Fig. (2) Components of the measurement system: (1) central unit (Microsoft Surface, Redmond, USA), (2) Ultrasound device (Echo Blaster 128, Telemed, Vilnius, Lithuania), (3) handheld tracking system (iPhone 6), (4) ultrasound transducer with attached rigid body, (5) reference rigid body (Aesculap AG, Tuttlingen, Germany).