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Biomedical subjects

I Bloch

Publications and source records attributed to I Bloch.

At least 19 recordsLinked to original sources
PubMed2003

Local reconstruction of stenosed sections of artery using multiple MRA acquisitions.

A method for reconstructing magnetic resonance angiography (MRA) volumes from successive acquisitions is described. The method is based on double oblique acquisitions of highly anisotropic MRA volumes, each of which corresponds to reduced k-space filling. These partial k-spaces are then combined to obtain a 3D k-space adapted to the frequency spread of the angiographic image of the stenosis. The SNR-resolution compromise of MRA is thus improved by focusing the acquisition on the most relevant k-space regions. The reconstruction is performed directly in k-space by averaging the partial k-spaces. The feasibility of the method was demonstrated in studies on a Lucite stenosis phantom, on MRAs of carotid arteries using three bolus injections, and on MRAs of renal arteries using a single contrast injection.

Carotid Arteries↗
PubMed2003

A primal sketch of the cortex mean curvature: a morphogenesis based approach to study the variability of the folding patterns.

In this paper, we propose a new representation of the cortical surface that may be used to study the cortex folding process and to recover some putative stable anatomical landmarks called sulcal roots usually buried in the depth of adult brains. This representation is a primal sketch derived from a scale space computed for the mean curvature of the cortical surface. This scale-space stems from a diffusion equation geodesic to the cortical surface. The primal sketch is made up of objects defined from mean curvature minima and saddle points. The resulting sketch aims first at highlighting significant elementary cortical folds, second at representing the fold merging process during brain growth. The relevance of the framework is illustrated by the study of central sulcus sulcal roots from antenatal to adult age. Some results are proposed for ten different brains. Some preliminary results are also provided for superior temporal sulcus.

Adult↗
PubMed2002

Distortion correction and robust tensor estimation for MR diffusion imaging.

This paper presents a new procedure to estimate the diffusion tensor from a sequence of diffusion-weighted images. The first step of this procedure consists of the correction of the distortions usually induced by eddy-current related to the large diffusion-sensitizing gradients. This correction algorithm relies on the maximization of mutual information to estimate the three parameters of a geometric distortion model inferred from the acquisition principle. The second step of the procedure amounts to replacing the standard least squares-based approach by the Geman-McLure M-estimator, in order to reduce outlier-related artefacts. Several experiments prove that the whole procedure highly improves the quality of the final diffusion maps.

Artifacts↗
PubMed2001

Exploring phase coherence in a 2D lattice of Bose-Einstein condensates.

Bose-Einstein condensates of rubidium atoms are stored in a two-dimensional periodic dipole force potential, formed by a pair of standing wave laser fields. The resulting potential consists of a lattice of tightly confining tubes, each filled with a 1D quantum gas. Tunnel coupling between neighboring tubes is controlled by the intensity of the laser fields. By observing the interference pattern of atoms released from more than 3000 individual lattice tubes, the phase coherence of the coupled quantum gases is studied. The lifetime of the condensate in the lattice and the dependence of the interference pattern on the lattice configuration are investigated.

Journal Article↗
PubMed2001

Optics with an atom laser beam.

We report on the atom optical manipulation of an atom laser beam. Reflection, focusing, and its storage in a resonator are demonstrated. Precise and versatile mechanical control over an atom laser beam propagating in an inhomogeneous magnetic field is achieved by optically inducing spin flips between atomic ground states with different magnetic moment. The magnetic force acting on the atoms can thereby be effectively switched on and off. The surface of the atom optical element is determined by the resonance condition for the spin flip in the inhomogeneous magnetic field. More than 98% of the incident atom laser beam is reflected specularly.

Journal Article↗
PubMed2001

Towards inference of human brain connectivity from MR diffusion tensor data.

