Rectangular collimation of the x-ray beam shall be
routinely used for periapical radiography. Each
dimension of the beam, measured in the plane of the
image receptor, should not exceed the dimension of
the image receptor by more than two percent of the
source-to-image receptor distance. Similar collimation
should be used, when feasible, for interproximal
(bitewing) radiography. Anatomy or the inability of
occasional specific patients to cooperate, including
some children, may make rectangular collimation
and beam-receptor alignment awkward or impossible
for some projections. The requirement may be
relaxed in these rare cases.