How low can you go? Minimum energy pathways for O2 dissociation on Pt(111).

Many DFT studies of O(2) dissociation on Pt(111) give conflicting information on preferred paths and final states. Here we report large p(4 × 4) unit cell minimum energy pathway evaluations and compare O(2) adsorption and dissociated states on Pt(111). Calculations reveal how the pathways for O(2) dissociation starting from top-fcc-bridge, top-hcp-bridge, and top-bridge-top sites are interconnected. They also provide a direct reaction pathway for the dissociation of an O(2) molecule from a top-fcc-bridge into an hcp and an fcc site, which is consistent with low temperature scanning tunneling microscope experiments. Such a pathway is shown to be considerably perturbed by the presence of co-adsorbed oxygen atoms. We quantify the coverage dependence through the construction of a Brønsted-Evans-Polanyi relationship relating the O(2) dissociation activation energies to the binding energies of the dissociated O atoms. We also show that all pathways starting from a top-fcc-bridge site give the smallest barriers for O(2) dissociation.

Degradation and drug release in calcium polyphosphate bioceramics: an MRI-based characterization.

Degradable, bioceramic bone implants made of calcium polyphosphate (CPP) hold potential for controlled release of therapeutic agents in the treatment of localized bone disease. Magnetic resonance imaging techniques for non-invasively mapping fluid distribution, T(1) and T(2) relaxation times and the apparent diffusion coefficient were performed in conjunction with a drug elution protocol to resolve free and bound water components within the material microstructure in two CPP formulations (G1 and G2). The T(2) maps provided the most accurate estimates of free and bound water, and showed that G1 disks contained a detectable free water component at all times, with drug release dominated by a Fickian diffusion mechanism. Drug release from G2 disks was characterized by a combined diffusional/structural relaxation mechanism, which may be related to the gradual infiltration of a free water component associated with swelling and/or chemical degradation.

Potential energy surfaces for oxygen adsorption, dissociation, and diffusion at the Pt(321) surface.

We report a first-principles, periodic supercell analysis of oxygen adsorption, diffusion, and dissociation at the kinked Pt(321) surface. Binding energies and binding site preferences of isolated oxygen atoms and molecules have been determined, and we show that both atomic and molecular oxygen prefer binding in bridge sites involving coordinatively unsaturated kink Pt atoms. Binding energies of atomic and molecular oxygen in different sites correlate well with the average metallic Pt coordination number of Pt atoms forming each site, although differences exist between adsorbates in symmetrically similar sites due to the inherent chirality of the surface. Atomic O in the strongest binding bridge sites experiences relatively small energy barriers for diffusion to neighboring sites compared to O on Pt(111). However, due to the structure of the surface, O diffusion is only rapid between different sites around the kink Pt atom, whereas the effective long-range tracer diffusion, as determined from a simple course-grain model, is shown to be anisotropic and slower than on the Pt(111) surface. Four dissociation pathways for O(2) at low coverage are also reported and found to be in agreement with experimental observations of facile dissociation, even at low temperature.

A truncated ultrasound screening procedure for atheroma of the cervical arteries in asymptomatic diabetic patients: evidence from a retrospective study.

AIM: To demonstrate that ultrasound screening of diabetic patients presenting with no cerebrovascular symptoms for evaluation of atheroma of the cervical arteries can be limited to the carotid arteries. METHODS: We retrospectively analyzed the results of cervical artery ultrasound imaging of diabetic patients with no cerebrovascular symptoms. This diabetic population was divided into two subpopulations according to whether or not the vertebral and subclavian artery findings were normal or abnormal. RESULTS: Of the 760 patients who fulfilled the criteria for study inclusion, the ultrasound imaging findings of the vertebral and subclavian arteries were normal in 712 cases. Review of the files of the 48 remaining patients showed that findings for either the vertebral or subclavian arteries did not lead to any changes in patient management because of associated risk factors, carotid atheroma or peripheral arterial disease. CONCLUSION: A vascular risk evaluation in diabetic patients could include ultrasound imaging assessment for cervical artery atheroma and our data suggest that such an evaluation could be focused solely on the carotid arteries.

