Portal imaging with amorphous selenium, a signal and noise analysis and comparison with fluoroscopic systems
Abstract
Conformal radiation therapy is being developed to increase cancer patient survival by closely tailoring the radiation dose distribution to match the shape of the tumour. For conformal therapy to be effective, improved patient position verification through portal imaging is required. Existing portal imaging systems produce poor images. By increasing the optical coupling, the x-ray quantum noise in a fluoroscopic portal imaging system was measured for the first time and compared to other sources of noise. It is shown that fluoroscopic systems are dominated by noise in the vacuum tube camera and although significant improvements in the SNR can be made by reducing this source of noise, practical clinical systems can only be made quantum noise limited up to a spatial frequency which is too low to optimize the dose distribution. Flat panel imaging systems currently show promise for improving image quality beyond that possible for fluoroscopic systems. Some of these systems use a photoconductor known as amorphous selenium (a-Se) as the sensor. Thus the signal and noise properties of a-Se for portal imaging are explored. Measurements of the charge signal produced by a-Se irradiated by x-rays showed that the x-ray sensitivity (charge produced per energy absorbed) increased somewhat with energy. This was interpreted as a linear energy transfer (LET) dependence. Two recombination models (geminate and columnar) were compared with the data but neither mechanism alone could explain the…
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