Keith S Cover

VU University Medical Center, Amsterdamo, North Holland, Netherlands

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Publications (26)95.81 Total impact

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    ABSTRACT: The back-to-back (BTB) acquisition of MP-RAGE MRI scans of the Alzheimer's Disease Neuroimaging Initiative (ADNI1) provides an excellent data set with which to check the reproducibility of brain atrophy measures. As part of ADNI1, 131 subjects received BTB MP-RAGEs at multiple time points and two field strengths of 3T and 1.5T. As a result, high quality data from 200 subject-visit-pairs was available to compare the reproducibility of brain atrophies measured with FSL/SIENA over 12 to 18 month intervals at both 3T and 1.5T. Although several publications have reported on the differing performance of brain atrophy measures at 3T and 1.5T, no formal comparison of reproducibility has been published to date. Another goal was to check whether tuning the SIENA options, including -B, -S, -R and the fractional intensity threshold (f) had a significant impact on the reproducibility. The BTB reproducibility for SIENA was quantified by the 50th percentile of the absolute value of the difference in the percentage brain volume change (PBVC) for the BTB MP-RAGES. At both 3T and 1.5T the SIENA option combination of “-B f=0.2”, which is different form the default values of f=0.5, yielded the best reproducibility as measured by the 50th percentile yielding 0.28 (0.23–0.39)% and 0.26 (0.20–0.32)%. These results demonstrated that in general 3T had no advantage over 1.5T for the whole brain atrophy measure – at least for SIENA. While 3T MRI is superior to 1.5T for many types of measurements, and thus worth the additional cost, brain atrophy measurement does not seem to be one of them.
    Psychiatry Research Neuroimaging 01/2014; · 3.36 Impact Factor
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    ABSTRACT: Background To measure hippocampal volume change in Alzheimer's disease (AD) or mild cognitive impairment (MCI), expert manual delineation is often used because of its supposed accuracy. It has been suggested that expert outlining yields poorer reproducibility as compared to automated methods, but this has not been investigated. Aim To determine the reproducibilities of expert manual outlining and two common automated methods for measuring hippocampal atrophy rates in healthy aging, MCI and AD. Methods From the Alzheimer's Disease Neuroimaging Initiative (ADNI), 80 subjects were selected: 20 patients with AD, 40 patients with mild cognitive impairment (MCI) and 20 healthy controls (HCs). Left and right hippocampal volume change between baseline and month-12 visit was assessed by using expert manual delineation, and by the automated software packages FreeSurfer (longitudinal processing stream) and FIRST. To assess reproducibility of the measured hippocampal volume change, both back-to-back (BTB) MPRAGE scans available for each visit were analyzed. Hippocampal volume change was expressed in μL, and as a percentage of baseline volume. Reproducibility of the 1-year hippocampal volume change was estimated from the BTB measurements by using linear mixed model to calculate the limits of agreement (LoA) of each method, reflecting its measurement uncertainty. Using the delta method, approximate p-values were calculated for the pairwise comparisons between methods. Statistical analyses were performed both with inclusion and exclusion of visibly incorrect segmentations. Results Visibly incorrect automated segmentation in either one or both scans of a longitudinal scan pair occurred in 7.5% of the hippocampi for FreeSurfer and in 6.9% of the hippocampi for FIRST. After excluding these failed cases, reproducibility analysis for 1-year percentage volume change yielded LoA of ± 7.2% for FreeSurfer, ± 9.7% for expert manual delineation, and ± 10.0% for FIRST. Methods ranked the same for reproducibility of 1-year μL volume change, with LoA of ± 218 μL for FreeSurfer, ± 319 μL for expert manual delineation, and ± 333 μL for FIRST. Approximate p-values indicated that reproducibility was better for FreeSurfer than for manual or FIRST, and that manual and FIRST did not differ. Inclusion of failed automated segmentations led to worsening of reproducibility of both automated methods for 1-year raw and percentage volume change. Conclusion Quantitative reproducibility values of 1-year microliter and percentage hippocampal volume change were roughly similar between expert manual outlining, FIRST and FreeSurfer, but FreeSurfer reproducibility was statistically significantly superior to both manual outlining and FIRST after exclusion of failed segmentations.
