Patrick Brigger

B-spline snakes: a flexible tool for parametric contour detection

We present a novel formulation for B-spline snakes that can be used as a tool for fast and intuitive contour outlining. We start with a theoretical argument in favor of splines in the traditional formulation by showing that the optimal, curvature-constrained snake is a cubic spline, irrespective of the form of the external energy field. Unfortunately, such regularized snakes suffer from slow convergence speed because of a large number of control points, as well as from difficulties in determining the weight factors associated to the internal energies of the curve. We therefore propose an alternative formulation in which the intrinsic scale of the spline model is adjusted a priori; this leads to a reduction of the number of parameters to be optimized and eliminates the need for internal energies (i.e., the regularization term). In other words, we are now controlling the elasticity of the spline implicitly and rather intuitively by varying the spacing between the spline knots. The theory is embedded into a multiresolution formulation demonstrating improved stability in noisy image environments. Validation results are presented, comparing the traditional snake using internal energies and the proposed approach without internal energies, showing the similar performance of the latter. Several biomedical examples of applications are included to illustrate the versatility of the method.

The watershed algorithm: a method to segment noisy PET transmission images

Attenuation correction is essential to PET imaging but often requires impractical acquisition times. Segmentation of short noisier transmission scans has been proposed as a solution. We report that a 3D morphological tool-the watershed algorithm-is well adapted for segmenting even 2-minute PET transmission images. The technique is non-iterative, fast and fully 3-D and inherently ensures class continuity and eliminates outliers. Pre-filtering the data induced smoother class edges, showing that a multi-resolution approach could be used to deal with partial volume effect and excessive noise in the data. The algorithm was tested on 2-minute scans of a torso phantom and of a human study.

Morphological shape representation for very low bit-rate video coding

Abstract Region-based coding schemes are among the most promising compression techniques for very low bit-rate applications. They consist of image segmentation, contour and texture coding. This paper deals with the use of the geodesic skeleton as a morphological tool for contour coding of segmented image sequences. In the geodesic case, already coded and known regions are taken into account for the coding of contours of unknown regions. A new technique is presented for the entropy coding of the coordinates of the skeleton points exploiting their special spatial distribution. Furthermore, a fast algorithm for the reconstruction of the skeleton points is given based on hierarchical queues. In the case of numerous isolated contour arcs (for example error coding in a motion prediction loop), the geodesic skeleton proofs higher efficiency than traditional methods. Results at very low bit-rates are presented and compared to standard methods confirming the validity of the chosen approach.

Multiresolution approximation using shifted splines

We consider the construction of least squares pyramids using shifted polynomial spline basis functions. We derive the pre and post-filters as a function of the degree n and the shift parameter /spl Delta/. We show that the underlying projection operator is entirely specified by two transfer functions acting on the even and odd signal samples, respectively. We introduce a measure of shift invariance and show that the most favorable configuration is obtained when the knots of the splines are centered with respect to the grid points (i.e., /spl Delta/=1/2 when n is odd and /spl Delta/=0 when n is even). The worst case corresponds to the standard multiresolution setting where the spline spaces are nested.

Multi-Scale B-Spline Snakes for General Contour Detection

Traditional snakes suffer from slow convergence speed (many control points) and difficult to adjust weighting factors for internal energy terms. We propose an alternative formulation using cubic B-splines, where the knot spacing is variable and controlled by the user. A larger knot spacing allows us to reduce the number of parameters, which increases optimization speeds. It also eliminates the need for internal energies, which improves user interactivity. The optimization procedure is embedded into a multi- resolution image representation, where the number of snake points is adapted to the image grid spacing by correctly adjusting the spline knot spacing. Hence, the proposed method provides a multi-scale approach in both the image and parametric contour domain. Our technique provides fast optimization of the initial snake curve and leads to more stable algorithms in noisy imaging environments. Several biomedical examples of applications are included to illustrate the versatility of the method.

