1. A method for converting projection data acquired by a nuclear medical imaging device using a first collimator into projection data as if the acquired data were acquired with a second collimator, said method comprising the steps of: a) modeling said first and second collimators with first and second mathematical models; b) receiving raw object projection data acquired using said first collimator; and c) iteratively projecting the raw object projection data backward and forward through the first and second mathematical models to generate estimated projection data as object projection data as if acquired with said second collimator.