Wavelet-based compression in electronic portal imaging can enhance objective as well as subjective image quality

This study attempts to assess the feasibility and application of an algorithm combining noise-reduction and lossy compression to electronic portal images in radiation therapy. The basis of the algorithm is universal hard- thresholding in the wavelet-domain. The approach for the assessment is two-fold: (1) an observer study using an image of a contrast-resolution phantom with different levels of compression; (2) a quantitative study using control phantom to determine contrast-to-noise (CNRs) and relative modulation transfer functions. All results are analyzed as a function of a quality factor which is linked to the universal threshold level and expresses the lossiness of the compression. Results show that there exists an optimal quality factor for which the noise-reduction is maximal. The contrast resolution study indicates that the noise reduction is relevant for the observer. Furthermore it is possible to determine the optimal level from the behavior of the CNR as a function of all quality factor in regions with essentially no contrast.