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The use of three-dimensional ultrasound (3DUS) imaging in clinical applications has contributed to more extensive information for medical diagnoses. Significant improvements in the generation of structural volumetric representation enable better visualization and more accurate measurement and analysis (Fenster & Downey, 2000; Solberg, Lindseth, Torp, Blake, & Nagelhus Hernes, 2007; Brunner, Obruca, Bauer, & Feichtinger, 2003; Scharf, Ghazwiny, Steinborn, & Sohn, 2000; Manini, Burton, Meixner, Eckert, Callstrom, Schmit, El-Youssef, & Camilleri, 2009). One of the advanced developments is the generation of 3DUS imaging out of two-dimensional ultrasound (2DUS) system. In this regard, this 3DUS system employs mechanical or freehand scanning techniques (Fenster & Downey, 2000; Campani, Bottinelli, Calliada, & Coscia, 1998; Candiani, 1998). The mechanical sweeps the region of interest by mounting the probe to a stepper motor to move the probe in a predefined manner where the relative position and angulation of each frame can be determined precisely. In contrast to the mechanical system, the freehand scanning acquires the region of interests by mounting a position tracking system to the probe. In this system, the scanning geometry is not predetermined (Fenster & Downey, 2000; Solberg, Lindseth, Torp, Blake, & Nagelhus Hernes, 2007; Gee, Prager, Treece & Berman, 2003; Huang, Zheng, Lu, & Chi, 2005). However, both have their own advantages and drawbacks.
In the evaluation of the spine, ultrasound imaging has been proven to capture spinal features from the reflection of the ultrasound signal (Suzuki, Yamamuro, Shikata, Shimizu, & Iida, 1989; Brendel, Winter, Rick, Stockheim, & Ermert, 2002; Purnama, Wilkinson, Veldhuizen, van Ooijen, Sardjono, Brendel, & Verkerke, 2006). In the study of Suzuki et al. (1989), ultrasound is capable of outlining the spinous process and the laminae to measure axial rotation of the vertebrae. This method, however, only applies 2DUS scanned to the marked skin of the back. In fact, what can be seen in the ultrasound images is not the bony structure, but only the reflection of some parts of the bone surface as investigated by Brendel et al. (2002). Purnama et al. (2006) has shown that the ultrasound signal reflects on the processi transversi and proved that such imaging system is an appropriate system to determine the shape of the human spine. Furthermore, to obtain the 3DUS volume of the spine, freehand scanning technique can be a good choice due to its flexibility to reach the whole spinal surface. However, imaging the spine using freehand 3DUS still suffers from an inadequate sampling process and speckle noise problems that may obscure the visibility of the spinal features causing inaccuracy in the extraction of the acquired vertebral features.