Fourteen symptomatic patients with severe extracranial internal carotid artery stenosis (≥70% of luminal narrowing) were monitored using long-term transcranial Doppler ultra-sonography to determine the rate of clinically silent embolism of the ipsilateral middle cerebral artery. Before carotid endarterectomy (all patients being treated with intravenous heparin), 462 such events occurred during a total monitoring time of 45 h. Statistical analyses of the inter-event intervals and of the relationship between events and cardiac cycle revealed random occurrence. While the ipsilateral events were found in each subject, silent embolism of the contralateral middle cerebral artery occured in only four patients each of whom had angiographically proven intracranial cross-flow from the symptomatic carotid territory (39 embolic events during 7 h monitoring time). The other 10 patients showed no contralateral embolism (10 h monitoring time). Five or more days (median 10 days) after surgery and cessation of intravenous anticoagulation the ipsilateral event rate had dropped to 13 in 33 h (P < 0.001) and the contralateral rate to zero. This suggests that the stenosed arterial segment is the main source of cerebral embolism detectable with ultrasound in symptomatic patients with high-grade internal carotid stenosis, and that carotid endarterectomy substantially reduces the rate of these events. Since reduction of ipsilateral stroke risk by successful endarterectomy is known to be of a similar degree in patients as those studied here, transcranially detected embolism may represent a new marker of disease activity of extracranial carotid artery stenosis.

Background and Purpose Previous work has shown that cerebral microembolism detected with transcranial Doppler sonography distal to internal carotid artery stenosis occurs more frequently in recently symptomatic compared with asymptomatic patients. It has remained unclear whether cerebral microembolism also indicates a higher risk of future cerebral or retinal ischemia. Summary of Report Sixty-four asymptomatic patients with unilateral 70% to 90% internal carotid artery stenosis were investigated prospectively (mean follow-up, 72 weeks). Five patients developed ischemic symptoms attributable to the stenosis (transient ischemic attack, 2 patients; stroke, 3 patients). A microembolic rate of ≥2 per hour in the ipsilateral middle cerebral artery was associated with a substantially increased risk of developing ischemia of the corresponding carotid territory (odds ratio, 31; 95% confidence interval, 3 to 302; P =.005). Conclusions This prospective pilot study suggests that cerebral microembolism detected with transcranial Doppler sonography may define a high-risk subgroup among patients with asymptomatic high-grade internal carotid artery stenosis.

Article abstract-We monitored breathing pattern and arterial oxygen saturation in 32 conscious patients with acute ischemic stroke. Seventeen (53%) had Cheyne-Stokes respiration with concomitant drops in oxygen saturation, unrelated to infarct location. The ventilatory disturbance promptly reversed after intravenous theophylline ethylenediamine or oxygen inhalation. The therapy is a simple way of improving arterial oxygenation in a large subgroup of patients with acute ischemic stroke. NEUROLOGY 1995;45: 820-821

A granular material exposed to shear shows a variety of unique phenomena: Reynolds dilatancy, positional order and orientational order effects may compete in the shear zone. We study granular packing consisting of macroscopic prolate, oblate and spherical grains and compare their behaviour. X-ray tomography is used to determine the particle positions and orientations in a cylindrical split bottom shear cell. Packing densities and the arrangements of individual particles in the shear zone are evaluated. For anisometric particles, we observe the competition of two opposite effects. On the one hand, the sheared granules are dilated, on the other hand the particles reorient and align with respect to the streamlines. Even though aligned cylinders in principle may achieve higher packing densities, this alignment compensates for the effect of dilatancy only partially. The complex rearrangements lead to a depression of the surface above the well oriented region while neighbouring parts still show the effect of dilation in the form of heaps. For grains with isotropic shapes, the surface remains rather flat. Perfect monodisperse spheres crystallize in the shear zone, whereby positional order partially overcompensates dilatancy effects. However, even slight deviations from the ideal monodisperse sphere shape inhibit crystallization.

When a granular material consisting of macroscopic elongated grains is exposed to shear, the individual grains align. We determine the particle distribution functions and orientational order parameters and study the collective dynamics as well as individual particle motion during shearing. X-ray computed tomography (CT) is used to obtain three-dimensional images of the shear zone. All individual particle positions and orientations are extracted by image processing software and the complete order tensor is determined. We compare the behavior of our ensembles of macroscopic grains with well-known continuum models for shear alignment and director dynamics of anisotropic liquids. Irrespective of the completely different particle interactions and size scales, analogies are found even on a quantitative level. Measurements of the local packing densities inside and outside the shear zone reveal a shear dilatancy, irrespective of the more efficient packing that can be expected for ordered ensembles of cylinders compared to randomly oriented samples.

Granular material flowing through bottlenecks, like the openings of silos, tend to clog and thus inhibit further flow. We study this phenomenon in a three-dimensional hopper for spherical and shapeanisotropic particles by means of x-ray tomography. The x-ray tomograms provide information on the bulk of the granular filling, and allows us to determine the particle positions and orientations inside the silo. In addition, it allows us to calculate local packing densities in different parts of the container. We find that in the flowing zone of the silo particles show a preferred orientation and thereby a higher order. Similarly to simple shear flows, the average orientation of the particles is not parallel to the streamlines but encloses a certain angle with it. In most parts of the hopper, the angular distribution of the particles did not reach the one corresponding to stationary shear flow, thus the average orientation angle in the hopper deviates more from the streamlines than in stationary shear flows. In the flowing parts of the silo, shear induced dilation is observed, which is more pronounced for elongated grains than for nearly spherical particles. The clogged state is characterized by a dome, i.e. the geometry of the layer of grains blocking the outflow. The shape of the dome depends on the particle shape.