摘要:

AbstractMultiecho sequences are widely used in MRI because they are an efficient means of acquiring data and they can be used to quantitatively estimateT2. However, the uniformity of each image and the accuracy of eachT2estimate may be corrupted by several instrumental factors such as rf pulse imperfections and static field inhomogeneities. We propose a scheme based on our prior knowledge of the effects of such factors and using the Lexenberg‐Marquardt method of nonlinear estimation that produces accurate estimates ofT2even in the presence of substantial system imperfections and improves the image quality of later echo images in a multiecho sequence. A further extension of this scheme permits mapping the distribution of the static magnetic field from a single multiecho imaging study of a uniform object. Simulations have been performed to test the efficacy of this scheme and the results are substantiated with experiments performed on a whole‐body imager operating at 6.35 MHz. © 1987 Academic Press,

ISSN:0740-3194    

DOI:10.1002/mrm.1910040302

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

年代:1987

数据来源: WILEY

摘要:

AbstractAs a result of the agonist stimulation of muscarinic receptors, myc‐inositol‐1 ‐phosphate accumulates in the presence of millimolar lithium concentrations. This accumulation ofmyo‐inositol‐1 ‐phosphate can be detected byin vivo31P NMR spectroscopy. Sincemyo‐inositol‐1‐phosphate is a breakdown product of phosphatidylinositol, this may provide a means of noninvasively monitoring phosphatidylinositol metabolismin vivo. © 1987 A

ISSN:0740-3194    

DOI:10.1002/mrm.1910040303

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

年代:1987

数据来源: WILEY

摘要:

AbstractNormally, cardiac triggering in MR spin‐echo imaging restricts repetition times (TR) to integral values of the cardiac period (TC), and introduces irregularities in TR due to variations in TC. We have investigated how much these restrictions decrease the accuracy and precision of spin‐lattice relaxation (T1) values of the myocardium calculated from two cardiac‐triggered spin‐echo images. By introducing additional excitation pulses, TR can effectively be reduced to a fractional value of TC and considerable improvement inT1precision is possible. For TC = 800 ms, the improvement inT1precision is 30% when two spin‐echo images of TR = 1/2 TC and TR = 2 TC are used to calculateT1instead of two images with TR = TC and TR = 2 TC. The irregularities in TR decrease bothT1precision and accuracy. Irregularities of the order of 15% in a mean TR of 800 ms produce a fourfold decrease in precision. Since irregularities in TC easily exceed 15%, MRI data should be acquired when individual TR values are approximately within ± 15% of the subject's mean TC. © 1987 Academic

ISSN:0740-3194    

DOI:10.1002/mrm.1910040304

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

年代:1987

数据来源: WILEY

摘要:

AbstractA technique to determine absolute metabolite concentrations of the kidneyin vivousing31P NMR is described. The technique is based on the use of methylphosphonic acid (MPA), which gives rise to a well‐resolved peak upfield fromin vivophosphorous metabolite resonances, as an “internal standard.” The method involves acquisition of a fully relaxed kidney spectrum with an implanted coil followed by intravenous infusion of MPA (4 ml of 150 mM) for a period of 30 min. The animal is then sacrificed to insure a steady state level of renal MPA and another spectrum is obtained. From these two spectra the ratio of intensities of MPA to β‐ATP was derived. In the method used here, no significant contribution from tissues outside the kidney was observed. In addition, a relatively homogenous distribution of MPA throughout the kidney was achieved. The amount of MPA per gram wet weight of kidney was also obtained through NMR methods by placing the excised organ in a phosphate‐calibrated solenoidal coil. The calibration spectra along with the ratio of intensities for MPA/β‐ATP were used to calculate the number of micromoles of ATP per gram wet weight of kidney. Infusion of a higher concentration of MPA (1.25M) produced a visible MPA resonance in other organs besides the kidney. Thus, MPA could be useful in determining phosphate metabolite concentrations in other tissues. © 1987 Acade

ISSN:0740-3194    

DOI:10.1002/mrm.1910040305

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

年代:1987

数据来源: WILEY

摘要:

