Free-running Sync stripes correction for MV-imaging with a-Si:H flat panels


Mooslechner Michaela

During radiation therapy, amorphous silicon (aSi:H) flat panel detectors are commonly used for megavoltage (MV) imaging due to quality assurance tasks. However, native image frames captured by these electronic portal imaging devices show a range of artifacts that have to be eliminated in order to guarantee high image quality for clinical and dosimetric applications. During irradiation with lower dose rates or at low MU imaging, moving vertical stripes (periodic synchronization artifacts) become a dominant feature of image frames, if the panel is operated in free-running mode.

Aim of the research project is to introduce a sync correction method, which is based on a theoretical model describing the interferences of the panel’s readout procedure with the linac’s pulse repetition frequency (PRF). The used amorphous silicon flat panel detectors comprise 2 x 8 subpanels with 128 x 512 pixel each (total 1024 x 1024 pixels), which are read column-wise from the outside to the inside of the active area. With the help of the theoretical interference model, the amplitude and the period of the sync stripes pattern can be analytically derived. The correction is done on a per frame basis without further information about the thyratron pulse timing.

Applying the correction, drastically improves clinical image quality. Especially some new developments in imaging applications can benefit from the correction, eg low-MU imaging at higher frame rates when tracking gold-fiducials in the lung, or simultaneous kV / MV acquisition.