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R Meuli

Publications and source records attributed to R Meuli.

At least 19 recordsLinked to original sources
PubMed2006

Cerebral vascular autoregulation assessed by perfusion-CT in severe head trauma patients.

PURPOSE: To use perfusion-CT technique in order to characterize cerebral vascular autoregulation in a population of severe head trauma patients with features of cerebral edema either on the admission or on the follow-up conventional noncontrast cerebral CT. MATERIAL AND METHODS: A total of 80 perfusion-CT examinations were obtained in 42 severe head trauma patients with features of cerebral edema on conventional noncontrast cerebral CT, either on admission or during follow-up. Perfusion-CT results, i.e. the regional cerebral blood volume (rCBV) and flow (rCBF), were correlated with the mean arterial pressure (MAP) measured during each perfusion-CT examination. Ratios were defined to integrate the concept of cerebral vascular autoregulation, and cluster analysis performed, which allowed identification of different subgroups of patients. MAP values and perfusion-CT results in these groups were compared using Kruskal-Wallis and Wilcoxon (Mann-Whitney) tests. Moreover, the functional outcome of the 42 patients was evaluated 3 months after trauma on the basis of the Glasgow Outcome Scale (GOS) score and similarly compared between groups. RESULTS: Three main groups of patients were identified: 1) 22 perfusion-CT examinations were collected in 13 patients, characterized by high rCBV and rCBF values and by significant dependence of perfusion-CT rCBV and rCBF results on MAP values (p<0.001), 2) 23 perfusion-CT examinations collected in 19 patients showing perfusion-CT results similar to control trauma subjects, and 3) 33 perfusion-CT collected in 16 patients, with low rCBV and rCBF values and near-independence of perfusion-CT results with respect to MAP values. The first group was interpreted as showing impaired cerebral vascular autoregulation, which was preserved in the third group. The second group was associated with the best functional outcome; it was linked to the first group, because eight patients went from one group to the other from admission to follow-up. CONCLUSION: Perfusion-CT in severe head trauma patients was able to provide direct and quantitative assessment of cerebral vascular autoregulation with a single measurement. It could hence be used as a guide for brain edema therapy, as well as to monitor the treatment efficiency.

Adult↗
PubMed2004

Spurious signals in DQF spectroscopy: two-shot stimulated echoes.

The most widely used technique for double-quantum filtered (DQF) single-voxel spectroscopy (SVS) is based on a symmetric PRESS sequence with two additional spatially unselective pi/2 pulses, one of which is usually frequency selective. The actual filtering, rejecting signals from all uncoupled resonances, can be done by suitable phase cycling of the rf pulses in successive shots, but in practice gradient filtering is always used. Under usual conditions the sequence repetition time is comparable to the spin-lattice relaxation time, and a stimulated echo is formed by five out of the ten rf pulses in two consecutive shots. This echo is not filtered out by the gradients, and additional phase cycling is needed to eliminate it. Its spatial origin is the full transverse slice selected by the last pulse of the PRESS sequence. The SVS shimming procedure may create an important field variation in this slice (outside the volume of interest VOI). Water singlet signals therefore appear in a band of frequencies other than 4.7 ppm, and remain unaffected by water suppression pulses. In practice phase-alternation schemes can reduce these spurious signals by several orders of magnitude, but even then they may mask the weak metabolite signals of interest. We describe a strategy to minimize these spurious signals and propose a 16-step phase cycling scheme that attenuates the stimulated echo in every two-step subcycle.

Algorithms↗
PubMed2004

Influence of detector collimation on SNR in four different MDCT scanners using a reconstructed slice thickness of 5 mm.

The purpose of this paper is to compare the influence of detector collimation on the signal-to-noise ratio (SNR) for a 5.0 mm reconstructed slice thickness for four multi-detector row CT (MDCT) units. SNRs were measured on Catphan test phantom images from four MDCT units: a GE LightSpeed QX/I, a Marconi MX 8000, a Toshiba Aquilion and a Siemens Volume Zoom. Five-millimetre-thick reconstructed slices were obtained from acquisitions performed using detector collimations of 2.0-2.5 mm and 5.0 mm, 120 kV, a 360 degrees tube rotation time of 0.5 s, a wide range of mA and pitch values in the range of 0.75-0.85 and 1.25-1.5. For each set of acquisition parameters, a Wiener spectrum was also calculated. Statistical differences in SNR for the different acquisition parameters were evaluated using a Student's t-test (P<0.05). The influence of detector collimation on the SNR for a 5.0-mm reconstructed slice thickness is different for different MDCT scanners. At pitch values lower than unity, the use of a small detector collimation to produce 5.0-mm thick slices is beneficial for one unit and detrimental for another. At pitch values higher than unity, using a small detector collimation is beneficial for two units. One manufacturer uses different reconstruction filters when switching from a 2.5- to a 5.0-mm detector collimation. For a comparable reconstructed slice thickness, using a smaller detector collimation does not always reduce image noise. Thus, the impact of the detector collimation on image noise should be determined by standard deviation calculations, and also by assessing the power spectra of the noise.

