NaI(Tl) camera crystals: imaging capabilities of hydrated regions on the crystal surface

Sodium iodide (NaI[Tl]) camera crystals may become hydrated. Hydration results in small spots on the crystal surface that at first are white but later turn yellowish. The appearance of these spots--as any artifact--in a diagnostic image is to be avoided, and the crystal should be replaced. The image of the hydrated spots of an aged NaI(Tl) camera crystal has been shown to appear in thallium-201 and xenon-133 flood-field images but not in cobalt-57, technetium-99m, iodine-131, and iodine-125 flood-field tests, giving an apparent energy dependence to the imaging capabilities of the hydrated spots. A simple method is presented, in the energy range represented by the isotopes above, that can be used as a quality-control test for any camera crystal, and an interpretation of the imaging process of the hydrated spots is proposed.