gms | German Medical Science

ESBS 2005: Skull Base Surgery: An Interdisciplinary Challenge
7th Congress of the European Skull Base Society held in association with
the 13th Congress of the German Society of Skull Base Surgery

18. - 21.05.2005, Fulda, Germany

New recently developed technologies for planning, registration and navigation with possible impact on further improved surgery on the skull

Meeting Contribution

  • M. Caversaccio - Department of ORL, Head and Neck Surgery, Inselspital, University of Bern, Switzerland
  • I. Pappas - ME Müller Institute for Surgical Technology, University of Bern, Switzerland
  • F. Langlotz - ME Müller Institute for Surgical Technology, University of Bern, Switzerland
  • G. Marti - Ecole Polytechnique fédérale, Lausanne, Switzerland
  • C. Baur - Ecole Polytechnique fédérale, Lausanne, Switzerland
  • L.P. Nolte - ME Müller Institute for Surgical Technology, University of Bern, Switzerland
  • R. Häusler - Department of ORL, Head and Neck Surgery, Inselspital, University of Bern, Switzerland

ESBS 2005: Skull Base Surgery: An Interdisciplinary Challenge. 7th Congress of the European Skull Base Society held in association with the 13th Congress of the German Society of Skull Base Surgery. Fulda, 18.-21.05.2005. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc05esbs49

doi: 10.3205/05esbs49, urn:nbn:de:0183-05esbs495

Published: January 27, 2009

© 2009 Caversaccio et al.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( You are free: to Share – to copy, distribute and transmit the work, provided the original author and source are credited.



Different new technologies developed in Bern will be presented:

1. Automatic method for mid-facial symmetry plane extraction

Recently developed computer applications provide tools for planning cranio-maxillofacial interventions, based on three-dimensional, virtual models of the patient’s skull obtained from computed-tomography scans. Precise knowledge of the location of the mid-facial plane is important for the assessment of deformities and for planning reconstructive procedures. We present a new method to automatically compute the mid-facial plane on the basis of a meshed surface model of the facial skeleton obtained from volumetric CT images. The method matches user-selected homologous surface areas of the left and right facial side using an iterative closest point optimisation algorithm and subsequently computes the optimal symmetry plane for this transformation. The new automatic method was compared in an experimental study including experienced and non-experienced users to a manual method, which is based on a selection of anatomical landmarks for the symmetry plane definition. The quality of the symmetry planes was evaluated by their ability to match homologous areas of the face. The results showed that the proposed automatic method is reliable, fast and leads to significantly higher accuracy than the manual method when performed by non-experts and in most cases also when performed by experts. The authors see a great potential for this method to become a standard tool in computer-aided cranio-maxillofacial planning applications (SPIE Medical Imaging, 403-410, 2004).

2. Automatic method to assess local CT-MR imaging registration accuracy on images of the head

Precise registration of CT and MR images is crucial in many clinical cases for poor diagnosis, decision macking or navigation in surgical interventions. Various algorithms can be used to register CT and MR datasets, but prior to clinical use the result must be validated. To evaluate the registration result by visual inspection is tiring and time-consuming. We propse a new automatic registration assessment method, which provides the user a color-coded fused representation of the CT and MR images, and indicates the location and extent of poor registration accuracy. The method for local assessment of CT-MR registration is based on segmentation of bone structures in the CT and MR images, followed by a voxel correspondence analysis. The result is represented as color-coded overlay. The algorithm was tested on simulated and real datasets with different levels of noise and intensitiy non-uniformity. Based on tests on simulated MR imaging data, it was found that the algorithm was robust for noise levels up to 7% and intensity non-uniformities up to 20% of the full intensity scale. Due to the inability to distinguish clearly between bone and cerebro-spinal fluids in the MR image (T1 weighted). The algorithm was found to be optimistic in the sense that a number of voxels are classified as well-registered although they should not. However, nearly all voxels classified as misregistered are correctly classified. The proposed algorithm offers a new way to automatically assess the CT-MR image registration accuracy locally in all the areas of the volume that contain bone and to represent the result with a user-friendly, intuitive color-coded overlay on the fused dataset [1].

3. Improved targeting device and Computer navigation for accurate placement of brachytherapy needles

Successful treatment of skull-base tumors with interstitial brachytherapy requires high targeting accuracy for the brachytherapy needles to avoid harming vital anatomical structures. To enable safe placement of the needles in this area, we developed an image-based planning and navigation system for brachytherapy, which includes a custom-made mechanical positioning arm that allows rough- and fine-adjustment of the needle position. The fine-adjustment mechanism consists of a XYZ microstage at the base of the arm and a needle-holder with two fine-adjustable inclinations. The rotation axes of the inclinations cross at the tip of the needle so that the inclinational adjustments do not interfere with the translational adjustments. A vacuum cushion and a non-invasive fixation frame are used for the head immobilization. To avoid mechanical bending of the needles due to the weight of attached tracking markers, which would be detrimental for targeting accuracy, only a single LED-marker on the tail of the needle is used. An experimental phantom-based targeting study with this set-up demonstrated that a positioning accuracy of 1.4 mm (RMS) can be achieved. The study showed that the proposed set-up allows brachytherapy needles to be easily aligned and inserted with high targeting accuracy according to a preliminary plan. The achievable accuracy is higher than if the needles are inserted manually. The proposed system can be linked to a standard afterloader and standard dosimetry planning module. The associated additional effort is reasonable for the clinical practice and therefore the proposed procedure provides a promising tool for the safe treatment of tumors in the skull base area [2].

4. Image guided microscope mounted with a minitracker

Surgical microscopy is used in many complex procedures in the area of otorhinolaryngology and neurosurgery. Here we present a new image-guided microscope system using augmented reality image overlays directly added onto one of the views of the operating microscope. With this system segmented objects, outlines of risk regions or access pathways, previously extracted from preoperative tomographic images can be directly displayed as augmented reality overlays on the microscope image.

Unlike other systems that uses an external tracking device, or based on the image information, the novelty of this design consists in mounting a precise mini-tracker directly on the microscope, in parallel to the optical axis, to track the movements of the surgical tools and the patient.(Patent Ref: 0299-001 B CH-P). The rigid connection of the tracker (Atracsys, Switzerland) with the microscope (M500, Leica, Switzerland) eliminates the need to externally track the position of the microscope, thereby bypassing the greatest source of error. Besides the gain in accuracy, this setup offers an improved ergonomy since the new location of the tracker makes it much easier for the surgeon to keep the line of sight to the tracked objects unobstructed (cf Figure 1 [Fig. 1]) (CARS 1311, 2004).

Acknowledgment: This work was supported by the Swiss National Science Foundation ( )


Pappas IP, Styner M, Malik P, Remonda L, Caversaccio M. Automatic method to assess local CT-MR imaging registration accuracy on images of the head. AJNR Am J Neuroradiol. 2005;26(1):137-44.
Pappas IP, Ryan P, Cossmann P, Kowal J, Borgeson B, Caversaccio M. Improved targeting device and computer navigation for accurate placement of brachytherapy needles. Med Phys. 2005;32(6):1796-801.