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We are developing an insert device that will improve image resolution within a smaller field-of-view for clinical whole-body PET scanners. We modified SimSET (Simulation System for Emission Tomography) to simulate the insert and a PET scanner. The system consists of two detector rings. The inner ring represents an insert (r=153 mm) with high-resolution detectors using 10 mm thick LSO. The outer ring represents a PET scanner (R=413 mm) with 25 mm thick LSO. Events were binned into three sets of sinograms assuming a 2.4 and 6.75 mm crystal-pitch for the insert and the PET scanner, respectively. The detectors in the insert are modeled as 1, 2, or 4 layers with different offset configurations to evaluate the corresponding system resolution with the depth-of-interaction (DOI) effect. Results show that image resolution at 1 cm radial offset is improved from 5.6 mm full-width-at-half-maximum (FWHM) of the original PET scanner to 2.0 mm with the insert. At 12 cm offset, the resolution of the original system is 5.9 and 5.5 mm for radial and tangential directions, respectively. With the insert, the radial resolution is 5.0 mm FWHM for a single-layer detector design, but improves to 2.7 and 2.2 mm for 2and 4-layer DOI detectors, respectively. Different offsets for multi-layer detectors have negligible effect on resolution. Sensitivity of the device is, assuming the insert has a 2 cm axial extend, estimated to be 3.3%, including coincidence events from the insert-alone and insert-to-scanner sinograms. In contrast, if the insert is used as a stand-alone microPET scanner, its sensitivity is 1.3%.