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particularly important. It must be remembered that a radiograph is
only a two dimensional representation of a three-dimensional object
and although roots may appear to have mesial or distal curves, they
are also likely to curve in buccal or lingual directions. Magnification
aids will assist interpretation of the radiograph.
Working length estimation – ‘diagnostic’ radiographs
These allow the clinician to determine the working length of the tooth,
that is, the distance between a fixed coronal reference point and the
apical limit of instrumentation. This must be determined for every
canal in the tooth to be treated. A combination of knowledge of the
expected working length and tactile sensation will assist in initial
placement of the diagnostic instrument.
When multiple canals in a single tooth or a multi-rooted tooth 
are being considered, more than one periapical radiograph may be
necessary. In some instances it is possible to take a single radiograph
with instruments in different canals. If it is suspected that there are
two canals in the same buccolingual plane, these may be disting-
uished by exposing the film with a mesial and/or a distal angulation
on the X-ray tube.
With experience, measurements may be taken from radiographic
images although allowances must be made for the inherent linear 
distortion and magnification present in all radiography. If the apex 
of the tooth and/or the end of the instrument is not visible then the
radiograph is of no diagnostic value.
During diagnostic radiography for working length estimation, 
it is customary to remove the rubber dam frame to facilitate film 
placement. The rubber dam and clamp should remain in place during
radiography.
Master cone radiographs
Although every care is taken during calculation of the working
length, errors can, and do, occur. As a final check before root filling/
obturation, a dry run, or ‘cone-fit’, radiograph may be taken. A 
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gutta-percha cone is positioned at the full working length and a 
radiograph taken to ensure that it is in the anticipated place. This
radiograph also allows the shape of the preparation to be assessed.
Any errors in the preparation or position of the gutta-percha can 
be corrected before final obturation. Some clinicians also advocate 
a radiograph part way through obturation, called a ‘mid-fill’ radio-
graph, the rationale being that the first cone of gutta-percha may
move in the initial stages of obturation.