摘要:

AbstractSpin‐lattice (T1) and spin‐spin (T2) relaxation times have been measured on perfused rat hearts under two experimental conditions.T1exhibits a monoexponential decay. On the other handT2has a decay with two components: a short oneT2sand a long oneT21. These facts have been discussed according to cross‐relaxation and a bicompartmen‐talization of tissue assuming a slow exchange model for spin‐spin relaxation and a fast exchange model for spin‐lattice relaxation. Increasing the osmotic pressure of the perfusion solution decreased the absolute density proton of theT2scompartment reflecting the loss of its water content. The paramagnetic ion manganese diminishes the values ofT1and those of the long componentT21without affceting its short component. Therefore the short component could be assigned to intracellular and the long component to extracellular free water. The extracellularT2(459 ms) is approximatively 10‐fold higher than the intracellularT2(45 ms) With images of “pureT2” such a difference could be useful to enhance the contrast between organs and the surrounding liquid or between organs with different water com

ISSN:0740-3194    

DOI:10.1002/mrm.1910020302

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

年代:1985

数据来源: WILEY

摘要:

AbstractA method of performing chemical‐shift imaging is described that avoids some of the disadvatages inherent in the techniques which use spin echos and phase encoding. A series of FIDs are recorded during the application of a series of static gradients and the spectral image is reconstructed using a conventional filtered back‐projection algori

ISSN:0740-3194    

DOI:10.1002/mrm.1910020303

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

年代:1985

数据来源: WILEY

摘要:

AbstractA new NMR imaging technique, aiming at a reduction of the data acquisition time, is described in detail. The method is applicable in all multidimensional imaging situations where at least one spatial coordinate is resolved. Discussed at some length are the point‐spread function, the requird bandwidth of the NMR spectrometer, and the sampling strategy used to diminish the loss in signal‐to‐noise ratio. Experiments reveal that a reduction of the data acquisition time by at least a factor of four is attainable in pra

ISSN:0740-3194    

DOI:10.1002/mrm.1910020304

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

年代:1985

数据来源: WILEY

摘要:

AbstractTo obtain separate NMR images of spatially resolved high‐resolution chemical‐shift information the effects of magnetic field inhomogeneity must be accounted for. A suitable correction method is described which relies on first generating a plot of the magnetic field distribution. Using these data the spectroscopic imaging data can be processed to display the spatial distributions of separate resonances without distortion from field inhomogeneity. The field plot is obtained by using the same data acquistion sequence while imaging a phantom object or in particular cases the plot may be derived from the spectroscopic imaging data set itself. The correction procedure is illustrated using protonin vivoimaging of a cat. Some additional data processing techniques are presented which offer alternative methods of displaying spectroscopic imaging d

ISSN:0740-3194    

DOI:10.1002/mrm.1910020305

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

年代:1985

数据来源: WILEY

摘要:

AbstractAn iterative linear interpolation is described for computingT1estimates from imagesignal ratios that are monotonous functions ofT1. Whether the function decreases or increases withT1, the same algorithm deseribed applies. The iterative process converges readily. Narrowing the whereabouts ofT1sought in lookup tables toT1regions just small enough to require only a few subsequent interpolations tends to shorten overallT1image computing time. Used with imaging methods for which the function is readily evaluated, the iterative interpolation allows a rapid and preciseT1computation compatible with variable choice of pulse‐sequence parameters. The flexibility is suitable to stochastic analysis ofT1measurement from signal ratios. Examples of applications to computing the stocbastic uncertainty represented either by crude indicators or by standard statistical quantities are given. A method of stochastic simulation yielding the statistical quantities is describe

ISSN:0740-3194    

DOI:10.1002/mrm.1910020306

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

年代:1985

数据来源: WILEY

摘要:

AbstractA new design for an axial magnetic field gradient is described. Implemented in a four‐coil configuration, it requires far less power than the conventional Maxwell pair, while maintaining the same field linearity. A practical design tool with a set of curves giving coil dimensions is proposed. Two realizations dedicated to NMR imaging are described and compared with the equivalent Maxwell pair. Substantial power reductions are achieved; in these cases, dc power is reduced by a factor of 5 and switching power by a factor of 1