This paper describes a method to infer the connectivity induced by white matter fibers in the living human brain. This method stems from magnetic resonance tensor imaging (DTI), a technique which gives access to fiber orientations. Given typical DTI spatial resolution, connectivity is addressed at the level of fascicles made up by a bunch of parallel fibers. We propose first an algorithm dedicated to fascicle tracking in a direction map inferred from diffusion data. This algorithm takes into account fan-shaped fascicle forks usual in actual white matter organization. Then, we propose a method of inferring a regularized direction map from diffusion data in order to improve the robustness of the tracking. The regularization stems from an analogy between white matter organization and spaghetti plates. Finally, we propose a study of the tracking behavior according to the weight given to the regularization and some examples of the tracking results with in vivo human brain data.

Axons↗
PubMed2001

Quantitative analysis of videostroboscopic images in presbylarynges.

OBJECTIVE/HYPOTHESIS: Age-related dysphonia, or presbylarynges, is often identified as the cause of voice disorders in older individuals. It is a diagnosis of exclusion that typically includes the subjective videostroboscopic findings of vocal fold bowing or atrophy and incomplete glottal closure. We hypothesize that vocal fold bowing correlates directly with glottal gap in patients with presbylarynges and that these characteristics may be quantified objectively using measures obtained from videostroboscopic images. STUDY DESIGN: Retrospective analysis of patient data and prospective analysis of control subjects. METHODS: The quantitative measures of bowing index and normalized glottal gap, as well as a novel measure, normalized laryngeal outlet, were calculated from the videostroboscopic examinations of 46 patients with presbylarynges and 20 normal control subjects. RESULTS: The mean bowing index values from the presbylarynges and control groups differed significantly, although there was overlap of individual values between groups. Bowing index values did not consistently predict normalized glottal gap values. Mean normalized laryngeal outlet values of the patients with presbylarynges were significantly smaller than those of the control group. CONCLUSIONS: Bowing does not consistently predict the extent of glottal gap. Therefore, other presbylaryngeal changes are probably contributing to incomplete glottal closure, which are not well-visualized stroboscopically, so the finding of bowing is not sufficiently specific to identify presbylarynges. The significantly smaller normalized laryngeal outlet values suggest that this identifies a compensatory strategy for glottal gap to optimize vocal function. This is a novel finding that contrasts with medial ventricular fold squeezing, which is more routinely sought and is typically considered a maladaptive compensatory strategy. Despite the advantages of quantitative stroboscopic measures, limitations in reliability limit their clinical utility.

Aging↗
PubMed2000

Measurement of the spatial coherence of a trapped Bose gas at the phase transition

The experimental realization of Bose-Einstein condensates of dilute gases has allowed investigations of fundamental concepts in quantum mechanics at ultra-low temperatures, such as wave-like behaviour and interference phenomena. The formation of an interference pattern depends fundamentally on the phase coherence of a system; the latter may be quantified by the spatial correlation function. Phase coherence over a long range is the essential factor underlying Bose-Einstein condensation and related macroscopic quantum phenomena, such as superconductivity and superfluidity. Here we report a direct measurement of the phase coherence properties of a weakly interacting Bose gas of rubidium atoms. Effectively, we create a double slit for magnetically trapped atoms using a radio wave field with two frequency components. The correlation function of the system is determined by evaluating the interference pattern of two matter waves originating from the spatially separated 'slit' regions of the trapped gas. Above the critical temperature for Bose-Einstein condensation, the correlation function shows a rapid gaussian decay, as expected for a thermal gas. Below the critical temperature, the correlation function has a different shape: a slow decay towards a plateau is observed, indicating the long-range phase coherence of the condensate fraction.

Journal Article↗
PubMed2000

Structural group analysis of functional activation maps.

We present here a new method for cerebral activation detection over a group of subjects. This method is performed using individual activation maps of any sort. It aims at processing a group analysis while preserving individual information and at overcoming as far as possible limitations of the spatial normalization used to compare different subjects. We designed it such that it provides the individual occurrence of the activations detected at a group level. The localization can then be performed on the individual anatomy of each subject. The analysis starts with a hierarchical multiscale object-based description of each individual map. These descriptions are then compared, rather than comparing the images directly. The analysis is thus performed at an object level instead of voxel by voxel. It is made using a comparison graph, on which a labeling process is performed. The label field on the graph is modeled by a Markov random field, which allows us to introduce high-level rules of interrogation of the data. The process has been evaluated on simulated data and real data from a PET protocol.

