We have designed and built a system to simulate emission computed tomography (ECT) devices and measure physical parameters of the component parts in such systems. It consists of a heavy duty X, Y, Z, ? scanning table under computer control through CAMAC with modular NIM electronics for signal processing. The modular approach allows ready reconfigurations of detectors and lead shielding to test new design concepts. Direct computer control allows simulation of complex scanning motions and detector geometries by motion of the object rather than the detectors as well as ready capability in data analysis. A study is presented in which the simulator shows a significant increase in signal to noise ratio in a commercial tomograph by small changes in lead shield geometry.