The dependence of 1/T1 on the magnetic field strength (the relaxation dispersion) has been measured at 37 degrees C on autopsy samples of human brain gray and white matter at field strengths corresponding to proton Larmor frequencies between 10 kHz and 50 MHz (0.0002-1.2 T). Additional measurements of 1/T1 and 1/T2 have been performed at 200 MHz (4.7 T) and 20 MHz (0.47 T), respectively. Absolute signal amplitudes are found to be proportional to the sample water content, not to the "proton density," and it is concluded that the myelin lipids do not contribute to the signal. Transverse magnetization decay data can be fitted with a triple exponential function, giving characteristic results for each tissue type, and are insensitive to variations of the pulse spacing interval. The longitudinal relaxation dispersion curves show characteristic shapes for each tissue type. The most striking difference is a large dispersion for white matter at very high fields. As a consequence, the relative difference in 1/T1 between gray and white matter shows a marked maximum around 10 MHz. Possible implications for MRI are discussed. A weighted least-squares fit of the dispersions has been performed using a four-parameter function of the form 1/T1 = 1/T1,w + D + A/(1 + (f/fc)beta'). The quality of the fit is superior to that of other functions proposed previously. The results of these fits are used to predict image contrast between gray and white matter at different field strengths.
An RF-only quadruple collision cell, fitted with retardation and acceleration lenses, has been installed in a field-free region of a large-scale tandem mass spectrometer. This new arrangement has allowed decelerated, mass-selected ions (ca. 5 eV kinetic energy) to react with reagent gases and reaccelerated, mass-selected products (cu. 8 keV) to be subsequently identified by collisional activation mass spectrometry. The system was tested by looking at ion/molecule reactions of cyelobutanone molecular ions, previously studied by FTICR mass spectrometry Collisional activation (CA) mass spectrometry is a well-established technique for the characterization of fast-ion beams.' Isomeric ions are usually clearly distinguished because the technique provides not only fingerprint spectra of isomers, but also direct structural information, since simple, site-specific cleavages are frequently induced. CA mass spectrometry has more recently been implemented as the powerful neutralization-reionization (NR) technique' which, due to a higher energy deposition in the ions,3 gives rise to spectra which are even more structurally significant.In some instances, however, the interpretation of CA spectra is not straightforward and a further stage of mass spectrometric analysis, i.e. an MS/MS/MS (MS3) experiment is then r e q~i r e d .~ In these experiments, collision-induced fragments are mass-selected and subjected to a further collisional activation step. It must be recognized that such experiments may just displace the initial problem and other kinds of information are therefore required. The occurrence of post-collisional isomerization of ions may also introduce ambiguities; such processes have been reported recently for some sulfur-containing organic ions.The reactivity of ions towards neutral reagents provides another method for isomer differentiation. Such reactions are most easily performed in a chemical ionization (CI) source and, if the source is installed on a sector instrument, mass-selected products can be identified by high-energy collisional activation.6 It is, however, often difficult to identify unambiguously the reaction which leads to a specific product, since different ions and neutrals are present at the same time. This difficulty can be overcome by the use of Fourier-transform ion cyclotron resonance (FTICR) methodology which allows mass-selected ions to be collisionally stabilized, to react with appropriate reagents or to be collisionally dec~mposed.~ Note, however, that fragmentation of these precursor ions takes place at low kinetic energy.Other recent instrumentation allowing inter uliu the study of ion/molecule reactions has been recently reviewed by Cooks et aL8 Among the different approaches, results obtained by these authors using a home-built pentaquadrupole instrument appear quite Authors for correspondence promi~ing.~ Again, collision-induced dissociations of the ion/molecule products are obtained in the low kinetic energy regime.We describe in this paper the modification of a large sector mass spectrom...
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Relaxometry between 10 kHz and 200 MHz (0.2 mT and 4.7 T) with a field-cycling device and a high-field-strength magnetic resonance (MR) unit permitted the determination of longitudinal relaxation rates of tissues and chemical compounds at numerous field strengths. The resulting nuclear magnetic relaxation dispersion profiles allowed the prediction of tissue contrast and efficacy of contrast agents at any field strength. Pure T1 contrast of normal brain tissue and pathologic lesions (multiple sclerosis, astrocytoma) increased from low field strengths to a maximum between 10 and 20 MHz and decreased afterward. Quadripolar dips reflecting the interaction between water and nitrogen atoms of the protein backbone appeared at 2.15 and 2.8 MHz, reducing T1 and opening the possibility of shorter imaging times and better tissue discrimination at these field strengths. Furthermore, it was shown that zero T1 contrast between normal and pathologic tissue samples may exist at certain field strengths. Gadolinium diethylenetriaminepentaacetic acid and gadolinium tetraazacyclododecanetetraacetic acid provided different contrast enhancement depending on the field strength.
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