contrast agent concentrations i.e., lowest relaxation times of blood (

ISSN:0740-3194    

DOI:10.1002/mrm.1910400404

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

年代:1998

数据来源: WILEY

摘要:

AbstractA simple model is presented that allows quantitative separation of the contributions of signals from water in blood and extravascular parenchyma due to changes in blood oxygenation, induced either by brain activation or by alteration of inspired oxygen. The separation is based on the progressive attenuation of the signals in the vasculature of different levels when bipolar field gradient pulses are applied. Diffusion‐weighted spin‐echo echo‐planar imaging sequences were used to measure signal changes under various conditions in both animals and human volunteers. Normoxic‐hyperoxic episodes were induced in rats before and after injection of a superparamagnetic iron oxide contrast agent. Signal changes produced by visual stimulation were measured in human volunteers, and in volunteers subject to alternating normoxic‐hyperoxic episodes, and with administration of Gd‐DTPA. Analysis of the results with our simple model suggests that the apparent diffusion coefficient increases and R2 ( = 1/T2) decreases upon brain activation, with a large component from extravascular water related to the decrease in the blood de‐oxyhemoglobin concentration. Furthermore, this study suggests that apparent diffusion coefficient of the extravascular component alone may provide localization of neuron

ISSN:0740-3194    

DOI:10.1002/mrm.1910400405

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

年代:1998

数据来源: WILEY

摘要:

AbstractTo quantify the extraction fraction, E, for gadopentetate across tumor capillaries, R3230 adenocarcinomas were implanted in the mammary fat pads of seven rats. The value of E was determined by using a two‐compartment tissue model in which the endothelial transfer coefficient, KPS(ml. min−1. cc−1of tissue), was estimated from the model fitted to changes in R1relaxation time (ΔR1; s−1) measured by dynamic three‐dimensional spoiled gradient recalled magnetic resonance imaging after injection of 0.1 mmol. kg−1of gadopentetate dimeglumine. The plasma flow rate through the tumor capillaries, Fp, (ml. min−1·g−1of tissue), was independently measured with fluorescent microspheres. E could be calculated by the relationship, E = KPS/Fp.The mean E for gadopentetate in the R3230 tumor was 0.197 ± 0.118 with a range of 0.123–0.454. The relatively small mean value of E for gadopentetate allows a fair approximation of the permeability surface area product by KPSin th

ISSN:0740-3194    

DOI:10.1002/mrm.1910400406

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

年代:1998

数据来源: WILEY

摘要:

AbstractThe hyperfine shift reagent, TmDOTP5−, was used to resolve the39K NMR resonances of intra‐ (Ki+) and extracellular (Ke+) potassium in isolated, perfused guinea pig hearts. [Ki+] as measured by39K NMR was 25.9 ± 10.3 mM, compared with 114.4 ± 10.8 mM as measured by atomic absorption spectros‐copy (AAS) using TmD0TP5‐as a marker of extracellular space. Thus, only approximately 23% of intracellular potassium was detected by39K NMR using our experimental conditions. The area of the Ki+signal increased during early ischemia then returned to baseline levels during reperfusion. In an effort to learn more about the Ki+not detected by39K NMR, hearts were perfused with a Rb+‐enriched, K+‐depleted buffer for an extended period. This resulted in loss of the entire39K NMR signal, and Ki+, as measured by AAS, decreased from ∼60 to ∼6 to 7 μmol/g wet weight. When K+‐depleted hearts were subjected to global ischemia, a small39K NMR signal reappeared, suggesting that at least a portion of the nonexchangeable Ki+becomes detectable by NMR during ischemia. This newly visible K+signal subsequently dissipated during reperfusion of ischemic hearts. We conclude that ischemia induces changes in the NMR visibility of39K in perfus

ISSN:0740-3194    

DOI:10.1002/mrm.1910400407

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

年代:1998

数据来源: WILEY

摘要:

