Quantitative evaluation of an automatic segmentation method for 3D reconstruction of intervertebral scoliotic disks from MR images

Background: For some scoliotic patients the spinal instrumentation is inevitable. Among these patients, those with stiff curvature will need thoracoscopic disk resection. The removal of the intervertebral disk with only thoracoscopic images is a tedious and challenging task for the surgeon. With computer aided surgery and 3D visualisation of the interverterbral disk during surgery, surgeons will have access to additional information such as the remaining disk tissue or the distance of surgical tools from critical anatomical structures like the aorta or spinal canal. We hypothesized that automatically extracting 3D information of the intervertebral disk from MR images would aid the surgeons to evaluate the remaining disk and would add a security factor to the patient during thoracoscopic disk resection.Methods: This paper presents a quantitative evaluation of an automatic segmentation method for 3D reconstruction of intervertebral scoliotic disks from MR images. The automatic segmentation method is based on the watershed technique and morphological operators. The 3D Dice Similarity Coefficient (DSC) is the main statistical metric used to validate the automatically detected preoperative disk volumes. The automatic detections of intervertebral disks of real clinical MR images are compared to manual segmentation done by clinicians.Results: Results show that depending on the type of MR acquisition sequence, the 3D DSC can be as high as 0.79 (+/- 0.04). These 3D results are also supported by a 2D quantitative evaluation as well as by robustness and variability evaluations. The mean discrepancy (in 2D) between the manual and automatic segmentations for regions around the spinal canal is of 1.8 (+/- 0.8) mm. The robustness study shows that among the five factors evaluated, only the type of MRI acquisition sequence can affect the segmentation results. Finally, the variability of the automatic segmentation method is lower than the variability associated with manual segmentation performed by different physicians.Conclusions: This comprehensive evaluation of the automatic segmentation and 3D reconstruction of intervertebral disks shows that the proposed technique used with specific MRI acquisition protocol can detect intervertebral disk of scoliotic patient. The newly developed technique is promising for clinical context and can eventually help surgeons during thoracoscopic intervertebral disk resection

Construction d'un modèle per-opératoire 3D du rachis pour la navigation en thoracoscopie.

RÉSUMÉ: Lors de discectomie par thoracoscopie, les outils de visualisation procurent peu d’information de profondeur et le champ de visualisation de la caméra miniature insérée dans le patient est relativement restreint. Aussi, le mouvement simultané de la caméra et des instruments chirurgicaux peut provoquer une désorientation. Ainsi, la courbe d’apprentissage pour l’utilisation de cette technologie est très abrupte et un nombre restreint de chirurgiens choisissent l’intervention minimalement invasive malgré les avantages qu’elle peut procurer aux patients. En effet la discectomie par thoracoscopie réduit les pertes sanguines, le traumatisme des tissus entourant le disque afin d’accéder à la zone d’intérêt et le temps d’hospitalisation. Les discectomies sont prescrites à certains patients scoliotiques afin de redonner de la flexibilité à la colonne avant l’instrumentation (pose de vis et tige pour corriger la déformation). La résection du disque intervertébral est faite partiellement et la quantité du disque réséqué dépend du degré de flexibilité que le chirurgien désire redonner au patient. En effectuant la discectomie par thoracoscopie, il est impossible pour le chirurgien de visualiser rapidement la quantité de disque restant en plus d’avoir les désavantages de désorientation et de petit champ de vision de la caméra miniature insérée dans le patient. Il est donc pertinent de tenter de réduire les problèmes de visualisation rencontrés lors des thoracoscopies en procurant au chirurgien la possibilité d’examiner en 3D les structures anatomiques du patient pendant la chirurgie sans ajouter de radiations supplémentaires au patient. Ce système d’assistance permettrait également d’accroître la sécurité du patient et la qualité de la chirurgie en donnant aux chirurgiens la possibilité de localiser en 3D la moelle épinière et en leur donnant également la possibilité de visualiser la quantité de disque restant. Ainsi, l’intérêt de fusionner les images vidéo avec un modèle pré-opératoire 3D est alors tout indiqué.---------- ABSTRACT: Visualization tools available while doing thoracoscopic diskectomy do not show depth information and the field of view of the miniaturized camera inserted into the patient is small. Also, simultaneous movement of the camera and surgical tools may result in disorientation. The learning curve for the use of this technology is very steep and numbers of surgeons choose not to use minimally invasive surgery despite important advantages for the patients. Indeed, thoracoscopic diskectomy reduce blood loss, trauma of surrounding soft tissues to access intervertebral disks and hospitalization time. Diskectomy are prescribed to specific scoliotic patients to gain flexibility of the spine before instrumentation surgery (fixation of screws and rod to correct the deformation). The intervertebral disk is partly resected depending on the level of flexibility the patient has to gain according to the surgeon. During thoracoscopic diskectomy, it is impossible for the surgeon to rapidly visualize the remaining disk tissue and this further increase the disadvantages for the surgeons. Hence, it is relevant to try to reduce visualization problems encountered during thoracoscopic diskectomy by providing to the surgeons a 3D view of the whole spine during the surgery, without adding supplementary radiation to the patient. The computer assisted surgery system would also increase the security of the patient by allowing the surgeons to localize rapidly in 3D the spinal canal as well as the remaining disk. The fusion of the video images with 3D spine of the patient is of great interest for the surgeons