SPECT imaging systems are used in nuclear medicine for diagnostic imaging of heart, brain, lung and other organ functions. Radiopharmaceuticals are injected in the blood stream of a patient. The propagation of the radio-pharmaceutical is monitored as a function of position and time using a gamma camera. In many tests, the monitoring occurs in real time. This technique has been extended for use in chemical process monitoring in our laboratory. In particular, a Siemens gamma camera with SPECT capabilities is routinely used for real time tracer studies, radioactive particle tracking and SPECT imaging. The same radio-pharmaceutical used in medical testing are used for process monitoring. The system has been used in determining hydrodynamic properties of gas/solid and liquid/solid fluidized beds and in visualizing and monitoring of multi-phase flow phenomena in porous media. (C)2000 Elsevier Science S.A. All rights reserved.