The property of being measured and described using two orthogonal directions. As applied to a PET scanner, it describes a measuring process with shielding septa restricting detectors’ coincidences. It is generally referred to as simply 2D. Detectors’ coincidences are restricted to just lines of response being those in the same ring of detectors. Reconstruction is then simplified with this directional restriction. However, there is a loss of count information due to the absorptions.