Investigation of an amorphous silicon flat-panel detector for ion radiography

Abstract

Using heavy ions in radiotherapy offers a good potential for targeted radiation of tumors and the ability to spare healthy tissue. Their characteristic interaction with matter holds the potential to employ ions for high-contrast radiographic imaging at a decreased dose in comparison to conventional X-ray imaging; however, it lacks simple detectors suitable for this purpose. In this study the performance of an amorphous silicon flat-panel detector, originally designed for photon imaging, was investigated for radiographic and tomographic imaging with ions. After the application of a newly-developed image correction routine, the flat-panel detector is capable of ion-radiographic imaging with high soft-tissue contrast. Further, the flat-panel detector was exploited to measure the water equivalent thickness (WET) of phantoms. To do so, the ambiguous correlation of detector signal to particle energy was overcome by active carbon ion beam energy variation and measurement of the signal-to-energy correlation. Using this method has enabled the study to determine the WET of the imaged object with an accuracy better than 1.5 mm WET. It is an improvement by a factor of about 2 with respect to the standard clinical method. Finally, the study has shown the feasibility of this imaging technique for high-resolution carbon ion computed tomography and determination of water equivalent path length (WEPL) accordingly. The developed imaging techniques present a method to measure the two-dimensional maps of WET and WEPL of complex phantoms with a simple and commercially available detector. The required doses in this configuration are too high (e.g. WET: 0.05 Gray, WEPL: 8 Gray) to be used for patient imaging, but this method presents a powerful tool to evaluate the performance of the treatment planning algorithm by studying range uncertainties.