The aim of this study is to evaluate a new Positron Emission Tomography (PET) geometry to improve the overall system sensitivity and design a patient/disease specific adjustable field of view (FOV). We proposed a PET system design which was simulated using Geant4 based Architecture for Medicine Oriented Simulations (GAMOS) software. We call this design "sandwich-PET" because the system with the size of 66.2 cm and 198.6 cm on the transaxial and axial direction, respectively, resembles a sandwich when the patient is between the detector heads. The system is intended for whole-body studies, while it can also be used for brain and cardiac studies. The system's transaxial FOV can be adjusted by moving the detector heads in the same or opposite direction, depending on the patient type. It employs lutetium yttrium oxyorthosilicate (LYSO) crystals with 2.76×2.76×18.1 mm pixel sizes. The performance evaluation test was conducted using two simulated line sources placed at the center on the axial and transaxial FOV. The coincidence events were detected with an energy window of 350-650 keV, and a coincidence time window of 20 ns. The result showed that the system's sensitivity increases exponentially when varying the transaxial FOV from 70 cm (regular openings for conventional PET scanners for all type of patients: slim, overweight, or obese) down to 20 cm (for slim or pediatric patients). This study shows that the system is capable of achieving much higher sensitivity comparing to the conventional whole-body scanners.