Introduction: For the pelvis, the left and right iliac anterior superior spine and the pubic symphysis define the anterior pelvic plane (APP) and are percutane-ously accessible. For cup-alignment in Total Hip Replacement (THR) these landmarks can be used to determine a pelvis coordinate system. Palpating their three-dimensional position leads to systematic APP malorientation due to the thicknesses of the tissues between skin and bone [1]. Since the pelvis landmarks are also visible in sonograms, we investigated whether their spatial positions can be determined more precisely with ultrasound (US) scans. The feasibility of automatic landmark detection in 3-D US volumes and the accuracy of the determined landmark positions was investigated using 2.5-D, interactive 3-D and automatic 3-D approaches. Methods: An ultrasound imaging system with a 5-10 MHz linear probe (Echo-Blaster 128, Telemed, Lithuania) was used together with an infra-red optical lo-calizer system (CamBar, Axios3D Services GmbH, Germany) and a Graphical User Interface (GUI) in order to realize a 2.5-D and 3-D ultrasound system.
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