摘要:

AbstractTwo‐dimensional shift‐correlated (COSY) andJ‐resolved NMR spectroscopy was used to identify and assign resonances in the aliphatic region (0.8 to 4.5 ppm) of the1H spectrum of acid extracts and tissue of rat brain. The chemical shift and spin‐spin coupling constants of several resonances, which could not be resolved in one‐dimensional spectra of tissue, were determined. These properties, together with the appropriate multiplet structure and scalar coupling patterns observed in 2DJ‐resolved and COSY spectra, were used to assign the resonances of lactate, threonine, alanine, γ‐amino butyrate,N‐acetyl aspartate, aspartate, glutamate, glutamine, taurine, and myo‐inositol. The single peak observed at 1.33 ppm in the ID1H spectrum of excised brain tissue, which is generally assigned to lactate, was shown instead to consist of two overlapping peaks in 2D COSY contour plots. The intensity of both resonances increased considerably after death. The second resonance was assigned to threonine and the assignment was confirmed by ion‐exchange chromatography. Several cross peaks were identified in 2D COSY spectra of intact brain tissue that were not present in similar spectra of the acid extract. These nonmetabolite resonances were assigned to macromolecules. Analysis of 2D COSY contour plots in conjunction with 1D spectra revealed that the total creatine (creatine + phosphocreatine) resonance at 3.0 ppm was overlapped by both GABA and macromolecules. This macromolecule resonance was present in a 2D COSY spectrum of the rat brain obtainedin situwith a surface coil following cardiac arrest, indicating that its presence in the 2D spectrum of excised brain tissue did not arise from tissue disruption. © 199

ISSN:0740-3194    

DOI:10.1002/mrm.1910170202

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

年代:1991

数据来源: WILEY

摘要:

AbstractA major factor contributing to proton (1H) spin‐lattice relaxation in biological tissues is believed to be magnetization transfer between 'H in free bulk water and1H in restricted motion associated with macromolecules. We have shown recently that saturation transfer is an effective approach for studying this magnetization transfer process. Herein the determination of magnetization transfer rates in biological tissues is further analyzed by considering the time and power dependencies of saturation transfer. Following these analyses, quantitative magnetization transfer rate constant image maps were collected from the kidneyin vivo.These rate constant images may prove useful in quantitative tissue characterization and in the determination of tissue‐specific1H relaxation mechanisms. © 1991 Academic Press,

ISSN:0740-3194    

DOI:10.1002/mrm.1910170203

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

年代:1991

数据来源: WILEY

摘要:

AbstractSpectroscopic imaging with a one‐dimensional phase‐encoding gradient and surface‐coil reception relies on the restricted range of sensitivity of the surface coil to provide localization in the dimensions transverse to the coil axis and consequently suffers from relatively poor localization in these dimensions. A two‐dimensional (2D) cylindrically selective excitation pulse with a large spectral bandwidth is presented here to remedy this problem. The gradient waveforms are derived from multiple spirals inkspace which form an overall pinwheel pattern, resulting in a pulse which is much shorter than the equivalent single‐spiral trajectory. Nonuniform traversal of the spirals further reduces the pulse width under conditions of gradient slew‐rate limitations, yielding overall gains in bandwidth of up to about 30 compared with the equivalent single‐spiral trajectory traversed at constant angular rate. The accompanying rf waveform is obtained by weighted 2D Fourier transformation of the desired sensitivity profile. A new weighting factor is introduced into the rf waveform to compensate for nonuniform sampling ofkspace by the pinwheel near the origin. This factor is independent of the weighting used to account for the rate of traversal of the trajectory and is applicable to 2D pulse design in general. Pulse sequences employing pinwheel excitation in conjunction with either phase‐encoding or slice‐selective inversion are used to produce multiple‐voxel and single‐voxel localization in a human heart and a phantom. Pinwheel pulses may be used to advantage on moieties with long spin‐lattice relaxation times and short transverse relaxation times and are therefore ideal for applications in phosphorus (13P) NMR. ©

