RAD 350 Chapter 7 X-ray Production
Primary function of the 3 important components of an x-ray unit (console, tube and high
voltage generator) is to accelerate electrons/projectile electrons across the x-ray tube from
cathode to anode (about 1 cm) and then suddenly stop, creating x-ray photons of varied
energies.
REMEMBER: 99% OF THE ELECTRICAL ENERGY IS DISSIPATED IN HEAT!
Kinetic energy is the energy of motion
mA is the source for the abundance of electrons to be accelerated across the tube
kVp is the controlling item for the amount of EMF (positive POTENTIAL energy) on
the anode of the tube
As kVp INCREASES, the efficiency of the x-ray tube to produce x-ray photons
INCREASES slightly
Only TWO processes convert the rapidly accelerated electrons into x-ray photons:
Characteristic Radiation occurs when an accelerated (projectile) electron “ejects” an orbital
electron from the tungsten target. The K-shell binding energy of tungsten is 69 kEv. Therefore,
if a K-shell electron was ejected (binding energy of 69 kEv) and an outer, more LOSELY bound
electron (say from the “P-shell” where it’s binding energy is only slightly more than .01 kEv)
electron “falls” into the void in the K-shell. The outer electron must “give up” some of it’s own
“on board energy” and let it be pulled into the K-shell void. THE DIFFERENCE IN BINDING
ENERGY IS THE ENERGY “GIVEN OFF” IN THE FORM OF AN X-RAY PHOTON. The energy “given
off” will be the difference in BINDING ENERGIES of the two orbital shell’s binding energy.
Characteristic x-rays can come from ANY shell, but only photons from the K-shell are useful.
Bremstrahlung (BREMS) or “braking radiation” is produced when a fast moving electron (one
negative charge) is affected by the SUM of positive charges contained in the nucleus of a
target (tungsten) atom. Since the nucleus has much MORE charge and the charge is positive,
the single negatively charged electron changes it’s course (vector) due to the opposite charge
and higher number of positive charges. This causes the “projectile electron” to slow down and
change direction, giving off an x-ray photon as it loses kinetic energy by “braking” or slowing
down.
As the kVp increases, the x-ray emission spectrum is “skewed” to the right of the emission
curve. As mA is increased there is NO CHANGE IN THE SHAPE OF THE CURVE – BUT the curve
is higher. If one doubles the mAs, the curve is TWICE as high.
Remember: As kVp goes up, wavelength is SHORTENED!
MOST PHOTONS IN THE DIAGNOSTIC IMAGING RANGE OF ENERGY ARE FROM BREMS!
The total number and energy of all the x-ray photons emitted is called the x-ray emission
spectrum. Since there are MANY energies, it is called a POLYCHROMATIC SPECTRUM!
Since the polychromatic beam has all energies ranging from the kVp set (if you set 80 kVp,
there will be at least one x-ray photon with an energy of 80 kEv and many more photons –
based upon mAs – at lower energy levels).
ADDED FILTRATION is used to absorb/ATTENUATE the LOWER ENERGY, NON-USEFULL
PHOTONS, thus saving the patient from receiving useless x-rays. This action INCREASES the
“average hardness of the beam” (by eliminating the LOWER ENERGY x-rays, the remaining xrays have a HIGHER AVERAGE ENERGY). This is referred to as “hardening the beam” – Added
filtration is usually made of aluminum because the CHARACTERISTIC RADIATION given off in
aluminum can be absorbed/dissipated in the air PRIOR to reaching the patient.