Analysis of the signal for high resolution digital X-ray detector under radiotherapy irradiation condition

In order to get a higher resolution of the image for a patient, the direct conversion mode which X-ray converts to electric charge are better than the indirect conversion mode which X-ray converts to light. But, the flat panel X-ray image detectors for direct conversion mode are not yet commercially available. In this study, we fabricated the samples for X-ray image film by using polycrystalline mercury iodide and carried out a feasibility study on the radiotherapy application. A sheet of copper (1 mm thickness) was attached onto the radiotherapy X-ray image film detector by insulating tape. The characteristics of the samples such as X-ray sensitivity, reproducibility, X-ray response on dose and dose rate for 6 MV beam was investigated. Dark currents were measured as a function of applied electric fields. The attenuated fraction of samples exposed with 6 and 10 MV beams was also measured as a function its thicknesses. By data analysis, the reproducibility of electric current measurements was 1.75% in the coefficient of variation. The measured values of electric charge revealed linearity with respect to the dose and dose rate, respectively. On the other hand, as the dose increases, the X-ray sensitivity decreased due to polarization effect. And the attenuated fraction for a 6 MV beam was 0.75% at the sample with the thickness of 380 ¿m. Therefore, the fabricated samples for X-ray image film provides the feasibility of radiotherapy imaging application. Moreover, it is found that the thickness of polycrystalline mercury iodide is very important factor for radiotherapy imaging, and a proper thickness having the attenuated fraction of about 1% should be greater than about 400 yW.