Duplex scanning diagnosis of internal carotid artery dissections. A case control study.

BACKGROUND: The reliability of duplex scanning (DS) for the diagnosis of internal carotid artery dissections (ICAD) is not clear. METHODS: Nine DS signs known to be suggestive for the diagnosis of ICAD were compared between 70 patients with ICAD and 70 matched patients without dissection. RESULTS: Visible internal tapering occlusion, regular eccentric narrowing channel, ectasia beyond the carotid bulb, resistive index asymmetry, blood flow slowdown, ophthalmic artery blood flow inversion, and biphasic flow are more frequent in cases than in controls (p < 0.001). Atheroma plaques were absent in 80% of ICAD. When DS direct signs and hemodynamic signs were studied, sensitivity was 90% and specificity 60%. CONCLUSION: Diagnosis of ICAD by DS could be improved if direct signs were combined with hemodynamic signs, giving a high sensitivity and a rather good specificity.

Fibromuscular dysplasia may herald symptomatic recurrence of cervical artery dissection.

BACKGROUND: The prevalence of fibromuscular dysplasia (FMD) in patients with cervical artery dissection (CAD) is unknown. Our objectives were to assess the risk of CAD recurring as a stroke or a transient ischemic attack and the association of these events with FMD. METHODS: We prospectively included and followed 103 consecutive patients who had been admitted for a CAD. The median follow-up was 4 years (range 4 months to 10 years). The main criteria for inclusion were a mural hematoma demonstrated by cervical magnetic resonance imaging and/or signs suggesting CAD on 2 other investigations. FMD was diagnosed on the so-called string of beads pattern by digital subtraction angiography. RESULTS: Five patients had CAD recurrence (60% occurred late). Four of these 5 patients had FMD. In 4 patients, CAD recurrence involved another cervical artery. CONCLUSION: The rate of symptomatic CAD recurrence was 1% per year and was often related to FMD.

[Pulsed transcranial Doppler examination in the acute phase of stroke]. / Le Doppler pulsé transcrânien à la phase aiguë de l'infarctus cérébral.

INTRODUCTION: Transcranial pulsed Doppler sonography with or without imagery is used in the acute phase of ischemic stroke in order to define stroke mechanism and intracranial hemodynamic consequences. STATE OF ART: This non-invasive low-cost investigation can be undertaken rapidly, within 10 minutes if guided by focal symptomatology. It detects middle cerebral artery stenosis in >or=50 p. cent of cases with a sensitivity between 75 and 90 p. cent, comparatively with conventional angiography, and middle cerebral artery occlusion with 90 p. cent sensitivity. When a temporal acoustic window is lacking, intravenous injection of a contrast agent allows good visualization of the intracranial vessels and the circle of Willis, in two-third of cases. Moreover, transcranial Doppler data have good prognostic value and facilitate analysis of recanalization after thrombolytic therapy. CONCLUSION: Beside its diagnostic and prognostic interest, transcranial Doppler sonography using a 2 MHz frequency appears to have therapeutic impact by favoring the thrombolysis process. Future developments may include early and prolonged insonation of patients suffering from stroke.

Accuracy of color-Doppler in the quantification of proximal vertebral artery stenoses.

BACKGROUND: Vertebrobasilar (VB) strokes appear to have the same causes as carotid strokes. Obstructive lesions of proximal vertebral arteries probably occur in about 30% of stroke patients. PURPOSE: Our aim was to assess the validity of color Doppler sonography compared to selective intra-arterial angiography in the quantification of proximal vertebral artery stenoses. MATERIALS AND METHODS: A prospective blind study of 316 vertebral arteries was undertaken between 1996 and 1998. One hundred and fifty-eight patients with cerebrovascular disorders without cerebral hemorrhage were studied consecutively by frequency or amplitude color Doppler flow imaging and intra-arterial angiography. The lesions were quantified by morphological and hemodynamic criteria and classified into 6 groups: 0% 207 arteries; 1-29% 32 arteries; 30-49% 29 arteries; 50-69% 13 arteries; 70-99% 23 arteries; 100% 12 arteries. RESULTS: Ten of the 12 occlusions were identified, the 2 false-negatives were due to 2 revascularized vessels. Moderate stenoses (<50%) were differentiated from tight stenoses (>50%) using hemodynamic criteria. The majority of false-negative stenoses (38) in the different groups were related to intrathoracic or very deep origin of the artery, anechogenic stenosis or a tortuous vessel. Stenoses greater than 70% were diagnosed in 71% of cases with a specificity of 99%. The kappa value was 0.80. CONCLUSION: Duplex sonography should be proposed first in VB attacks or stroke to detect and quantify vertebral artery stenoses for surgery and angioplasty.

