Biomedical subjects

Reto Meuli

Publications and source records attributed to Reto Meuli.

At least 19 recordsLinked to original sources

PubMed2006

Human auditory belt areas specialized in sound recognition: a functional magnetic resonance imaging study.

The human primary auditory cortex is surrounded by at least six other, anatomically distinct areas that process auditory information. We have investigated their specialization with respect to sound recognition or sound localization with triple epoch functional magnetic resonance imaging paradigm (recognition-localization-rest) in 18 normal individuals. In each study participant, the pattern of selective activation by the recognition or by the localization tasks was superimposed on the map of the nonprimary auditory areas, as identified in previous anatomical studies. Two areas, anterior lateral and anterior areas, were activated bilaterally in significantly more individuals by the recognition than by the localization task. They are proposed to be human homologues of macaque anterolateral auditory belt area.

Adult↗

PubMed2006

Fibertract segmentation in position orientation space from high angular resolution diffusion MRI.

In diffusion MRI, standard approaches for fibertract identification are based on algorithms that generate lines of coherent diffusion, currently known as tractography. A tract is then identified as a set of such lines selected on some criteria. In the present study, we investigate whether fibertract identification can be formulated as a segmentation task that recognizes a fibertract as a region where diffusion is intense and coherent. Indeed, we show that it is possible to segment efficiently well-known fibertracts with classical image processing methods provided that the problem is formulated in a five-dimensional space of position and orientation. As an example, we choose to adapt to this newly defined high-dimensional non-Euclidean space, called position orientation space, an algorithm based on the hidden Markov random field framework. Structures such as the cerebellar peduncles, corticospinal tract, association bundles can be identified and represented in three dimensions by a back projection technique similar to maximum intensity projection. Potential advantages and drawbacks as compared to classical tractography are discussed; for example, it appears that our formulation handles naturally crossing tracts and is not biased by human intervention.

Algorithms↗

PubMed2006

Extinction of auditory stimuli in hemineglect: Space versus ear.

Unilateral extinction of auditory stimuli, a key feature of the neglect syndrome, was investigated in 15 patients with right (11), left (3) or bilateral (1) hemispheric lesions using a verbal dichotic condition, in which each ear received simultaneously one word, and a interaural-time-difference (ITD) diotic condition, in which both ears received both words lateralised by means of ITD. Additional investigations included sound localisation, visuo-spatial attention and general cognitive status. Five patients presented a significant asymmetry in the ITD diotic test, due to a decrease of left hemispace reporting but no asymmetry was found in dichotic listening. Six other patients presented a significant asymmetry in the dichotic test due to a significant decrease of left or right ear reporting, but no asymmetry in diotic listening. Ten of the above patients presented mild to severe deficits in sound localisation and eight signs of visuo-spatial neglect (three with selective asymmetry in the diotic and five in the dichotic task). Four other patients presented a significant asymmetry in both the diotic and dichotic listening tasks. Three of them presented moderate deficits in localisation and all four moderate visuo-spatial neglect. Thus, extinction for left ear and left hemispace can double dissociate, suggesting distinct underlying neural processes. Furthermore, the co-occurrence with sound localisation disturbance and with visuo-spatial hemineglect speaks in favour of the involvement of multisensory attentional representations.

Acoustic Stimulation↗

PubMed2006

Interhemispheric integration at different spatial scales: the evidence from EEG coherence and FMRI.

The early visual system processes different spatial frequencies (SFs) separately. To examine where in the brain the scale-specific information is integrated, we mapped the neural assemblies engaged in interhemispheric coupling with electroencephalographic (EEG) coherence and blood-oxygen-level dependent (BOLD) signal. During similar EEG and functional magnetic resonance imaging (fMRI) experiments, our subjects viewed centrally presented bilateral gratings of different SF (0.25-8.0 cpd), which either obeyed Gestalt grouping rules (iso-oriented, IG) or violated them (orthogonally oriented, OG). The IG stimuli (0.5-4.0 cpd) synchronized EEG at discrete beta frequencies (beta1, beta2) and increased BOLD (0.5 and 2.0 cpd tested) in ventral (around collateral sulcus) and dorsal (parieto-occipital fissure) regions compared with OG. At both SF, the beta1 coherence correlated with the ventral activations, whereas the beta2 coherence correlated with the dorsal ones. Thus distributed neural substrates mediated interhemispheric integration at single SF. The relative impact of the ventral versus dorsal networks was modulated by the SF of the stimulus.

