An inertial-optical tracking system for portable, quantitative, 3D ultrasound

Freehand 3D ultrasound imaging has been growing in popularity. However, the unavoidable reconstruction errors introduced by freehand motion have limited its usefulness. To overcome this, freehand ultrasound systems have been augmented with external tracking sensors to produce accurate 3D images in mainly experimental settings, but these systems have yet to be accepted for general clinical use. In addition, the use of external tracking sensors limits the portability of the system. This paper presents a 5 degree of freedom, low cost, integrated tracking device for quantitative, freehand, 3D ultrasound. It uses a combination of optical and inertial sensors to track the position and orientation of the ultrasound probe during a 3D scan. These sensors can be attached to or contained completely within the ultrasound transducer. Stradwin 3D ultrasound software acquires 2D image frames from the ultrasound system and position and orientation data from the tracking system to generate 3D ultrasound images in real-time. 3D reconstruction performance was evaluated by freehand scanning cylindrical inclusions in a tissue mimicking ultrasound phantom. Different scan patterns were tested to provide performance data for errors introduced in individual degrees of freedom. 3D images were formed from the data with and without the use of the tracking information, and then manually segmented. The volume and surface accuracy of the segmented regions were then compared to the ground truth. The mean volume error was 3.84% with the position information and 18.57% without. The mean RMS surface error was .381 mm with the position information and .843 mm without.