A method and apparatus for performing correlated double sampling to remove low frequency noise. The method and apparatus includes an active pixel of an array of active pixels comprising a sensor circuit for collecting radiation induced charges and transducing them to a measurement signal corresponding to the amount of charge collected, two memory elements for storing the measurement signal at the beginning and the end of a first integration period respectively, and at least one further memory element for storing at least the measurement signal at the beginning of a next integration period.