摘要:

AbstractIt has been observed that the signal amplitude of multiplet resonances such as the 1H doublet resonance of lactate varies with pulse sequence timing when echo‐driven volume selective methods such as point resolved spectroscopy are used. Herein a standard vectorial description is presented for the mechanism of this artifact, which results from the chemical shift between homonuclear scalar‐coupled (i.e.,Jcoupled) nuclei. The chemical shift causes the extent of a signal phase modulation to vary for different spatial regions of the excited voxel. This variation results in spatial interference effects that can lead to marked loss of signal intensity as well as corruption of the size and shape of the voxel from which signal is obtained. The phenomenon is substantial at an imaging field of 1.5 T and becomes especially pronounced at higher field strengths. Several strategies to avoid the artifact are provi

ISSN:0740-3194    

DOI:10.1002/mrm.1910390202

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

年代:1998

数据来源: WILEY

摘要:

AbstractFlow‐sensitive functional MRI (fMRI) was performed using steady‐state arterial water labeling (SS‐AWL). Arterial water labeling was accomplished by flow induced adiabatic fast passage. The signal intensity of the visual cortex in arterial water labeled images decreased by ˜1.4% during visual stimulation of the brain. Acquisition of arterial water unlabeled and labeled images allows measurement of relative cerebral blood flow increase during brain activation. During visual stimulation, cerebral blood flow in the visual cortex increased by 17 to 35% as measured by SS‐AWL. Quantitation of brain activation in terms of a physiological parameter using SS‐AWL will facilitate comparative fMRI studies under different

ISSN:0740-3194    

DOI:10.1002/mrm.1910390203

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

年代:1998

数据来源: WILEY

摘要:

AbstractAccumulation of lipids in atherosclerotic plaques causes progressive narrowing of the arterial lumen, often followed by thrombosis and ischemia. Currently several different methods, most requiring disruption of the plaque, are used to study the physical properties of lipids accumulated in plaques, and lipid composition is typically determined by chemical analysis of completely disrupted plaques. In this study,13C magic angle spinning NMR spectroscopy (MAS NMR) was used to determinein situthe lipid composition and molecular organization of all lipid phases in human carotid artery plaques (exvivo). Protocols were developed to observe signals from one lipid phase without interference from other phases. In addition,31P MAS NMR detected calcification in plaques by the signals from inorganic phosphate complexed to calcium. Together,13C and31P MAS NMR comprise a powerful nondisruptive approach for determining the quantity and phase state of components in arterial plaques.

ISSN:0740-3194    

DOI:10.1002/mrm.1910390204

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

年代:1998

数据来源: WILEY

摘要:

AbstractThe effects of endotoxin injury on lung NMR relaxation times (T1, CPMGT2, and Hahn decay constant (HahnT2)) were studied in excised unperfused rat lungs. Blinded histologic examination showed no clear‐cut separation between endotoxin and control lungs. Morphometric lung tissue volume density and gravimetric lung water content did not differ significantly between the two groups. In contrast, the values of the fast, intermediate, and slowT2components, obtained by multiexponential analysis of the CPMG decay curve, increased markedly after endotoxin administration, with minimal overlap between endotoxin and control values. The response of HahnT2was, in general, in the same direction as that of CPMG 72; however, HahnT2may be more affected by measurement errors and may be less sensitive to the presence of lung injury.t1showed minimal changes after injury. The present data suggest that CPMG 72measurements can consistently detect the presence of lung injury even when conventional histologic, morphometric, and gravimetric studies provide negative or equivocal results, and that the CMPGT2method is superior, in this respect, to the Hahn decay method.T1does not appear to be sensitive to lung injury in the absence of significant lung water accumulatio

ISSN:0740-3194    

DOI:10.1002/mrm.1910390205

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

年代:1998

数据来源: WILEY

摘要:

AbstractDetailed measurements of theT1‐weighted,T2‐weighted, and MT‐weighted signal were performed for exvivomuscle samples heated to various temperatures for different times. Consistent, monotonic increases in signal intensity were observed with progressive thermal coagulation, corresponding to an increase inT2relaxation time and an increase in MT‐weighted signal for temperatures above 60°C. The relationship for T1relaxation was more complex, showing a decrease in T1relaxation from 40 to 60°C and an increase above 60°C. These techniques provide a more direct measure of tissue thermal coagulation than that provided by MR thermometry and suggest MR imaging strategies for the optimization and monitoring of thermal coagulation therapy protocols that create thermal damage in targ

ISSN:0740-3194    

DOI:10.1002/mrm.1910390206

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

年代:1998

数据来源: WILEY

摘要:

