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At least 829 records · Page 46Linked to original sources
PubMed1991

The effects of different steam-sterilization programs on material properties of poly(L-lactide).

As-polymerized poly(L-lactide) test rods were sterilized by seven different specially designed computer-operated autoclaving programs. As a control, common hospital sterilization was performed. In all cases, the molecular weight decreased after sterilization. A short time high-temperature sterilization led to less molecular weight decrease than a low sterilization temperature cycle with a longer sterilization time. Regular hospital sterilization significantly reduced the elongation at break and also resulted in a decrease of 35% in tensile strength. The program causing minimal damage to the material properties was studied in detail. This program, with a sterilization period of 60 s and 129 degrees C, was effective for PLLA sterilization and also looks very promising for sterilization of other thermo- and moisture-labile polymers.

Humans↗
PubMed1990

Single-shot localized echo-planar imaging (STEAM-EPI) at 4.7 tesla.

The resolution and homogeneity limitations of echo-planar imaging (EPI) are overcome by zoom imaging of an easily shimmed localized volume. Use of the stimulated echo enables single-shot localization. In vivo 0.5-mm resolution EPI images of selected regions of a cat brain at 4.7 T are presented.

Animals↗
PubMed1995

Limitations of stimulated echo acquisition mode (STEAM) techniques in cardiac applications.

Stimulated echoes are widely used for imaging functional tissue parameters such as diffusion coefficient, perfusion, and flow rates. They are potentially interesting for the assessment of various cardiac functions. However, severe limitations of the stimulated echo acquisition mode occur, which are related to the special dynamic properties of the beating heart and flowing blood. To the well-known signal decay due to longitudinal relaxation and through-plane motion between the preparation and the read-out period of the stimulated echoes, additional signal loss is often observed. As the prepared magnetization is fixed with respect to the tissue, this signal loss is caused by the tissue deformation during the cardiac cycle, which leads to a modification of the modulation frequency of the magnetization. These effects are theoretically derived and corroborated by phantom and in vivo experiments.

Blood Flow Velocity↗
PubMed1997

STEAM-Burst: a single-shot, multi-slice imaging sequence without rapid gradient switching.

The stimulated-echo acquisition mode-Burst sequence is a single-shot, multi-slice imaging technique that does not involve rapid gradient switching. A Burst excitation pulse train is followed by a 90 degrees hard pulse and, after a mixing time, by a 90 degrees slice-selective pulse. A read gradient refocuses a set of stimulated echoes, which can be phase-encoded to form an image. By repeating the selective pulse N times, each time with the carrier frequency offset differently, it is possible to sample N slices in a single-shot. A comparison is made of the sequence with other three-dimensional single-shot methods. Experiments implementing the technique on a 3 T whole-body imaging system and a 2 T, 31-cm bore animal imager are described. Both phantom and brain images are presented. The principal advantages of the new sequence are its speed, the absence of rapid gradient switching and corresponding freedom from artifacts, its insensitivity to static magnetic field inhomogeneities, and its low acoustic noise. The main disadvantages are the low signal-to-noise ratio of the images produced and the concomitant limitation in resolution.

Animals↗
PubMed2000

Application of LCModel for quality control and quantitative in vivo 1H MR spectroscopy by short echo time STEAM sequence.

The linear combination of model spectra (LCModel) calculation of a parameter for long-term quality control, kT, was introduced, representing the ratio of the temporal and nominal intensities of CH3 groups of lactate and acetate in a quality control phantom. This procedure is a part of the quality assurance of the scanner using fully automatic measurement and calculation of kT parameters, and utilizing Shewhart regulation control charts for continuous evaluation of the magnetic resonance (MR) scanner setting. The application of the kT parameter for the correction of in vivo data increases the precision of molar concentration determination by about 4%. This was tested by the quantitative in vivo MR determination of the molar concentrations of 13 prominent metabolites (N-acetylaspartate (NAA), N-acetylaspartylglutamate, creatine and phosphocreatine (Cr), choline-containing compounds (Cho), myo-inositol, scyllo-inositol, gamma-aminobutyric acid, glutamine, glutamate, glucose, lactate, alanine, taurine) in the white matter and hippocampus of the brain in groups of volunteers, using a short echo time stimulated echo acquisition mode sequence (echo time = 10 ms) and the LCModel technique. The repeatability of the measurement of prominent metabolites such as NAA, Cr and Cho was found to be around 10% (relative standard deviation, n = 6); precision in a group of volunteers (n = 20 and 28, respectively) was in the range of approximately 13-20%. For other metabolites, which are measured with a lower signal-to-noise ratio, the precision can be much lower.

Acetates↗