AbstractIt was recently reported (C‐W. Chenet al., FEBS Lett.168, 70 (1984)) that water solutions of Mn3+(TPPS4) have a surprisingly high relaxivity at 20 MHz and 37oC, greater than most Mn2+complexes including the hexaaquoion. Because Mn3+(TPPS4) is highly stable, and porphyrins in general are tumor‐seeking, we have sought to understand the origin of the large relaxivity by comparing the 1/TlNMRD profiles (magnetic field dependence of 1/T1of solvent protons) of Mn3+(TPPS4) solutions with those of a number of other small Fe3+and Mn2+complexes. By relating the measured NMRD profiles to the theory of relaxation by magnetic dipolar interactions, in a form appropriate for small paramagnetic solute molecules, we establish that the theory affords an excellent quantitative description of the relaxation behavior of all the samples, and confirm that the relaxivity of Mn3+(TPPS4) is anomalously high. The effect is attributed, in part, to the anisotropy of the ground‐state wavefunction of Mn3+in the porphyrin complex, effectively bringing the spin density of the Mn3+ions closer to the protons of the coordinated water molecules than would a spherically symmetricS‐state ion. In addition, the paramagnetic relaxation time of the Mn3+spins, though short, is longer than would be anticipated for a non‐S‐state ion, and increases substantially with magnetic field above about 2 MHz. In this regard, Mn3+(TPPs4) may be one of a class of molecules with properties particularly favorable for use as contrast‐enhancing agents in magnetic resonance imaging. © 1987 Acade

ISSN:0740-3194    

DOI:10.1002/mrm.1910040306

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

年代:1987

数据来源: WILEY

摘要:

AbstractWe have used a combination of a shift reagent and mathematical filtering or presaturation of the extracellular sodium resonance for the quantitative investigation of the intracellular sodium and lithium relaxation times in the perfused frog heart. While theT1of the intracellular sodium was found to consist of a single‐exponential time constant (approximately 23 ms), theT2, was better fit as a double‐exponential decay with time constants of approximately 2 and 17 ms. However, the relative amplitudes of the two time constants in theT2decay were found to be inconsistent with those which would be expected from a homogeneous pool of nuclei undergoing quadrupolar interactions. The relaxation times were not changed by a fivefold increase in the intracellular sodium level (due to perfusion with a ouabain‐containing buffer). TheT1andT2, of the intracellular lithium (after perfusion with lithium‐containing buffer) were both well fit by single exponentials (700‐ and 31‐ms time constants, respectively). © 1987 Academi

ISSN:0740-3194    

DOI:10.1002/mrm.1910040307

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

年代:1987

数据来源: WILEY

摘要:

AbstractTwo NMR experiments are described that use ac field gradients to measure flow velocities. Both experiments perform no spacial localization; instead, fluid velocity is converted to a resonance frequency shift. Thus, by Fourier transformation of a single FID or echo train thedistributionof velocities is available. The first experiment uses only one of pulse but requires a homogeneous static field. The second experiment uses the CPMG multiple‐pulse sequence to refocus the effects of field inhomogeneity and chemical shifts. Results are shown for both experiments using a simple flowing phantom. Applications of the techniques to time‐resolved blood flow measurement are discussed. © 1987 Academic Press,

ISSN:0740-3194    

DOI:10.1002/mrm.1910040308

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

年代:1987

数据来源: WILEY

摘要:

AbstractThe intensity of multiple echoes separated by a time 2τ has been modeled using the closed form of a finite geometric series. This eliminates long exponential series, introduces the number of echoes as an independent parameter, and corrects for the netT1relaxation during the echo train. Other sequences having echo trains can be modeled similarly. © 1987 Academic Press, In

ISSN:0740-3194    

DOI:10.1002/mrm.1910040309

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

年代:1987

数据来源: WILEY

摘要:

AbstractAn NMR imaging technique applicable to high‐velocity flow imaging is described. By subtraction of two phase images obtained by varying the flow‐encoding gradient, it is possible to extract much larger ranges of velocity components. This differential phase‐encoding technique is tested by experiments in a phantom as well as a human volunteer. © 1987 Academic Pres

ISSN:0740-3194    

DOI:10.1002/mrm.1910040310

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

年代:1987

数据来源: WILEY

摘要:

AbstractSuppression of water and fat in proton spectra of rat skeletal muscle was achieved by acquisition of the spin echoes formed in the steady state after a series of frequency‐selective composite pulses of constant amplitude, phase, and interpulse interval. The method enables observation of lactate in the presence of fat without resorting to difference spectroscopy, and on spectrometers which permit data acquisition immediately prior to a pulse, enables much faster data accumulation compared to standard echo sequences. © 1987 Academic Press, I

ISSN:0740-3194    

DOI:10.1002/mrm.1910040311

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

年代:1987

数据来源: WILEY