Forensic Dentistry, Second Edition

Forensic dental radiography 189

direct current (DC) or constant potential units. These units are more effi-
cient and provide more high-energy, diagnostically useful photons and cut
exposure times roughly in half. Older units also have difficulty in producing
the extremely short exposure times (usually tenths of a second) required by
digital x-ray sensors, which require significantly less radiation than film. One
very simple but effective method to accomplish this is to cover the opening
of the tube head collimator in an old unit with round sections of rare earth
screen material until the beam is weakened sufficiently to allow longer expo-
sure settings comparable to the unit’s timer capabilities.
X-rays emanate from the target of the x-ray source in a diverging pattern.
Therefore, there is always a varying amount of magnification of the object in
any plane film image. The degree of magnification is determined by the ratio
of the x-ray source-to-object distance and source-to-film distance. The larger
the distance from the source to the image receptor, the less magnification
occurs. Likewise, the closer the object to the receptor, the less the magnifi-
cation and the sharper the image will be. The diverging pattern also affects
the radiation safety involved for the operator. That is because the energy
of the quickly diverging beam will weaken mathematically as a square of
its distance. Therefore, changing the distance of an individual to the x-ray
source from 1 foot to 4 feet reduces the dose or intensity of the radiation to
1/16th of the original dose.^4 This should be considered when attempting to
configure operator safety in the mass disaster morgue.
New technology in the form of handheld generators that are truly pow-
ered by direct current from rechargeable batteries is now in great use in
forensic dentistry (Aribex™ Nomad™) but will be discussed further later in
this chapter and in other chapters in this textbook.

10.3 The Radiographic Image and Image Receptors

Unlike photography, the radiographic image is not created by energy reflected

back to a film but by the transmissive energy of the beam after passing

through an object. Therefore, the total of the external and internal structures

of the object is represented in the image and not simply the surface area. This

is significant in that a radiographic image reveals objects that cannot be per-

ceived with the naked eye. This also means, however, that dental radiographic

images require interpretation by the observer because the image is presented

as a two-dimensional representation of a three-dimensional object.

Radiographic images of the teeth and maxillofacial structures can only

be created due to the fact that the beam of electromagnetic energy is attenu-

ated in varying degrees, depending on the absorption characteristics of dif-

ferent structures through which it passes, and that recording media will react

differently depending on the energy received. Thus, an amalgam restoration