ISSN:0740-3194    

DOI:10.1002/mrm.1910170204

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

年代:1991

数据来源: WILEY

摘要:

AbstractWe have developed a novel contrast agent, ytterbium‐diethylenetriamine pentaacetic acid (Yb‐DTPA), for magnetic resonance imaging (MRI) and its mechanism of action is predominantly by the susceptibility mechanism. We have applied this contrast agent for renal MRI studies. We also demonstrate that Yb‐DTPA has a renal clearance similar to that of inulin in rats and may therefore be useful for the estimation of glomerular filtration rate. © 1991 Academic Pres

ISSN:0740-3194    

DOI:10.1002/mrm.1910170205

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

年代:1991

数据来源: WILEY

摘要:

AbstractWe calculate the effects of subvoxel variations in magnetic susceptibility on MR image intensity for spin‐echo (SE) and gradient‐echo (GE) experiments for a range of microscopic physical parameters. The model used neglects the overlap of gradients from one magnetic inclusion to the next, and so is valid for low volume fractions and weak perturbations of the magnetic field. Transverse relaxation is predicted to deviate significantly from linear exponential decay in both SE and GE at a particle radius of 2.5 μm. Calculated changes in transverse relaxation rates for SE and GE increase linearly with volume fraction of high‐susceptibility regions of 5 μm diameter, but increase with about the 3/2 power of volume fraction of regions with 15 μm spacing between centers. This sensitivity to the actual size and spacing of magnetized regions may allow them to be measured on the basis of contrast, without being resolved in images. GE and SE decay rates are approximately twice as sensitive to long cylinders of 5 μm diameter than to spheres of the same size, for diffusion constants of 2.5 μm2/ms. Calculated changes in transverse decay rates increase with approximately the square of field and susceptibility variation for 5‐μm spheres and a diffusion constant of 2.5 μm2/ms. This exponent is smaller for cylindrical magnetized regions of the same size, and also depends on the diffusion constant. We discuss possible applications of our theoretical results to the analysis of the effects of high‐susceptibility contrast agents in brain. Experimental data from the literature are compared with calculated signal changes according to the model. The monotonic dependence of decay rates on the volume of distribution of the contrast agent suggests that cerebral blood volume and flow could be measured using MR contrast. © 1991 Ac

ISSN:0740-3194    

DOI:10.1002/mrm.1910170206

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

年代:1991

数据来源: WILEY

摘要:

AbstractThe design of magnetic particles as a magnetic resonance contrast agent will rely on the prediction of their ability to induce transverse relaxation among the surrounding protons. There exists several divergent predictions of the contribution of these agents to 1 /T2. This article points out a problem, commonly overlooked, in the development of expressions for the relaxation enhancement which has led some workers to the derivation of results inappropriate for large magnetic particles. The size of the magnetic inhomogeneity created by the particle precludes the averaging of the interaction over a single proton unless it experiences an average field in the timeTbetween pulses. Computer simulations following the trajectories of diffusing water molecules in inhomogeneous fields are shown to be the correct approach to dealing with large inhomogeneities. © 1991 Academic Press, Inc

ISSN:0740-3194    

DOI:10.1002/mrm.1910170207

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

年代:1991

数据来源: WILEY

摘要:

AbstractLeakage of Gd‐DTPA through a defective blood‐brain barrier is measured quantitatively using dynamic MRI scanning, in which repeated scans are made after a bolus injection. Image registration artifacts are minimized; a dose of 0.1 mM/kg and an IR sequence enable enhancement to be measured quantitatively. The triexponential enhancement curve is fitted to a theoretical model based on compartmental analysis. The transfer constant, or permeability surface area product per unit volume of tissue(k), and leakage space per unit volume of tissue (y) are measured. Estimates for a quickly enhancing multiple sclerosis lesion arek= 0.050 min−1,v1= 21%; for a slow onek= 0.013 min−1,v1= 49%. This implies permeability in the range 4‐17 × 10−6cm s−1, in broad agreement with other physiological methods. The method is noninvasive and can be used to make serial measurements in patients and in experimental animal models. The time course of pathological aspects of diseases with blood‐brain barrier breakdown, such as multiple sclerosis, tumors, and infections (e.g., HIV) can be studied, along with their response to therapy. The measurements are of physiological variables and are therefore independent of imaging equipment and field. © 1991 Ac

