5.3 - On Board kV Imaging (OBI)

For on board imaging devices, describe the principles of acquisition, assessment, and consequences of a positioning error being found.

OBI devices are standard in modern linear accelerators. They are placed on the gantry at a 90o offset to the primary beam. They have the benefit of using kilovoltage x-rays, which provide improved contrast over standard portal images. They have a similar detection system to electronic portal imaging devices.
OBI devices increase the workload needed for quality assurance as their isocentre must be checked in addition to the primary beam. The machine must also rotate 90o in order to acquire images with the OBI device.
In terms of assessment and consequences, OBI functions in a similar way to EPI devices but with increased discrimination of bony landmarks.