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Sub-Image Sampling Method to Improve Temporal Resolution of Time-Resolved EPID Applications

C Cassell1*, D Lee1 , P Greer1,2 , (1) School of Mathematical and Physical Sciences, University of Newcastle, Callaghan, NSW, Australia (2) Radiation Oncology, Calvary Mater Newcastle Hospital, Newcastle, NSW, Australia


(Tuesday, 7/16/2019) 1:15 PM - 1:45 PM

Room: Exhibit Hall | Forum 3

Purpose: To correct for lag introduced to images taken with an EPID panel by the raster scanning read out time over the whole panel during dynamic beam delivery applications. This will improve the accuracy of EPID for applications including time-resolved dosimetry and dynamic MLC verification.

Methods: An analytical method has been developed to correct for time lag introduced by the raster scanning of the EPID panel during dynamic Linac operation. This method uses linear interpolation to determine how the signal from each row will be integrated. In order to compensate for lag, the method considers each row individually. The signal is then assigned to the correct control point, as dictated by the treatment plan. Measurements were taken using an AS1200 EPID panel for a MLC sliding window test plan and for patient treatment plans. The panel was read out in CINE mode using a frame grabber PC. The images were then analysed offline to find the integrated image for each control point, using simple integration and the correction method. The measured and corrected were compared to predicted images, to determine the effectiveness of the correction method.

Results: The analysis of the simple integration and correction methods used on the MLC test plan data show that the method is capable of correcting the lateral shift introduced by the time lag. The lag introduces a gradient of 0.0055 ± 0.4% to the MLC aperture (corresponding to an offset of 1.87 ± 0.0075 mm), which is reduced to 0.0013 ± 0.3% (0.34 ± 0.001 mm) by the correction method.

Conclusion: The results show that it is possible to improve the performance of the EPID by modelling an analytical method correcting the time lag of the read out.


Electronic Portal Imaging, Image Artifacts, Quality Assurance


TH- External beam- photons: portal dosimetry

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