The crossover surface coil is constructed with two turns of copper foil using a unique transposition construction which incorporates an explicit center tap ground of the widened bottom layer. The coil reduces dielectric and inductive losses by effective shielding of electric fields and uniform distribution of magnetic fields for minimum Q losses upon loading of the coil. Flexible, copper foil construction permits easy conformation to tissues of interest, enhancing coil performance. Construction details of the crossover coil and Q data for coils of various sizes operating at a number of frequencies are given.
In vivo 19F NMR at 4.7 T has shown that the biphasic elimination of the vapor anesthetic isoflurane from rat brain is ca 15% slower in old (23-24 months) animals compared with young (5-6 months) animals. The fast kinetic component has a t1/2 of ca 7-9 min and the slow event, 100-115 min. Gas chromatographic measurement of arterial blood elimination displays age attenuation to the same extent, although a monophasic kinetic process (6-7 min). The slow wash-out from brain is thought to involve elimination from intracranial fatty tissue as postulated by others in rabbit brain. Longitudinal relaxation time measurements show monoexponential recovery and essentially identical values for young (1.09 + 0.11 s) and old (1.04 +/- 0.09 s) animals. For dipalmitoylphosphatidylcholine vesicles the monoexponential recovery also suggests rapidly exchanging averaged homogeneous lipid environments for the anesthetic, but the longer T1s (2.75 +/- 0.25 s) imply less restricted mobility compared with brain. Single T2 values were obtained in vivo, indicating either a single compartment or rapid exchange between multiple environments. These measurements were inconsistent, undoubtedly as a result of B1 inhomogeneity. The age-attenuated elimination kinetics for isoflurane are consistent with poorer cardiopulmonary function, whereas the T1 data suggest similar environments for the anesthetic in young and old brain tissue.
Apparent 31P spin-lattice relaxation times have been measured in vivo for brain phosphates in young adult, mature adult, and aged rats at 4.7 T and 35 degrees C. Statistically significant differences were found for most phosphate species, except PCr and gamma-ATP, among the three age groups, particularly between the young and mature adults. Age-related changes in tissue composition and exchange reactions are discussed as possible contributors to these results.
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