The majority of EPID dosimetry literature discusses
response linearity and the so-called image lag and ghosting effects despite the
lack of a common definition of these quantities. However, the results of these
studies are generally not consistent, and it is often difficult to compare the
results from different studies. We present here a detailed study of the
acquisition and readout characteristics of a-Si EPID and its dosimetric
performance. EPID response was assessed over the range of 1 - 500 MU using different
dose rates and integration times. In addition, a computer model was designed to
simulate the EPID image formation with different dose, dose rate, and
integration time combinations. All aspects of image processing and readout
simulation were carried out using custom written MatLab codes. Two distinct
signal profiles were observed depending on the delivered dose, dose rate and
integration time combination. Total integrated signal (ST)
is linear with the delivered dose. For dosimetry, image lag and ghosting
effects mainly result in the residual signal (SR) that
appears as delayed signal after the end of irradiation. At its maximum, SR
is less than 2.5% of ST. The readout technique is such
that it is impossible to measure SR accurately. SR
is definable only when readout equilibrium occurs. Signal profiles provide a
through and reliable description of the EPID response incorporating the dose,
dose rate, integration time, and the residual signal. The definition of EPID
signals based on this method shall provide an accurate universal EPID dosimetry
framework.
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