In order to build an animal PET system with high sensitivity and resolution, we propose a four-layer PET system which provides the DOI (depth of interaction) information. Each crystal layer has a relative offset of half the crystal pitch with each other and uses an identical type of crystals simplifying readout electronics. The proposed system PET system consists of four LYSO (Lu1.8Y0.2SiO5=Ce) or L0.9GSO (Lu1.8Gd0.2SiO4:Ce) crystal layers with a crystal dimension of 1.5×1.5×5.0 mm3. The animal PET system has a diameter of 84 mm with one Hamamatsu H9500 flat panel PMT ring, and can be upgraded to two PMT ring configuration. An estimated efficiency using GATE simulation is ∼15% at the center of one detector ring system. We present flood images obtained with a 22Na radiation source to verify the concepts of the proposed system, and obtained promising results.