Brain↗
PubMed2000

Regularization of diffusion-based direction maps for the tracking of brain white matter fascicles.

Magnetic resonance diffusion tensor imaging (DTI) provides information about fiber local directions in brain white matter. This paper addresses inference of the connectivity induced by fascicles made up of numerous fibers from such diffusion data. The usual fascicle tracking idea, which consists of following locally the direction of highest diffusion, is prone to erroneous forks because of problems induced by fiber crossing. In this paper, this difficulty is partly overcomed by the use of a priori knowledge of the low curvature of most of the fascicles. This knowledge is embedded in a model of the bending energy of a spaghetti plate representation of the white matter used to compute a regularized fascicle direction map. A new tracking algorithm is then proposed to highlight putative fascicle trajectories from this direction map. This algorithm takes into account potential fan shaped junctions between fascicles. A study of the tracking behavior according to the influence given to the a priori knowledge is proposed and concrete tracking results obtained with in vivo human brain data are illustrated. These results include putative trajectories of some pyramidal, commissural, and various association fibers.

Algorithms↗
PubMed2000

Segmentation of the skull in MRI volumes using deformable model and taking the partial volume effect into account.

Segmentation of the skull in medical imagery is an important stage in applications that require the construction of realistic models of the head. Such models are used, for example, to simulate the behavior of electro-magnetic fields in the head and to model the electrical activity of the cortex in EEG and MEG data. In this paper, we present a new approach for segmenting regions of bone in MRI volumes using deformable models. Our method takes into account the partial volume effects that occur with MRI data, thus permitting a precise segmentation of these bone regions. At each iteration of the propagation of the model, partial volume is estimated in a narrow band around the deformable model. Our segmentation method begins with a pre-segmentation stage, in which a preliminary segmentation of the skull is constructed using a region-growing method. The surface that bounds the pre-segmented skull region offers an automatic 3D initialization of the deformable model. This surface is then propagated (in 3D) in the direction of its normal. This propagation is achieved using level set method, thus permitting changes to occur in the topology of the surface as it evolves, an essential capability for our problem. The speed at which the surface evolves is a function of the estimated partial volume. This provides a sub-voxel accuracy in the resulting segmentation.

Computer Simulation↗
PubMed1998

Electroporation therapy of head and neck cancer.

The purpose of this paper is to introduce the concept of electroporation therapy and present our results from using this new technique combined with intralesional bleomycin in head and neck cancer patients. Electroporation therapy is a technique wherein high-voltage electric impulses delivered into a neoplasm transiently increase cell membrane permeability to large molecules, including cytotoxic agents. In this phase I/II study, extremely low-dose bleomycin sulfate was electroporated into head and neck malignant neoplasms in 10 patients. Tumor responses included 2 nonresponders, 3 partial responders, and 5 complete responders, with a mean follow-up of 40 weeks. We conclude that this technique offers promising possibilities in the local treatment of head and neck cancer.

Adult↗
PubMed1997

Prognostic significance of Bcl-2 expression in localized squamous cell carcinoma of the head and neck.

This study was conducted to determine whether Bcl-2 overexpression in localized squamous cell carcinoma of the head and neck (SCCHN) might serve as a marker for tumors unlikely to respond to standard treatment. Tissue samples from 33 patients undergoing surgery or irradiation for early-stage SCCHN during the year 1977 to 1992 were stained for Bcl-2. All patients had either T1N0 lesions of the oral cavity, pharynx, or larynx or T1N0 or T2N0 lesions of the true vocal cords. Of the 33 patients, 26 remained disease-free after at least 3 years of follow-up; the remaining 7 patients developed either tumor recurrence or a second primary tumor, 4 of which were fatal. Twelve patients had tissue specimens staining positive for Bcl-2; 6 of these patients had a poor outcome, and 6 had a good outcome. The relationship between poor outcome and overexpression of Bcl-2 in tumor cells was statistically significant (p = .0047 by Fisher's exact test). For tumors overexpressing Bcl-2, there was no significant difference in recurrence rate between those undergoing surgery and those undergoing radiotherapy as the primary mode of treatment. The overexpression of Bcl-2 in early lesions in this study predicted a cure rate of 50%, as opposed to the generally expected 90%, suggesting that Bcl-2 is a significant prognostic indicator in early SCCHN. Future studies will determine if altering the treatment will improve outcome in these patients.

