Calibration of a detector array through beam profile reconstruction with error‐locking

Abstract

An iterative method is proposed to calibrate radiation sensitivities of an arbitrary two‐dimensional (2D) array of detectors. The array is irradiated with a wide open‐field beam at the central position, as well as at laterally and longitudinal shifted positions; the 2D beam profile of the wide field is reconstructed iteratively from the ratios of shifted images to the central image. The propagation errors due to output variation and inaccurate array positioning are estimated and removed from the reconstructed beam profile by an error‐locking scheme with narrow open‐field irradiations. The beam profile is interpolated when necessary and then compared to raw detector responses to determine sensitivities. Two additional methods were implemented for comparison: 1) the commercial iterative calibration method for MapCHECK2 with translation and rotation operations; 2) a labor‐intensive noniterative method without the issue of error propagation. A MapCHECK2 2D detector array was used to validate the proposed method with the 6 MV photon beam from a Varian iX linear accelerator. All calibration methods were repeated three times. A total of 5, 9, and 29 irradiations were required to implement the commercial method, the proposed method and the noniterative method respectively. Moreover, a 5 mm positioning error was intentionally introduced into the calibration procedures of the commercial and the proposed method to test their robustness. Under the normal operation condition of the linear accelerator and with careful alignment of the MapCHECK2, the deviations of the calibrated sensitivities of the proposed method and commercial method with respect to the noniterative method were 0.30%±0.29% and 0.92%±0.63% respectively; when the 5 mm positioning error was presented, these two methods resulted in deviations of 0.40%±0.36% and 3.58%±1.94%, respectively. A patient study suggested that, due to this 5 mm positioning error, the mean DTA (dose to agreement) passing rate by the commercial method was 2.7% lower than that by the noniterative method, whereas the proposed method led to a comparable passing rate. It is evident from this study that the proposed iterative method leads to within 1% mean calibration results to established methods. It requires much fewer number of measurements than noniterative method and is more robust against the positioning error than the commercial iterative method. The method also eliminates the need of rotation operations and, therefore, is applicable to inline detector arrays without rotation function, such as electronic portal imager device (EPID).PACS number: 87.56.Fc

Determination of the electronic portal imaging device pixel‐sensitivity‐map for quality assurance applications. Part 1: Comparison of methods

“…For such EPIDs, the WashU method has been demonstrated to be accurate. 13 While not proven, it is theorized that the Monte Carlo method would likely be best for properly characterizing the backscatter non‐uniformity in the PSM, followed by the WashU method. This is because the WashU method utilizes only small EPID panel shifts (4 mm shift) compared to the CMN and Varian methods, and hence backscatter is more consistent throughout the data collection process.…”

“…Because of the problems associated with the EPID flood field calibration a number of authors have attempted to develop alternate EPID calibration procedures where the non‐uniformity of the imager response is corrected without disturbing the non‐uniformity of the incident beam. 10 , 11 , 12 , 13 Such a calibration procedure was first attempted by Greer 10 who named this new calibration procedure the pixel‐sensitivity‐map (PSM). The PSM is analogous to the array type calibrations used in commercial 2D‐array detectors to correct for response differences between individual detectors.…”

“…A fourth method is also included based on data obtained from Monte Carlo simulations (the Monte Carlo method). Other methods of determining the PSM have been published in the literature, 12 , 13 but only the locally available methods are included in this study. The aim of the study is to evaluate each method and help identify which method of PSM determination is superior in terms of accuracy, efficiency, and usability so that the method of PSM determination can be standardized and used universally for QA applications using EPID.…”

Determination of the electronic portal imaging device pixel‐sensitivity‐map for quality assurance applications. Part 2: Photon beam dependence

“…A fourth method is also included based on data obtained from Monte Carlo simulations (the Monte Carlo method). Other methods of determining the PSM have been published in the literature, 3 , 4 , 11 but are not included in this study. Details on the methodology of the four OSM methods investigated were presented in Part 1 of this study.…”

“…Because of the problems associated with the EPID flood‐field calibration for QA applications a number of authors have attempted to develop alternate EPID calibration procedures where the non‐uniformity of the imager response is corrected without disturbing the non‐uniformity of the incident beam. 1 , 2 , 3 , 4 Such a calibration procedure was first attempted by Greer 1 who named this new calibration procedure the pixel‐sensitivity‐map (PSM). The PSM is analogous to the array‐type calibrations used in commercial 2D‐array detectors to correct for response differences between individual detectors.…”