Abstract
Today, electronic portal imaging devices (EPIDs) are widely used as a replacement to portal films for patient position verification, but the image quality is not always optimal. The general aim of this study was to optimize the acquisition parameters of an amorphous silicon EPID commercially available for clinical use in radiation therapy with the view to avoid saturation of the system. Special attention was paid to selection of the parameter corresponding to the number of rows acquired between accelerator pulses (NRP) for various beam energies and dose rates. The image acquisition system (IAS2) has been studied, and portal image acquisition was found to be strongly dependent on the accelerator pulse frequency. This frequency is set for each “energy — dose rate” combination of the linear accelerator. For all combinations, the image acquisition parameters were systematically changed to determine their influence on the performances of the Varian aS500 EPID system. New parameters such as the maximum number of rows (MNR) and the number of pulses per frame (NPF) were introduced to explain portal image acquisition theory. Theoretical and experimental values of MNR and NPF were compared, and they were in good agreement. Other results showed that NRP had a major influence on detector saturation and dose per image. A rule of thumb was established to determine the optimum NRP value to be used. This practical application was illustrated by a clinical example in which the saturation of the aSi EPID was avoided by NRP optimization. Moreover, an additional study showed that image quality was relatively insensitive to this parameter.PACS numbers: 87.53.Oq; 87.59.Jq
Highlights
Portal imaging is currently used to verify patient position during radiation treatment.[1,2,3,4] A specific subject of interest with electronic portal imaging devices (EPIDs) is their ability to determine patient dose
Energy and frequency dependence For the four X-ray beam energies studied in this work, different monitor unit (MU) rates may be selected ranging from 100 MU/min to 400 MU/min for the 6- and 20-MV X-ray beams, from 50 MU/min to 250 MU/min for 4 MV, and from 80 MU/min to 400 MU/min for the 10 MV beam
The acquisition parameters of the aS500 EPID have been studied, and number of rows/pulses (NRP) has been optimized for clinical use in radiation therapy
Summary
Portal imaging is currently used to verify patient position during radiation treatment.[1,2,3,4] A specific subject of interest with electronic portal imaging devices (EPIDs) is their ability to determine patient dose. Portal imaging systems are developed to provide both geometrical and dosimetric information.[5,6,7,8] Compared to previous systems, the amorphous silicon-based EPID provides better quality portal images,(9,10) but in some cases with a high dose rate or short source-to-detector distance (SDD), saturation can occur.[11] This saturation can be avoided by optimizing the software acquisition parameters. The influence on portal image acquisition and dose per image of a major acquisition parameter, the number of rows acquired between two consecutive accelerator pulses, called the number of rows/pulses (NRP), was studied. The influence of this parameter on the EPID response was investigated as a function of the accelerator pulse rate repetition. The influence of NRP and of the SDD on the detector saturation was assessed
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