Adult↗
PubMed1994

Fast nonsupervised 3D registration of PET and MR images of the brain.

We propose a fully nonsupervised methodology dedicated to the fast registration of positron emission tomography (PET) and magnetic resonance images of the brain. First, discrete representations of the surfaces of interest (head or brain surface) are automatically extracted from both images. Then, a shape-independent surface-matching algorithm gives a rigid body transformation, which allows the transfer of information between both modalities. A three-dimensional (3D) extension of the chamfer-matching principle makes up the core of this surface-matching algorithm. The optimal transformation is inferred from the minimization of a quadratic generalized distance between discrete surfaces, taking into account between-modality differences in the localization of the segmented surfaces. The minimization process is efficiently performed via the precomputation of a 3D distance map. Validation studies using a dedicated brain-shaped phantom have shown that the maximum registration error was of the order of the PET pixel size (2 mm) for the wide variety of tested configurations. The software is routinely used today in a clinical context by the physicians of the Service Hospitalier Frédéric Joliot (> 150 registrations performed). The entire registration process requires approximately 5 min on a conventional workstation.

Algorithms↗
PubMed1992

[A contribution to the controversial discussion on a preoperative orthodontic treatment for infants with unilateral cheilognathopalatoschisis].

A longitudinal study of 39 casts of pre-operative orthodontically treated unilateral cleft lip and palate children from birth to palate operation was carried out with a special coordinate measuring technique to quantify changes in the maxilla. A significant reduction in width of the alveolar cleft was found. The anterior alveolar arch width remained constant, while the posterior region slightly increased. These results seem to justify treatment with the infant appliance. After measuring casts from 39 preoperatively treated and casts from 62 untreated patients prior to lip operation, a comparison of the mathematical average values between the two groups revealed few differences. This can be explained by referring to the original morphological findings and the different measurements during the preoperative orthodontic treatment, which lead to different changes in segment position prior to lip operation. In a cross section examination this difference could not be found. A sub-classification of the subject matter in primary and secondary clefts revealed that between both types clear differences in the resulting parameters can be observed. This leaves the question, whether the width of the cleft palate is caused by tissue deficiency or by an embedded tongue. Furthermore, how will the orthodontic appliance therapy influence the growth? A greater dislocation of the segments in secondary clefts may be expected in cases where the rest position of the tongue appears caudal. Improved measuring methods to record cleft morphology and more emphasis on the study of soft tissue reactions may help to understand the differing results after preoperative or orthodontic treatment.

Cleft Lip↗
PubMed1992

Site-specific incorporation of [125I]iododeoxyuridine into DNA.

A procedure for the incorporation of [125I]IdU into specific sites in DNA is described. The approach depends upon attachment of radioiododeoxyuridine to a controlled pore glass support which is then used for automated synthesis of an oligomer. The resulting oligomer, containing a terminal 3'[125I]iododeoxyuridine, is used as a primer during DNA synthesis catalyzed by the Taq polymerase employing thermal cycling. The product formed includes the radioiodonucleotide at a single internal site determined by the length of the oligomer.

Base Sequence↗
PubMed1986

Mathematical morphology for automatic detection of brain lesions at magnetic resonance imaging.

An original method of image analysis has been developed, using 'mathematical morphology', in order to detect brain tumors at magnetic resonance imaging of the head following injection of gadolinium-DPTA. The main steps of the analysis are as follows: 1) Spatial reduction by automatic determination of a region of interest, 2) elimination of noise by morphologic filtering (increasing alternate closing-opening sequence), 3) localization of tumor by searching for r-h maxima, and 4) extraction of tumor contours using three-dimensional structuring elements. The preliminary results of this automatic and robust method encourage further studies of the potential of tissue analysis as an aid to tumor diagnosis.

Brain↗