MR systems

These structures are unique for several reasons. First, they represent three new multidimensional 12-MR systems, which are rare even among zeolites. Second, the amount of framework substitution by metals such as Mn2+ and Mg2+ was unknown prior to this series. Also, the ease of forming both gallium and aluminum phosphates appear to be comparable. Finally, it would appear the charge-matching approach has proven to be a successful strategy for the synthesis of new molecular sieves. It is not clear whether these materials are thermally or hydrothermally stable but they do represent novel pore structures that should impart some unusual properties. 

The only aluminophosphate known having a 20-MR system is JDF-20 (162, 163). The structure of JDF-20 viewed along the 001 direction is shown in Fig. 27. The large 20-MR elliptical channels are 14.5 x 7.9 x 6.2 A in diameter. Like Cloverite, JDF-20 is an interrupted framework that is reflected in the formula [Al5P6024H][N(Et3)H]2.2Fl20, where there is less A1 than P. This results in four P=0 and two O-H-O groups that protrude into the pore window. There are also intersecting channels defined by 10- and 8-MR structures. 

After the advent of MR systems with powerful gradient hardware, dynamic scans with a repetition rate of at least one image or image stack per 2 s have become possible. By use of such sequences it has become feasible to visualize the transit of a bolus of paramagnetic or superparamagnetic contrast agent through the brain (Moseley et al. 1990 Rosen et al. 1990). For DSC, the bolus is injected intravenously. 

It has been shown that single ring aromatic alkylation reactions such as benzene to ethylbenzene take place primarily within the 12- ring (12-MR) system, and that the 10-ring (10-MR) system contributes little to the ethylbenzene reaction. A key feature of MCM-22 is its ability to operate stably at low benzene-to-ethylene ratios with minimal production of polyethylbenzenes (PEBs) or ethylene oligomers. The excellent ethylbenzene selectivity of the MCM-22 catalyst is likely due to confinement effects within this 12-MR pore system and to the very facile desorption... 

There are other costs associated with utilizing high magnetic fields. In general, the price of the MR system increases linearly with the magnetic field strength (e.g., a 7T... 

Broad resonance lines, accompanied by Tj relaxation times much longer than the T2 relaxation times, are indications that the fluorine nucleus is in a restricted-mobility environment [14, 20, 21, 71]. The specific MR characteristics of a compound in its in vivo state must be evaluated and the acquisition and analysis adjusted to account for the values of these characteristics prior to performing human 19F spectroscopy studies. Effective T2 values (T2 ) that are shorter than 2 ms have spectra that are difficult to measure using whole-body MR systems. In general, acquisition of spectra that contain short T2 -value species require the use of short (<0.5 ms) excitation RF pulses and careful attention... 

Klomp et al. [24] were able to double the SNR of their 19F MR spectra at 1.5 T by systematically reducing the noise factor of each component in the signal detection of the MR system. With this increase, a 4 cm x 4 cm x 4 cm (64 cm3) 3D CSI acquisitions were acquired in 4 min without polarization transfer or decoupling and these acquisitions were subsequently used to evaluate five patients receiving 5-FU chemotherapy for treatment of superficial and central liver metastases. The in vivo T for 5-FU in the liver following a bolus injection was 380 + 80ms, which was considerably lower than in earlier publications (see Table 19.3). 

The reason for that was that the effect of lowering the membrane thickness and permeability quickly saturated out, because the operation of the MR systems became limited by the catalyst activity. 

The Munsell system arranges color samples according to a definite systematic process. The differences between the individual colors have been revised many times over the years the improved system is known as the Munsell Renotation System. Two different MR systems are used, one for glossy, the other for mat surfaces. 

All spectrometers for absorption measurements require a light source, a wavelength selection device, a sample holder, and a detector. Complete UVA IS and UVA IS/near-IR (MR) systems are available from many commercial instrument companies, including Agilent Technologies, PerkinElmer, Shimadzu Scientific Instruments, and Thermo Fisher Scientific. These large companies each offer from 5 to 8 versions of their systems with varied capabilities. Their websites offer applications, videos on using the instruments, tutorials, and a host of other useful information. 

Schoenenberger AW, Debatin JF,Guldenschuh l,et al. (1998) Dynamic MR defecography with a superconducting, open-configuration MR system. Radiology 206 641-646... 

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