The purpose of this work is to study the feasibility of using a PEImager scanner in positron emission mammography (PEM). PEM was performed using two opposite block detectors of the PEImager without rotating. One projection image or several focal-plane images were obtained by dual-head imaging, because the number of line of response (LOR) to be reconstructed into tomograms by conventional sinogram-based reconstruction methods is limited. In this work, an iterative algebraic reconstruction was employed to reconstruct projection data to obtain a tomogram. The locations and sizes of hot spots in a breast phantom can be interpreted using such planar tomography, considering the efficiency of detector elements, attenuation and geometric factors. Spheres with diameters of 3 mm, 5 mm, 8 mm and 10 mm were inserted into a cylindrical phantom simulated tumors embedded in the background with different radioactivities, to evaluate the tumor detection capacity capability of tumor detection. For an 80-mm-depth phantom with a 20:1 tumor-to-normal tissue radioactivity ratio, a tumor with a diameter of 3 mm can be detected using planar tomography, while a 5-mm-diameter and 10-mm-diameter tumors can be detected in the ratios 10:1 and 5:1, respectively. The images of planar tomography have better contrast than focal-plane reconstruction, and have greater tumor-detection capacity ability.