2D Translation from a 6-DOF MEMS IMU’s Orientation for Freehand 3D Ultrasound Scanning

Abstract

MEMS (Micro Electro-Mechanical Systems) based technology has made it possible to fabricate accelerometers and gyroscopes that are small enough to fit onto chips and thus all the components for a 6-DOF (6 degrees of freedom) IMU (Inertial Measurement Unit) can be enclosed in one compact device. Examples in the market include those from Xsens Technologies B.V. and GLI Interactive LLC. The IMU that is used in this paper is the MT9-B from Xsens where we sought to obtain translational data for use in freehand 3D Ultrasound (3D-US) Scanning. A freehand 3D-US image is constructed from a series of 2D Ultrasound B-scans made with a conventional B-mode scanner scanning across a subject of interest. From the 3D position and orientation (i.e. the 6-DOF parameters) of those B-scans a 3D image can be constructed. The conventional methods of obtaining those parameters in freehand 3D-US scanning are from optical, magnetic, acoustic and articulated-arm tracking. All those methods have some form of limitation and an inertial sensor solution is sought to be an alternative that is not physically intrusive (in the case of articulated- arm tracking) and less dependent of environmental conditions (unlike the remaining three methods). In this paper, orientation output alone is used as translation calculated from acceleration output suffers from drift. The calculation of translation from orientation change is made possible by housing the sensor in a cube of known geometry. The cube is then rolled and ‘walked’ across a plane and its planar translation can then be calculated. We then evaluate the results and assess the feasibility of this method to be applied in freehand 3D-US. Its limitations are expounded upon and improvements are suggested. We hope that this paper would be a good introduction to research using 6-DOF inertial MEMS sensors that have a potentially wide range of applications.