AbstractAdaptive near‐optimal MRI spatial encoding entails, for the acquisition of each image update in a dynamic series, the computation of encodes in the form of a linear algebra‐derived orthogonal basis set determined from an image estimate. The origins of adaptive encoding relevant to MRI are reviewed. Sources of error of this approach are identified from the linear algebraic perspective where MRI data acquisition is viewed as the projection of information from the field‐of‐view onto the encoding basis set. The definitions of ideal and non‐ideal encoding follow, with nonideal encoding characterized by the principal angles between two vector spaces. An analysis of the distribution of principal angles is introduced and applied in several example cases to quantitatively describe the suitability of a basis set derived from a specific image estimate for the spatial encoding of a given field‐of‐view. The robustness of adaptive near‐optimal spatial encoding for dynamic MRI is favorably shown by results computed using singular value decomposition encoding that simulates specific instances of worst case data acquisition when all objects have changed or new objects have appeared in the field‐of‐view. The mathematical analysis and simulations presented clarify the applicability and efficiency of adaptively determined near‐optimal spatial encoding throughout a range of circumstances as may typically occur duri

ISSN:0740-3194    

DOI:10.1002/mrm.1910390207

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

年代:1998

数据来源: WILEY

摘要:

AbstractThe problem of automatically tuning and matching a probe is considered, and attention is drawn to the main cause of difficulty: the interaction of tuning and matching mechanisms. The use of a doubly resonant, inductively coupled probe is therefore advocated, for to first order, tuning, and matching are then orthogonal for small perturbations. It is shown that when such a probe is equipped with negative feedback control of tuning and matching elements, driven by error signals derived from a bridge and the NMR spectrometer, stable and rapid automatic tuning and matching are achieved. The importance of approximately correct phasing of the error signals is emphasized, but the restriction of having to have specific cable lengths to achieve this task is removed. The effects of small errors of phasing are then considered and shown to be unimportant for the chosen probe, in contrast to the situation for other, nonorthogonal probe types. Suggestions for orthogonalization of two common capacitively coupled probe designs are made, and a practical instrument design is then given that, with the addition of external components, can also be used if the NMR receiver cannot be pressed into service.

ISSN:0740-3194    

DOI:10.1002/mrm.1910390208

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

年代:1998

数据来源: WILEY

摘要:

AbstractAn analytical model of tissue relaxation and restricted diffusion in human blood is presented. The blood tissue model is composed of three different compartments: red blood cells, plasma, and macromolecular protons. The relaxation rate constants and free diffusion coefficients of intracellular and extracellular water may differ. Analytical formulas for signal loss due to relaxation and diffusion in the Carr‐Purcell Meiboom‐Gill and pulsed‐field‐gradient multispin echo experiments for this tissue model are derived. The model is fitted to the experimental data for human blood with various concentrations of Gadolinium contrast agent. The obtained model parameters are realistic. The validity and sensitivity of the model are also di

ISSN:0740-3194    

DOI:10.1002/mrm.1910390209

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

年代:1998

数据来源: WILEY

摘要:

AbstractGlobal head motion occurring between successive image acquisitions during a functional MRI time series can corrupt the signal of physiologic brain activation, potentially invalidating interpretation of the final activation map from that particular fMRI time series. By approximating the head as a rigid body, multiaxial global head motion can be decomposed into orthogonal linear and rotational components. This paper describes a method using orbital navigator echoes to provide prospective correction for both through–plane and in–plane inter–image head rotation in functional MRI. The dynamic detection and correction of rotation can be performed in<100 ms. Phantom experiments demonstrate accurate correction of rotational motion over a range of ±0.36° to ±12°. Imaging studies in volunteers document the feasibility of real–time prospective correction of rotational motionin vivo.Using a modified receiver operating characteristic method, motion–corrected functional MRI sensorimotor studies incorporating deliberate head rotations are shown to be superior to functional MRI time series acquired under similar conditions but without moti

ISSN:0740-3194    

DOI:10.1002/mrm.1910390210

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

年代:1998

数据来源: WILEY

摘要:

AbstractAn algorithm is proposed which improves the spatial resolution of difference or effect images acquired with a keyhole sampling strategy. This new reconstruction algorithm usesa priori informationabout sharp structures in the observed signal changes from a high resolution reference scan. The potential of this algorithm even being able to deal with noisy effect images is illustrated by application to functional MRI data.

ISSN:0740-3194    

DOI:10.1002/mrm.1910390211

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

年代:1998

数据来源: WILEY