Spin Echo

The effect of substituting two Na ! by one Ca 2! in a rare-earth aluminoborosilicate glass is investigated by multinuclear magic-angle spinning (MAS) and multiple-quantum (MQ)MAS nuclear magnetic resonance (NMR) spectroscopy. Quantitative analysis of the 23 Na and 27 Al MAS/MQMAS data along with the 11 B MAS NMR data provides complementary information enabling to cast light on different structural key points. A strong decrease of the N 4 " BO 4 /(BO 3 ! BO 4 ) ratio is observed consecutively to this substitution, indicating that sodium is more favorable than calcium to the formation of BO 4 units. The experimental N 4 ratio is compared to the Dell and Bray model prediction and it is shown that several adjustments, due to the presence in our glass of Nd and Zr, are necessary to obtain acceptable agreement with experimental data. 29 Si MAS NMR data also put in evidence an effect of the substitution on the polymerization degree. Glass in glass phase separation is clearly detected when the ratio of CaO to Na 2 O is greater than 1 and a different evolution of NMR parameters is observed for the ratio of CaO to Na 2 O being less than or equal to 1. Concerning aluminum charge compensation, it is demonstrated that, as long as no phase separation is detected, the negative charge of AlO 4 # entities is almost exclusively balanced by sodium cations. Finally, changes of the sodium ions organization within the glass network are also evidenced by spinlattice relaxation and spin echo decay measurements.

The procedure proposed by Packer and Rees (J. Colloid Interface Sci. 40 (1972) 206) to interpret pulsed field gradient spin-echo (PGSE) experiments on emulsions is commonly used to resolve for the distribution of droplet sizes via nuclear magnetic resonance (NMR). Nevertheless, such procedure is based on several assumptions that may restrict its applicability in many practical cases. Among such constrains, (a) the amplitude of the spin-echo (signal) must be influenced solely by the drop phase, and not by the continuous phase; and (b) the shape of the drop size distribution must be assumed a priori. This article discusses new theory to interpret results from PGSE experiments and a novel procedure that couples diffusion measurements (PGSE) with transverse relaxation rate experiments (the so-called CPMG sequence) to overcome the above limitations. Results from experiments on emulsions of water dispersed in several crude oils are reported to demonstrate that the combined CPMG-PGSE method renders drop size distributions with arbitrary shape, the wateryoil ratio of the emulsion and the rate of decay of magnetization at the interfaces, i.e. the surface relaxivity. It is also shown that the procedure allows screening if the dispersion is oil-inwater (oyw) or water-in-oil (wyo) in a straightforward manner and that it is suitable to evaluate stability of emulsions. ᮊ

When polyelectrolyte-neutral block copolymers are mixed in aqueous solutions with oppositely charged species, stable complexes are found to form spontaneously. The mechanism is based on electrostatics, and on the compensation between the opposite charges. Electrostatic complexes exhibit a core-shell microstructure. In the core, the polyelectrolyte blocks and the oppositely charged species are tightly bound and form a dense coacervate microphase. The shell is made of the neutral chains and surrounds the core. In this paper, we report on the structural and magnetic properties of such complexes made from 6.3 nm diameter superparamagnetic nanoparticles (maghemite -Fe 2 O 3 ) and cationic-neutral copolymers. The copolymers investigated are poly(trimethylammonium ethylacrylate methylsulfate)-b-poly(acrylamide), with molecular weights 5000-b-30000 g·mol -1 and 110000-b-30000 g·mol -1 . The mixed copolymer-nanoparticle aggregates were characterized by a combination of light scattering and cryo-transmission electron microscopy. Their hydrodynamic diameters were found in the range 70 -150 nm and their aggregation numbers (number of nanoparticles per aggregate) between several tens to several hundreds. In addition, Magnetic Resonance Spin-Echo measurements show that the complexes have a better contrast in Magnetic Resonance Imaging than single nanoparticles, and that these complexes could be used for biomedical applications.

