Tongue position errors

The tongue was not positioned in the palate leading to a radiolucency obscuring the apices of maxillary teeth.

Air is a negative contrast substance.  If the tongue is not firmly placed against the palate air is trapped between the tongue and palate.  Radiation passing through this air is not dampened.  Structures in the path of the central ray are over-irradiated.  This causes a "burn-out effect".
This error can be eliminated if the tongue is kept firmly pressed against the palate during exposure.  The tongue acts as a radiation diminishing filter.

Radiograph above re-taken with the tongue in proper position. Have the patient practice the tongue position before placing the patient in the unit.