The next generation of discoveries in molecular imaging requires positron emission tomography (PET) systems with high spatial resolution and high sensitivity to visualize and quantify low concentrations of molecular probes. The goal of this work is to assemble and explore such a system. We use cadmium zinc telluride (CZT) to achieve high spatial resolution, three-dimensional interaction positioning, and excellent energy resolution. The CZT crystals are arranged in an edge-on configuration with a minimum gap of in a four-sided panel geometry to achieve superior photon sensitivity. The developed CZT detectors and readout electronics were scaled up to complete significant portions of the final PET system. The steering electrode bias and the amplitude of the analog signals for time measurement were optimized to improve performance. The energy resolution (at 511 keV) over 468 channels is full-width-at-half-maximum (FWHM). The spatial resolution is FWHM. The time resolution of six CZT crystals in coincidence with six other CZT crystals is 37 ns. With high energy and spatial resolution and the relatively low random rate for small animal imaging, this system shows promise to be very useful for molecular imaging studies.