W. Skalli

3D/2D registration and segmentation of scoliotic vertebrae using statistical models

We propose a new 3D/2D registration method for vertebrae of the scoliotic spine, using two conventional radiographic views (postero-anterior and lateral), and a priori global knowledge of the geometric structure of each vertebra. This geometric knowledge is efficiently captured by a statistical deformable template integrating a set of admissible deformations, expressed by the first modes of variation in Karhunen-Loeve expansion, of the pathological deformations observed on a representative scoliotic vertebra population. The proposed registration method consists of fitting the projections of this deformable template with the preliminary segmented contours of the corresponding vertebra on the two radiographic views. The 3D/2D registration problem is stated as the minimization of a cost function for each vertebra and solved with a gradient descent technique. Registration of the spine is then done vertebra by vertebra. The proposed method efficiently provides accurate 3D reconstruction of each scoliotic vertebra and, consequently, it also provides accurate knowledge of the 3D structure of the whole scoliotic spine. This registration method has been successfully tested on several biplanar radiographic images and validated on 57 scoliotic vertebrae. The validation results reported in this paper demonstrate that the proposed statistical scheme performs better than other conventional 3D reconstruction methods.

Personalized body segment parameters from biplanar low-dose radiography

Body segment parameters are essential data in biomechanics. They are usually computed with population-specific predictive equations from literature. Recently, medical imaging and video-based methods were also reported for personalized computation. However, these methods present limitations: some of them provide only two-dimensional measurements or external measurements, others require a lot of tomographic images for a three-dimensional (3-D) reconstruction. Therefore, an original method is proposed to compute personalized body segment parameters from biplanar radiography. Simultaneous low-dose frontal and sagittal radiographs were obtained with EOS/spl trade/ system. The upper leg segments of eight young males and eight young females were studied. The personalized parameters computed from the biplanar radiographic 3-D reconstructions were compared to literature. The biplanar radiographic method was consistent with predictive equations based on /spl gamma/-ray scan and dual energy X-ray absorptiometry.

Vertebral wedging characteristic changes in scoliotic spines.

Study Design. A morphometric analysis of vertebral wedging in vertebrae from scoliotic specimens. Objective. To quantify the vertebral body changes in 30 anatomic specimens affected by a scoliotic deformity. Summary of Background Data. Only a few studies have evaluated the exact changes occurring at the level of the vertebral body in scoliotic spines. Most are observational studies of rare scoliotic specimens presenting major curvatures. The orientation of vertebral wedging is important for the planning of corrective surgery, performing vertebral osteotomy, and the evaluation of possible growth modulation. Materials and Methods. Thirty scoliotic specimens with curves presenting various degrees of severity were studied using a three-dimensional digitizing protocol developed to create a precise three-dimensional reconstruction of the vertebrae. Every scoliotic specimen was then matched with a normal specimen, and comparisons were made on the vertebral body parameters both for thoracic and lumbar vertebrae. Analysis of variance and t test calculations were performed to identify significant differences with P = 0.05. Results. A total of 471 vertebrae from scoliotic spines and 510 vertebrae from normal specimens were measured. Vertebral wedging increased progressively towards the apex of the curve and was maximal at the apex. Vertebral wedging was more prominent in the frontal plane, and there was minimal wedging in the sagittal plane. Vertebral heights were significantly different at T3 and T4 for the upper adjacent curve and at T6–T8 for a typical right thoracic curve, with smaller heights located on the concavity of the curve. No changes were observed on the convexity of the curve. Conclusion. Vertebral wedging is an essential component of the scoliotic deformity. The present study provides critical information for corrective surgery and vertebral osteotomy, as vertebral wedging occurs primarily in the frontal plane. Accurate knowledge of this deformityshould also provide new insight into corrective surgical strategies aiming at growth modulation and more efficient surgical correction.

A hierarchical statistical modeling approach for the unsupervised 3D reconstruction of the scoliotic spine

In this paper, we propose a new and accurate 3D reconstruction technique for the scoliotic spine from a pair planar and conventional radiographic images (postero-anterior and lateral). The proposed model uses a priori hierarchical global knowledge, both on the geometric structure of the whole spine and of each vertebra. More precisely, it relies on the specification of two 3D templates. The first, a rough geometric template on which rigid admissible deformations are defined, is used to ensure a crude registration of the whole spine. 3D reconstruction is then refined for each vertebra, by a template on which nonlinear admissible global deformations are modeled, with statistical modal analysis of the pathological deformations observed on a representative scoliotic vertebra population. This unsupervised coarse-to-fine 3D reconstruction procedure is stated as a double energy function minimization problems efficiently solved with a stochastic optimization algorithm. The proposed method, tested on several pairs of biplanar radiographic images with scoliotic deformities, is comparable in terms of accuracy with the classical CT-scan technique while being unsupervised and requiring a lower amount of radiation for the patient.

