An electronic portal imaging device has been designed and constructed. It consists of an array of 128 CsI scintillation crystals coupled to photodiodes which is scanned across the field in 4 seconds. The linac is operated at a dose rate of 400 cGy min-1 and the dose delivered for image acquisition is approximately 27 cGy. The data acquisition controller is a stand-alone STE computer located within the scan arm. Sample images are presented showing contrast and spatial resolution of the system together with a humanoid phantom image and a clinical image of a breast cancer patient. The phantom images show the detector has a contrast resolution of 0.3% (at 15 mm diameter) and a spatial resolution of 2.5-3.2 mm. Images of uniform Perspex blocks have also been calibrated for thickness, indicating the system can measure radiological thickness to an accuracy of 2-3 mm of water. These results indicate the detector may be used for transit dosimetry applications including compensator design.
Biophysical Phenomena Biophysics