OBI
On-Board Imager — Varian's on-linac kV imaging system. A kV X-ray tube and flat-panel detector are mounted on retractable arms orthogonal to the MV treatment beam. Enables daily kV-kV setup verification and kV cone-beam CT (CBCT) for image-guided radiotherapy (IGRT). Elekta's analogous system is XVI; Accuray uses orthogonal kV imaging on CyberKnife.
Why it matters to buyers: Any modern linac has this or equivalent. Required for IGRT protocols, SRS / SBRT setup, and 6 DoF couch-shift verification. Daily Winston-Lutz tests and CBCT reconstruction quality depend on OBI calibration. A linac without functional OBI cannot run modern image-guided programs.
Why it matters to engineers: Arm motor, flat-panel detector, kV tube are separate failure points — diagnose individually before component swap. Dead-pixel maps on the kV imager flag end-of-life. Daily Winston-Lutz on each arm position validates geometric calibration before clinical use.