Adult↗

PubMed2006

Perfusion-CT assessment of infarct core and penumbra: receiver operating characteristic curve analysis in 130 patients suspected of acute hemispheric stroke.

BACKGROUND AND PURPOSE: Different definitions have been proposed to define the ischemic penumbra from perfusion-CT (PCT) data, based on parameters and thresholds tested only in small pilot studies. The purpose of this study was to perform a systematic evaluation of all PCT parameters (cerebral blood flow, volume [CBV], mean transit time [MTT], time-to-peak) in a large series of acute stroke patients, to determine which (combination of) parameters most accurately predicts infarct and penumbra. METHODS: One hundred and thirty patients with symptoms suggesting hemispheric stroke < or =12 hours from onset were enrolled in a prospective multicenter trial. They all underwent admission PCT and follow-up diffusion-weighted imaging/fluid-attenuated inversion recovery (DWI/FLAIR); 25 patients also underwent admission DWI/FLAIR. PCT maps were assessed for absolute and relative reduced CBV, reduced cerebral blood flow, increased MTT, and increased time-to-peak. Receiver-operating characteristic curve analysis was performed to determine the most accurate PCT parameter, and the optimal threshold for each parameter, using DWI/FLAIR as the gold standard. RESULTS: The PCT parameter that most accurately describes the tissue at risk of infarction in case of persistent arterial occlusion is the relative MTT (area under the curve=0.962), with an optimal threshold of 145%. The PCT parameter that most accurately describes the infarct core on admission is the absolute CBV (area under the curve=0.927), with an optimal threshold at 2.0 ml x 100 g(-1). CONCLUSIONS: In a large series of 130 patients, the optimal approach to define the infarct and the penumbra is a combined approach using 2 PCT parameters: relative MTT and absolute CBV, with dedicated thresholds.

Adult↗

PubMed2006

Hand preference and sex shape the architecture of language networks.

In right-handed subjects, language processing relies predominantly on left hemisphere networks, more so in men than in women, and in right- versus left-handers. Using DT-MRI tractography, we have shown that right-handed men are massively interconnected between the left-hemisphere language areas, whereas the homologous in the right hemisphere are sparse; interhemispheric connections between the language areas and their contralateral homologues are relatively strong. Women and left-handed men have equally strong intrahemispheric connections in both hemispheres, but women have a higher density of interhemispheric connections.

Adult↗

PubMed2006

X-ray phase contrast imaging of metallized and non-metallized biological samples.

Imaging of biological samples has been performed with a variety of techniques for example electromagnetic waves, electrons, neutrons, ultrasound and X-rays. Also conventional X-ray imaging represents the basis of medical diagnostic imaging, it remains of limited use in this application because it is based solely on the differential absorption of X-rays by tissues. Coherent and bright photon beams, such as those produced by third-generation synchrotron X-ray sources, provide further information on subtle X-ray phase changes at matter interfaces. This complements conventional X-ray absorption by edge enhancement phenomena. Thus, phase contrast imaging has the potential to improve the detection of structures on images by detecting those structures that are invisible with X-ray absorption imaging. Images of a weakly absorbing nylon fibre were recorded in in-line holography geometry using a high resolution low-noise CCD camera at the ESRF in Grenoble. The method was also applied to improve image contrast for images of biological tissues. This paper presents phase contrast microradiographs of vascular tree casts and images of a housefly. These reveal very fine structures, that remain invisible with conventional absorption contrast only.

Animals↗

PubMed2006

A cross validation study of deep brain stimulation targeting: from experts to atlas-based, segmentation-based and automatic registration algorithms.