The purpose of this study was to compare the patient tolerance and efficacy, as magnetic resonance imaging negative oral contrast agents, of a mixture of clay compound bentonite and low density barium sulfate suspension with that of higher density barium sulfate. Twenty patients were randomized into two groups: 10 patients receiving a mixture of low concentration 60% w/v barium sulfate plus 2.5% w/v bentonite, and 10 patients receiving 220% w/v barium sulfate Liuqid-HD (E-Z-EM, Westbury, NY). Post-contrast Spin-echo (SE) Tl-and T2-weighted images (WI) were obtained on a l.OT magnet. Two independent readers scored the overall intraluminal signal intensity and delineation of the gastrointestinal tract and adjacent organs. Patient acceptance was evaluated via a short questionnaire, by recording spontaneous comments and documenting the quantity of contrast agent ingested. There was greater intraluminal bowel signal reduction and organ delineation with 220% w/v barium than with the barium-bentonite mixture on both SE TlWI (p = 0.03) and SE T2WI (JJ = 0.42). With both agents there was greater signal reduction on SE T2WI than SE TlWI. Higher scores for organ delineation for both contrast agents were seen with SE TlWI. With 220% w/v barium, there was significantly better delineation of the pancreatic body @ = 0.02) and pancreatic tail (p = 0.02) on TlWI compared with SE T2WI. With the barium-bentonite mix-, SE TlWI showed improved delineation of jejunum compared with SE T2WI (JJ = 0.03). There were no statistically significant differences between the volume of contrast ingested in the two groups. Abdominal cramps were recorded for one patient in each group. These results suggest that bariumbentonite mixture, although useful as a negative gastro-intestinal contrast agent, is not as effective as 220% w/v barium. Further studies with a larger patient population and concentration optimization studies are needed. 0 1997 Elsevier Science Inc.

Hyperintensity in the posterior limb of the internal capsule at T2-weighted MR imaging, consistent with corticospinal tract (CST) degeneration, is described in amyotrophic lateral sclerosis (ALS). However, the lack of specific tests or biological markers hinders confirmation of the diagnosis, especially in the early stages. We investigated the CST in ALS with MR imaging. We examined 25 patients (14 men, 11 women; mean age, 49.1 years; range, 29-68 years) and 21 age- and sex-matched control subjects without upper motor neuron signs. According to the revised El Escorial criteria, 22 patients had definite ALS; two, probable ALS; and one, suspected ALS. Fluid-attenuated inversion recovery (FLAIR; TR/TE/TI, 11,000/140/2600) and T1-weighted spin-echo (SE)/magnetization transfer contrast-enhanced (MTC; TR/TE, 510/12) imaging was performed at 1 T. Two experienced neuroradiologists blinded to the patients' history independently evaluated the CST. T1-weighted SE MTC imaging allowed visual...

Objective: To determine the association between the self-report of pain and disability and findings on lumbar MR images, and to compare two different health care providers in Spanish patients with low back pain (LBP). Design: Cross-sectional Material and methods: A total of 278 patients, 137 men and 141 women aged 44±14 years submitted with low back pain (LBP) were studied. One hundred and nine patients were from the National Health System (NHS) and 169 from private practice. Patients with previous discitis, surgery, neoplasm or traumatic episodes were excluded. Every patient completed a disability questionnaire with six core items, providing a score of disability from 2 to 28. All patients had sagittal spin-echo T1 and turbo spin-echo T2, axial proton-density and MR myelography weighted images. MR images of the two most affected disc levels were read, offering an MR imaging score from 0 to 30. Results: Patients with a combination of LBP and sciatica showed the highest levels of disability (p=0.002). MR imaging scores only correlated with pain interference with normal work (p=0.04), but not with other disability questions. Patients from the NHS showed greater disability scores than private ones (p=0.001) and higher MR imaging scores (p=0.01). Conclusion: In patients with LBP, MR imaging only correlates with pain interference with work but not with other disability questions. Differences are found between private and NHS patients, the latter being more physically affected.