[Trismus, pseudobulbar syndrome and cerebral deep venous thrombosis]. / Trismus, syndrome pseudobulbaire et thrombose veineuse profonde cérébrale.

We report a case of cerebral deep venous thrombosis that manifested clinically by a pseudobulbar syndrome with major trismus, abnormal movements and static cerebellar syndrome. To our knowledge, only three other cases of deep cerebral venous thrombosis associated with cerebellar or pseudobulbar syndrome have been published since 1985. The relatively good prognosis in our patient could be explained by the partially intact internal cerebral veins as well as use of early anticoagulant therapy. There was a spontaneous hyperdensity of the falx cerebri and the tentorium cerebelli on the brain CT scan, an aspect highly contributive to diagnosis. This hyperdensity of the falx cerebri was found in 19 out of 22 cases of deep venous thrombosis detailed in the literature.

[Doppler ultrasonography of carotid atheromatous lesions]. / Echographie-Doppler des lésions athéromateuses des artères carotides.

Carotid stenosis is the most frequently factor involved in stroke. The diagnosis of these lesions in routinely based on ultrasonic examination which allows to detect the carotid stenoses which were essentially atheromatous. B-mode evaluates the fine echostructure of plaques and Doppler helps to quantify the degree of stenosis using direct and indirect hemodynamic criteria. Echography with the help of color Doppler imaging identifies the site of stenosis in order to determine the degree of stenosis as a percentage of normal carotid area. Moreover echography is an essential method to appreciate the surface as well as the echostructure of the carotid lesion which have to be taking into account for a well-adapted therapy. The maximal velocities determined at the tight site of the lesion, the spectral changes downstream the stenosis and the hemodynamic changes observed upstream and downsteam of the carotid stenosis allow a precise quantification of this stenosis. The information obtained using sonographic methods (including 2 direct hemodynamic criteria, 2 indirect hemodynamic criteria and, if possible, a B-mode assessment of the area) are essential to appreciate the neurological risk in these patients.

MRI study of transient cerebral ischemia in the gerbil: interest of T2 mapping.

RATIONALE AND OBJECTIVES: The aim of this study was to evaluate the diagnostic use of MRI and, more precisely, the use of quantitative T2 imaging at 7 T for the early detection of neuronal cerebral alterations after transient ischemia in the gerbil. METHODS: One hundred forty-seven Mongolian gerbils were separated into four groups for which a bicarotid artery occlusion lasted for 4, 6, 8, or 10 minutes, respectively. The animals were scanned before carotid artery occlusion and at 3, 6, 10, 24, and 48 hours and 5 days after the ischemic incident. MR images were acquired on a Bruker Avance DRX300 mini-imaging system. RESULTS: Our results show that T2 mapping is able to localize brain damage induced by transient ischemia and to detect early perturbations in water content (as early as 6 hours after ischemia). CONCLUSIONS: T2 measurements in the striata are correlated with the severity of the ischemic incident, since the changes observed on the T2 images are directly proportional to the duration of occlusion.

[Floating thrombus in the carotid]. / Thrombus flottant dans une carotide.

The cardiogenic origin of stroke is often established on the basis of documented arrhythmia, computed tomographic evidence of cerebral ischemia, duplex Doppler findings ruling out an atheromatous carotid origin, and sometimes visualization of an intracavitary thrombus at transesophageal echocardiography. In case of a large thrombus as observed in our present patient, duplex Doppler provides an important means of visualizing the thrombus despite the lack of a parietal lesions suggestive of atheromatosis.

