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Hot or cold spots on uniformity flood fields can be caused from improper gain, peak or window adjustments to the affected photomultipler tube or surrounding tubes. Poor coupling of the photomultipler tubes to the crystal can also cause this artifact.
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Measles is caused by the loss of the hermetic seal on the crystal. Moisture enters and migrates across the crystal until it is trapped by some imperfection in the crystal surface. As more moisture enters, the imperfection increases in size because the moisture dissolves the crystal. This process continues until it becomes apparent in an off peak flood and eventually a corrected flood and clinical studies. | |||
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