Validation of image registration algorithms is a difficult task and open-ended problem, usually application-dependent. In this paper, we focus on deep brain stimulation (DBS) targeting for the treatment of movement disorders like Parkinson's disease and essential tremor. DBS involves implantation of an electrode deep inside the brain to electrically stimulate specific areas shutting down the disease's symptoms. The subthalamic nucleus (STN) has turned out to be the optimal target for this kind of surgery. Unfortunately, the STN is in general not clearly distinguishable in common medical imaging modalities. Usual techniques to infer its location are the use of anatomical atlases and visible surrounding landmarks. Surgeons have to adjust the electrode intraoperatively using electrophysiological recordings and macrostimulation tests. We constructed a ground truth derived from specific patients whose STNs are clearly visible on magnetic resonance (MR) T2-weighted images. A patient is chosen as atlas both for the right and left sides. Then, by registering each patient with the atlas using different methods, several estimations of the STN location are obtained. Two studies are driven using our proposed validation scheme. First, a comparison between different atlas-based and nonrigid registration algorithms with a evaluation of their performance and usability to locate the STN automatically. Second, a study of which visible surrounding structures influence the STN location. The two studies are cross validated between them and against expert's variability. Using this scheme, we evaluated the expert's ability against the estimation error provided by the tested algorithms and we demonstrated that automatic STN targeting is possible and as accurate as the expert-driven techniques currently used. We also show which structures have to be taken into account to accurately estimate the STN location.

Algorithms↗

PubMed2006

Understanding diffusion MR imaging techniques: from scalar diffusion-weighted imaging to diffusion tensor imaging and beyond.

The complex structural organization of the white matter of the brain can be depicted in vivo in great detail with advanced diffusion magnetic resonance (MR) imaging schemes. Diffusion MR imaging techniques are increasingly varied, from the simplest and most commonly used technique-the mapping of apparent diffusion coefficient values-to the more complex, such as diffusion tensor imaging, q-ball imaging, diffusion spectrum imaging, and tractography. The type of structural information obtained differs according to the technique used. To fully understand how diffusion MR imaging works, it is helpful to be familiar with the physical principles of water diffusion in the brain and the conceptual basis of each imaging technique. Knowledge of the technique-specific requirements with regard to hardware and acquisition time, as well as the advantages, limitations, and potential interpretation pitfalls of each technique, is especially useful.

Body Water↗

PubMed2005

Imaging of a synchronous neuronal assembly in the human visual brain.

Perception, motion, and cognition involve the formation of cooperative neuronal assemblies distributed over the cerebral cortex. It remains to explore what characterizes the assemblies, their location, and the structural substrate of assembly formation. In this EEG/fMRI study, we describe the response of the visual areas of the two hemispheres in subjects who viewed bilateral iso-oriented (IG) or orthogonally-oriented (OG) moving gratings projected in the two hemifields. The IG stimulus synchronized activity across the hemispheres, as shown by an increased EEG coherence. The increase was restricted to the occipital electrodes and to the beta band. Compared with OG, IG increased the BOLD signal in a restricted territory corresponding to area VP/V4. Within this territory, a linear relation was found between the increased interhemispheric EEG coherence and BOLD. Thus, the increased BOLD localized a trans-hemispheric, synchronous neuronal assembly probably achieved by a callosal cortico-cortical connection. This assembly might reflect an early stage of perceptual grouping since the IG stimulus conforms to Gestalt psychology principles of collinearity and common fate.

Adult↗

PubMed2005

Detection of liver metastases under 2 cm: comparison of different acquisition protocols in four row multidetector-CT (MDCT).

This study compared different acquisition protocols performance to detect small liver metastases (<2 cm). Thirty consecutive patients with histologically proven hepatic metastases were explored by MDCT at the liver equilibrium phase by four successive acquisitions. We compared the following protocols (1-4): 5/30/1.5 (section thickness/table speed/pitch); 5/15/0.75; 5/11.25/0.75; and 2.5/15/1.5 with the same X-ray dose. The gold standard was based on patient radiological follow-up. Evolutive lesions were considered as true positive (TP). The described lesions, not found on the follow-up exams despite tumoral progression, were considered as false positive (FP). Stable lesions could not be considered as metastasis and were eliminated. One hundred and seventy-six lesions were detected: 61 TP and 91 FP. Twenty-four lesions were eliminated. The mean kappa values for protocols 1, 2, 3 and 4 were, respectively, 0.43, 0.68, 0.73 and 0.51 (0.61-0.80: substantial agreement) and the mean areas under the ROC curve were, respectively, 0.76, 0.87, 0.86 and 0.80. The results of protocols 2 and 3 were significantly superior to those of protocols 1 and 4. MDCT protocols using thin sections or an increased table speed are less efficient in detecting small metastases.

Adult↗

PubMed2005

Cross validation of experts versus registration methods for target localization in deep brain stimulation.