    NeuroImage 01/2014; 92:169–181. · 6.25 Impact Factor
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    ABSTRACT: Purpose To compare the reproducibility of the hippocampal atrophy rates over one year generated by the fully automated FreeSurer/ReconAll and FSL/FIRST software packages using a back-to-back (BTB) reproducibility test based on the ADNI1 data set [1] . Conclusions Study funding was provided by neuGRID4you (N4U), an European Community FP7 project (grant agreement 283562), and the VU University Medical Center, Amsterdam, The Netherlands ADNI1 Back-to-Back MPRAGEs While rarely mentioned in the literature, as part of the first Alzheimer's Disease Neuroimaging Initiative (ADNI1) study, the 3D T1-weighted MRI scans (also know as MPRAGE scans) were acquired in duplicate during each patient visit -with the acquisition of the second MPRAGE starting within seconds of completion of the first. As ADNI has over 800 subjects -with an average of 6 visits each – spread over several years, roughly 9,000 back-to-back (BTB) MPRAGE were available to probe the performance of brain atrophy measures [1]. Table 1. The reproducibility of hippocampal atrophy rates for FreeSurfer/ReconAll and FSL/FIRST using back-to-back (BTB) ADNI1 MPRAGEs MRI scans . Units are percentage difference in the atrophy rate over 1 year. Smaller is better.
    Alzheimer's & dementia: the journal of the Alzheimer's Association 07/2013; 9:592-P592. · 14.48 Impact Factor
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    ABSTRACT: Background: In many retrospective studies and large clinical trials, high-resolution, good-contrast 3DT1 images are unavailable, hampering detailed analysis of brain atrophy. Ventricular enlargement then provides a sensitive indirect measure of ongoing central brain atrophy. Validated automated methods are required that can reliably measure ventricular enlargement and are robust across magnetic resonance (MR) image types. Aim: To validate the automated method VIENA for measuring the percentage ventricular volume change (PVVC) between two scans. Materials and Methods: Accuracy was assessed using four image types, acquired in 15 elderly patients (five with Alzheimer's disease, five with mild cognitive impairment, and five cognitively normal elderly) and 58 patients with multiple sclerosis (MS), by comparing PVVC values from VIENA to manual outlining. Precision was assessed from data with three imaging time points per MS patient, by measuring the difference between the direct (one-step) and indirect (two-step) measurement of ventricular volume change between the first and last time points. The stringent concordance correlation coefficient (CCC) was used to quantify absolute agreement. Results: CCC of VIENA with manual measurement was 0.84, indicating good absolute agreement. The median absolute difference between two-step and one-step measurement with VIENA was 1.01%, while CCC was 0.98. Neither initial ventricular volume nor ventricular volume change affected performance of the method. Discussion: VIENA has good accuracy and good precision across four image types. VIENA therefore provides a useful fully automated method for measuring ventricular volume change in large datasets. Conclusion: VIENA is a robust, accurate, and precise method for measuring ventricular volume change. Hum Brain Mapp, 2013. © 2013 Wiley Periodicals, Inc.
    Human Brain Mapping 01/2013; · 6.88 Impact Factor
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    ABSTRACT: Background Brain atrophy studies often use FSL-BET (Brain Extraction Tool) as the first step of image processing. Default BET does not always give satisfactory results on 3DT1 MR images, which negatively impacts atrophy measurements. Finding the right alternative BET settings can be a difficult and time-consuming task, which can introduce unwanted variability.AimTo systematically analyze the performance of BET in images of MS patients by varying its parameters and options combinations, and quantitatively comparing its results to a manual gold standard.Methods Images from 159 MS patients were selected from different MAGNIMS consortium centers, and 16 different 3DT1 acquisition protocols at 1.5 T or 3 T. Before running BET, one of three pre-processing pipelines was applied: (1) no pre-processing, (2) removal of neck slices, or (3) additional N3 inhomogeneity correction. Then BET was applied, systematically varying the fractional intensity threshold (the “f” parameter) and with either one of the main BET options (“B” — bias field correction and neck cleanup, “R” — robust brain center estimation, or “S” — eye and optic nerve cleanup) or none. For comparison, intracranial cavity masks were manually created for all image volumes. FSL-FAST (FMRIB's Automated Segmentation Tool) tissue-type segmentation was run on all BET output images and on the image volumes masked with the manual intracranial cavity masks (thus creating the gold-standard tissue masks). The resulting brain tissue masks were quantitatively compared to the gold standard using Dice overlap coefficient (DOC). Normalized brain volumes (NBV) were calculated with SIENAX. NBV values obtained using for SIENAX other BET settings than default were compared to gold standard NBV with the paired t-test.ResultsThe parameter/preprocessing/options combinations resulted in 20,988 BET runs. The median DOC for default BET (f = 0.5, g = 0) was 0.913 (range 0.321–0.977) across all 159 native scans. For all acquisition protocols, brain extraction was substantially improved for lower values of “f” than the default value. Using native images, optimum BET performance was observed for f = 0.2 with option “B”, giving median DOC = 0.979 (range 0.867–0.994). Using neck removal before BET, optimum BET performance was observed for f = 0.1 with option “B”, giving median DOC 0.983 (range 0.844–0.996). Using the above BET-options for SIENAX instead of default, the NBV values obtained from images after neck removal with f = 0.1 and option “B” did not differ statistically from NBV values obtained with gold-standard.Conclusion Although default BET performs reasonably well on most 3DT1 images of MS patients, the performance can be improved substantially. The removal of the neck slices, either externally or within BET, has a marked positive effect on the brain extraction quality. BET option “B” with f = 0.1 after removal of the neck slices seems to work best for all acquisition protocols.