[Color Doppler ultrasonography of short para-optic ciliary arteries in vascular glaucoma]. / Echo-Doppler couleur des artères ciliaires courtes para-optiques dans le glaucome vasculaire.

PURPOSE: To study color Doppler imaging in orbital vessels, especially para-optic short ciliary arteries, in patients with vascular glaucoma. PATIENTS AND METHODS: Twenty vascular normal or moderate elevated pressure glaucoma patients (6 with bilateral glaucoma, 9 with unilateral glaucoma et 5 with asymmetric glaucoma) underwent a color Doppler imaging (Accusson 128XP) in orbital arteries: common carotid, ophthalmic artery, central retinal artery, short para-optic ciliary arteries. Systolic and diastolic blood flow velocities were measured, and resistance index of Pourcelot and ratios between different parameters were calculated. RESULTS: In patients with unilateral glaucoma, ratios of systolic velocities between para-optic ciliary artery and common carotid, and ratio of Pourcelot index between para-optic ciliary artery and ophthalmic artery were increased in glaucomatous eyes. And Pourcelot index were statistically significant increased in all glaucomatous patients of the study compared with normal subjects. DISCUSSION AND CONCLUSION: Comparison of Pourcelot index and ratios between parameters makes color Doppler imaging more reliable, especially when analyse short para-optic ciliary arteries, which vascularize optic nerve. In these arteries Pourcelot index was increased in vascular glaucoma patients. Vascular risk factor should be taken into consideration in this form of glaucoma.

Introduction of the closed cranial window technique in gerbils and verification by observation of the effects of specific drugs.

The exact mechanisms of cerebral arterial hypoxia are not perfectly defined. Our purpose is to adapt and validate, with drugs well known in rats and rabbits, a closed cranial window technique in gerbils. The method was used with seventeen gerbils to measure diameter changes of the pial arterioles under normoxia (after the topical application of agonists and antagonists of ATP-sensitive and Ca2+-dependent potassium channels), as well as under hypoxia. In normoxia, aprikalim (10(-6) M), a direct activator of ATP-sensitive potassium channels, increases the diameter of pial arterioles by 10+/-2% (N = 17). This effect is inhibited by glibenclamide (10(-6) M), but not affected by iberiotoxin (10(-6) M), a specific inhibitor of Ca2+-dependent potassium channels. The adenosine-induced dilation by 19+/-5% (N = 17) is reduced by 59+/-16% with iberiotoxin, by 33+/-23% with glibenclamide and inhibited by theophylline (10(-5) M). In hypoxia (15% O2), pial arteriole diameters are increased by 24+/-5% (N = 17) and partially decreased by the application of glibenclamide and iberiotoxin to 59+/-11% and 54+/-5%, respectively. These data are similar to those obtained in other species and validate the closed cranial window technique on gerbils. They indicate that, as for rats and rabbits, both ATP-sensitive and Ca2+-dependent potassium channels are present in gerbil pial vessels and play a role in hypoxia.

Ultrasound characterization and quantification of carotid atherosclerosis lesions.

The development of ultrasonography in vascular applications has entailed research of ultrasound parameters leading to precise the diagnosis and quantification of carotid lesions in routine. The use of recent colour Doppler imaging techniques (velocity or power encoding) is recommended as they allow a better definition of the lesions and recesses. At present, features of plaque that could be related to increased incidence of stroke are: echogenicity, with a more frequent observation of anechoic or hypoechoic lesions in the case of clinical signs; texture, with frequent haemorrhage; surface contour, with a high rate of ulcerations which were accompanied by stroke; plaque motion, with a significant alteration in plaque motility in symptomatic patients; progression or regression in size, with a faster progression of anechoic and hypoechoic plaques; an increase in plaque volume is associated with a greater risk of stroke; a significant relationship between the presence of "ulcers" and embolic activity. The quantification of stenosis degree could be made using velocity criteria and/or morphological criteria. Velocity criteria could be obtained at the site of the stenosis (direct criteria) or downstream the carotid stenosis using Duplex systems. Morphological criteria could be obtained using B-mode imaging or colour Doppler but this quantification remains difficult in case of diffuse carotid stenoses or very severe stenoses.