I. Introduction
Most of the commercially available positron emission tomography (PET) systems are intended for whole-body imaging and are combined with computerized tomography (CT) or magnetic resonance (MR) imaging capabilities in order to obtain anatomical information, which is also used to produce the attenuation image [1] for attenuation correction (AC) during PET image reconstruction. In recent years, organ-dedicated PET systems have been proposed as alternatives to whole-body scanners, aiming at systems that are less expensive, require less space, and/or provide easier patient access, higher resolution and/or better sensitivity [2]–[8]. Some of these systems, such as those consisting of two opposing detector panels, acquire only incomplete tomographic data, due to limited angular coverage. In addition, many of these systems are not combined with a modality providing anatomical images (CT or MR) and, therefore, the conventional approaches to AC cannot be applied.