Technique Factors

back arrow       forward arrow

kVp

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.

mA

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 10 mA.

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.

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.

Total current

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.


back arrow       forward arrow