title: An iterative algorithm for reconstructing incident beam distributions from transmission measurements using electronic portal imaging.
abstract: The problem of reconstructing incident radiotherapy beam profiles from electronic portal images recorded behind a phantom is addressed. To this end an iterative algorithm is presented, which is able to extract the input beam profile from a portal image by compensating for the attenuation of the beam and subtracting the amount of scatter emitted by the phantom. The algorithm requires only a thickness map of the phantom. Scatter is estimated using a superposition method based on precalculated Monte Carlo scatter kernels. The method is tested for a homogeneous water-equivalent slab phantom for simple rectangular and complex multileaf collimated fields. It is shown that the method produces a stable result within four iterations yielding an accuracy for the incident beam distribution of better than 3%.
authors.affiliation: Philips Research Laboratories, Aachen, Germany. lothar.spies@philips.com
authors.forename: L
authors.initials: L
authors.lastname: Spies
authors.forename: M
authors.initials: M
authors.lastname: Partridge
authors.forename: B A
authors.initials: BA
authors.lastname: Groh
authors.forename: T
authors.initials: T
authors.lastname: Bortfeld
mesh_terms: D000465:Algorithms; D004581:Electronics; D007091:Image Processing, Computer-Assisted / Q000379:methods; D019047:Phantoms, Imaging; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D011882:Radiotherapy, High-Energy / Q000379:methods; D012542:Scattering, Radiation