Performance of electronic portal imaging devices (EPIDs) used in radiotherapy: Image quality and dose measurements

Abstract

The aim of our study was to compare the image and dosimetric quality of two different imaging systems. The first one is a fluoroscopic electronic portal imaging device (first generation), while the second is based on an amorphous silicon flat-panel array (second generation). The parameters describing image quality include spatial resolution [modulation transfer function (MTF)], noise [noise power spectrum (NPS)], and signal-to-noise transfer [detective quantum efficiency (DQE)]. The dosimetric measurements were compared with ionization chamber as well as with film measurements. The response of the flat-panel imager and the fluoroscopic-optical device was determined performing a two-step Monte Carlo simulation. All measurements were performed in a 6 MV linear accelerator photon beam. The resolution (MTF) of the fluoroscopic device (f 1/2 = 0.3 mm(-1)) is larger than of the amorphous silicon based system (f 1/2 = 0.21 mm(-1)), which is due to the missing backscattered photons and the smaller pixel size. The noise measurements (NPS) show the correlation of neighboring pixels of the amorphous silicon electronic portal imaging device, whereas the NPS of the fluoroscopic system is frequency independent. At zero spatial frequency the DQE of the flat-panel imager has a value of 0.008 (0.8%). Due to the minor frequency dependency this device may be almost x-ray quantum limited. Monte Carlo simulations verified these characteristics. For the fluoroscopic imaging system the DQE at low frequencies is about 0.0008 (0.08%) and degrades with higher frequencies. Dose measurements with the flat-panel imager revealed that images can only be directly converted to portal dose images, if scatter can be neglected. Thus objects distant to the detector (e.g., inhomogeneous dose distribution generated by a modificator) can be verified dosimetrically, while objects close to a detector (e.g., a patient) cannot be verified directly and must be scatter corrected prior to verification. This is justified by the response of the flat-panel imaging device revealing a strong dependency at low energies.

Enhancement of megavoltage electronic portal images for markerless tumor tracking

“…Megavoltage (MV) electronic portal imaging device (EPID) has been widely used as an on‐line verification tool for treatment field in radiation therapy 1, 2, 3, 4. While the weight of the verification tool appears to be decreased after the emergence of kilovoltage (kV) imager mounted on a linear accelerator, EPID images still have their own advantages.…”

Patient Specific Pre-Treatment QA Verification Using an EPID Approach

“…Several researchers have investigated the use of electronic portal imaging devices for quality assurance purposes such as MLC position verification (16)(17)(18) and IMRT patient specific dose verification (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29). Renner et al (24) reported on four different methods that have been introduced for the purpose of using the EPID for treatment verification: 1) Compute the dose from each beam to the EPID and compare to the actual EPID image of each beam (4-15); 2) Use the EPID to verify the leaf positions for intensity modulated fields (16)(17)(18); 3) Reconstruct the dose to the patient using the exit image acquired during treatment (20)(21)(22)(23); and 4) Convert the EPID images to an incident fluence distribution and use it for dose calculation employing the patient CT anatomy (24,25).…”

Filmless Evaluation of the Mechanical Accuracy of the Isocenter in Stereotactic Radiotherapy

“…Such alternative techniques are electronic portal imaging devices [1,3,5,7,11,20,23,35] or the digital luminescence radiography [2,4,13,16,17,32,37].…”