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The current theoretical understanding of the linear and nonlinear evolution of resistive tearing instabilities in sheared magnetic fields is reviewed. The physical mechanisms underlying this instability are emphasized. Some of the problems which are encountered in developing a model of magnetic energy dissipation in coronal loops are discussed and possible solutions are suggested.
The fourth season of the Fezzan Project continued the interdisciplinary approaches of previous seasons. Geographical and environmental work focused principally in sampling sediments for scientific dating and with integrating ground observation with remote sensing data. Excavations continued at Old Germa, where the site has now reached Garamantian levels. In a separate development, the tentative identification has been made of an early mosque at the site, in an area adjacent to the G1 excavation trench. Substantial results were gained from work aimed at enhancing the important data recorded by Charles Daniels in his earlier excavations and survey in the Wadi al-Hayat. The enhancement of the Daniels' survey archive was integrated with completion of the wider prospection being undertaken by the new project. This survey included fieldwalking, standing building survey, analysis of the foggara irrigation systems and recording of rock art scenes. Finds work comprised the finalisation of a pottery type series for the Germa area, the study of small finds from the recent survey work, botanical analysis and completion of lithics recording. A programme of radiocarbon dating is now being undertaken to improve the phasing of sites and monuments. The first two volumes of final reports are now in preparation.
Electron escape over a one-dimensional potential barrier is treated with a Monte Carlo method that incorporates simple models for the electron-phonon interaction. The consequences of these models are considered here through the calculation of the escaping electron velocity distribution and the electron energy distribution before escape. Effective temperatures are derived from both distributions. The numerical results are compared with those from the classical model of thermionic emission.
The escape rate is calculated for an electron in a one-dimensional potential well. First-order time-dependent perturbation theory is used with solutions of Schrödinger's equation and a set of coupled rate equations is numerically solved. The time evolution of an ensemble of one-electron systems is followed and the fraction of systems that remain in a bound state is found to decay exponentially as time passes. The characteristic time constant for the decay grows exponentially with an increase in the well depth. This is analogous to Kramers' result for the classical escape problem.
Many electron devices and chemical reactions depend on the escape rate of particles confined by potential wells. When the diffusion coefficient of the particle is small, the carrier continuity or the Smoluchowski equation is used to study the escape rate. This equation includes diffusion and field-aided drift. In this work solutions to the Smoluchowski equation are probed to show how the escape rate depends on the potential well shape and well depth. It is found that the escape rate varies by up to two orders of magnitude when the potential shape differs for a fixed well depth.
We have conducted a real time, two-dimensional light scattering study of the nonlinear dynamics of field-induced structures in an electrorheological fluid subjected to oscillatory shear. We have developed a theoretical description of the observed dynamics by considering the response of a fragmenting/aggregating particle chain to the prevailing hydrodynamic and electrostatic forces. This structural theory is then used to describe the nonlinear rheology of ER fluids.
The quantum mechanical escape rate is calculated for an electron in a one-dimensional potential well. First-order time-dependent perturbation theory is used for the bound-to-bound and the bound-to-free transitions. The bound-to-free transition probability decays exponentially with bound energy. The fraction of one-electron systems in a bound state decays exponentially with time. The characteristic time constant grows exponentially with an increasein the depth of the potential well.
We present limits on transit timing variations and secondary eclipse depth variations at 8 microns with the Spitzer Space Telescope IRAC camera. Due to the weak limb darkening in the infrared and uninterrupted observing, Spitzer provides the highest accuracy transit times for this bright system, in principle providing sensitivity to secondary planets of Mars mass in resonant orbits. Finally, the transit data provides tighter constraints on the wavelength-dependent atmospheric absorption by the planet.
Cerebrospinal fluid (CSF) shunt malfunction is one of the most common life-threatening neurosurgical conditions. In the emergency department (ED), imaging techniques to identify shunt malfunction include the shunt series (SS) and CT scanning of the head. We sought to determine the test characteristics of the SS and CT scan for identifying children with shunt malfunction.
