A flood image performed without the collimator (intrinsic) reveals a subtle semicircular region of nonuniform decreased activity along the upper edge of the camera head (circle).

Differential Diagnosis

Areas of nonuniformity on a flood image can be due to an abnormality in the collimator, crystal, or electronics of the camera.

•  Faulty photomultiplier tube: In this case, the area of nonuniformity has a half-hexagonal configuration and thus follows the pattern of photomultiplier tubes, which are hexagonal in cross-section.

•  Damaged collimator: Although this may cause a similar appearance, this is an intrinsic flood, which is performed with the collimator off.