  Tracking of rotation and translation of medical instruments plays a
substantial role in many modern interventions. Traditional external optical
tracking systems are often subject to line-of-sight issues, in particular when
the region of interest is difficult to access or the procedure allows only for
limited rigid body markers. The introduction of inside-out tracking systems
aims to overcome these issues. We propose a marker-less tracking system based
on visual SLAM to enable tracking of instruments in an interventional scenario.
To achieve this goal, we mount a miniature multi-modal (monocular, stereo,
active depth) vision system on the object of interest and relocalize its pose
within an adaptive map of the operating room. We compare state-of-the-art
algorithmic pipelines and apply the idea to transrectal 3D Ultrasound (TRUS)
compounding of the prostate. Obtained volumes are compared to reconstruction
using a commercial optical tracking system as well as a robotic manipulator.
Feature-based binocular SLAM is identified as the most promising method and is
tested extensively in challenging clinical environment under severe occlusion
and for the use case of prostate US biopsies.
