Portal Imaging with Flat-panel Detector and CCD Camera

摘要

This paper provides a comparison of imaging parameters of two portal imaging systems at 6 MV: a fiat panel detector and a CCD-camera based portal imaging system. Measurements were made of the signal and noise and consequently of signal-to-noise per pixel as a function of the exposure. Both systems have a linear response with respect to exposure, and the noise is proportional to the square-root of the exposure, indicating photon-noise limitation. The fiat-panel detector has a signal-to-noise ratio, which is higher than that observed with the CCD-Camera based portal imaging system. This is expected because most portal imaging systems using optical coupling with a lens exhibit severe quantum-sinks. The paper also presents data on the screen's photon gain (the number of light-photons per interacting x-ray photon), as well as on the magnitude of the Swank-noise, (which describes fluctuation in the screen's photon gain). Images of a Las Vegas-type aluminum contrast detail phantom, located at the ISO-Center, were generated at an exposure of 1 MU. The CCD-camera based system permits detection of Aluminum-holes of 0.01194 em diameter and 0.228 mm depth while the Flat-panel detector permits detection of Aluminum holes of 0.01194 em diameter and 0.1626 mm depth, indicating a better signal-to-noise ratio Rank Order Filtering was applied to the raw images from the CCD-based system in order to remove the direct hits. These are camera responses to scattered x-ray photons which interact directly with the CCD of the CCD-Camera and generate "Salt and Pepper type noise", which interferes severely with attempts to determine accurate estimates of the image noise.