Forensic Dentistry, Second Edition

188 Forensic dentistry

cathode rays produced florescence and heat within and without the tube.
Without doubt, Crookes and Hertz were unknowingly producing x-rays at
the time. However, Röentgen did, in fact, discover that other invisible rays
emanating from the device possessed the ability to penetrate solid objects
and produce photographic shadows of flesh and bones. “This discovery,
marking as it did a distinct epoch in the Science of Medicine, was received
by the world with incredulity and amazement, for its reported possibilities
savored almost of the occult.”^1
Radiographs of nondiagnostic quality were made of teeth within days of
Röentgen’s discovery, but the first practical and routine use of dental radiog-
raphy in a dental practice is attributed to C.E. Kells of New Orleans.^2 The first
published use of dental radiography as a means of achieving a human iden-
tification was in 1943.^3 Today it remains as the leading and most reliable tool
for human identification through dental findings. When there is a conflict
between the written dental record and antemortem radiographs of a subject,
deference is given to the radiographs as the gold standard having less potential
for human error than charted dental information. This chapter on the basic
theory of dental radiography is presented at a level such that the principles
pertinent to the topics and themes most important to forensic dental investi-
gations are emphasized.

10.2 Dental X-ray Generators

X-ray photons are generated when electrons in an evacuated tube are sub-

jected to an extremely high voltage (kV) potential and bombard a tungsten

target at a high rate of speed. Thus, electrical energy is converted to kinetic

energy, which is then converted to electromagnetic energy. Only 1% or less

of the bombarding electron energy is converted into x-radiation, with the

remaining energy resulting in a very large gain of heat, which is the greatest

cause of tube failure. This fact accounts for the absolute need to follow the

manufacturer’s recommended duty cycle by waiting the specified amount of

time between exposures. The setting of a mass disaster morgue is more likely

to destroy a tube head than working on typical dental patients, where the

time for placing films after each exposure helps protect the duty cycle.

The resulting x-ray beam is comprised of millions of photons of vary-

ing energy (wavelengths) and is referred to as having a continuous or poly-

chromatic spectrum. Older x-ray units produce even more variation in the

uniformity of the beam as the alternating current rises and falls. Photons in

these units are also produced only when the alternating current (AC) flows

in one of two directions. Newer x-ray generators remedy this by utilizing

sophisticated electronics to rectify the AC so that the device functions at a

constant and continuous kilovolt (kV) potential and are often described as