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Session: Dosimetric Tools [Return to Session]

Using Wavelet Transform to Compute Pixel Sensitivity Maps of MV Portal Imaging Devices

M Ahmed1*, (1) Vanderbilt University Medical Center, Nashville, TN

Presentations

MO-H345-IePD-F3-3 (Monday, 7/11/2022) 3:45 PM - 4:15 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 3

Purpose: To compute the pixel sensitivity map (PSM) for amorphous silicon electronic portal imaging devices (EPID) using single flood field measurements.

Methods: Analysis of MV EPID signals shows that they are composed of energy-fluence based signals, induced by incident radiation, superimposed with pixels gain based signals due to the inherent response of device diode detectors. Energy-fluence induced signal features slowly varying amplitudes, while pixels gain induced signal features fast and abrupt changing amplitudes. The separation of these two signals enables the calculation of a device PSM. PSM is used to normalize the response of individual pixel diode detectors for proper usage of EPIDs as a dosimeters. Wavelets transform is a powerful tool that may be used to decompose and reconstruct measured EPID signals without PSM spatial features. Here, we utilized Wavelet transform to computed PSMs from single flood field EPID measurements. Computed PSMs are compared with PSMs generated using other published techniques such as the regression-based and the shifting-based PSM calculation methods.

Results: Discrete approximation of Meyer wavelet at 10 levels was used to reconstruct the energy-fluence induced signal and extract as1000 EPID's PSM. Soft thresholding was applied with coefficients 6.8, 5.7, 9.1, 5, 9.0 for levels L1 to L5, respectively. +9.0 thresholding values were used for L6 to L10. No distinction between horizontal, vertical, and diagonal detail coefficients was considered in the Wavelet model. The mean square error between PSMs generated with Wavelet transforms and other techniques were < 1.3E-4.

Conclusion: In this work, we present an approach to calculate pixel sensitivity maps of MV portal imaging devices. This approach utilizes Wavelet transform to deconvolve the device-specific pixels gain map from unattenuated flood field EPID measurements. The method does not require shifting the imager device, thus requires single flood field measurement to generate device pixel sensitivity map.

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