We retrospectively reviewed the medical records of children with a CSF shunt who presented to our tertiary care pediatric emergency department and received an SS during a 2-year period from Jan. 1, 2001, to Dec. 31, 2002. A pediatric neuroradiologist reviewed all SS and CT scans. We defined shunt malfunction as present if the child underwent operative shunt revision.
We identified 437 ED visits by 280 children. Forty-seven SS were read as abnormal. A CT scan was performed in 386 (88.3%) cases and 80 were abnormal. Shunt malfunction was identified in 131 (30.0%) children. Sensitivity, specificity, positive predictive value, negative predictive value, positive likelihood ratio and negative likelihood ratio of the SS for identifying cases of shunt malfunction were 30.0%, 95.8%, 72.3%, 75.1%, 7.1 and 0.7, respectively; for the CT scan, they were 61.0%, 82.7%, 64.5%, 80.5%, 3.5 and 0.5, respectively.
Neuroimaging has a low sensitivity for identifying shunt malfunction. Neurosurgical consultation should be sought if shunt malfunction is clinically suspected, despite normal imaging.
The consequences of congenital brain disorders for adult cognitive
function are poorly understood. We studied different forms of memory in 29
young adults with spina bifida meningomyelocele (SBM), a common and
severely disabling neural tube defect. Nondeclarative and semantic memory
functions were intact. Working memory was intact with low maintenance and
manipulation requirements, but impaired on tasks demanding high
information maintenance or manipulation load. Prospective memory for
intentions to be executed in the future was impaired. Immediate and
delayed episodic memory were poor. Memory deficits were exacerbated by an
increased number of lifetime shunt revisions, a marker for unstable
hydrocephalus. Memory status was positively correlated with functional
independence, an important component of quality of life. (JINS,
2007, 13, 312–323.)
There is a range of psychological interventions for established schizophrenia. These include family interventions, motivational interventions for substance misuse and for non-adherence to medication, cognitive remediation for neurocognitive deficits and cognitive–behavioural therapy for symptoms. Psychological interventions may explicitly target risk factors for poor outcome, such as substance use, or protective factors, such as adherence to medication, or be directed at specific symptoms or deficits. There is emerging evidence for efficacy of psychological treatments during, following and even prior to the first episode. Important areas for further study are how different treatment modalities can interact productively and patient and carer preferences for treatment. Many trials of psychological treatments have design flaws and this tends to overestimate the treatment effect.
This study is a comparative analysis and meta-analysis of three randomized clinical trials. Children with spastic diplegia received either ‘selective’ dorsal rhizotomy (SDR) plus physiotherapy (SDR+PT) or PT without SDR (PT-only). Common outcome measures were used for spasticity (Ashworth scale) and function (Gross Motor Function Measure [GMFM]). Baseline and 9- to 12-month outcome data were pooled (n=90). At baseline, 82 children were under 8 years old and 65 had Gross Motor Function Classification System level II or III disability. Pooled Ashworth data analysis confirmed a reduction of spasticity with SDR+PT (mean change score difference –1.2; Wilcoxon p<0.001). Pooled GMFM data revealed greater functional improvement with SDR+PT (difference in change score +4.0, p=0.008). Multivariate analysis in the SDR+PT group revealed a direct relationship between percentage of dorsal root tissue transected and functional improvement. SDR+PT is efficacious in reducing spasticity in children with spastic diplegia and has a small positive effect on gross motor function.
Malignant spinal cord astrocytomas are rare tumors and their specific MR characteristics have not been previously described. We present a detailed MR analysis of four children with malignant astrocytoma.
A review of the clinical database at the Hospital for Sick Children, Toronto revealed four patients with histologically-verified malignant spinal cord astrocytomas (WHO Grade 3 or 4) with pre-operative MR available for retrospective review.