    NeuroImage 07/2012; 61(4):1484–1494. · 6.25 Impact Factor
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    Keith S. Cover
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    ABSTRACT: Color intensity projections (CIPs) have been shown to improve the visualisation of greyscale angiography images by combining greyscale images into a single color image. A key property of the combined CIPs image is the encoding of the arrival time information from greyscale images into the hue of the color in the CIPs image. A few minor improvements to the calculation of the CIPs image are introduced that substantially improve the quality of the visualisation. One improvement is interpolating of the greyscale images in time before calculation of the CIPs image. A second is the use of hue cycling - where the hue of the color is cycled through more than once in an image. The hue cycling allows the variation of the hue to be concentrated in structures of interest. An angiogram of a brain is used to demonstrate the substantial improvements hue cycling brings to CIPs images. A third improvement is the use of maximum intensity projection for 2D rendering of a 3D CIPs image volume. Other potential applications of CIPs are also mentioned.
    06/2012;
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    Keith S. Cover
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    ABSTRACT: While fully automated methods for detecting faint moving objects in astronomical images - such as Kuiper belt objects (KBOs) - are constantly improving, visual detection still has a role to play especially when the fixed background is cluttered with stars. Color intensity projections (CIPs) using hue cycling - which combines a sequence of greyscale images into a single color image - aids in the visual detection of moving objects by highlighting them using color in an intuitive way. To demonstrate the usefulness of CIPs in detecting faint moving objects a sequence of 16 images from the SuprimeCam camera of the Subaru telescope were combined into a CIPs image. As well has making even faint moving objects easier to visually detect against a cluttered background, CCD artefacts were also more easily recognisable. The new Hyper SuprimeCam for the Subaru telescope - which will allow many short exposure images to be acquired with little dead time between images - should provide ideal data for use with the CIPs algorithm. In addition, the current search for KBOs to be targeted by the New Horizon's spacecraft after its flyby of Pluto provides an excellent test case for the state of the art in faint moving object detection against a cluttered background.
    02/2012;
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    ABSTRACT: SienaX and Siena are widely used and fully automated algorithms for measuring whole brain volume and volume change in cross-sectional and longitudinal MRI studies and are particularly useful in studies of brain atrophy. The reproducibility of the algorithms was assessed using the 3D T1 weighted MP-RAGE scans from the Alzheimer's Disease Neuroimaging Initiative (ADNI) study. The back-to-back (BTB) MP-RAGE scans in the ADNI data set makes it a valuable benchmark against which to assess the performance of algorithms of measuring atrophy in the human brain with MRI scans. A total of 671 subjects were included for SienaX and 385 subjects for Siena. The annual percentage brain volume change (PBVC) rates were -0.65±0.82%/year for the healthy controls, -1.15±1.21%/year for mild cognitively impairment (MCI) and -1.84±1.33%/year for AD, in line with previous findings. The median of the absolute value of the reproducibility of SienaX's normalized brain volume (NBV) was 0.96% while the 90th percentile was 5.11%. The reproducibility of Siena's PBVC had a median of 0.35% and a 90th percentile of 1.37%. While the median reproducibility for SienaX's NBV was in line with the values previously reported in the literature, the median reproducibility of Siena's PBVC was about twice that reported. Also, the 90th percentiles for both SienaX and Siena were about twice the size that would be expected for a Gaussian distribution. Because of the natural variation of the disease among patients over a year, a perfectly reproducible whole brain atrophy algorithm would reduce the estimated group size needed to detect a specified treatment effect by only 30% to 40% as compared to Siena's.