Artifacts↗
PubMed2004

Quantitative characterization of edge enhancement in phase contrast x-ray imaging.

The aim of this study was to model the edge enhancement effect in in-line holography phase contrast imaging. A simple analytical approach was used to quantify refraction and interference contrasts in terms of beam energy and imaging geometry. The model was applied to predict the peak intensity and frequency of the edge enhancement for images of cylindrical fibers. The calculations were compared with measurements, and the relationship between the spatial resolution of the detector and the amplitude of the phase contrast signal was investigated. Calculations using the analytical model were in good agreement with experimental results for nylon, aluminum and copper wires of 50 to 240 microm diameter, and with numerical simulations based on Fresnel-Kirchhoff theory. A relationship between the defocusing distance and the pixel size of the image detector was established. This analytical model is a useful tool for optimizing imaging parameters in phase contrast in-line holography, including defocusing distance, detector resolution and beam energy.

Biophysical Phenomena↗
PubMed2003

Magnetic resonance artifact induced by the electrode Activa 3389: an in vitro and in vivo study.

BACKGROUND: The electrode Activa 3389 is widely implanted for deep brain stimulation (DBS) and MRI is often used to control the position of the electrode. However, induced distorsion artifacts may result in imprecise localization and may lead to misinterpretations of the clinical effects and mechanisms of DBS. METHODS: In vitro 3D MR study: the proximal and distal contacts of one electrode were spotted by two localizers. The maximal artifact height (MAH) and width (MAW: measured on distal contact), and the distances between the artifact and the localizers (proximal, distal and lateral) were measured on 2 transverse and sagittal MR sequences with 90 degrees rotation of frequency-encoded gradient and phase direction. In vivo 3D MR study: coronal and sagittal reconstructions along the main axis of the electrode were performed on 10 postoperative MR (20 electrodes) to measure MAH and MAW. A Student t test was used to compare in vitro and in vivo measurements. FINDINGS: In vitro study: A MAH of 10.35 mm (+/-0.23) and MAW of 3.6 mm (+/-0.2) were found. We measured symmetrical extensions of the artifact over the distal contact. In vivo study: A MAH of 10.36 mm (+/-0.44) and MAW of 3.56 mm (+/-0.30) were obtained. No significant different artifact dimensions were measured between in vitro and in vivo studies (p<0.0001). INTERPRETATION: Precise 3D localization of the electrode in implanted patients is provided by MR identification of the limits of the distal contact artifact. The position of the other contacts is deduced given the size of the contacts and the intercontact distance.

Artifacts↗
PubMed2003

Synchrotron radiation in radiology: novel X-ray sources.

X-rays, the key ingredient of radiology, are still primarily produced with the mechanism discovered by Röntgen over one century ago. A different approach, however, is becoming increasingly important: the use of synchrotron sources based on fast-moving electrons and on their relativistic properties. We discuss the elements of this new strategy, its practical implementation, and some new types of synchrotron sources under development. The present and potential impact on radiology of different classes of synchrotron sources is briefly and realistically analyzed.

Humans↗
PubMed2003

Spontaneous bilateral subdural haematomas in the posterior cranial fossa revealed by MRI.

A 52-year-old woman treated for acute myeloproliferative disease developed progressive stupor. CT showed obstructive hydrocephalus resulting from unexplained mass effect on the fourth ventricle. MRI revealed bilateral extra-axial collections in the posterior cranial fossa, giving high signal on T1- and T2-weighted images, suggesting subacute subdural haematomas. Subdural haematomas can be suspected on CT when there is unexplained mass effect. MRI may be essential to confirm the diagnosis and plan appropriate treatment.

Cranial Fossa, Posterior↗
PubMed2003

DTI mapping of human brain connectivity: statistical fibre tracking and virtual dissection.

Several approaches have been used to trace axonal trajectories from diffusion MRI data. If such techniques were first developed in a deterministic framework reducing the diffusion information to one single main direction, more recent approaches emerged that were statistical in nature and that took into account the whole diffusion information. Based on diffusion tensor MRI data coming from normal brains, this paper presents how brain connectivity could be modelled globally by means of a random walk algorithm. The mass of connections thus generated was then virtually dissected to uncover different tracts. Corticospinal, corticobulbar, and corticothalamic tracts, the corpus callosum, the limbic system, several cortical association bundles, the cerebellar peduncles, and the medial lemniscus were all investigated. The results were then displayed in the form of an in vivo brain connectivity atlas. The connectivity pattern and the individual fibre tracts were then compared to known anatomical data; a good matching was found.

