For IGRT, the use of FPDs for kV CBCT presented an advance over early embodiments of MV CT (Mosleh-Shirazi et al., 1998) as well as kV CBCT based on XRIIs (Fahrig and Holdsworth 2000) and CCDs (Jaray et al., 1999). e rst decade of the 21st century saw increasingly widespread clinical use of such systems, with a variety of clinical platforms described in sections later in this chapter.