    Psychiatry Research 09/2011; 193(3):182-90. · 2.68 Impact Factor
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    ABSTRACT: Use of planning 4-dimensional CT (4DCT) scans often permits use of smaller target volumes for thoracic tumors but this assumes a reproducible pattern of motion during radiotherapy. We compared cranio-caudal (CC) motion on MV cine-images acquired during treatment with that seen on planning 4DCT. A pre-programmable respiratory motion phantom and a software tool for motion assessment were used to validate the use of MV cine-images for motion detection. MV cine-images acquired in 20 patients with node-positive lung cancer were analyzed using the same software. Intra-fraction CC motion on 6 MV cine-images from each patient was compared with CC motion on their planning 4DCT. Software-based motion measurement on MV cine-images from the phantom corresponded to actual motion. Mean CC motion of primary tumor, carina and hilus on 4DCT was 7.3mm (range 2-13.8mm), 6.8mm (1.8-21.2) and 11.0mm (4.2-15.1), respectively. Corresponding intra-fraction motion on MV cine was 4.1mm (0.6-13.6mm); 2.7mm (0-10mm) and 6.0mm (1.8-14.4mm), respectively. The tumor, hilus and carina could be tracked in 95%, 88% and 38% of the MV cine-images, respectively. Intra-fraction motion can be reliably measured using MV-cine images from a phantom. Motion discrepancies identified on MV cine-images can identify patients in whom planning 4DCT scans are not representative.
    Radiotherapy and Oncology 05/2011; 99(2):155-60. · 4.52 Impact Factor
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    ABSTRACT: We investigated progression of atrophy in vivo, in Alzheimer’s disease (AD), and mild cognitive impairment (MCI). We included 64 patients with AD, 44 with MCI and 34 controls with serial MRI examinations (interval 1.8 ± 0.7years). A nonlinear registration algorithm (fluid) was used to calculate atrophy rates in six regions: frontal, medial temporal, temporal (extramedial), parietal, occipital lobes and insular cortex. In MCI, the highest atrophy rate was observed in the medial temporal lobe, comparable with AD. AD patients showed even higher atrophy rates in the extramedial temporal lobe. Additionally, atrophy rates in frontal, parietal and occipital lobes were increased. Cox proportional hazard models showed that all regional atrophy rates predicted conversion to AD. Hazard ratios varied between 2.6 (95% confidence interval (CI) = 1.1–6.2) for occipital atrophy and 15.8 (95% CI = 3.5–71.8) for medial temporal lobe atrophy. In conclusion, atrophy spreads through the brain with development of AD. MCI is marked by temporal lobe atrophy. In AD, atrophy rate in the extramedial temporal lobe was even higher. Moreover, atrophy rates also accelerated in parietal, frontal, insular and occipital lobes. Finally, in nondemented elderly, medial temporal lobe atrophy was most predictive of progression to AD, demonstrating the involvement of this region in the development of AD.
    European Radiology 12/2009; 19(12):2826-2833. · 4.34 Impact Factor
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    Keith S. Cover
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    ABSTRACT: The recent release of the first light sky map of the cosmic microwave background (CMB) from the Planck satellite provides an initial opportunity for comparison with the WMAP and COBE sky maps and their reconstruction algorithms. The precision of the match between Planck's and WMAP's anisotropies below several degrees in size, which corresponds to spherical harmonics with high l, provides confidence that the differences between the anisotropies at low l are substantial. If the Planck first light sky map is taken as the gold standard, the results seem to suggest the low l components of the WMAP map and a considerable part of the COBE sky map have a similar reconstruction artefact. As the Planck first light sky map covers only about 10% of the sky, any conclusions drawn from this comparison are speculative but deserving of further investigation. Comment: 10 pages and 3 figures in manuscript - added some references and improved the discussion
    10/2009;
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    ABSTRACT: Diffusion tensor imaging (DTI) measures have shown to be sensitive to white matter (WM) damage in multiple sclerosis (MS), not only inside focal lesions but also in user-defined regions in the so-called normal-appearing white matter (NAWM). New analysis techniques for DTI measures are now available that allow for hypothesis-free localization of damage. We performed DTI measurements of 30 MS patients selected for low focal lesion loads, and of 31 age-matched healthy controls and analyzed these using tract-based spatial statistics (TBSS). Patients were found to have a lower fractional anisotropy (FA) compared to controls in a number of brain regions, including the fornices, the left corona radiata, the inferior longitudinal fasciculus in both hemispheres, both optic radiations, and parts of the corpus callosum. In the regions of reduced FA, an increase in radial diffusivity and a less pronounced increase of axial diffusivity were found. Neurocognitive assessment showed that patients had normal visuospatial memory performance, just-normal attention, and impaired processing speed; the latter was associated with abnormal FA in the corpus callosum, an area which was relatively devoid of lesions visible on proton density-weighted images in our patients. TBSS can be useful in future studies with other MS patient samples to provide an unbiased localization of damage and generate location-specific hypotheses.