ISSN:0740-3194    

DOI:10.1002/mrm.1910170208

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

年代:1991

数据来源: WILEY

摘要:

AbstractPhosphorus NMR can measure myocardial tissue pH from the chemical shift of inorganic phosphate (Pi) in isolated buffer‐perfused hearts, butin vivothePipeak originating from the myocardium is obscured by the resonance of 2, 3‐diphosphoglycerate (DPG) in the blood, making pH difficult to determine. Taking advantage of the fact that most of the interfering DPG is within the cardiac chambers and is rapidly flowing out of the sensitive volume of our coil, we developed a pulse sequence which would separate myocardialPisignal from interfering DPG. We tested this method on a flow phantom and in living rat heart, using exogenous glycerol phosphate as a blood‐pool marker. The results indicated that signal from moving and nonmoving substances could be separated, and derived values for myocardial pH and PCr/Piratio were consistent with previous estimates. This method should be useful for studying myocardial acid‐base physiology with NMR. © 1991 Academic Pr

ISSN:0740-3194    

DOI:10.1002/mrm.1910170209

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

年代:1991

数据来源: WILEY

摘要:

AbstractAcute myocardial infarction is associated with an accumulation of lipids. Spectroscopic and chemical‐shift imaging strategies which can depict the spatial distribution of these chemical species are evolving. The present study was undertaken to test whether the Dixon method could detect spatially lipids known to accumulate in myocardium after an ischemic insult. Seven dogs underwent a 24‐h coronary artery occlusion (LAD = 4, Cx = 3). Post mortem, hearts were removed and imagedex vivo.Myocardial samples were also evaluated by high‐resolution1H NMR spectroscopy. Lipid images revealed regions of increased signal intensity, in the regions corresponding to the myocardial infarction, particularly in the periphery of the infarction. An increase in mobile lipids was observed by1H NMR spectroscopy of myocardial samples with moderately reduced blood flow and corresponding to regions with increased signal intensity on the lipid image. This study shows that chemical‐shift imaging may be useful for detecting alterations in myocardial lipid levels following an ischemic insult. © 1991 Academic Pr

ISSN:0740-3194    

DOI:10.1002/mrm.1910170210

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

年代:1991

数据来源: WILEY

摘要:

AbstractA rapid‐profiling NMR pulse sequence has been designed to provide an interactive, real‐time cardiac probe analogous to M‐mode ultrasound. The pulse sequence employs a two‐dimensional (2D) selective NMR pulse to excite a narrow (nominally 1 ‐cm‐diameter) cylinder of magnetization intersecting the heart. This procedure is followed by a readout gradient applied along the length of the cylinder, or “beam,” to yield an M‐mode type profile with a one‐dimensional Fourier transform reconstruction.k‐space techniques were used to design 2D pulses which excite cylinders characterized by either Gaussian or square radial excitation profiles. Images of phantoms acquired at 1.5 T confirm the predictions of thek‐space analysis. The cylinder can be displaced interactively by modulating the rf excitation and the beam axis can be reoriented to any oblique direction by changing the relative mixing of the gradient waveforms. Flow compensation using bipolar gradient waveforms inverts the contrast of flowing blood and suppresses flow artifacts. A gated cardiac image is acquired as a reference to locate the excitation axis. A series of cardiac experiments was performed on several healthy volunteers. As the beam is moved and rotated to probe the myocardium, the profile plots resemble an M‐mode echocardiogram. Unlike in M‐mode echocardiography, however, the axis of interrogation is not limited to specific windows, and there is distinct flexibility of contrast. However, the temporal resolution is currently less than that achieved by ultrasound. NMR M‐mode profiling provides a direct, fast method of measuring heart motion to assess cardiac function as part of an MR cardiac exa

ISSN:0740-3194    

DOI:10.1002/mrm.1910170211

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

年代:1991

数据来源: WILEY