On the nature and reduction of plaque-mimicking flow artifacts in black blood MRI of the carotid bifurcation

Cardiac-gated black blood MRI of the carotid artery bifurcation in normal human subjects shows signal within the lumen suggesting wall thickening or atherosclerotic plaque. This signal was believed to be artifactual, arising from complex flow patterns present at the carotid bifurcation. Computer simulation of the hemodynamics and black blood multislice image acquisition in a model of the carotid bifurcation showed that these artifacts arise from spins recovering their signal within the slow, recirculating flow of the carotid bulb. The computed hemodynamics also suggested that these artifacts could be minimized or eliminated entirely by gating the acquisition of slices in the most artifact-prone region of the carotid bulb within a 250-ms window after peak systole. Application of these predictions to studies of normal volunteers showed that, in most cases, these flow artifacts in black blood MRI can be eliminated simply by altering the phase of the cardiac cycle to which the image acquisition is gated. The observation that the size and placement of the saturation slabs had little effect on these artifacts suggested that, in those cases in which recirculation persists throughout the cardiac cycle, either inversion-recovery or presaturation within the bulb itself would be required to suppress them.     

A Comparison of the Forecast Performance of Markov-switching and Threshold Autoregressive Models of US GNP

While there has been a great deal of interest in the modelling of non-linearities in economic time series, there is no clear consensus regarding the forecasting abilities of non-linear time-series models. We evaluate the performance of two leading non-linear models in forecasting post-war US GNP, the self-exciting threshold autoregressive model and the Markov-switching autoregressive model. Two methods of analysis are employed: an empirical forecast accuracy comparison of the two models, and a Monte Carlo study. The latter allows us to control for factors that may otherwise undermine the performance of the non-linear models.     

统计模拟分光光度法测定氨基酸注射液诸组分含量

应用统计模拟分光光度法同时测定复方制剂组分含量．在复方制剂合格限的（８７±１７）％范围内，以２％浓度单位递增，利用均匀设计法设计实验方案配制１７水平的合成样品液，绘制其溶液的紫外吸收光谱，选择组分含量随吸光度变化大的灵敏波长，并测出对应的吸光度，用逐步回归法构造反映该复方制剂在灵敏波长下吸光度与组分含量关系的数学模型；根据构造的数学模型，在计算机上算出诸组分含量在含量合格限范围内的详尽组合．在测定未知样品时，只须在灵敏波长下测得其溶液的吸光度，采用全组合法寻优，求出未知样品诸组分含量．     

统计模拟分光光度法同时测定肺保三效片中4组分含量

前文［１，２］报道了统计模拟分光光度法用于复方制剂２组分、３组分的含量测定。本文进一步对复方制剂４组分进行分析测定。肺保三效片含对乙酰氨基酚、苯妥英、茶碱、咖啡因、盐酸麻黄碱、氯苯那敏、颠茄流浸膏等，法定方法［３］只对前２种组分进行了定量测定，本文用...     

Training medical students in HIV/AIDS test counselling: results of a randomized trial

The study assessed the effectiveness of a programme aimed at increasing medical students' skills in counselling patients presenting for HIV testing/AIDS information. Senior medical students were randomly assigned to receive a short course in pre- and post-test counselling, or to a control group which received the usual curriculum. The students' performance in counselling simulated patients was videotaped at baseline and after 3 months. A subsample was also assessed at 12 months. Students receiving the programme showed significantly greater improvement in pre- and post-test counselling skills over 3 months than did the controls. For the subsample continuing to 12 months, a significant effect over time was found; however, there was no significant difference between the groups. This may have been influenced by the small sample sizes used for the 12-month assessment. General interactional skills improved for the overall sample over 3 and 12 months, but again there were no significant differences between groups. Those exposed to the programme did not show significantly greater changes in either knowledge or attitude scores over either time frame, compared with controls. When taught in addition to the usual undergraduate curriculum at Newcastle University, this short interactional skills course significantly enhanced students' ability to provide pre- or post-test counselling for HIV/AIDS.     

Simulation study on bypass and therapeutic occlusion in the posterior circulation

Summary A hydraulic vascular model of the vertebrobasilar artery with an autoregulatory mechanism was constructed. The haemodynamic effect of superficial temporal artery (STA)-posterior cerebral artery (PCA) bypass was investigated in cases of bilateral vertebral artery (VA) occlusion and basilar artery (BA) occlusion. Assuming therapeutic BA occlusion for basilar bifurcation aneurysms, the flow volume through the P 1 segment of the PCA was determined in relation to diameters of the posterior communicating artery (PCom).The bypass increases both flow volume and intraluminal pressure in the pressure range below 60 mm Hg, while it increases only pressure in the autoregulatory range above 60 mm Hg. Its haemodynamic effect is more marked in BA occlusion than in VA occlusion. The averaged values of the increase in flow volume are 29.0±4.3% (mean±SD) and 16.5+1.0%, respectively. The total flow volume increase of the vertebrobasilar system is usually lower than the flow volume measured at the bypassed STA. The difference between the two is equal to the simultaneous decrease in flow volume through the PComs. Flow volume through the P1 segment can be expressed as a function of the PComs diameter ratio squared.A hydraulic simulation study will be useful for speculating on the haemodynamic effects of these operative procedures.     

