Ex vivo evaluation of force data and tissue elasticity for robot-assisted FESS.

The objective of this study was to describe and evaluate soft tissue and bone properties of nasal cavity and paranasal sinuses in ex vivo preparations for a safe robot-assisted endoscope movement during functional endoscopic sinus surgery (FESS). In a first experiment we measured forces exerted by the endoscope during FESS with a force/torque sensor. In a second experiment we used a purpose built device to exert forces on chosen tissue structures. The experiment was monitored by a custom software, which records force of the endoscope and the deformation and the breaking point of tissue. All tests were performed on five formalin fixed cadaver heads. In the first experiment we found that the average force during FESS is 2.21 N and the maximal force is 7.96 N. The force-way-ratio measurement shows highest elasticity for the ethmoidal bulla, followed by the lamina papyracea; however, they break at low forces (> or =6 N). Furthermore the carotid canal seems to have the lowest elasticity but it can tolerate forces up to 30 N. Based on these measurements force thresholds can be defined for robot-assisted endoscope guidance. All thresholds have to be assigned to subregions of the nasal cavity and paranasal sinuses.

Analysis of manual segmentation in paranasal CT images.

Manual segmentation is often used for evaluation of automatic or semi-automatic segmentation. The purpose of this paper is to describe the inter and intraindividual variability, the dubiety of manual segmentation as a gold standard and to find reasons for the discrepancy. We realized two experiments. In the first one ten ENT surgeons, ten medical students and one engineer outlined the right maxillary sinus and ethmoid sinuses manually on a standard CT dataset of a human head. In the second experiment two participants outlined maxillary sinus and ethmoid sinuses five times consecutively. Manual segmentation was accomplished with custom software using a line segmentation tool. The first experiment shows the interindividual variability of manual segmentation which is higher for ethmoidal sinuses than for maxillary sinuses. The variability can be caused by the level of experience, different interpretation of the CT data or different levels of accuracy. The second experiment shows intraindividual variability which is lower than interindividual variability. Most variances in both experiments appear during segmentation of ethmoidal sinuses and outlining hiatus semilunaris. Concerning the inter and intraindividual variances the segmentation result of one manual segmenter could not directly be used as gold standard for the evaluation of automatic segmentation algorithms.