The Siemens Microsystems (Knoxville, TN) MicroPET Focus 220 is dedicated to pre-clinical studies. It has 4 rings of detectors comprised of 12×12 arrays of lutetium oxyorthosilicate (LSO) crystals and acquires 95 slices using all cross-coincidence planes (full 3D acquisition). Gantry aperture is 22 cm, transaxial FOV is 19 cm, and axial FOV is 7.6 cm with 0.8 mm slice spacing. Image resolution is <2.0 mm for all 3 dimensions within the central 5 cm of the FOV. Absolute system sensitivity is 3.4% for the center slice. NEC peaks at 44 kcps with ~6 mCi of activity within a 10-cm phantom (using 6 ns coincidence windows, 350-650 keV).