Table 1.

The geometries and image acquisition methods of commonly used IGRT systems studied here. The correlation-based 3D target position estimation methods applicable to each image acquisition method are shown in the last column (details are explained in sections 2.1.3 and 2.1.4).

Commonly used IGRT systems	Geometry (figure 1)	Image acquisition method (figure 2)	Applicable 3D target position estimation methods
CyberKnife	Coplanar fixed dual kV imager	Synchronous stereoscopic imaging	Synchronous image-based estimation or Sequential image-based estimation
BrainLab ExacTrac	Non-coplanar fixed dual kV imager with 60° angular separation	Alternate stereoscopic imaging	Sequential image-based estimation
Elekta, Siemens, Varian linacs	Coplanar rotating single kV imager	Rotational monoscopic imaging	Sequential image-based estimation