*2.3.2 Optical-based position tracking*

The freehand 3D ultrasound imaging system with optical tracking sensor involves two important equipments: the markers mounted on the probe and one or multiple cameras to track the marker. Currently, the Polaris Optical Tracking System and Optotrak Certus are the two commercial optical trackers for 3D ultrasound imaging system and both are the product of Northern Digital (NDI). However, the problems found in the optical tracking system are that the marker mounted on the probe is large and caused the ultrasound scanning session to be inconvenient [12] and the line of sight of cameras must not be obstructed [13]. In order to counter this problem, the work in [13] created an optical tracking system with inertial sensor for freehand 3D ultrasound imaging, without external reference such as cameras. As for the cost-effective feature, in [11], the authors had introduced the use of PlayStation (PS) Move and PS Eye in the conventional 2D ultrasound probe for the 3D ultrasound reconstruction. This method is also able to offer portability and extensibility to the ultrasound imaging system.

#### *2.3.3 Sensorless*

The untracked freehand system or sensorless method requires the operator to move the transducer in a steady and regular motion at a constant linear or angular velocity, while 2D ultrasound frames are captured to generate a 3D ultrasound image [14]. Recently, a sensorless reconstruction method has been designed using a regression-based distance measurement, interpolation techniques, and unconstrained

freehand data without any limitation on the trajectory [15]. In recent study, the image-based algorithm makes use of the adaptive speckle decorrelation to learn relative position and orientation between the acquired 2D ultrasound image pairs [16]. Since the sensorless freehand ultrasound does not need any position tracking sensor, it is considered the most portable 3D freehand ultrasound system [16]. However, the inconsistency scan rate and angle can cause the reconstruction result to be not smooth and also results in less quality 3D image during 3D visualization step [14].
