Computer assisted navigation systems that combine real-time endoscopy images with pre-operative volumetric data sets
aim at improving the physician's understanding of the underlying anatomical structures. To achieve accurate and safe
guidance these systems are required to provide a consistent representation of the physical world. This implies that all
data streams are synchronized. In our case, we are dealing with synchronization of tracking data and a video stream
obtained by a tracked endoscope. Previously, such synchronization was obtained pre-operatively using phantoms. This
type of approach assumes a constant latency between the data streams and is less desirable for clinical use due to the
required additional hardware. In this work we describe an online temporal synchronization method. The method is based
on the observation that in clinical practice the endoscope is not in constant motion. By identifying corresponding
stationary points in the video and tracking streams temporal synchronization can be performed online in a manner that is
transparent to the user. Initial evaluation of our approach in a laboratory study has shown that it provides comparable
estimates to a phantom based approach we had previously proposed.
In a detailed laboratory investigation we performed a series of experiments in order to assess the validity of the widely
used TRE concept to predict the application accuracy. On base of 1mm CT scan a plastic skull, a cadaver head and a
volunteer were registered to an in house navigation system. We stored the position data of an optical camera (NDI
Polaris) for registration with pre-defined CT coordinates. For every specimen we choose 3, 5, 7 and 9 registration and 10
evaluation points, respectively, performing 10 registrations. The data were evaluated both with the Arun and the Horn
approaches. The vectorial difference between actual and predefined position in the CT data set was stored and evaluated
for FRE and TRE. Evaluation and visualization was implemented in Matlab. The data were analyzed, specifically for
normal distribution, with MS Excel and SPSS Version 15.0.
For the plastic skull and the anatomic specimen submillimetric application accuracy was found experimentally and
confirmed by the calculated TRE. Since for the volunteer no Titanium screws were implanted anatomic landmarks had to
be used for registration and evaluation; an application accuracy in the low millimeter regime was found in all
approaches. However, the detailed statistical analysis of the data revealed that the model predictions and the actual
measurements do not exhibit a strong statistical correlation (p < 0.05). These data suggest that the TRE predictions are
too optimistic and should be used with caution intraoperatively.
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