Biomedical subjects
F Langevin
Publications and source records attributed to F Langevin.
Two-point method for T1 estimation with optimized gradient-echo sequence.
Relaxation times estimation methods play a central role in various problems, such as magnetic resonance (MR) hardware calibration, tissue characterization, or temperature measurement. Previous studies have proposed optimization criteria to estimate the relaxation time T1 faster than with a multipoint method leading to two-point methods. In this paper, the class of optimized two-point methods is extended to gradient-echo (GE) sequence offering new advantages over spin-echo (SE) or inversion recovery (IR) sequences. Two GE acquisitions, with optimal flip angles theta1 and theta2 minimizing both the total scan time and the variance in the computed T1 image were applied to estimate T1, and the results were compared with those of SE sequence with optimized paired repetition times T(R1) and T(R2). First, phantom studies were carried out with five tissue-like samples on a 0.5T scanner. Then in vivo, human brain T1 image were calculated using both optimized GE and SE two-point methods. More precise T1 GE estimates than those for SE were found thanks to high signal-to-noise ratio (SNR) per unit of time, but with a small bias. These results also concern the temperature variation measurement methods, based on T1 estimation. Preliminary experimental data for temperature measurement are given.
Fast 3D registration of MR brain images using the projection correlation registration algorithm.
Cross-correlation can be used to match 2D images in translation and in rotation. An extension to 3D mono-modality matching is presented. This process allows comparison of two sets of data along the same orientation. To decrease computation time, oblique projections of a 'wandering slice' are used. The precision is about +/- 0.2 degree in rotation and +/- 0.2 mm in translation. Some examples, applied to pre- and post-therapy comparison, are given.
Brain evoked potential topographic mapping based on the diffuse approximation.
Evoked potential mapping is a convenient technique to describe brain electrical activity using pictorial representation. A new interpolation method based on the diffuse approximation is applied to represent evoked potential distribution over the skull. The method retains most of the attractive features of the finite-element method but does not require explicit elements. In the simulation examples, the human head is assumed to be a single-layer sphere with homogeneous conductivity, and Ary eccentricity transformation is considered to approximate the more realistic three-shell model. The patterns shown in the computed maps suggest the ability of the proposed method to extract coherent information from the data from different electrodes. In the application protocol, visual evoked potentials are used to test the method with a realistic head shape.
[MRI of cerebral function].
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[Imaging of cerebral function: new trends in PET, MEG and MRI].
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Three dimensional representation of brain electrical activity.
Brain topography mapping is a useful technique for the representation of electrical activity recorded on the scalp. It clarifies spatial and temporal relationships between different cortical areas. In this work we propose a system which includes several enhancements over those previously proposed, such as an optimised interpolation method and a three dimensional reconstruction of maps. This system is available in a personal computer environment. Results clearly show a superiority of the 3D representation over 2D maps obtained with different projections. The performance of this system in terms of speed and precision is comparable to that of dedicated image processing and image synthesis workstations proposed for brain mapping.
Topographical characterization of normal visual evoked responses.
A study of visual evoked responses in a normal population was carried out using a high-precision programmable stimulator and an innovative brain mapping system which presents the overall activity on a three dimensional surface representing the human head. The interpolation method used gives a more precise computation of the potential value between electrodes. Two normal distributions of visual responses are presented which were identified while trying to topographically characterize visual evoked responses in order to construct a comparative data base. The proposed classification of two normal groups was verified by means of a statistical topographic analysis with the significance probability mapping technique.
Scalp potential and current density mapping with an enhanced spherical spline interpolation.
A method for producing rapid and accurate scalp electroencephalography, evoked potential and current density mapping is described. This method is based on the spherical spline interpolation, which has been validated as the most suitable function for this application. In comparison with the basic algorithm, the proposed technique is accurate and fast: it reduces the computation time by a factor of 10, while preserving the desired precision level. Furthermore, we study the choice of an optimal configuration and number of electrodes in order to produce a good reconstruction of brain electrical activity with this method. The implementation and application of this enhanced method in a three-dimensional brain mapping system is shown.