A quality assurance phantom for electronic portal imaging devices

Abstract

Electronic portal imaging device (EPID) plays an important role in radiation therapy portal imaging, geometric and dosimetric verification. Consistent image quality and stable radiation response is necessary for proper utilization that requires routine quality assurance (QA). A commercial ‘EPID QC’ phantom weighing 3.8 kg with a dimension of 25×25×4.8 cm3 is used for EPID QA. This device has five essential tools to measure the geometric accuracy, signal‐to‐noise ratio (SNR), dose linearity, and the low‐ and the high‐contrast resolutions. It is aligned with beam divergence to measure the imaging and geometric parameters in both X and Y directions, and can be used as a baseline check for routine QA. The low‐contrast tool consists of a series of holes with various diameters and depths in an aluminum slab, very similar to the Las Vegas phantom. The high‐resolution contrast tool provides the modulation transfer function (MTF) in both the x‐ and y‐dimensions to measure the focal spot of linear accelerator that is important for imaging and small field dosimetry. The device is tested in different institutions with various amorphous silicon imagers including Elekta, Siemens and Varian units. Images of the QA phantom were acquired at 95.2 cm source‐skin‐distance (SSD) in the range 1–15 MU for a 26×26 cm2 field and phantom surface is set normal to the beam direction when gantry is at 0° and 90°. The epidSoft is a software program provided with the EPID QA phantom for analysis of the data. The preliminary results using the phantom on the tested EPID showed very good low‐contrast resolution and high resolution, and an MTF (0.5) in the range of 0.3–0.4 lp/mm. All imagers also exhibit satisfactory geometric accuracy, dose linearity and SNR, and are independent of MU and spatial orientations. The epidSoft maintains an image analysis record and provides a graph of the temporal variations in imaging parameters. In conclusion, this device is simple to use and provides testing on basic and advanced imaging parameters for daily QA on any imager used in clinical practices.PACS number: 87.57 C‐, 87.57 N‐

Gamma analysis dependence on specified low‐dose thresholds for VMAT QA

“…It had a pixel pitch of 0.039 × 0.034 cm 2 with an active detector area of 40 × 30 cm 2 . (18)(19)(20) The EPID response was calibrated by both a dark field to correct the background signal and a flood field to adjust the variation in pixel sensitivity. Then, the EPID was scaled such that one calibrated unit corresponded to 100 monitor units of a 10 × 10 cm 2 field size at a source-to-detector distance (SDD) of 100 cm.…”

Verification of Positionalaccuracy of Electronic Portal Imaging Device by Using Graticule Imrt Phantom

“…A study conducted by Das et al "A quality assurance phantom for electronic portal imaging device" have studied the appropriate procedure to carry out the Quality Assurance of an EPID. [8] They conduct their studies on PTW EPID QC phantom with associated image analysis software provides a convenient, automated and easy process for periodic verication of performance of EPID with regard to baseline values.…”

A simple quality assurance test tool for the visual verification of light and radiation field congruent using electronic portal images device and computed radiography

“…It has an active imaging area of 40 cm × 30 cm (at focus to detector distance of 100). The image matrix is created from an array of 1024 × 768 photodiodes, giving an effective pixel size of 390 μm at FDD of 150 cm [ 12 ]. The maximum frame acquisition rate is 9.574 frames/second, the permitted dose range is 4 - 25 MV and the permitted dose rates are 50-600 mu/min [ 13 ].…”