There were three boys and one girl with a mean age at presentation of four years (range 7 months - 12 years). Mean duration of symptoms prior to presentation was six weeks (range 3 days - 5 months). Pre-operative MR analysis revealed that all tumors were located in the cervical or cervico-thoracic regions and expanded the cord over an average of 6.5 vertebral levels. The signal was usually hypointense on T1-weighted and hyperintense or mixed intensity on T2-weighted images. In the three cases where gadolinium was given, all demonstrated enhancement (one rim enhancement with a discrete border and two with inhomogeneous central enhancement). One tumor appeared to be exophytic, one had a significant cystic component, and none showed evidence of hemorrhage. Pre-operative leptomeningeal spread of tumor was documented in two of four cases and involved intracranial spread in both cases.
There did not appear to be any specific MR characteristics to help differentiate a malignant astrocytoma from a low-grade tumor, except for the high rate of leptomeningeal spread at presentation. It is recommended that full neuraxis MR imaging be performed pre-operatively in children in whom a rapidly progressive clinical course suggests a malignant lesion. This will likely have a high positive yield and provide valuable information prior to surgical intervention.
Assembly is a basic device of nature where a broad range of entities, spanning many levels of scale, interact across time and space to produce oberserved pattern. Entities such as individuals, phenotypes, populations, guilds, and higher levels of organization like hierarchical structures are all subject to the processes of assembly. Because all biological systems are assembled in a dynamical sense, any generality in the process would prove valuable to our understanding of nature. Arguably, such an understanding is essential to fully appreciate the action of mechanisms as they are played out in evolutionary and ecological time.
Community assembly is ultimately driven by the invasion (e.g., speciation, immigration) and extinction of species played out against a complex background of environmental constraint. While the environment acts as a filter, eliminating some species and promoting others, it also provides spatio-temporal complexities which serve as a resource upon which ecological strategies can be built. Assembly processes and rules which operate within one environment may exhibit entirely different outcomes as a function of even minor environmental variation. Despite such obvious descriptions of the course of nature, the essence of the assembly trajectory remains little more than an elusive metaphor. Here, the character of the assembly trajectory is evaluated and a general framework offered which serves to interface the operation of ecological mechanisms and the mechanics of community assembly within the more general realm of complex systems.
One of the difficulties with lumboperitoneal (LP) shunts has been non-invasively ascertaining shunt function. It has been previously reported that in the presence of a functioning LP shunt the perimesencephalic cisterns become obliterated – the “absent cistern sign”. In order to more rigorously test this association we performed a retrospective analysis of LP shunt patients at the Hospital for Sick Children, Toronto.
The CT scans of all patients undergoing LP shunting over a 17 year period were reviewed. The “absent cistern sign” and ventricular size were compared against the results of either an isotope shunt study or surgical findings performed within 2 days of the CT.
There were 38 CT scans (27 patients) performed within 2 days of an isotope shunt study and 15 CT scans (14 patients) performed within 2 days of a surgical intervention. These results give the absent cistern sign a sensitivity of 75% and a specificity of 57% when compared to the shunt isotope findings and a sensitivity of 100% and a specificity of 50% when compared to the surgical findings. Over 30% of the CT scans showed ventriculomegaly in the presence of a functioning shunt and, conversely, nearly 45% of the CT scans had normal or small lateral ventricles in the presence of a malfunctioning shunt.
The “absent cistern sign” appears to reliably rule out a completely blocked shunt, but is less reliable in detecting a normal or partially obstructed shunt. Ventricular size correlates poorly with LP shunt function.
