A collimator with a magnetic personality?

We report an unusual technical problem with a gamma camera fitted with a pi nhole collimator. An unexpected decrease of approximately 20 keV in the ene rgy of the Tc-99m photopeak is seen if the (circular) collimator is rotated relative to the camera head. The centreline of the photopeak varied over t he range 118-141 keV depending on the orientation of the collimator. The co unt rate and photopeak width changed concurrently. Image uniformity was sev erely degraded for lower peak energies. The reason for this behaviour is th at the pinhole collimator had become magnetized. The magnetic field was mea sured and had maximum values of 1.0 mT at the edge and 0.1 mT near the came ra face. The magnetic field strongly influenced the response of the photomu ltiplier tubes. We conclude that acceptance testing of gamma cameras should include a check for collimator magnetization. ((C) 2001 Lippincott William s & Wilkins).