Algorithms↗
PubMed2003

Isolated absence of optic chiasm revealed by congenital nystagmus, MRI and VEPs.

Congenital nystagmus is a rare condition mainly characterised by rhythmic, conjugate, and horizontal oscillations of both eyes that persist in the vertical gaze. This disorder is usually noticed in the neonatal period and persists throughout life. It can be of sensory origin, associated with low visual acuity of various causes, or of motor origin, caused by a defect in the slow eye movement system. The former can be genetically determined. It can also be associated with several conditions, the most frequent being albinism. The achiasma syndrome has recently been recognized in two patients as an autosomal recessive inherited cause of congenital nystagmus. We report the case of the so far youngest reported baby having been diagnosed with the isolated achiasmatic condition, which presented with congenital nystagmus and see-saw nystagmus, and discuss its clinical findings and 18 months follow-up. The achiasmatic syndrome should be included in the differential diagnosis of congenital nystagmus, as all the described cases presented like that. Complete investigations should be performed to allow the best evolution and follow-up of these children.

Evoked Potentials, Visual↗
PubMed2003

[Perfusion-CT guided acute stroke management].

The easily accessible and available PCT reliably identifies reversible and irreversible ischaemia in acute stroke patients. These knowledge will allow treatment strategies to become more appropriate and individualized. Patients with significant penumbra may be candidates for treatment with dangerous or costly medication, and patients without may not, independently of duration of stroke symptoms. Furthermore, PCT also has the scientific potential to identify appropriate patients for therapeutic trials. Finally, salvage of PCT-defined penumbra could be used as a surrogate marker for effectiveness of interventions.

Cerebrovascular Circulation↗
PubMed2002

Stroke or transient ischemic attacks with basilar artery stenosis or occlusion: clinical patterns and outcome.

BACKGROUND: Basilar artery occlusion (BAO) is associated with a high mortality rate, although cases with spontaneous favorable outcomes have recently been reported, and basilar artery stenosis (BAS) has received little consideration until now. OBJECTIVE: To study the prognostic clinical factors by testing numerous combinations of admission status characteristics of patients with brain ischemia caused by BAO or BAS. METHODS: We conducted a retrospective review from the Lausanne Stroke Registry (group 1) of patients with stroke or transient ischemic attack caused by BAS less than 50% or BAO as diagnosed by magnetic resonance angiography who were not treated by thrombolysis. Neurologic findings on admission were correlated with outcomes. We compared clinical patterns associated with poor outcomes in group 1 with those in patients with stroke who died from BAO or BAS (confirmed at autopsy) (group 2). RESULTS: Eighty-eight patients were studied. The outcomes of patients with stroke in group 1 (35/43) was poor (severe disability or death) in 54% of cases. A statistical analysis revealed that 4 factors-dysarthria, pupillary disorders, lower cranial nerve involvement, and consciousness disorders on admission-were strongly (P<.001) associated with poor outcomes. The multivariate analysis showed that the outcome was poor in 100% of cases in which consciousness disorders or the combination of the remaining 3 factors were present, whereas in the absence of these factors, a poor outcome was reported in only 11%. In 87% of the 45 patients with stroke in group 2, the same clinical patterns were present on admission. CONCLUSIONS: The prognosis of BAS greater than 50% or BAO is diverse and certain clinical characteristics seem to predict a lower risk of poor outcome. Their presence may help to decide the most suitable therapy.

Aged↗
PubMed2002

Performance of multidetector computed tomography colonography compared with conventional colonoscopy.

BACKGROUND AND AIMS: This was a prospective blinded study to compare computed tomography (CT) colonography, performed with multidetector arrays CT scan (MDCT), with conventional colonoscopy for the detection of colorectal neoplasia. METHODS: Fifty patients were examined by MDCT after standard bowel preparation and rectal air insufflation in the supine and prone positions. Data sets were examined by one radiologist and one gastroenterologist blinded to the patient's history and colonoscopy results. Patients subsequently underwent colonoscopy on the same day, which served as the gold standard. RESULTS: Nine of 11 lesions >10 mm (82%), 5/15 lesions of 6-9 mm (33%), and 1/42 polyps <5 mm (3%) were detected by MDCT colonography. One false positive result for a structure larger than 10 mm was described. Nineteen of 21 patients who had no lesions during conventional colonoscopy were considered free of lesions by MDCT colonography, yielding a per patient specificity of 90%. CONCLUSION: MDCT colonography provides good data quality and has good sensitivity and specificity for the detection of colonic lesions of 10 mm or more.

Aged↗
PubMed2002

Comparison of admission perfusion computed tomography and qualitative diffusion- and perfusion-weighted magnetic resonance imaging in acute stroke patients.

