Siemens Medical Solutions and Gamma Medica Inc., of Northridge, Calif., announced they have signed an agreement that gives Siemens exclusive US marketing rights for Gamma Medica's X-SPECT molecular imaging product. X-SPECT, Gamma Medica's second-generation MicroSPECT system, combines single photon emission computed tomography (SPECT) with anatomical imaging via x-ray computed tomography. It is designed for in-vivo imaging of mice and other small animals in the laboratory setting, primarily in the study of diseases and for drug development. Siemens Medical Solutions is part of Siemens AG, based in Malvern, Penn., and Erlangen, Germany.   . . .   L-3 Communications has appointed Adrienne L. Stevens as president of its recently acquired avionics systems division, which has facilities in Michigan, Ohio and Florida.   . . .   US Navy Rear Admiral (Ret.) Thomas Paulsen has been appointed to the board of eMagin Corp., a developer of organic light emitting diode-on-silicon technology that combines integrated circuits, microdisplays and optics to create a virtual image.   . . .   Eutimio "Lou" Sena has been appointed president and CEO of Cell Robotics International Inc., a medical laser products company based in Albuquerque, N.M.