Biomedical subjects

Keith S Cover

Publications and source records attributed to Keith S Cover.

4 recordsLinked to original sources

PubMed2006

Disturbed functional connectivity in brain tumour patients: evaluation by graph analysis of synchronization matrices.

OBJECTIVE: Cerebral functions are based on the functional interactions between multiple distinct specialized regions of the brain. Functional interactions require anatomical connections as well as the synchronization of brain oscillations. The present work aims at evaluating the impact of brain tumours on spatial patterns of functional connectivity of the brain measured at rest by MEG. METHODS: We analyzed the statistical dependency (by computing the synchronization likelihood (SL, a measure of generalized synchronization)) between MEG signals at rest, in 17 patients with a brain tumour and in 15 healthy controls. Following an approach that derives from graph theory, we also analyzed the architectural properties of the networks by computing two parameters from the SL matrix, the cluster coefficient C and the characteristic path length L. RESULTS: Alterations in synchronization levels were found in the patients and were not focal but involved intra-hemispheric connectivity. Effects were different considering the frequencies sub-bands, predominating in a decrease in high frequencies bands for long-distance connections and an increase in slower bands for local connectivity. In addition, graph analysis reveals changes in the normal "small-world" network architecture in addition to changes in synchronization levels with some differences according to the studied frequency sub-bands. CONCLUSIONS: Brain tumours alter the functional connectivity and the "network" architecture of the brain. These alterations are not focal and effects are different considering the frequencies sub-bands. SIGNIFICANCE: These neurophysiological changes may contribute to the cognitive alterations observed in patients with brain tumours.

Adult↗

PubMed2006

Color intensity projections: a rapid approach for evaluating four-dimensional CT scans in treatment planning.

PURPOSE: Four-dimensional computerized tomography scans (4DCT) enable intrafractional motion to be determined. Because more than 1500 images can be generated with each 4DCT study, tools for efficient data visualization and evaluation are needed. We describe the use of color intensity projections (CIP) for visualizing mobility. METHODS: Four-dimensional computerized tomography images of each patient slice were combined into a CIP composite image. Pixels largely unchanged over the component images appear unchanged in the CIP image. However, pixels whose intensity changes over the phases of the 4DCT appear in the CIP image as colored pixels, and the hue encodes the percentage of time the tissue was in each location. CIPs of 18 patients were used to study tumor and surrogate markers, namely the diaphragm and an abdominal marker block. RESULTS: Color intensity projections permitted mobility of high-contrast features to be quickly visualized and measured. In three selected expiratory phases ("gating phases") that were reviewed in the sagittal plane, gating would have reduced mean tumor mobility from 6.3 +/- 2.0 mm to 1.4 +/- 0.5 mm. Residual tumor mobility in gating phases better correlated with residual mobility of the marker block than that of the diaphragm. CONCLUSION: CIPs permit immediate visualization of mobility in 4DCT images and simplify the selection of appropriate surrogates for gated radiotherapy.

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PubMed2006

How do brain tumors alter functional connectivity? A magnetoencephalography study.

OBJECTIVE: This study was undertaken to test the hypothesis that brain tumors interfere with normal brain function by disrupting functional connectivity of brain networks. METHODS: Functional connectivity was assessed by computing the synchronization likelihood in a broad band (0.5-60Hz) or in the gamma band (30-60Hz) between all pairwise combinations of magnetoencephalography signals. Magnetoencephalography recordings were made at rest in 17 brain tumor patients and 15 healthy control subjects. For a given threshold of synchronization likelihood values, graphs of the suprathreshold connections between each magnetoencephalography channel and the others channels were built. RESULTS: In some regions, a variable number of channels without connectivity (missing connective points) at this threshold was found. The number of missing connective points was higher in patients with brain tumors than in control subjects (p < 0.0001, broad and gamma band) and was higher for left-sided than right-sided tumors (p = 0.008, broad band; p < 0.0001, gamma band). Individual results analysis indicates that the majority of brain tumor patients display several regions with missing connective point alterations in the affected and in the contralateral hemisphere. INTERPRETATION: Our findings suggest that brain tumors induce a loss of functional connectivity that affects multiple brain regions, and that left side brain tumors have the more severe consequences in this respect.

Adult↗

PubMed2005

Multiple sclerosis patients show a highly significant decrease in alpha band interhemispheric synchronization measured using MEG.

MEG data were acquired from a group of relapsing-remitting multiple sclerosis (MS) patients and a group of healthy controls, using an eyes-closed no-task condition. An interhemispheric coherence measure (IHCM), reflecting the synchronization between the left and right hemispheres, showed a decrease in the patients, particularly in the alpha band. No comparable differences were seen in the alpha band power or its distribution over the head. The observed difference is in agreement with a reduced long-range connectivity in the brains of MS patients. The IHCM was found to be reproducible in controls over a period of more than 15 months. Further studies should investigate whether MEG derived synchronization measures may be useful as markers for MS disease load.

Adult↗