ISSN:0740-3194    

DOI:10.1002/mrm.1910020307

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

年代:1985

数据来源: WILEY

摘要:

AbstractAn investigation has been undertaken to study the effects of chemical shift on NMR image obtained by the projection reconstruction technique. Mathematical simulation studies indicate that chemical shift causes blurring in NMR images regardless of the image reconstruction technique. A systematic study of such artifacts is presented and possible correction schemes are proposed. Exemplary results with and without correction of the chemical‐shift artifact and pulse schemes employed for projection reconstruction are presented. The present chemical‐shift correction scheme is basically the same as a spectroscopic imaging technique such as the existing 4‐D NMR imaging method except that the encoding steps required can be reduced substantially in the case of modest spectroscopic resol

ISSN:0740-3194    

DOI:10.1002/mrm.1910020308

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

年代:1985

数据来源: WILEY

摘要:

AbstractThe problem of minimizing the space needed to house two or three superconducting magnets was studied. Dipolar fields were used to approximate the stray fields. Field contours of a single dipole μ are reviewed. The three‐dimensional contour surface is discussed in terms of its intersection with planes parallel to a base plane containing μ. The area within a field contourHin the base plane was numerically determined to be 142.55 m2{(m̈/m̈0)(H0/H)}2/3where μ0= 109erg/G andH0= 5 G. For the contours within planes parallel to the base plane, the contour area and its intercepts with the planarxandyaxes are given as a function of the distance from the base plane. For two dipoles at one site and for a given mechanical stress, the torque‐free orientations allow closer spacing. Field contours for the four torque‐free orientations are given for μ1= μ2. For one zero‐torque configuration (μ1and μ2antiparallel, but perpendicular to the line joining them) and for a mutual force of 667 N (150 lb), the 5‐G contour area is only 36% greater than that for a single dipole. General expressions for the forces, torques, and field gradients for two dipoles are given. Torque‐free cases of three superconducting magnets (two of which are identical) at one site are described. Contour plots of the linear arrangements of the magnets as well as of two symmetrical triangular arrangements (corner angle is either 70.5

ISSN:0740-3194    

DOI:10.1002/mrm.1910020309

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

年代:1985

数据来源: WILEY

摘要:

AbstractThe advantages of electron paramagnetic resonance (EPR) imaging at L‐band frequencies are discussed. The construction and calibration of a low‐field L‐band EPR imaging spectrometer is described with capillary phantoms containing aqueous nitroxides as paramagnetic imaging agents. The peak separation induced by the magnetic field gradient is related to the object separation. The linewidth of each (first‐derivative) line is used to calculate the dimensions of each paramagnetic object, which is the sum of an intrinsic and a gradient‐induced component. By extrapolating the linewidth back to zero field gradient, one obtains the intrinsic linewidth correction factor in computing image size. The nonuniformity caused by deterioration of biological substructure was examined with plant stems where one capillary vessel had become leaky. Spin‐label destruction, particularly by biological reducing agents, was compared for three species of

ISSN:0740-3194    

DOI:10.1002/mrm.1910020310

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

年代:1985

数据来源: WILEY

摘要:

AbstractIt might appear that the Fe3+ion would be particularly useful as an agent for enhancing contrast in NMR images since it has a relatively large magnetic moment and occursin vivoin a variety of forms. Moreover, the concentration of Fe3+changes locally in certain disease states (e. g., β‐thalassemia) and in trauma (formation of methemoglobin), and can be altered in the gastrointestinal tract by the ingestion of readily available dietary supplements. However, the Fe3+ion is insoluble above pH ∼ 4, and soluble chelate and protein complexes of Fe3+tend to sequester the ions from solvent; hence, the efficacy of Fe3+ions for relaxing water protons ought to be low under typical physiological conditions. We report the magnetic field dependence of the relaxation rate of solvent protons (NMRD profiles) for solutions of a variety of Fe3+complexes to demonstrate the phenomenology relevant to NMR imaging. From these data we make some estimates to show that, deposite the low relaxation rates of solvent protons in solutions of Fe3+complexes, certain observed changes in image contrast are consistent, quantitatively, with inferences that can be drawn from solution

ISSN:0740-3194    

DOI:10.1002/mrm.1910020311

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

年代:1985

数据来源: WILEY