Abstract: We have identified two new noise sources in the real-time portal imaging chain which cause the temporal noise to be higher than the spatial noise. When a vacuum tube camera is used for portal imaging with pulsed linear accelerators, a horizontal banding artifact due to beam pulsation as well as an interface artifact (zig-zag type variation in intensity along vertical direction in a single frame) appears in the images. Frame transfer CCD cameras are only sensitive to the interface artifact. We found that noise due to the X-ray beam pulsation artifacts dominates over other temporal and spatial noise sources in real time portal images, which results in a signal independent SNR. However, when 256 frames are averaged, the pulsation artifact averages out. Further, we found that long term drift in the electronics also contributes excess temporal noise. The SIT camera takes 3 - 4 hours to stabilize its dark current and the frame grabber requires a similar time to stabilize its reference voltages. When sufficient time is allowed for the system to stabilize, and a continuous X-ray source is used, spatial and temporal noise in real time portal images are found to be equal at the 1 standard deviation level up to 256 added frames.!5
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