Modélisation vertébrale et squelettique par le système EOS

La durée du balayage est de l’ordre de 15 s pour un adulte et bien sûr décroît avec la taille de l’individu. L’immobilité requise pendant la durée du balayage est cependant une certaine limitation chez les tout jeunes enfants dont certains ne réalisent pas cette immobilité. Des systèmes de contention souple sont à l’étude pour palier à cet inconvénient. Les clichés obtenus sont numériques, non distordus (puisque le rayon est toujours perpendiculaire à l’objet). Ils peuvent être traités numériquement pour avoir un effet zoom sur une articulation précise par exemple. Facilement stockés informatiquement dans l’ordinateur, ils peuvent être délivrés sur fi lm et traités selon la pénétration (zones peu visibles en radio conventionnelle) cela évite la répétition des clichés. Par ailleurs grâce aux logiciels de reconstruction tridimensionnelle mis au point à l’ENSAM en collaboration avec LIO Montréal, une reconstruction 3D surfacique semi automatique de toutes les pièces squelettiques peut être effectuée. La validité de ces reconstructions 3D a été évaluée par rapport à celle obtenue par les coupes jointives obtenues au scanner et vérifi ée tout à fait comparable. Tout cela s’obtient à partir de la seule paire de clichés initiaux avec l’avantage donc d’une diminution considérable des doses d’irradiation (de l’ordre de 800 à 1000 fois moins que les reconstructions 3D scanner). Si l’on se souvient que les dangers de l’irradiation sont d’autant plus importants que l’enfant est jeune, on comprend l’intérêt d’un tel appareil en pédiatrie. Enfi n comme l’examen se fait en position debout, l’infl uence de la gravité est donc bien exprimée grâce à EOS. Le corollaire est un inconvénient : EOS en position couchée n’existe pas encore ce qui l’exclut pour les examens faits obligatoirement en position couchée (traumatismes par exemple).

3D biplanar reconstruction of scoliotic vertebrae using statistical models

This paper presents a new 3D reconstruction method of the scoliotic vertebrae of a spine, using two conventional radiographic views (postero-anterior and lateral), and global prior knowledge on the geometrical structure of each vertebra. This geometrical knowledge is efficiently captured by a statistical deformable template integrating a set of admissible deformations, expressed by the first modes of variation in the Karhunen-Loeve expansion of the pathological deformations observed on a representative scoliotic vertebra population. The proposed reconstruction method consists in fitting the projections of this deformable template with the segmented contours of the corresponding vertebra on the two radiographic views. The 3D reconstruction problem is stated as the minimization of a cost function for each vertebra and solved with a gradient descent technique. The reconstruction of the spine is then made vertebra by vertebra. This 3D reconstruction method has been successfully tested on several biplanar radiographic images, yielding very promising results.

A computer-based classifier of three-dimensional spinal scoliosis severity

ObjectiveThis article describes a computer-based method for the classification of spine scoliosis severity. This is a first step toward an effective computerized tool to assist general practitioners diagnose spine scoliosis. The method progresses away from Cobb angles toward pattern and magnitude categorization based upon 3D configurations.Materials and methodsThe purpose is to classify spine shapes reconstructed from a pair of calibrated X-ray images into one of three categories, namely, normal spine, moderate scoliosis, and severe scoliosis. The spine shape is represented by the three-dimensional coordinates of a sequence of equidistant points sampled by interpolation on the reconstructed spine shape. Classification is carried out using a self- organizing Kohonen neural network trained using this representation.ResultsThe tests were performed using a database of 174 spine biplane X-rays. The classification accuracy was 97%.ConclusionThe results demonstrate that classification of 3D spine descriptions by a Kohonen neural network affords a solid basis for an effective tool to assist clinicians in assessing scoliosis severity.

3D reconstruction of the scapula from biplanar radiographs

Access to 3D bone models is critical for applications ranging from pre-operative planning to biomechanics studies. This work presents a method for 3D reconstruction of the scapula from biplanar radiographs, which is based on the combination of a parametric model approach in conjunction with a Moving Least Squares (MLS) deformation technique. A parametric scapula model was created by fitting geometric primitives (with their descriptive parameters) to the CT reconstruction of a dry scapula. These geometric primitives were then used to define a set of handles which allow the user to control the as-rigid-as-possible deformation of the template model in real-time, until optimal correspondence between the actual X-ray images and the retro-projection of the deformed model. When applied to 10 dry scapulae, the presented method allowed obtaining reconstructions which were on average within 1mm of the CT-derived model at scapula regions of interest. Morphological parameters such as the glenoids dimensions and orientation were determined with errors of 1° and less than 1mm, on average. This is of great interest as the current methods used in clinical practice, which are based on 2D-CT, are subject to uncertainties of the order of 5° for glenoid version. This method is of particular interest as it further reduces our dependence to CT for 3D reconstruction of bones and clinical parameter estimation.

A patient specific model for gait analysis

INTRODUCTION In gait analysis, joint forces and moments are computed from force plate data, kinematics data and body segment parameters (BSP). BSP are estimated from predictive or scaling equations, such as de Leva’s equations [1], which are limited to the three principal moments of inertia and to the center of mass’ position along a line joining two adjacent joints. Dumas proposed a method to compute personalized BSP of the thigh thanks to stereoradiography [2]. The object of this work is to combine stereoradiography and gait analysis to set up a patient specific model of the lower limb in order to enhance the accuracy of computed kinetics and improve the clinical analysis.