  Purpose. Ability to locate and track ultrasound images in the 3D operating
space is of great benefit for multiple clinical applications. This is often
accomplished by tracking the probe using a precise but expensive optical or
electromagnetic tracking system. Our goal is to develop a simple and low cost
augmented reality echography framework using a standard RGB-D Camera.
  Methods. A prototype system consisting of an Occipital Structure Core RGB-D
camera, a specifically-designed 3D marker, and a fast point cloud registration
algorithm FaVoR was developed and evaluated on an Ultrasonix ultrasound system.
The probe was calibrated on a 3D-printed N-wire phantom using the software PLUS
toolkit. The proposed calibration method is simplified, requiring no additional
markers or sensors attached to the phantom. Also, a visualization software
based on OpenGL was developed for the augmented reality application.
  Results. The calibrated probe was used to augment a real-world video in a
simulated needle insertion scenario. The ultrasound images were rendered on the
video, and visually-coherent results were observed. We evaluated the end-to-end
accuracy of our AR US framework on localizing a cube of 5 cm size. From our two
experiments, the target pose localization error ranges from 5.6 to 5.9 mm and
from -3.9 to 4.2 degrees.
  Conclusion. We believe that with the potential democratization of RGB-D
cameras integrated in mobile devices and AR glasses in the future, our
prototype solution may facilitate the use of 3D freehand ultrasound in clinical
routine. Future work should include a more rigorous and thorough evaluation, by
comparing the calibration accuracy with those obtained by commercial tracking
solutions in both simulated and real medical scenarios.
