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Linac kV Imaging System (OBI / XVI)

The diagnostic-energy (kV) X-ray imaging subsystem mounted on the linac gantry — Varian markets it as the On-Board Imager (OBI), Elekta as XVI (X-ray Volume Imaging), and Accuray / Siemens platforms have analogous configurations. The kV imager is mounted at 90° offset from the treatment-energy (MV) beam, with its own kV X-ray tube, generator, and flat-panel detector. It enables image-guided radiotherapy (IGRT) workflow: patient-setup verification via 2D / orthogonal kV imaging, and cone-beam CT (CBCT) acquisition by rotating the gantry while collecting kV projection data for 3D volume reconstruction.

The kV imaging system is operationally distinct from the MV EPID panel — different X-ray source (kV tube vs the MV linac itself), different detector (kV-energy-tuned panel vs MV-tuned), different clinical use case (setup verification + CBCT vs treatment-verification imaging + in-vivo dosimetry). Both retract / extend during treatment delivery to clear the beam path, and both age via similar mechanical-arm and panel-aging patterns.

For modern radiation-therapy programs, the kV imaging system is essentially mandatory — IGRT-based setup verification is standard of care for all treatment techniques above conventional 3DCRT. Sites running SBRT / SRS without daily CBCT or daily kV imaging would be considered behind current practice.

Fits

Platform-specific. Representative entries:

MR-linac platforms (Elekta Unity, ViewRay MRIdian) use integrated MR imaging for guidance instead of kV imaging — no kV tube, no kV detector, no CBCT in the conventional sense.

Distinctive technology

  • kV X-ray tube — diagnostic-energy rotating-anode tube mounted on the gantry, separate from the linac's MV target.
  • Flat-panel kV detector — amorphous-silicon CsI panel similar in architecture to diagnostic DR detectors but mounted at 90° to the MV beam path.
  • CBCT acquisition mode — gantry rotates ~360° collecting kV projections; reconstruction produces a 3D volume image of the patient in treatment position.
  • 2D kV pair / orthogonal imaging — alternative setup-verification mode without full CBCT.
  • Real-time kV imaging during treatment ("ExacTrac"-class workflows, integrated treatment imaging) — used for motion management on some platforms.

Failure modes

  • Panel dead pixels — same general pattern as detector dead-pixel growth applied to the linac's kV panel.
  • Retract / extend arm wear — the kV tube and the kV panel are both mounted on retractable arms. Cumulative retract / extend cycles wear the support mechanisms; this is often the limiting factor before the panel itself ages out. Similar pattern to EPID panel arm wear.
  • kV tube end-of-life — the diagnostic kV tube exhibits the same general failure modes as any rotating-anode X-ray tube: bearing wear, arcing, filament wear.
  • Calibration drift — geometric calibration between the kV imaging frame and the MV treatment frame must remain within tolerance for accurate IGRT. Drift outside tolerance produces patient-setup errors.
  • Cable / connector wear at the gantry-mounted kV tube and detector.
  • CBCT reconstruction-quality drift — degraded panel uniformity or kV output stability shows up as CBCT image-quality degradation before clinical setup is affected.

Diagnosis

  • Daily QA isocentricity verification — Winston-Lutz test for MV / kV agreement.
  • Bad-pixel-map trending on the kV panel.
  • kV tube service-log review — same indicators as for any X-ray tube.
  • CBCT phantom imaging at scheduled QA intervals — TG-179 phantom or equivalent.
  • Visual inspection of retract arms at PM intervals.
  • TG-142 quality-assurance protocol for IGRT compliance.

Replacement path

  • Panel-level swap for kV detector failures.
  • Tube swap for kV tube end-of-life.
  • Arm / mechanism service for retract-arm issues.
  • Calibration suite post-swap: kV / MV isocenter verification, CBCT image-quality recommissioning, TG-179 phantom acceptance.

Field notes

  • kV imaging system maintenance is a meaningful operating-cost line on a high-volume IGRT-driven linac program.
  • Refurb-linac due-diligence on IGRT-equipped systems — kV panel age + bad-pixel count + retract-arm cycle history + most-recent isocentricity-QA report.
  • CBCT software-license tier is sometimes a separate variable from the hardware — refurb buyers should verify license inheritance.

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