Morphological shape representation of segmented images based on temporally modeled motion vectors

Region based coding schemes are among the most promising compression techniques for very low bit-rates. They consist of image segmentation, contour and texture coding. In this paper, a new shape representation for segmented images based on the geodesic morphological skeleton is presented. It is used for the coding of contour prediction residues obtained after motion compensation based on temporally modeled motion vectors. A non-reversible pre-skeletonization filter removes contour noise. A reversible post-skeletonization filter allows progressive transmission of contour information. It is based on the spatial distribution of the skeleton points, which are found to be close to known contours.<<ETX>>

Segmentation of gated TI-SPECT images and computation of ejection fraction: A different approach

BackgroundWe describe a set of image processing algorithms and mathematical models that can be advantageously used in schemes for the segmentation of thallium-201-single photon emission computed tomography (SPECT) images and for computation of left ventricular ejection fraction (EF).MethodsThe system consists of two independent blocs for image segmentation and computation of function. The former is based on a multiresolution elliptical coordinate transformation and dynamic contour tracking. Computation of EF is formulated on the basis of both the endocardial and epicardial contours, and we compare this formulation with that using only the endocardial border for images with low signal-to-noise ratios. The accuracy of border detection was validated against manual border tracing on FDG-PET images, simulated TI-201-SPECT images where the true underlying borders were known, and actual TI-201-SPECT images. Finally, we compared EFs computed for FDG-PET, technetium-99m-SPECT and Tl-201—SPECT with those obtained from planar gated blood pool imaging.ResultsThe automatically obtained results always were within the manual uncertainty range. Agreement between myocardial volumes from positron emission tomography and automatically obtained values from the simulated Tl-201-SPECT images was excellent (r=0.95, n=32). Agreement between EFs from planar gated blood pool imaging and the other image modalities was good (FDG-PET: y=5.89+1.21x, r=0.92, see=6.24, n=19, Tc-99m-SPECT: y=−3.86+1.06x, r=0.88, see=7.78, n=9, Tl-201-SPECT: y=17.8+0.81x, r=0.77, see=7.44, n=26). For noisy input data the combined use of information from epicardial and endocardial contours gives more accurate EF values than the traditional formula on the basis of the endocardial contour only.ConclusionsAlternate approaches for segmentation and computation of function have been presented and validated. They might also be advantageously incorporated into other existing techniques.

B-spline snakes and a JAVA interface: an intuitive tool for general contour outlining

We present a novel formulation for B-spline snakes that can be used as a tool for fast and intuitive contour outlining. The theory is implemented in a platform independent JAVA interface, which allows real time computation of the snake curve. In this paper, our main focus is on two points. First, we propose a novel B-spline snake formulation, where the intrinsic scale of the spline model is adjusted a priori. It leads to a reduction of the number of parameters to be optimized and eliminates the need for internal energies. The approach solves the two main drawbacks of traditional snakes (slow convergence speed and difficult to adjust weighting factors for internal energy terms). Second, we comment on our experience using JAVA for the implementation of the snake and for the design of the graphical user interface. Our technique provides a very intuitive, user-friendly, and platform independent tool for contour outlining, generally applicable to a vast range of images. Several biomedical examples of applications are included to illustrate the versatility of the method.

Morphological operators for very low bit rate video coding

This paper deals with the use of some morphological tools for video coding at very low bit rates. Rather than describing a complete coding algorithm, the purpose of this paper is to focus on morphological connected operators and segmentation tools that have proved to be attractive for compression.

Segmentation of gated SPECT images for automatic computation of myocardial volume and ejection fraction

Describes an image processing system for the automatic assessment of ejection fraction (EF) from noisy SPECT left ventricular myocardial perfusion images. A segmentation scheme consisting of an elliptical coordinate transformation, matched filtering and dynamic contour tracking detects the endo- and epicardial boundaries. Computation of EF is performed based on the epi- rather than the endocardial boundary, which proves to be more robust for images with low signal-to-noise ratios. The computation incorporates anatomical constraints of constant myocardial mass and smooth cardiac variation. The algorithm was tested on different image modalities and shows good linear agreement with EFs obtained from conventional approaches based on planar gated blood pool imaging (PET: y=8.7+1.07x, r=0.84, Technetium-99m MIBI SPECT: y=5.0+0.80x, r=0.90, Thallium-201 SPECT: y=23.5+0.82x, r=0.77, RMS error=10.0). The proposed scheme may be an alternative for EF computation without the need for additional image acquisitions.