Sources of γ-camera nonuniformity. (A) Mistuning (or detuning), meaning that photopeak of radionuclide does not coincide with photopeak energy window of camera, perhaps because energy window (as shown) or high voltages of PMTs are not set correctly. (B) Uncoupling of PMT from crystal, resulting in loss of all or part of light signal in resulting air gap between PMT entrance window and crystal. (Courtesy of Dr. Barbara Binkert, New York Presbyterian Hospital, New York, NY.) (C) Cracked crystal, either because of mechanical trauma (impact) or temperature excursion (i.e., temperature increase or decrease at rate faster than 5C per hour, causing crystal to expand or contract, respectively, to point of cracking). Note that it is rate of temperature change that is critical. Photographs on right show cracked crystal that produced corresponding image. Even though cracks are grossly imperceptible, artifacts produced are dramatic. (Courtesy of Dr. Barbara Binkert, New York Presbyterian Hospital, New York, NY.) (D) Corrupted, deleted, or switched-off software correction tables. Even perfectly functioning γ-cameras have some nonuniformity due to point-to-point variations in energy spectra, greater sensitivity at and lower sensitivity between PMTs, and residual nonuniformity due to ill-defined factors such as variations in crystal thickness. Associated nonuniformities are measured and used to create energy, linearity, and uniformity (or sensitivity) correction tables. Note that linearity correction table has biggest effect on uniformity: if corrupted, deleted, or switched off, PMT pattern becomes grossly apparent, and IU approaches 20%. Fortunately, in contrast to uniformity correction table and, to lesser extent, energy correction table, linearity correction table rarely needs to be updated once γ-camera is installed; if updating becomes necessary, it is almost always done by field-service personnel of manufacturer, not by end-user.
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