Dosimetric properties of the liquid ionization chamber electronic portal imaging device (EPID)

抄録

In the relation between the pixel values of the electronic portal imaging device (EPID) and dose rate, pixel values have been reported to be proportional to the square roots of the dose rates although some researchers have reported that each EPID pixel value was almost proportional to the dose rates when they were more than 1 Gy/min. To resolve this contradiction, in this study we have investigated the dosimetric properties of the liquid ionization chamber EPID based on a two-dimensional matrix of liquidfilled ionization chambers. Our results show that the pixel values of this EPID were proportional to the square roots of the dose rates when they were less than 0.5Gy/min, and the values were proportional to the dose rates when they were more than that. In addition, the survival time of electrons in the liquid of the EPID is reportedly dependent on the amount of impurities in the liquid ionizing chamber of EPID. The pixel values of the EPID acquired in rapid succession, increased. In the results, the pixel values were constantly increased to approximately 0.2% of pre-images for less than 15 s of the rest interval. When changes in the sensitivity of each pixel value were approximately 1%, approximately 5% changes were observed against the transformed dose rate. To avoid this difference, we developed the following formula relating the pixel value, P, and dose rate, D.
P=a. D+b-k⋅ h
(a, b, h: constants, k: variable)