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141.
Michael E. Moseley Merry C. Nishimura Lawrence H. Pitts Henry M. Bartkowski Thomas L. James 《Magnetic resonance imaging》1984,2(3):205-209
Proton nuclear magnetic resonance relaxation times, T1 and T2, of water in unfixed gray and white matter from normal and edematous rabbit brain tissues were measured in vitro at 23°C and 100 MHz to evaluate the effects of the temperature (?25°C to 37°C) and duration (0 to 96 h) of tissue storage on relaxation times. T1 and T2 tended to decrease during storage, probably from slow dehydration of the tissue. This effect was greatest in tissues stored at 37°C and least in those stored at 4 and ?25°C; decreases in T1 and T2 were greater in white matter than in gray matter. Freezing brain tissue to ?25°C caused a sudden decrease in the T2 of normal white matter. Relaxation times were constant for 5 h in tissues stored at 23°C and for 40 h at 4°C. These results correlated well with corresponding tissue water loss. 相似文献
142.
Relative rate constants for the reaction of OH radicals with a series of n-alkanes have been determined at 299 ± 2 K, using methyl nitrite photolysis in air as a source of OH radicals. Using a rate constant for the reaction of OH radicals with n-butane of 2.58 × 10?12 cm3 molecule?1s?1, the rate constants obtained are (X1012 cm3 molecule?1 s?1): propane 1.22 ± 0.05, n-pentane 4.13 ± 0.08, n-heptane 7.30 ± 0.17, n-octane 9.01 ± 0.19, n-nonane 10.7 ± 0.4, and n-decane 11.4 ± 0.6. The data for propane, n-pentane, and n-octane are in good agreement with literature values, while those for n-heptane, n-nonane, and n-decane are reported for the first time. These data show that the rate constant per secondary C—H bond is ∽40% higher for —CH2— groups bonded to two other —CH2— groups than for those bonded to a —CH2— group and a —CH3 group. 相似文献
143.
The inclusion of a flat metric tensor in gravitation permits the formulation of a gravitational stress-energy tensor and the formal derivation of general relativity from a linear theory in flat spacetime. Building on the works of Kraichnan and Deser, we present such a derivation using universal coupling and gauge invariance.Next we slightly weaken the assumptions of universal coupling and gauge invariance, obtaining a larger "slightly bimetric" class of theories, in which the Euler-Lagrange equations depend only on a curved metric, matter fields, and the determinant of the flat metric. The theories are equivalent to generally covariant theories with an arbitrary cosmological constant and an arbitrarily coupled scalar field, which can serve as an inflaton or dark matter.The question of the consistency of the null cone structures of the two metrics is addressed. 相似文献