“…These systems, along with the added expense of additional hardware, require careful calibration of the tracker to the ultrasound image (Mercier et al, 2005). Some of these systems also make use of image-based registration methods to refine the registration and account for other movements that cannot be tracked by the probe (Xiao et al, 2002; Gee et al, 2003; Poon and Rohling, 2006; Yao et al, 2009; Zhuang et al, 2010). For systems that do not use image-based refinement, to account for the displacement of anatomy due to respiration, either patients are put on breath-hold, images are respiration-gated (Makela et al, 2002), or respiration is tracked and accounted for using an additional tracker on the chest or abdomen (Wein et al, 2008).…”