In the last five years, Deep Brain Stimulation (DBS) has become the most popular and effective surgical technique for the treatent of Parkinson's disease (PD). The Subthalamic Nucleus (STN) is the usual target involved when applying DBS. Unfortunately, the STN is in general not visible in common medical imaging modalities. Therefore, atlas-based segmentation is commonly considered to locate it in the images. In this paper, we propose a scheme that allows both, to perform a comparison between different registration algorithms and to evaluate their ability to locate the STN automatically. Using this scheme we can evaluate the expert variability against the error of the algorithms and we demonstrate that automatic STN location is possible and as accurate as the methods currently used.

Deep Brain Stimulation↗

PubMed2005

White matter fiber tract segmentation in DT-MRI using geometric flows.

In this paper, we present a 3D geometric flow designed to segment the main core of fiber tracts in diffusion tensor magnetic resonance images. The fundamental assumption of our fiber segmentation technique is that adjacent voxels in a tract have similar properties of diffusion. The fiber segmentation is carried out with a front propagation algorithm constructed to fill the whole fiber tract. The front is a 3D surface that evolves with a propagation speed proportional to a measure indicating the similarity of diffusion between the tensors lying on the surface and their neighbors in the direction of propagation. We use a level set implementation to assure a stable and accurate evolution of the surface and to handle changes of topology of the surface during the evolution process. The fiber tract segmentation method does not need a regularized tensor field since the surface is automatically smoothed as it propagates. The smoothing is done by an intrinsic surface force, based on the minimal principal curvature. This segmentation can be used for obtaining quantitative measures of the diffusion in the fiber tracts and it can also be used for white matter registration and for surgical planning.

Adult↗

PubMed2005

Acute brain perfusion disorders in children assessed by quantitative perfusion computed tomography in the emergency setting.

OBJECTIVE: Perfusion computed tomography (CT) is a simple imaging technique that allows accurate quantitative assessment of brain perfusion. Perfusion CT is an ideal imaging technique to be used in the emergency setting and has thus gained recognition in the early management of adult acute stroke patients. Perfusion CT can be applied to children successfully by using adequate imaging protocols. The goal of this article is to provide a pictorial essay of the perfusion CT features of diseases that affect brain perfusion as depicted in a population of children who were evaluated in the emergency CT unit of our institution. METHODS: During the period of September 2001 to October 2002, all the children, who were evaluated in the emergency CT unit of our institution and who were prescribed with a cerebral CT and an intravenous administration of iodinated contrast material, underwent a perfusion-CT examination. Perfusion-CT maps were reviewed in the patients diagnosed as abnormal on the basis of follow-up clinical/radiological examinations and correlated with the results of these tests. RESULTS: Brain perfusion-CT examinations have been performed in 77 children. Fifty-three patients were considered as normal, based on normal conventional cerebral CT and normal clinical/radiological follow-up. Perfusion-CT results showed major abnormalities in 14 cases among the 24 remaining patients, related to brain ischemia in 2, head trauma in 9, brain infection in 2, and sickle cell disease in 1. These abnormalities consisted in low regional cerebral blood flow and volume values, and in high mean transit time values. They demonstrated typical anatomical distribution, depending on the considered pathological condition. CONCLUSIONS: Perfusion CT provides quantitative assessment of child brain perfusion disorders. Its ability to be easily performed upon admission makes it an ideal emergency tool that advantageously competes with other imaging techniques such as perfusion-weighted magnetic resonance imaging, despite its limited spatial coverage. Its usefulness with respect to the impact on treatment and outcome, however, remains to be established in further studies.

Adolescent↗

PubMed2005

Primary intracranial dural lymphoma of mucosa-associated lymphoid tissue (MALT) type: report of one case and review of the literature.

Primary indolent leptomeningeal lymphoma is a rare entity, and corresponds in most cases to mucosa-associated lymphoid tissue (MALT) type lymphoma. We are reporting a case of a 75 years old woman, who presented with a 2-year history of behavioral disorder, progressive memory loss and aphasia. Neuroimaging showed a mass infiltrating the frontal circumvolutions and the roof of the orbit. The biopsy revealed an infiltration of the dura by an indolent lymphoma, characteristic of a MALT-type lymphoma. Complete staging work-up did not show any evidence of systemic involvement. A treatment with systemic methotrexate, combined with intrathecal chemotherapy and followed by radiotherapy (30,6 Gy) of the primary site, was conducted. The 3-year follow-up confirms the persistent remission, the patient remaining well and free of symptoms. The review of the literature highlights the importance to recognize the indolent PLML as a distinct clinical entity, which exhibits a rather good prognosis following a relatively non-toxic therapy.

