The strength of the high voltage determines the energy of the x-rays. Higher
voltages result in higher energy photons, which are more penetrating. This is
why in general radiography, higher energy x-rays are used for larger patients.
The high voltage is expressed in units of kilovolt peak (kVp). In dental
radiography, typical values for high voltage are between 70 and 90 kVp.
The number of electrons traveling between the cathode and anode is known as
the tube current. As the tube current increases, more electrons are slammed into
the target, and more x-ray photons are produced. The tube current is expressed
in units of milliampere (mA). Typical values of tube current are between 3 and
One milliampere represents a very large number of electrons. In fact, if the
current in an x-ray tube is 1 mA, then every millisecond, 6.3 trillion electrons
collide with the target.
Many modern dental intra-oral x-ray systems do not allow selection of kVp and
mA. To make a change in the appearance of an x-ray image, one must alter the
other variable that is present on these machines: time.
The duration of the x-ray exposure is determined by the timer setting of the
machine. A longer timer setting will result in a darker image on the film. Timer
settings are usually in units of seconds (s). On older equipment, the timer
values may be expressed in units of pulses. 60 pulses correspond to one second.
Therefore, 6 pulses = 6/60 second = 1/10 second.
It is the total current that determines how many x-ray photons are produced
in a given exposure, and how dark the film will become for a given kVp. The
total current is expressed as the product of tube current (mA) and time (s). It
is represented as mAs.