Mendocino Channel lies at the base of Gorda Escarpment on the southern edge of Gorda deep-sea fan about 40 km off Cape Mendocino, California. The channel marks the seaward extension of the Mattole and Mendocino Submarine Canyons and contains several prominent meanders along its length. Based on water-gun and 3.5-kHz seismic records, the channel-levee geometry and structure are similar to deepsea meandering channels reported elsewhere. Over the approximately 50-km section of the channel studied, channel sinuosity and channel slope are about 1.7 and 4.2 m/km, respectively. The valley slope is likely time-variable because of the tectonically induced movements of the southern Gorda Plate upon which the channel is situated. Changes to the valley slope will lead to changes in the meander development during future episodes of turbidity currents. Five (possibly six) turbidite units that have 14C ages ranging from 960 yBP to 3,595 yBP were recovered in box cores from the channel floor. These ages overlap dates of great earthquakes that are estimated to have occurred along the region north of Cape Mendocino. Paleoseismic events are proposed to be the triggering mechanisms for the turbidity currents that deposited the turbidites in Mendocino Channel.
With the increased use of swath surveying of the ocean floor, meandering (senso latto) channels have become common features found on the surface of many deep-sea fans.
Neurenteric cysts are rare spinal lesions of congenital origin. They usually present insidiously with a long history of local spinal pain, radiculopathy and myelopathy. We report a 14-year-old male with a high cervical neurenteric cyst who developed a progressive myelopathy after minor neck trauma. Full recovery followed a partial cyst excision and decompressive procedure.
Significance and conclusion:
The possible pathogenic mechanisms for this unusual presentation include hemorrhage into the cyst, sudden mechanical compression from abnormal spinal movement of a chronically distorted and compressed spinal cord, or an increase in the size of the cyst secondary to accumulation of cyst fluid. In this case a small increase in the cyst size may have resulted in increased mechanical distortion and spinal cord dysfunction on a compressive and ischemic basis.
The neonatal cat model of kaolin-induced hydrocephalus is associated with progressive and severe ventriculomegaly. In this experiment we studied the evolution of the histopathological changes in hydrocephalic (n = 23) cats from 5–168 days after the induction of hydrocephalus along with age-matched controls (n = 10). In the periventricular white matter, extracellular edema and axonal damage were present within days of the onset of hydrocephalus. This was followed by reactive gliosis, white matter atrophy, and in some animals gross cavitation of the white matter. Even in the chronic, apparently compensated state there was ongoing glial cell death. Six cats were shunted an average of 23.6 ± 6.5 days after the induction of hydrocephalus because they were no longer able to feed independently. In spite of clinical improvement the white matter changes persisted. Overt cortical changes were minimal except where areas of white matter destruction encroached upon the deep layers. The white matter changes are very similar to those seen in periventricular leukomalacia and suggest that ischemia plays a role in neonatal brain injury caused by hydrocephalus.
Despite the great capacity for the pediatric brain to recover from stroke, the morbidity and mortality in children who harbor an arteriovenous malformation (AVM) remains high. This study examines the clinical data and management experience with 132 patients with brain AVM from 1949 to 1989. Although the high tendency for a childhood AVM to present with hemorrhage (79%) remained constant for the forty year study period, the associated morbidity and mortality of hemorrhage changed. The mortality rate from hemorrhage for the entire series was 25%, which was reduced from 39% to 16% after the introduction of computed tomography. The mortality from AVM hemorrhage since 1975 was dependent on location; 8 of 14 patients (57%) with a cerebellar AVM died from hemorrhage while only 2 of 44 patients (4.5%) with a cerebral hemisphere AVM died (p < 0.0001). Sixteen children (12%) presented with a chronic seizure disorder. Surgical excision of the malformation resulted in complete seizure control off anti-convulsant medication in 73% of patients. Although 21% of patients were treated non-operatively (many with terminal poor-grade hemorrhage), 79% had a surgical procedure with total AVM excision achieved in 70 patients (53.1%). Complete AVM resection was followed by a normal neurological outcome in 47 children (67%). Most partial excisions (n=9) and clipping of feeding arteries (n=7) were performed in the early years of this study, and did not provide protection from rehe-morrhage. Although conservative management has been advocated for selected non-hemorrhagic AVMs, we conclude that essentially all children with an AVM should be treated in order to eliminate the risk of hemorrhage. Long-term conservative management in pediatric patients is warranted only in patients with large AVMs not amenable to treatment using current multimodality techniques.