BACKGROUND AND PURPOSE: Besides classic criteria, cerebral perfusion imaging could improve patient selection for thrombolytic therapy. The purpose of this study was to compare quantitative perfusion CT imaging and qualitative diffusion- and perfusion-weighted MRI (DWI and PWI) in acute stroke patients at the time of their emergency evaluation. METHODS: Thirteen acute stroke patients underwent perfusion CT and DWI or PWI on admission. The size of infarct and ischemic lesion (infarct plus penumbra) on the admission perfusion CT was compared with that of the MR abnormalities as shown on the DWI trace and on the relative cerebral blood volume, cerebral blood flow, time to peak, and mean transit time maps calculated from PWI studies. RESULTS: The most significant correlation was found between infarct size on the admission perfusion CT and abnormality size on the admission DWI map (r=0.968, P<0.001). A significant correlation was also observed between the size of the ischemic lesion (infarct plus penumbra) on the admission perfusion CT and the abnormality size on the mean transit time map calculated from admission PWI (r=0.946, P<0.001). Information about cerebral infarct and total ischemia (infarct plus penumbra) carried by both imaging techniques was similar, with slopes of 0.913 and 0.905, respectively. CONCLUSIONS: An imaging technique may be helpful in the identification of cerebral penumbra in acute stroke patients and thus in the selection of patients for thrombolytic therapy. Perfusion CT and DWI/PWI are equivalent in this task.

Acute Disease↗
PubMed2002

[Chronic progressive myeloradiculopathy in a 61-year-old man].

A painful worsening of known difficulties in walking led us to investigate a man who presented a spastic paraparesis. Radiological investigations had to be repeated three times before making a diagnosis of a right C6 spinal dural arteriovenous fistula after a 22-month follow-up. Knowing the mechanisms leading to spinal venous hypertension may explain the low yield of the early radiological investigations that should be repeated. The efficiency of the treatment depends on the severity of the presurgical neurologic manifestations.

Central Nervous System Vascular Malformations↗
PubMed2001

Wrist ligament injuries: value of post-arthrography computed tomography.

OBJECTIVE: To evaluate the use of post-arthrography high-resolution computed tomography in wrist ligament injuries. DESIGN AND PATIENTS: Thirty-six consecutive patients who had a history and clinical findings suggestive of ligamentous injuries of the wrist were prospectively studied. The findings of three-compartment arthrography and post-arthrography computed tomography (arthro-CT) were compared with those of arthroscopy. The evaluation concentrates on the detection and precise localization of ligament lesions in the triangular fibrocartilage (TFC), the scapholunate ligament (SLL) and the lunotriquetral ligament (LTL). RESULTS: For TFC, SLL and LTL lesions, standard arthrography responded with a sensitivity and specificity of 85% and 100%, 85% and 100%, 80% and 100% respectively, while arthro-CT showed a sensitivity and specificity of 85% and 100%, 100% and 100%, 80% and 100% respectively. The precise localization of the lesions was possible only with arthro-CT. CONCLUSION: The sensitivity and specificity of standard arthrography and arthro-CT are similar, although the latter shows the site of tears or perforation with greater precision, while conventional arthrography demonstrates them indirectly. This precision is essential and may have clinical implications for the success of treatment procedures.

Adolescent↗
PubMed2001

2D and 3D CT imaging correlated to rigid endoscopy in complex laryngo-tracheal stenoses.

The aim of this study was to compare 2D and 3D CT imaging in the pre- and postoperative evaluation of complex benign larynges-tracheal airway stenoses with rigid endoscopy, considered as the gold standard. Six patients (aged 5-72 years) with a total of nine complex laryngo-tracheal stenoses underwent non-contrast helical CT scans (slice thickness 3 mm, pitch 1.3, reconstruction interval 1.5 mm) before and after surgical resection. With prototype software, virtual endoscopy (VE) post-processing algorithms were applied to the imaging data sets. The VE and multiplanar 2D findings were compared with rigid endoscopy, considered as standard of reference. All nine stenoses were correctly identified on 3D images and their anatomical locations correctly assessed on 2D reconstructions. Artifacts were met when patients were unable to suspend their breath, leading to one false-positive result. Two-dimensional images and 3D VE of tracheal stenoses proved to be efficient and complementary to the rigid endoscopy, permitting a reliable endoluminal 3D view and evaluation of the surrounding anatomical structures. Limitations of this technique relate to the maximal spatial resolution of 1.5 mm, the lack of color, and the inability to assess the mucosa. Virtual endoscopy is for complex laryngo-tracheal stenoses an excellent complement for rigid endoscopy, remaining the method of reference, and may be indicated with complicated pathological structures.

Adolescent↗