AbstractID: 6704 Title: Simple Quantitative QA of Amorphous Silicon EPID Image
Quality
Introduction Amorphous Silicon Electronic Portal Imaging Devices (aSi EPIDs) can
deliver superior image quality with minimal maintenance. However, the characteristics
of these devices should be monitored routinely to ensure optimal performance. A simple,
quantitative procedure for daily QA of EPID image quality has been developed.
Methods and Materials The spatial resolution of an aSi EPID is fixed by the detector
pixel size and the contrast resolution is fixed by the efficiency of the screen and diode
arrays. Therefore, we evaluate image quality through measurement of image noise, or
variation in pixel value across a flat image. QA is performed on a daily basis by the
radiation therapist who acquires a flood field image. A software module has been
developed which is accessible through the commercial EPID software. It measures noise
throughout the image, excluding a small region which contains a simple phantom. The
software returns a pass/fail condition to the therapist, who confirms that the phantom is
visible in the image. The phantom ensures that a low noise value is not the result of a
catastrophic failure. The noise level is automatically recorded in a log file on the
acquisition computer.
Results Threshold for intervention is a noise level 1.5 times that which was measured at
acceptance. Several examples of compromised image quality will be presented, all of
which show a noise level above the QA threshold.
Conclusion A quantitative QA system has been developed which ensures optimal
performance from the EPID.
Research supported by Varian Medical Systems