Image Quality of CBCT Using Inspiration Breath Hold (IBH)
The most common cause of CBCT image degradation on the TrueBeam are stripe artefacts caused by patient breathing. If gated CBCT is no option, a simple technique with some help from the patient can bring big improvements:
The following GIF animation shows the blending of the planning CT and the CBCT in ten steps. The CBCT (Thorax protocol) was acquired with inspiration breath hold (IBH) in two parts, because the whole gantry rotation takes about a minute, which is too long to hold breath.
The kV beam was manually paused at about 50% to let the patient breath freely, and then resumed. The improvement is dramatic. There are nearly no stripe artefacts anymore in the CBCT image, only some ring artefacts of minor importance:
(click to enlarge)
Note that this is an animated GIF rendered from screenshots. GIF compression is not lossless, which means that image quality is affected.
Spotlight Mode
On the next day, for the same patient we acquired the CBCT image using Spotlight mode. This mode leaves the kV detector centered, therefore the body is cropped. But it requires only half a rotation of the gantry. The patient was able to hold his breath during the whole acquisition:
