Recent work has suggested that diffusion-weighted functional magnetic resonance imaging (FMRI) with strong diffusion weighting (high b value) detects neuronal swelling that is directly related to neuronal firing. This would constitute a much more direct measure of brain activity than current methods and represent a major advance in neuroimaging. However, it has not been firmly established that the observed signal changes do not reflect residual vascular effects, which are known to exist at low b value. This study measures the vascular component of diffusion FMRI directly by using hypercapnia, which induces blood flow changes in the absence of a change in neuronal firing. Hypercapnia elicits a similar diffusion FMRI response to a visual stimulus including a rise in percent signal change with increasing b value, which was reported for visual activation. Analysis of the response timing found no evidence for an early response at high b value, which has been reported as evidence for a nonhemodynamic response. These results suggest that a large component of the diffusion FMRI signal at high b value is vascular rather than neuronal.brain activation ͉ diffusion MRI ͉ functional MRI ͉ neuronal swelling F unctional neuroimaging has enabled major advances in the study of normal and pathological brain function. However, the methods that provide the greatest coverage and spatial resolution, including positron emission tomography (1, 2) and magnetic resonance imaging (MRI) (3, 4), are indirect measures of neuronal activity based on metabolically driven changes in blood flow. These hemodynamic measures suffer from spatial and temporal confounds (5, 6), are nonlinearly related to neuronal firing (7,8), and depend on baseline hemodynamics that are uncoupled from the activity of interest (9, 10). An imaging method that detects neuronal activity more directly while achieving whole-brain coverage would therefore represent a significant advance for neuroscience.One alternative method is diffusion-weighted functional MRI (DFMRI), which attenuates the MRI signal in a manner that depends on the amount of motion (diffusion and flow) in the underlying tissue. In general, this attenuation is described by a factor exp(ϪbD), where D is the apparent diffusion coefficient of the local tissue, and b is an acquisition parameter that describes the strength of diffusion contrast. At low b value, true diffusive motion is more difficult to detect, and the ''apparent diffusion'' is dominated by local blood flow (11)(12)(13)(14). In general, diffusion weighting is directional, so that attenuation depends on the diffusion and flow parameters along the applied direction. Different signal sources have been proposed in DFMRI depending on the b value. Early work used low b value, which is thought to reflect tissue perfusion (11-16). Recent work suggested that DFMRI at high b value detects cellular swelling that is a direct consequence of neural firing (17, 18), which would constitute a more direct measure of neuronal activity.A recent study at high b valu...
SK Fung, H Dick, H Devlin, E Tullis. Transmissibility and infection control implications of OBJECTIVE:To describe the microbiology and potential virulence factors of Burkholderia cepacia; to discuss the studies that have investigated its mode of transmission among cystic fibrosis patients; and to identify the major risk factors associated with acquisition of this pathogen inside and outside of the hospital environment. DATA SOURCES: MEDLINE search of the literature published between 1986 and 1997 using the key words/subject words Pseudomonas cepacia, Burkholderia cepacia, cystic fibrosis, infection control and transmissibility, and the bibliography of selected papers. DATA EXTRACTION: Selected studies examining epidemiology, microbiology, virulence factors and mode of transmission of B cepacia in cystic fibrosis. DATA SYNTHESIS AND CONCLUSIONS: B cepacia is a multidrug-resistant Gram-negative bacillus that has recently been recognized as a major respiratory pathogen in patients with cystic fibrosis. Colonization by this organism can lead to rapid pulmonary deterioration and premature death. Recent studies based on genomic subtyping techniques have suggested that it can be transmitted from person to person. Close social contact and hospitalization have been identified as risk factors for cross-infection. With the implementation of strict infection control policies such as segregation according to colonization status, the rate of new colonization has substantially decreased in most cystic fibrosis treatment centres. Key Words: Burkholderia cepacia, Cystic fibrosis, Infection control, Pseudomonas cepacia, Pulmonary infection, TransmissibilityTransmissibilité et implications du contrôle de l'infection à Burkholderia cepacia dans la fibrose kystique OBJECTIF : Décrire la microbiologie et les facteurs potentiels de virulence de Burkholderia cepacia ; examiner les études portant sur son mode de transmission parmi les patients atteints de fibrose kystique ; et identifier les facteurs de risque majeur associés à une contamination par ce pathogène en milieu hospitalier ou extrahospitalier. SOURCES DES DONNÉES : Recherche dans Medline des articles publiés entre 1986 et 1997 en utilisant les mots clés/mots sujets Pseudomonas cepacia, Burkholderia cepacia, fibrose kystique, contrôle de l'infection et transmissibilité, et la bibliographie d'articles choisis. EXTRACTION DES DONNÉES : Études choisies examinant l'épidémiologie, la microbiologie, les facteurs de virulence et le mode de transmission de B cepacia dans la fibrose kystique. SYNTHÈSE DES DONNÉES ET CONCLUSIONS : B cepacia est un bacille Gram négatif résistant à l'action de plusieurs médicaments et que l'on a récemment identifié comme un des plus importants pathogènes respiratoires chez les patients voir page suivante C ystic fibrosis (CF) is the most common lethal genetic disease affecting Caucasians (1). The most frequent mutation that gives rise to this autosomal recessive condition involves the deletion of a phenylalanine residue at position 508 of the gene encodin...
Brucella melitensis was identified in an aspirate obtained from a patient’s hip joint during a procedure at a hospital in Canada. We conducted an investigation into possible exposures among hospital workers; 1 worker who assisted with the procedure tested positive for B. melitensis. Aerosol-generating procedures performed outside the laboratory may facilitate transmission of this bacterium.
Campylobacterfetus subsp. fetus was isolated from the stools of two homosexual males. One was asymptomatic at the time of isolation. The other presented with diarrhea. Both isolates were initially grown at 42°C. This organism should be included among the list of organisms that are found in homosexual males.
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