    NeuroImage 12/2008; 44(4):1397-403. · 6.25 Impact Factor
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    Keith S Cover
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    ABSTRACT: The concept of rejecting the null hypothesis for definitively detecting a signal was extended to relaxation spectrum space for multiexponential reconstruction. The novel test was applied to the problem of detecting the myelin signal, which is believed to have a time constant below 40 ms, in T2 decays from magnetic resonance imagining of the human brain. It was demonstrated that the test allowed the detection of a signal in a relaxation spectrum by using only the information in the data, thus avoiding any potentially unreliable prior information. The test was implemented both explicitly and implicitly for simulated T2 measurements. For the explicit implementation, the null hypothesis was that a relaxation spectrum existed that had no signal below 40 ms and that was consistent with the T2 decay. The confidence level by which the null hypothesis could be rejected gave the confidence level that there was signal below the 40 ms time constant. The explicit implementation assessed the test's performance with and without prior information where the prior information was the non-negative relaxation spectrum assumption. The test was also implemented implicitly with a data conserving multiexponential reconstruction algorithm that used left invertible matrices and that has been published previously. The implicit and explicit implementations demonstrated similar characteristics in detecting the myelin signal in both the simulated and experimental T2 decays, providing additional evidence to support the close link between the two tests. When the relaxation spectrum was assumed to be non-negative, the novel test required signal to noise ratios (SNRs) approaching 1000 in the T2 decays for detection of the myelin signal with high confidence. When the relaxation spectrum was not assumed to be non-negative, the SNR requirements for a detection with high confidence increased by a factor of 25. The application of the test to a T2 decay from human white matter, measured in vivo with a SNR of 650, demonstrated a solid detection of the signal below 40 ms believed to be due to the myelin water. This study demonstrated the robustness and reliability of extending the concept of rejecting the null hypothesis to relaxation spectrum space. The study also raised serious questions about the susceptibility to false positive detection of the myelin signal of the multiexponential reconstruction algorithms currently in use.
    Review of Scientific Instruments 06/2008; 79(5):055106. · 1.60 Impact Factor
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    Keith S Cover
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    ABSTRACT: To assess its usefulness, the peak version of color intensity projections (CIPs) was used to display a summary of the grayscale images composing a renogram as a single color image. Method For each pixel in a renogram, the time point with the maximum intensity was used to control the hue of the color of the corresponding pixel in the CIPs image. The hue ranged over red-yellow-green-light blue-blue with red representing the earliest time. Results For subjects with normal appearing kidneys, the injection site shows up in red, the kidneys in a red-yellow and the bladder in a green-blue. A late fill kidney typically appeared greener or bluer than a normal kidney indicating it reached its peak intensity at a later time point than normal. Conclusions Having the time and intensity information summarized in a single image promises to speed up the initial impression of patients by less experienced interpreters and should also provide a valuable training tool.
    05/2008;
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    Keith S Cover
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    ABSTRACT: While the multiexponential nature of T2 decays measured in vivo is well known, characterizing T2 decays by a single time constant is still very useful when differentiating among structures and pathologies in MRI images. A novel, robust, fast and very simple method is presented for both estimating and displaying the average time constant for the T2 decay of each pixel from a multiecho MRI sequence. The average time constant is calculated from the average of the values measured from the T2 decay over many echoes. For a monoexponential decay, the normalized decay average varies monotonically with the time constant. Therefore, it is simple to map any normalized decay average to an average time constant. This method takes advantage of the robustness of the normalized decay average to both artifacts and multiexponential decays. Color intensity projections (CIPs) were used to display 32 echoes acquired at a 10ms spacing as a single color image. The brightness of each pixel in each color image was determined by the intensity of the corresponding pixel in the earliest image and the hue was determined by the normalized decay average. Examples demonstrate the effectiveness of using CIPs to display the results of a multiecho sequence for a healthy subject and a multiple sclerosis patient.
