We have previously developed a hollow cylindrical bolt-clamped Langevin-type transducer with a high mechanical quality factor (Q) for excitation of high-amplitude torsional vibrations. In this paper, we propose a construction of vibrating systems that comprise a `stepped horn' (a mechanical transformer of torsional vibration) and the stumpy `twisting' transducer developed by us. The distinctive feature of the construction is that it achieves compactness without sacrificing the volume of piezoelectric ceramics that is important for high-power ultrasonic applications. Through experiments, we have found some crucial points in the procedure for constructing torsional-vibration systems that can be operated at a relatively high frequency. In addition, we have conducted some experiments of ultrasonic plastic welding using one of the vibrating systems for evaluation of the feasibility of the construction in this manufacturing technique
We investigated age-related changes in the visibility of intracranial arteries on magnetic resonance angiography (MRA) and the influence of risk factors for stroke. We studied 230 adult patients without specific neurological deficits. MRA was performed using the three-dimensional time-of-flight technique with a spoiled gradient-recalled acquisition sequence. We classified internal carotid artery (IC) and the horizontal (M1) and distal (beyond M2) middle cerebral segments into 4 grades. Linear regression revealed a significant negative relation between age and the quality of demonstration on MRA. For IC and M1, the score was significantly lower in subjects with risk factors than in those without. The distal MCA was poorly seen in patients without a history of hypertension or lacunar infarcts. A marked correlation was observed between visibility and age patients with no history of hypertension, diabetes mellitus and hyperlipidaemia. We suggest that atherosclerotic change and decline in flow velocity with normal ageing are factors leading to decreased visibility on MRA.
ABSTRACT:To obtain low polymeric polystyrene (PS), pyrolysis of high polymeric PS in solution was studied in the temperature range from 290 to 400°C by using additives or acid catalysts. The low polymeric PS targeted here was that with average molecular weight of 10 4 . When the feed PS was pyrolyzed in tetralin by adding sulfur or diphenyl disulfide, the molecular weight of PS decreased greatly, even at lower temperatures, and the desired low polymeric PS was formed in a relatively large amount at the temperatures below 350°C. The degradation behavior was able to be explained in terms of a random polymer chain scission mechanism initiated by sulfur radicals formed from the additives.
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