title: Portal imaging: Performance improvement in noise reduction by means of wavelet processing.
abstract: This paper discusses the suitability, in terms of noise reduction, of various methods which can be applied to an image type often used in radiation therapy: the portal image. Among these methods, the analysis focuses on those operating in the wavelet domain. Wavelet-based methods tested on natural images--such as the thresholding of the wavelet coefficients, the minimization of the Stein unbiased risk estimator on a linear expansion of thresholds (SURE-LET), and the Bayes least-squares method using as a prior a Gaussian scale mixture (BLS-GSM method)--are compared with other methods that operate on the image domain--an adaptive Wiener filter and a nonlocal mean filter (NLM). For the assessment of the performance, the peak signal-to-noise ratio (PSNR), the structural similarity index (SSIM), the Pearson correlation coefficient, and the Spearman rank correlation (ρ) coefficient are used. The performance of the wavelet filters and the NLM method are similar, but wavelet filters outperform the Wiener filter in terms of portal image denoising. It is shown how BLS-GSM and NLM filters produce the smoothest image, while keeping soft-tissue and bone contrast. As for the computational cost, filters using a decimated wavelet transform (decimated thresholding and SURE-LET) turn out to be the most efficient, with calculation times around 1 s.
authors.affiliation: Hospital Universitario Virgen de la Arrixaca, ctra. Madrid-Cartagena s/n, 30120 El Palmar (Murcia), Spain. Electronic address: antonio.gonzalez7@carm.es.
authors.forename: Antonio
authors.initials: A
authors.lastname: González-López
authors.affiliation: Departamento de Tecnologías de la Información y las Comunicaciones, Universidad Politéctnica de Cartagena, Cartagena, Spain.
authors.forename: Juan
authors.initials: J
authors.lastname: Morales-Sánchez
authors.affiliation: Departamento de Tecnologías de la Información y las Comunicaciones, Universidad Politéctnica de Cartagena, Cartagena, Spain.
authors.forename: Jorge
authors.initials: J
authors.lastname: Larrey-Ruiz
authors.affiliation: Departamento de Tecnologías de la Información y las Comunicaciones, Universidad Politéctnica de Cartagena, Cartagena, Spain.
authors.forename: María-Consuelo
authors.initials: MC
authors.lastname: Bastida-Jumilla
authors.affiliation: Departamento de Tecnologías de la Información y las Comunicaciones, Universidad Politéctnica de Cartagena, Cartagena, Spain.
authors.forename: Rafael
authors.initials: R
authors.lastname: Verdú-Monedero
mesh_terms: D000465:Algorithms; D016477:Artifacts; D001499:Bayes Theorem; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D016018:Least-Squares Analysis; D015233:Models, Statistical; D016011:Normal Distribution; D010315:Particle Accelerators; D056888:Patient Positioning / Q000379:methods*; D011878:Radiotherapy / Q000379:methods*; D015203:Reproducibility of Results; D012306:Risk; D059629:Signal-To-Noise Ratio; D058067:Wavelet Analysis
keywords: Denoising; Image statistics; Portal imaging; Wavelet processing