    04/2008;
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    ABSTRACT: Dynamic susceptibility-weighted contrast-enhanced (DSC) MRI or perfusion-MRI plays an important role in the non-invasive assessment of tumor vascularity. However, the large number of images provided by the method makes display and interpretation of the results challenging. Current practice is to display the perfusion information as relative cerebral blood volume maps (rCBV). Color intensity projections (CIPs) provides a simple, intuitive display of the perfusion-MRI data so that regional perfusion characteristics are intrinsically integrated into the anatomy structure the T2 images. The ease of use and quick calculation time of CIPs should allow it to be easily integrated into current analysis and interpretation pipelines.
    12/2007;
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    ABSTRACT: Reliable and rapid delineation of arteriovenous malformations enables the application of effective treatments such as stereotactic radiosurgery. We describe a new method to improve the speed and reliability of visualizing the flow of contrast images with digital subtraction angiography. In line with current practices, digital subtraction angiography was used to produce a sequence of grayscale images. The new method combines the standard grayscale images produced by digital subtraction angiography into a single composite color image that encodes the contrast arrival time at each point of the brain's circulatory system. The algorithm is simple, fast, and easy to implement. The technique allows the flow of contrast from a series of angiography images to be summarized in a single color image. This visualization method promises to improve the speed of manual delineation of arteriovenous malformations. Further studies are required to evaluate the clinical value of the use of color intensity projection images, supplemented by grayscale images as necessary, in comparison with contouring on grayscale images only.
    Neurosurgery 04/2007; 60(3):511-4; discussion 514-5. · 2.53 Impact Factor
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    ABSTRACT: To detect changes in repeated astronomical images of the same field of view (FOV), a common practice is to stroboscopically switch between the images. Using this method, objects that are changing in location or intensity between images are easier to see because they are constantly changing. A novel display method, called arrival time color intensity projections (CIPs), is presented that combines any number of grayscale images into a single color image on a pixel by pixel basis. Any values that are unchanged over the grayscale images look the same in the color image. However, pixels that change over the grayscale image have a color saturation that increases with the amount of change and a hue that corresponds to the timing of the changes. Thus objects moving in the grayscale images change from red to green to blue as they move across the color image. Consequently, moving objects are easier to detect and assess on the color image than on the grayscale images. A sequence of images of a comet plunging into the sun taken by the SOHO satellite (NASA/ESA) and Hubble Space Telescope images of a trans-Neptunian object (TNO) are used to demonstrate the method. Comment: 9 pages, 2 figures. Accepted for publication in Publications of the Astronomical Society of the Pacific. The quality of figure 1 been improved from the previous posted version
    Publications of the Astronomical Society of the Pacific 03/2007; · 3.69 Impact Factor
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    ABSTRACT: 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. 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. 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. 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. These neurophysiological changes may contribute to the cognitive alterations observed in patients with brain tumours.
    Clinical Neurophysiology 10/2006; 117(9):2039-49. · 3.14 Impact Factor
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    Keith S Cover
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    ABSTRACT: The COBE time ordered data (TOD) was reanalysed for anisotropies in the cosmic microwave background with a novel technique that is much simpler and more robust than used in the official analysis of the COBE TOD. The technique extends the statistical concept of rejecting the null hypothesis to image reconstruction by asking if the TOD is consistent with no anisotropies outside the galactic band. If it is assumed that the instrumentation noise is ideal, as appears to be the case in the official analysis, the null hypothesis can be rejected to 28 sigma, a highly significant detection. However, the official analysis presents no information supporting the ideal noise assumption to the level required to reject the null hypothesis. The interaction of COBE's calibration error of roughly 3.0% with the brightest regions of the galactic band in combination with the reconstruction algorithm may have introduced artefacts roughly of the size and spectral signature of the 10ppm anisotropies reported by COBE. Data from follow up missions have similar problems with calibration errors that may have resulted in similar sky maps to COBE. It is pointed out that either a few hours of repetitive circular scans using the WMAP satellite or the single beam antenna of the soon-to-be-launched Planck satellite should provide definitive evidence as to the existence or non existence of the 10ppm anisotropies without the use of image reconstruction.
    08/2006;

Publication Stats

421 Citations
95.81 Total Impact Points

Institutions

  • 2004–2014
    • VU University Medical Center
      • • Department of Physics and Medical Technology (FMT)
      • • Department of Radiation Oncology
      Amsterdamo, North Holland, Netherlands
  • 2008
    • University of British Columbia - Vancouver
      • Department of Physics and Astronomy
      Vancouver, British Columbia, Canada