the long axis of the tooth being X-rayed. A tube head
with a 16-inch X-ray source to cylinder end distance
(long cone) should be used with the paralleling
technique. The tube head must be positioned so that the
central X-ray beam is projected perpendicular to the
tooth and the film packet. To properly position the film
and the tube head, use paralleling devices.
There are two different paralleling devices. One is
used for radiographs of the anterior teeth; the other is
used for radiographs of the posterior teeth. Each
paralleling device consists of a bite-block, an indicator
rod, and locator ring (fig. 1-10). The bite-block has a
slot and a film backing support to hold the X-ray film
Assembling The Anterior Device
Figure 1-11 shows a fully assembled anterior
paralleling device. Refer to this figure during the
following explanation on assembling the paralleling
Grasp the periapical film packet between the
thumb and first two fingers of your right hand. The
printed surface of the packet should be facing you, and
the side with the raised dot should be in the film
positioning slot of the paralleling device.
Figure 1-11.Assembled anterior paralleling device.
Hold the base of the anterior bite-block between
the thumb and first two fingers of your left hand. Ensure
that the plastic film support is pointed upward and the
film positioning slot is away from you.
Holding the film packet in position, press it
against the plastic support and slide the film down into
the positioning slot. The printed side of the packet
should be facing the plastic support, and the raised dot
should be located toward the positioning slot.
The two prongs of the indicator rod are inserted
into the openings in the bite-block.
Slide the anterior locator ring onto the indicator
rod. Look through the locator ring. If the bite-block and
Figure 1-10.Anterior and posterior paralleling devices.