模拟失重和辐射对大鼠免疫功能和垂体β—内啡肽含量的影响

本文研究的目的的是探讨短期模拟失重、电离辐射及两者复合因素对大鼠免疫的功能的影响及其免疫功能改变的原因。大鼠为实验动物，尾悬吊为模拟失重动物模型＾６０Ｃｏ为辐射源。     

Zero and first moment area estimation from microdialysis data

Summary The calculation of classical pharmacokinetic parameters from microdialysis data has been described in a previous paper. In this paper I have derived methods for calculating AUMC and AUC from the time-integral type of data that are generated in microdialysis pharmacokinetics experiments. The method derived to estimate AUC is elementary, but is given a theoretical basis using principles of mathematical real analysis, clearly stating the assumptions.The method derived to estimate AUMC is a numerical approximation method based on the linear trapezoidal method. A simulation study was performed to evaluate the precision of the methods and to compare them with corresponding methods for analysis of blood sample data.The estimates from the presently derived methods have a small bias and a small variance. In the simulation study I investigated the influence of model parameters, number of samples, size of statistical error, and the size of the AUC beyond the last sample. Finally, I have given numerical examples from real data to illustrate the use of the method.     

A role for computer simulation in solving the riddles of autoreceptor-mediated regulation of GABA release

Summary The autoreceptor-mediated control of GABA release was simulated on a personal computer using commercially available software (STELLA^TM/ITHINK^TM). The experimental data to be matched were taken from previous publications. A basic model was able to fairly accurately reproduce frequency dependencies of GABA release in the presence and absence of uptake inhibition as well as concentration-response curves for changes in release produced by the agonist, (–)-baclofen, or by relatively low concentrations of the antagonists, phaclofen and CGP 35348. Obvious mismatch was observed at high concentrations of a potent antagonist, at a stimulation frequency of 2 Hz. Whereas the experimental data indicate a 3-fold increase in release as compared to controls, simulation predicts a 7-fold increase. By adaptation of the model, simulation data were obtained indicating that this mismatch was not due to (a) the autoreceptor occurring as two subtypes with different affinities for antagonists, (b) the occurrence of an agonist and antagonist state of the autoreceptor, with the latter prevailing at low synaptic concentrations of endogenous GABA, and (c) overruling of uptake inhibition by markedly elevated synaptic GABA concentrations. On the other hand, a simple restriction of the amount of transmitter able to be released per time unit produced much better matching data. A refined model assuming a restricted replacement capacity for exocytotically emptied synaptic vesicles at their docking sites gave similar results. As a consequence, we shall attempt to address this possibility experimentally.Simulation can never prove a case in the positive sense. It can, however, help to exclude ill-matching solutions of a problem and to prioritize among possible ones, which then must be experimentally addressed. We found simulation with this user-friendly software extraordinarily useful, also and not least because it necessitates and stimulates very intense dealing with a subject.Correspondence to: P. C. Waldmeier at the above address     

两种中药对模拟失重兔耳微循环的影响

失重或模拟失重可引起血循环系统的紊乱,它是引起航天中生理功能失调的原因之一。通过实验及对有关资料的分析,我们发现失重时血循环系统的紊乱和中医“血瘀症”时的变化十分相似。因此,设想采用中药来改善失重或模拟失重时的血循环可能是一种很好的防护措施。本实验观察了两种中药(川芎和丹黄合剂)对模拟失重兔耳微循环的影响。实验方法是将51只兔分四组。第一组,11只,自由活动,实验中喂水;第二组,13只,头低位20°,悬吊期间喂水;第三组,13只,悬吊中喂川芎(10g/kg);第四组,14只,悬吊中喂丹黄合剂(15g/kg),水和药物量均为5ml/kg。实验时间6天,观察实验前后耳微循环的变化,结果是川芎和丹黄合剂均有改善模拟失重兔耳微循环的作用。表现为与实验前比,悬吊组兔耳微血管径和流量增加,流速减慢,微血管中出现血细胞聚集和血球血浆分离。提示悬吊可引起血液的头向分布,血液在头部微血管中潴留和微循环紊乱。在悬吊过程中喂中药的两组,耳微循环得到较大的改善,两种中药具有降低悬吊兔耳微血管径和流量,保持或增加微血管流速,减轻微血管中血细胞聚集和血球—血浆分离的作用。丹黄合剂改善悬吊兔耳微循环的作用优于川芎。丹黄合剂由丹参和黄芪组成,它是我们根据航天对人体的影响而特意配制的一种新药。结果证明,它在改善悬吊兔血液流变性和主要器官组织结构上的作用也优于川芎。因此,丹黄合剂可能成为防止失重对人体影响的一种有效药物。