Aged↗

PubMed2004

Postcarotid endarterectomy hyperperfusion or reperfusion syndrome.

BACKGROUND AND PURPOSE: Hyperperfusion syndrome (HS) after carotid endarterectomy (CEA) has been related to impaired cerebrovascular autoregulation in a chronically hypoperfused hemisphere. Our aim was to provide new insight into the pathophysiology of the HS using magnetic resonance imaging (MRI) studies with diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI). METHODS: Five out of 388 consecutive patients presented 2 to 7 days after CEA, partial seizures (n=5), focal deficits (n=5), and intracerebral hemorrhage (n=3). In 4 patients, using sequential examinations, we identified vasogenic or cytotoxic edema by DWI; we assessed relative interhemispheric difference (RID) of cerebral blood flow (CBF) by PWI; and we measured middle cerebral artery mean flow velocities (MCA Vm) by transcranial Doppler (TCD). RESULTS: None of the patients presented pathological DWI hyperintensities, consistent with the absence of acute ischemia or cytotoxic edema. In 2 patients, we found an MRI pattern of reversible vasogenic edema similar to that observed in the posterior leukoencephalopathy syndrome. Middle cerebral artery (MCA) mean flow velocities (Vm) were not abnormally increased at any time. PWI documented a 20% to 44% RID of CBF in favor of the ipsilateral to CEA hemisphere. CONCLUSIONS: HS can occur in the presence of moderate relative hyperperfusion of the ipsilateral hemisphere. MCA Vm values may not accurately reflect RID of CBF over the cortical convexity. We suggest that the hemodynamic pathogenetic mechanisms of the HS are more complicated than hitherto believed and that they may be more accurately described by the term "reperfusion syndrome."

Aged↗

PubMed2004

Synchrotron radiation in radiology: radiology techniques based on synchrotron sources.

The characteristics of synchrotron X-ray sources--quite different from those of conventional sources--are exploited by several new imaging techniques. These techniques expand the capabilities of conventional radiology and find interesting application in special cases. We briefly review the basic principle, applications and limitations of the most important of them: monochromatic mammography, two-wavelength digital subtraction angiography, phase-contrast/edge-enhancement imaging, diffraction-enhanced imaging and microtomography.

Angiography↗

PubMed2004

Relationship between brain perfusion computed tomography variables and cerebral perfusion pressure in severe head trauma patients.

OBJECTIVE: To compare brain perfusion-computed tomography (CT) results with invasive cerebral perfusion pressure (CPP) monitoring in severe head trauma patients. DESIGN: Prospective cohort study. SETTING: Emergency room and surgical intensive care unit of our hospital. PATIENTS: Sixty-one severe head trauma patients. INTERVENTIONS: We prospectively collected 103 perfusion-CT examinations with simultaneous measurement of mean arterial pressure and intracranial pressure, affording calculation of CPP. The statistical relationship between perfusion-CT results and the corresponding CPP values was evaluated using Wilcoxon (Mann-Whitney) and generalized F-tests. The functional outcome of the 61 patients was evaluated 3 months after trauma on the basis of the Glasgow Outcome Scale score and compared between groups using Fisher's exact tests. MEASUREMENTS AND MAIN RESULTS: Perfusion-CT enabled us to distinguish between two groups of patients. Within each group, a significant correlation (p <.001) between the CPP values and the corresponding perfusion-CT results was demonstrated. There was also a significant correlation (p <.001) between the CPP values and the extent of the abnormal perfusion-CT areas (R up to.817). The first group was characterized by a weak dependence of perfusion-CT results on the corresponding CPP values (low slope) and the second group by a strong dependence (steep slope). These groups were interpreted as having preserved (or pseudo) and impaired cerebral vascular autoregulation, respectively. The functional outcome was better in the second group of patients. CONCLUSIONS: Intermittent perfusion-CT measurements plus continuous CPP measurement provide more information than continuous CPP alone. Perfusion-CT gives unique information regarding regional heterogeneity of brain perfusion. It might allow clinicians to distinguish between patients with preserved auto-regulation (or pseudoautoregulation) and those with impaired autoregulation and could therefore guide interpretation of CPP measurements and therapy.

Adult↗