AbstractThe purpose of this work was to investigate the temporal relationship between intensity changes inT2*‐weighted NMR images and tissue oxygen content, measured by myoglobin proton NMR spectroscopy, in the skeletal muscle. During an ischemic stress test, the calf muscles of five healthy volunteers were studied at 3 Tesla. An interleaved NMRI‐NMRS sequence was used, which made it possible to recordT2*‐weighted images and myoglobin spectra simultaneously. During ischemia, rapid changes in muscle signal intensity were observed onT2*‐weighted images, which immediately preceded myoglobin desaturation. Bearing in mind the respective P50of hemoglobin and myoglobin, this observation clearly favored the hypothesis that hemoglobin desaturation was responsible for the changes inT2*. This interpretation was further supported by the temporal coincidence between the experimental NMR data and a model of hemoglobin desaturation solely derived from physiological conside

ISSN:0740-3194    

DOI:10.1002/mrm.1910400408

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

年代:1998

数据来源: WILEY

摘要:

AbstractMRI ultra‐fast imaging techniques are used to characterize flow emerging from streamlined and abrupt stenoses inside cylindrical channels. Reattachment lengths of the shear boundary to the channel wall are measured using rotating ultra‐fast imaging sequence (RUFIS) in‐flow imaging. Velocity profiles of flow are created using velocity (sine and cosine)‐encoded RUFIS sequences. The sine‐encoded images permit one to identify reverse flow (i.e., eddies) that arise within the region of flow reattachment. The ratios of peak velocities (downstreamhpstream of the stenosis) derived from the cosine‐encoded images are used to identify the transition from the laminar to the turbulent regimen. Based on these experiments, the transition from the laminar to turbulent regimen occurs at a stenotic Reynolds Number of 350, whereas fully developed turbulence occurs at a stenotic Reynolds Number of 2600. These results are compared with the results from invas

ISSN:0740-3194    

DOI:10.1002/mrm.1910400409

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

年代:1998

数据来源: WILEY

摘要:

AbstractThe 3D TRICKS method for contrast‐enhanced, time‐resolved MR DSA has been recently described. In this paper, computer simulations are used to investigate the relative frame rate, temporal window, artery‐vein temporal separation, contrast‐to‐noise ratio, and spatial resolution of TRICKS and conventional scans for breath‐hold and non‐breath‐hold applications. For non‐breath‐hold applications, TRICKS can be coofigured to provide increased CNR or spatial resolution at an increased frame rate, but with a longer temporal window when compared with a series of conventional scans in which the central portion of k‐space is sampled at the same rate as for the TRICKS scans. For breath‐hold applications, TRICKS typically provides three images with 75% of the conventional single acquisition spatial resolution and is more tolerant of variations in contrast curve shape w

ISSN:0740-3194    

DOI:10.1002/mrm.1910400410

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

年代:1998

数据来源: WILEY

摘要:

AbstractIt has been observed that fast spin‐echo (FSE) images with a large field of view (>40 cm) in certain directions exhibit unusual ghosting artifacts that cannot be eliminated with existing ghost removal methods. These artifacts have been related to a higher‐order magnetic field perturbation (known as the concomitant field, or Maxwell field) concomitant to the linear i‐maging gradient, in accordance with the Maxwell equations Several methods have been developed to eliminate or minimize the effects of the concomitant magnetic field by redesigning the FSE pulse sequences. In the slice‐selection direction, the gradient waveforms are made symmetrical about the refocusing RF pulses wherever possible. Surrounding the first refocusing pulse, such symmetry cannot be achieved due to the slice‐refocusing gradient, which is often combined with the left crusher. In this case, it is shown how crusher gradients can be reshaped to nullify the phase due to the concomitant field. In the phase‐encoding direction, the gradient amplitude is reduced and its duration is prolonged. Artifacts due to the readout gradient are eliminated by reshaping the prephasing lobe, while keeping its area fixed. In all the three directions, the gradient waveforms are adjusted so that they have minimal overlap. Selected methods have been implemented on a clinical scanner, and typically reduce the ghost intensities in phantom and human images by a

ISSN:0740-3194    

DOI:10.1002/mrm.1910400411

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

年代:1998

数据来源: WILEY