摘要:

AbstractThe tailored radio frequency gradient echo (TRFGE) technique that has been used in venography (Choet al., Magn. Reson. Med.28, 25‐38, 1992; Ro, Cho,Magn. Reson. Mod.28, 237, 1992) is applied to functional imaging. The TRFGE technique has the advantage that it is sensitive to the field gradient created by the susceptibility effect, thereby enhancing only the signals from the regions with local field gradient In addition, the method is insensitive to the in‐flow effect, especially from the arterial blood. The latter further simplifies the functional MRI data analysis. The TRFGE sequence is, therefore, suitable for functional MR imaging for which true susceptibility effect measurements are of prime importance. To examine the TRFGE functional imaging systematically, experiments with various imaging parameters such as flip angle a, repetition time, and echo time were performed and the results were compared with the data obtained from the conventional gradient echo functional MR imaging. The experimental results shown were all obtained from human volunteers with a 2.0T whole body MRI sys

ISSN:0740-3194    

DOI:10.1002/mrm.1910350102

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

年代:1996

数据来源: WILEY

摘要:

AbstractNMR lineshapes were calculated for a model of lung, and NMR proton spectra were measured for individual voxels in an excised inflated rat lung. NMR lines for parenchymal lung regions containing alveoli, alveolar ducts, and capillaries were calculated using a computer simulation of the NMR signal from a three‐dimensional honeycomb‐like structure, a collection of modified Wigner‐Seitz cells. These cells were modified by rounding the comers and increasing the thickness of the boundaries to model various degrees of lung inflation and lung water. NMR lineshapes were also calculated for the central or nonparenchymal lung regions containing bronchi and large blood vessels. A comparison of theoretical lineshapes with those measured in individual voxels both in the parenchymal and in the central (largely nonparenchymal) regions in excised rat lungs at an inflation pressure of 30 cm of water shows excellent agreement These results indicate that the NMR lineshape reflects the underlying lung geometry. This research constitutes the first calculations and measurements of NMR lineshapes in lung. The appendix describes a new method for calculating the magnetic field inside a weakly diamagnetic material of arbitrary shape placed in an otherwise uniform external magnetic field. This new method does not require either solution of simultaneous equations or evaluation of integral expres

ISSN:0740-3194    

DOI:10.1002/mrm.1910350103

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

年代:1996

数据来源: WILEY

摘要:

AbstractThese experiments assessed and quantitated the release of free manganese Mn++from the hepatobiliary contrast agent Mn‐DPDP (manganese dipyridoxal diphosphate), using several magnetic resonance techniques (EPR spectroscopy,31P‐NMR spectroscopy, and relaxometry) to differentiate between free Mn++and Mn++in complexes in various preparations. The presence of calcium and magnesium in physiological concentrations in aqueous solutions induced the release of Mn++from the complex, as did incubation of the complex in liver homogenates. After intravenous injection of 15 μmol/kg of Mn‐DPDP, both EPR and31P‐NMR spectroscopy demonstrated that Mn‐DPDP is partly dissociated (approximately 25%) in the liver. By comparingin vitroandex vivodata from the liver, we concluded that the dissociation of Mn‐DPDP occurs primarily in the liver, whereas a minor portion of the dissociated Mn found in the liver comes from dissociation of the complex in the blood. Most of the dissociated Mn in liver becomes bound to macromolecules and is responsible for the enhancement of relaxivity observed wit

ISSN:0740-3194    

DOI:10.1002/mrm.1910350104

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

年代:1996

数据来源: WILEY

摘要:

AbstractThis article discusses the applicability to a living animal of the temperature mapping method using the water proton chemical shift obtained with three‐dimensional magnetic resonance spectroscopic imaging (3D‐MRSI). There are several sources of error in obtaining the spectra with 3D‐MRSI: signal noise, limitation in the frequency resolution due to the finite signal length, intravoxel inhomogeneity in the static magnetic field, and variation in the magnetic field due to the eddy current magnetic field. A spectral estimation method called phase deduction complex Lorentzian fitting (PD‐CLF) was proposed. Numerical simulations demonstrated that this method reduces the error in the chemical shift to one third of that obtained with the simple frequency subtraction method that uses zero‐padded first Fourier transformation (FFT). The temperature images obtained using 3D‐MRSI with PD‐CLF clearly visualized the changes and distribution of temperature in an ane

ISSN:0740-3194    

DOI:10.1002/mrm.1910350105

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

年代:1996

数据来源: WILEY

摘要:

AbstractThe proton exchange rates between water and the hydroxyl protons of threonine, serine, tyrosine, the amino protons of lysine, and the guanidinium protons of arginine were measured in the pH range 0.5 to 8.5 and for the temperatures 4°C, 10°C, 20°C, 30°C, and 36°C. The intrinsic exchange rates of the hydroxyl and amino protons at pH 7.0°C and 36°C were found to be in the range 700 to about 10,000 s−1. In addition, the exchange catalysis by phosphate, carbonate, carboxyl‐, and amino‐groups was investigated. The presence of these exchange catalysts at physiological concentrations increased the proton exchange rates from hydroxyl and amino groups several fold. The proton exchange rates are sufficiently fast that the total magnetization transfer between biomolecules and free bulk water is not rate limited by the proton exchange rate, but by the intramolecular cross‐relaxation rates between the exchangeable and nonexchangeable protons of the biomolecules. Since the cross‐relaxation rates between surface hydration water molecules and biomolecules are usually vanishingly small because of too rapid exchange with the free bulk water, it is proposed that the contrast in MR images is a fingerprint of the number of the exchangeable protons from OH and NH groups of the tissue, as far as the contrast depends on the magnetization transfer between biomol

