Image quality improvements of electronic portal imaging devices by multi-level gain calibration and temperature correction

Abstract

Amorphous silicon (aSi:H) flat panel detectors are prevalent in radiotherapy for megavoltage imaging tasks. Any clinical and dosimetrical application requires a well-defined dose response of the system to achieve meaningful results. Due to radiation damages, panels deteriorate and the linearity of pixel response to dose as well as the stability with regard to changing operating temperatures get worse with time. Using a single level gain correction can lead to an error of about 23% when irradiating a flood field image with 100 MU min(-1) on an old detector. A multi-level gain (MLG) correction is introduced, emending the nonlinearities and subpanel-related artifacts caused by insufficient radiation hardness of amplifiers in the read-out electronics. With rising temperature, offset values typically increase (up to 300 gray values) while the response at higher dose values per frame remain constant for a majority of pixels. To account for temperature-related image artifacts, two additional temperature correction methods have been developed. MLG in combination with temperature corrections can re-establish the aSi:H image quality to the performance required by reliable medical verification tools. Furthermore, the life span and recalibration intervals of these costly devices can be prolonged decisively.

Use of local noise power spectrum and wavelet analysis in quantitative image quality assurance for EPIDs

“…Huber et al reported noticeable image quality improvements with the MLG calibration over the SLG calibration. 10 We also observed better image quality after the MLG calibration, especially the reduction of shadow banding artifacts. With the Las Vegas phantom, it led to the conclusion that both calibration methods were satisfactory.…”

“…In reality, however, the actual response, particularly at low-dose region, is nonlinear due to poorer pixel sensitivity and its characteristics may have changed since the last calibration as the panel ages. 10 The SLG calibration usually works well for a relatively new panel; however, it may lead to poor image quality in the lowdose region as the panel ages (Fig. 4).…”

“…1(A) and 1(B) demonstrate inter-subpanel flat-fielding artifacts as a result of the response variation across subpanels. 10 Figure 1(C) shows the "shadow banding" artifacts which occur due to insufficient light signals from irradiated photons during data readouts. 11 The loss of image information due to defective pixels or the fault with electronics (chip on board) can lead to artifacts at the junction of subpanels as shown in Fig.…”

“…9 The imaging performance of EPIDs is also subject to temperature variation as reported in other studies. 10,12 Moreover, the nonlinearity of a-Si EPID response depends on dose rate as well as linac output variation, particularly at low dose, further increasing variations in noise level. 9 All these phenomena can hamper the observer's ability to discern the desired signal from imaging noise.…”

An open‐source software for monitoring intrafraction motion during external beam radiation therapy based on superimposition of contours of projected ROIs on cine‐MV images

“…The two sections are read synchronously, column by column, from the outside to the inside of the panel. 19,20 Therefore, for the section on the linac A-side, the readout starts at column c ¼ 0 at t ¼ 0 and ends at col-…”