ISSN:0740-3194    

DOI:10.1002/mrm.1910350106

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

年代:1996

数据来源: WILEY

摘要:

AbstractThe effect of perfusion on relaxation time in tissue has only been considered for first‐pass kinetics of NMR‐signal after application of contrast agents. The importance of perfusion on relaxation has not yet been studied for steady state conditions, i.e., when the intravascular relaxation rate is constant in time. The aim of this study is to develop a model in which T, relaxation is derived as a function of perfusion and intracap‐illary volume fraction (regional blood volume). Tissue is considered to be two‐compartment system, which consists of intracapillary and extravascular space. Intracapillary relaxation differs from relaxation in the arterial system due to diffusion‐exchange of magnetization from extravascular to intracapillary space. Perfusion tends to attenuate this difference and thus counteracts the effect on intracapillary relaxation. Relaxation in the extravascular space becomes a function of perfusion because extravascular and intracapillary magnetization are linked by diffusion. This dependence is presented in analytical form and a generic equation is derived. A T1experiment is considered in which all spins of tissue and blood are inverted at the beginning. Calculations are performed for thefast exchangemodel of tissue. Perfusion increases relaxation enhancement of intravascular contrast agents. This effect is considerable in highly perfused tissue like myocardium. The dependence of relaxation on perfusion implies an overestimation of theregional blood volumewhen the calculation of the latter is based on tissue models that neglect perfusion. The model presented here is applied to predict the effect of perfusion on T1imaging with FLASH‐pulse sequences because this technique has been proven to be a powerful method to obtain T1maps within a short time interval. For thefast exchangemodel, two algorithms are suggested that determine perfusion andregional blood volumefrom T1imaging in the presence and absence of intravascular cont

ISSN:0740-3194    

DOI:10.1002/mrm.1910350107

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

年代:1996

数据来源: WILEY

摘要:

AbstractT1andT2relaxation times and iron concentrations were measured in 24 specimens of gray matter from fresh human and monkey brains at magnetic fields from 0.05 to 1.5 Tesla. Three different effects were found that correlate with iron content: aT1‐shortening that falls off somewhat at high fields, aT2‐shortening that is field‐independent and thus important at low fields, and a contribution to 1/T2that increases linearly with field strength. This linear field dependence has been seen only in ferritin and other ferric oxyhydroxide particles. Our results are in agreement within vivoMRI studies and are generally consistent with values for ferritin solution, except for differences such as clustering of ferritin in tissue. A cerebral cavernous hemangioma specimen showed similarT2‐shortening, but with a 2.7 times larger magnitude, attributed to larger clusters of hemosiderin in macrophages. The dependence on interecho time 2Twas measured in three brains; 1/T2increased significantly for T up to 32 ms, as expected from the size of the ferritin clusters. These findings support the theory that ferritin iron is the primary determinant of MRI contrast in normal gray

ISSN:0740-3194    

DOI:10.1002/mrm.1910350108

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

年代:1996

数据来源: WILEY

摘要:

AbstractMuscle performance is markedly influenced by tissue perfu‐sion. Techniques that allow quantification of microvascular flow are limited by the use of ionizing radiation. In this investigation, we apply an NMR model previously developed by Detre ef al. to the measurement of human muscle perfusion during reactive hyperemia. We compare our results with conventional plethysmography adapted to NMR. Using echo‐planar imaging,T1andT2were measured in 14 subjects during rest, ischemia, and reactive hyperemia. Mean leg muscleT1in healthy volunteers is 850 ms at rest and 834 ms at reperfusion, leading to a calculated reactive hyperemia flow increase (T1flow) of 103 ± 40 ml/100 ml/min.T1flows correlate well with NMR‐plethysmography values. Changes inT2, which are sensitive to both deoxyhemoglobin content and vessel diameter variations, are also correlated with perfusion measurements.T1changes allow quantification of regional perfusion in human muscle during reactive hyp

ISSN:0740-3194    

DOI:10.1002/mrm.1910350109

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

年代:1996

数据来源: WILEY

摘要:

AbstractThe effects of off‐resonance radio‐frequency irradiation on the intensity of the MR signal from water protons in the cat brain are asymmetric around the chemical shift of the water signal. This asymmetry, which could arise from a shift in the magnetization transfer spectrum ˜1.5 ppm upfield from the solvent water signal, must be taken into account to compensate for magnetization transfer effects inherent in arterial spin tagging approaches that use a single radio‐frequency coil. Two approaches that either correct for, or circumvent, the apparent upfield shift of the magnetization transfer spectrum are presented, and a perfusion image of the cat brain, using flow‐induced adiabatic inversion of arterial water protons, is presented. Other problems in obtaining quantitative cerebral blood flow values using the arterial spin tagging approach are d

ISSN:0740-3194    

DOI:10.1002/mrm.1910350110

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

年代:1996

数据来源: WILEY

摘要:

AbstractImprovements in the echo‐volumar imaging (EVI) technique and its application in whole‐body studies are described. Using an in‐house built 0.5 T echo‐planar imaging system, complete modulus three‐dimensional image data are acquired in 102 ms with real‐time display. Hardware limitations have restricted the maximum array size achievable to 64 × 32 × 8 voxels. Representative voxel dimensions areX= 6.0 mm;y= 4.0 mm, and z = 10.0 mm. Results on human volunteers are presented, showing cardiac, liver, and bladder images. Also shown are the first EVI gastric filling and emp

ISSN:0740-3194    

DOI:10.1002/mrm.1910350111

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

年代:1996

数据来源: WILEY